Geophysical monitoring of the injection and postclosure phases at the Ketzin pilot site

CHAPTER

Geophysical monitoring of the injection and postclosure phases at the Ketzin pilot site

6.2

Stefan Lu¨th1, Jan Henninges1, Monika Ivandic2, Christopher Juhlin2, Thomas Kempka1, Ben Norden1, Dennis Rippe1 and Cornelia Schmidt-Hattenberger1 1

German Research Centre for Geosciences, Potsdam, Germany Department of Earth Sciences, Uppsala University, Uppsala, Sweden

2

Chapter Outline 6.2.1 The Ketzin pilot site—site infrastructure, CO2 injection, closure and postclosure operation....................................................................................524 6.2.1.1 CO2 injection ...........................................................................525 6.2.1.2 CO2 back-production ................................................................526 6.2.1.3 Brine injection .........................................................................527 6.2.2 Site characterization—site geology and reservoir model .................................528 6.2.2.1 General setting.........................................................................528 6.2.2.2 The reservoir model of the Ketzin site.........................................531 6.2.3 Geophysical monitoring .................................................................................531 6.2.3.1 Introduction .............................................................................531 6.2.3.2 Well logging and permanent monitoring ......................................532 6.2.3.3 Seismic monitoring...................................................................537 6.2.3.4 Geoelectric monitoring ..............................................................545 6.2.3.5 Ketzin monitoring system design and deployment........................ 546 6.2.3.6 Data acquisition, processing, and inversion.................................546 6.2.3.7 Key results from crosshole and surface-downhole measurements..........................................................................548 6.2.3.8 Lessons learned from geoelectric monitoring...............................550 6.2.4 Numerical simulations of multiphase flow.......................................................550 6.2.5 Conclusion ...................................................................................................555 References .............................................................................................................555

Active Geophysical Monitoring. DOI: https://doi.org/10.1016/B978-0-08-102684-7.00025-X © 2020 Elsevier Ltd. All rights reserved.

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6.2.1 The Ketzin pilot site—site infrastructure, CO2 injection, closure and postclosure operation Even though large-scale CO2 storage has been demonstrated at several sites for already more than two decades, pilot-scale. storage sites are important cornerstones for establishing geological CO2 storage since they facilitate the development and assessment of specifically designed monitoring and verification approaches. To this end, the Ketzin pilot site for CO2 storage was developed and operated within a 13year period, covering all life-cycle phases of a storage site. The Ketzin project was initiated in 2004, as the first European onshore storage project, with the first site characterization and risk assessments, as well as the first field tests for geophysical surveys, being performed at this time (Wu¨rdemann et al., 2010). The pilot site is located near the town of Ketzin/Havel, B25 km west of Berlin (Germany). On the site, seasonal storage of natural gas had been performed for 30 years in aquifer formations at 250
400 m depths. The development of the pilot-site infrastructure started in 2007. The site infrastructure included these main elements:

• Injection well (CO2 Ktzi 201/2007), monitoring wells (CO2 Ktzi 200/2007, CO2 Ktzi 202/2007, Hy Ktzi P300/2011, CO2 Ktzi 203/2012);

• Storage tanks and injection facility at the surface; • Monitoring portakabin and surface and downhole installations for reservoir monitoring. The first three of the injection and monitoring wells were drilled in 2007, reaching maximum depths of between 750 and 800 m. These three wells were designed with the same casing layout, including stainless production casings equipped with preperforated sand filters in the reservoir section and wired on the outside with a fiber-optical cable, a multiconductor copper cable, and a polyurethane (PU) heating cable to surface. The reservoir casing section was externally coated with a fiberglass resin wrap for electrical insulation (Prevedel et al., 2008). The wells were designed as important components of the monitoring concept and they were equipped with various geophysical sensors in order to facilitate continuous or repeated active and passive geophysical monitoring of the storage reservoir. These components were installed in the boreholes:

• Fiber-optic-sensor cable loop for distributed temperature sensing (DTS, all wells);

• Two-line electrical heater cable (Ktzi 201/2007, Ktzi 202/2007); • Vertical electrical resistivity array (VERA) consisting of 15 ring-shaped stainless-steel electrodes, 15-line surface connection cable (all wells);

• Fiber-optic pressure/temperature (P/T) sensor; • Fiber-optic surface connection cable (at injection string only). In 2011 and 2012, two further monitoring wells were drilled. The well Hy Ktzi P300/2011 was drilled as a shallow observation well, reaching 446 m depth

6.2.1 The Ketzin pilot site—site infrastructure

(Martens et al., 2013). This well was drilled into the first aquifer above the Stuttgart formation and its main purpose was above-zone monitoring of the storage site. This above-zone monitoring concentrated on pressure sensing and geochemical monitoring using a U-tube system for fluid and gas sampling in order to detect any potential leakage through the first caprock. The well CO2 Ktzi 203/2012 was drilled in 2012, reaching a maximum depth of 700 m, penetrating the storage formation at 630 m depth (Prevedel et al., 2014). This well was also equipped with a fiber-optic-sensor cable loop for DTS and a heater cable, as well as with two fiber-optic P/T sensors at 305 and 610 m depths, respectively. It penetrated the Stuttgart formation after more than 4 years of CO2 injection and one of its main tasks was to facilitate retrieval core samples from the Stuttgart sandstone after 4 years of CO2 contact.

6.2.1.1 CO2 injection CO2 injection at Ketzin lasted from June 30, 2008, through August 29, 2013 (Martens et al., 2014). During this period, a total amount of 67 kt of CO2 was injected into a saline aquifer (Upper Triassic sandstone layers of the Stuttgart Formation) at a depth of 630
650 m. The CO2 used was mainly of food-grade quality (purity . 99.9%). In May and June 2011, 1.5 kt of CO2 captured from the Vattenfall oxyfuel pilot plant Schwarze Pumpe (power plant CO2 with purity .99.7%) was injected. The injection history (cumulative mass of CO2 injected) and related reservoir pressure during the injection and postinjection phases are shown in Fig. 6.2.1. In July and August 2013, a coinjection experiment with

FIGURE 6.2.1 Pressure in the injection well and cumulative mass of CO2 injected.

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CHAPTER 6.2 Geophysical monitoring

combined CO2 and N2 injection was performed to test and demonstrate the technical feasibility of a continuous impure CO2 injection scenario. A total of 613 t of CO2 and 32 t of N2 were continuously mixed on site and injected, resulting in an average CO2 to N2 mass ratio of approximately 95 to 5. The CO2 was delivered in liquid state by road tankers to the Ketzin pilot site and stored in two intermediate storage tanks at about 218 C and 18 bar on site. Prior to injection, the CO2 was preconditioned: plunger pumps raised the pressure to the necessary injection pressure and CO2 was preheated to 45 C by ambient air heaters and an electrical heater in order to avoid liquid
vapor phase transition of the injected CO2 and associated pressure build-up within the reservoir. The CO2 was transported via a pipeline to the well Ktzi 201. Typical injection rates ranged between 1400 and 3250 kg CO2/h, with a maximum monthly injection rate of 2300 t. Within a so-called “cold-injection” experiment between March and July 2013, the injection temperature was stepwise reduced from 45 C down to 10 C to study the thermodynamics in the wellbore for an injection at ambient temperatures and its impact on the reservoir (Mo¨ller et al., 2014). A total amount of 3 kt of CO2 was injected throughout this experiment where monitoring included injection wellhead and downhole pressure, temperature point information, and profiles. Down to an injection temperature of 25 C the entire injection process continued as single-phase with gaseous CO2. At an injection temperature of 20 C, the CO2 started to condense liquid CO2 droplets and the injection process proceeded under two-phase conditions in the surface installations and the upper parts of the injection well Ktzi 201. Due to increasing temperature with increasing depth, these liquid CO2 droplets re-evaporated in the injection well and the lower part of the well was again single-phase. Decreasing the injection temperature to 15 C and finally 10 C resulted in two-phase conditions throughout the entire injection process and pressure fluctuations within the wellbore Ktzi 201. Corresponding pressure fluctuations at the nearby well Ktzi 200 located 50 m from the injection point could not be observed. The injection facility was finally dismantled in December 2013.

6.2.1.2 CO2 back-production From the 15th to 27th October 2014, a part of the formerly injected CO2 was retrieved from the reservoir via the former injection well (Ktzi 201) and vented to the atmosphere (back-production experiment). A total amount of 240 t of CO2 and 55 m3 of brine were safely back-produced from the CO2 storage reservoir. This field experiment addressed the following three main questions (Martens et al., 2015): 1. How do the reservoir and the wellbore behave during back-production of CO2? 2. What is the composition of the retrieved gas? 3. How much is the atmospheric gas composition and distribution?

6.2.1 The Ketzin pilot site—site infrastructure

Electrical resistivity tomography (ERT) monitoring shows that the geoelectric array at the production well was capable of tracking the back-production process, for example, the backflow of brine into the parts formerly filled with CO2. Preliminary results also show that the back-produced CO2 at Ketzin had a purity of .97%. The secondary component in the CO2 stream was N2, probably originating from former field tests. The results are expected to help to verify geochemical laboratory experiments which are typically performed in simplified synthetic systems.

6.2.1.3 Brine injection Prior to the injection of CO2, the sandstones in the reservoir were filled with brine with a salinity of about 240 g/L NaCl equivalent (dominantly NaCl and subordinate CaCl2; Zimmer et al., 2011). This brine had been displaced partially by the injected CO2. With the brine injection experiment, carried out roughly 2 years after the end of the CO2 injection phase, the area between the wellbores Ktzi 201 (injector and observation well) and Ktzi 200 (observation well) was intended to be partially refilled with brine (Mo¨ller et al., 2016). This process was monitored using a geoelectrical system of vertically installed electrodes within these wells (VERA; Schmidt-Hattenberger et al., 2014). With VERA, the electrical resistivity distribution in the observation area and its changes during the different operational phases and field tests were determined and evaluated. The test aimed at: 1. Quantification of the residual gas saturation: After the injection of CO2 into the saline aquifer, brine was reinjected into the same horizon. It partially displaced the gas and part of the CO2 remained within the pore space (residual gas saturation). The brine injection stimulated the natural back flow of brine into the formation in time-lapse mode and was monitored using the VERA system. Thus it was possible to compare the results from this monitoring phase before and during CO2 injection (displacement of brine by CO2) with those during and after the brine injection (displacement of CO2 by brine). 2. Evaluating brine injection as a means for leakage remediation: The test was also intended to give an insight on the feasibility of using brine injection as a means for leakage remediation. Upon completion of the test, pressure within the well was further monitored. Thus it was possible to determine the timespan until pressure rebuild within the well occurred. Injection of brine (or water) may be a cost-effective measure to “kill” (i.e., turn it into a neutral pressure state) and shield a leaking well quickly for a certain timespan to allow for a work-over. The test was carried out between October 12, 2015 and January 6, 2016. The well (Ktzi 201) was killed with a plunger pump within B20 minutes at about 250 L/min with a total mass of approximately 4.7 t of brine. Subsequently, a rotary pump was used to continuously inject brine. During the first 48 hours of

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CHAPTER 6.2 Geophysical monitoring

injection, a high injection rate ( . 2.5 t/h) was targeted to avoid possible backflow of the CO2 into the vicinity of the well. From October 14, the target injection rate was reduced to B1.4 t/h. The pressure in the formation gradually increased during the experiment (see Fig. 6.2.1). Maximum allowed reservoir pressure for the Ketzin site was 85 bar. To leave a safety margin, it was then decided to switch the injection regime to be pressure driven. From December 12 onwards, the operations crew adjusted the injection rate continuously so that the formation pressure was stabilized at around 81 bar. Continuous injection ceased on January 6 and the remaining 1.8 t of brine left in the surface tanks and in the hoses was pumped discontinuously into the well in the following days during the dismounting of the surface facilities.

6.2.2 Site characterization—site geology and reservoir model 6.2.2.1 General setting The Ketzin pilot site is located within the Central European Basin System, which formed during the latest Carboniferous to earliest Permian times (Fig. 6.2.2, inset map). The Northeast German Basin constitutes a subbasin of this system, comprising a sedimentary thickness of up to 6500 m that contains Permian to Quaternary deposits (Hoth et al., 1993). At Ketzin, the sedimentary succession exhibits a thickness of about 4000 m, including different reservoir rocks and potential storage formations. A striking feature of sedimentary Permian basins, and of the study area, is the formation of salt structures due to diapirism of Permian (Zechstein) salt. At the Ketzin site, this diapirism caused the deformation of Triassic and Lower Jurassic formations generating a gently dipping, east-northeast (ENE)-west-south-west (WSW) striking double anticline (“Roskow
Ketzin anticline,” Fig. 6.2.2). The transgressive sediments of the Tertiary Oligocene form the first formation unaffected by anticlinal uplift (Fo¨rster et al., 2006). The Tertiary deposits are in turn overlain by unconsolidated Quaternary sediments. Although deeper aquifers may also be suitable as potential CO2 storage formations in Ketzin, based on operational and funding motives a sandstone reservoir of the Middle Keuper (Upper Triassic) section was chosen, located in the Stuttgart Formation at depths of 625
700 m. This formation is overlain by the Weser Formation, which acts as the immediate caprock of the reservoir. Further up in the sequence, the Jurassic age Sinemurian/Hettangian reservoir sandstones, situated at depths of 250
400 m, had been used as a storage facility for coal gas and natural gas for about 30 years at the project site. From this former exploration period of the 1960s, a huge amount of geological information concerning the general site structure and the uppermost hundreds of meters was available. These data were invaluable for the site characterization and risk analysis and for the development of risk mitigation concepts for the Ketzin site. For example, in order

6.2.2 Site characterization—site geology and reservoir model

FIGURE 6.2.2 Location of the Roskow
Ketzin anticlinal structure in the Northeast German Basin (NEGB), highlighted by the isolines (meters below ground level) of the strongest seismic reflector of the Triassic (“K2 horizon,” uppermost Weser Formation). Shown are the location of the Ketzin storage site (star), the extension of the 3D seismic data (dotted lines), and the reservoir model domain size (black square). For geographic orientation, main waters and the location of the villages Roskow and Ketzin are given. Coordinate system: UTM WGS 1984, Zone 33. Adjacent Permian subbasins are denoted (PT: Polish Trough, NWGB: Northwest German Basin) in the inset map. Modified from Norden, B., Frykman, P., 2013. Geological modelling of the Triassic Stuttgart Formation at the Ketzin CO2 storage site, Germany. Int. J. Greenhouse Gas Control 19, 756
774.

to establish a meaningful near-surface monitoring program, the shallow groundwater flow system needed to be addressed (Norden, 2011). Based on the drilling data (Fig. 6.2.3) and the 3D baseline seismic campaign of the Ketzin project (Juhlin et al., 2007), different structural models of the Ketzin site were developed. The models were set-up according to the focus of investigation, addressing the near- and far-field reservoir behavior, as well as to study geomechanical (stress and fault reactivation) aspects (Norden et al., 2013). For this review, we focus on the reservoir formation: the Middle Triassic Stuttgart formation (Norden and Frykman, 2013). Geologically, the storage formation is embedded in formations which are dominated by continental playa-type sediments (i.e., the Grabfeld Formation below the Stuttgart Formation and the Weser and Arnstadt Formations above the reservoir zone). Only the Stuttgart Formation indicates a fluvial environment (Beutler et al., 1999), represented by floodplain siltstones and mudstones and embedded channel sandstones. The lateral extent of the channel belts, formed by amalgamation of individual fluvial channels, is highly variable. Basin-wide, the Stuttgart

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FIGURE 6.2.3 Drilled lithological profiles and casing design (gray: cement; blue: water/uncemented; white small hatches denote slotted casing intervals with connection to the reservoir sandstone) for the CO2 Ktzi boreholes. Inset base map shows the drilling locations (distance between lines: 25 m).

Formation is on average only 20
100 m thick (Beutler and Tessin, 2005) and consists of immature sandstones with a basin-wide homogeneous grain-size, indications for rapid transport and deposition (Aigner and Bachmann, 1992). At Ketzin, all drilled boreholes show fine- to medium-grained sandy, silty, and clayey sediments of variable thickness in the lower and middle parts of the Stuttgart formation and thicker sandstone units toward the top of the section (Fig. 6.2.3). The thickest sandstone units (9
20 m) represent the typical channel facies and provide the CO2 storage section. The thinner (dm- to m-thick) sandstone layers below these channel deposits, interbedded with thin mudstone and coal layers, are considered as overbank (floodplain) facies (Fo¨rster et al., 2010).

6.2.3 Geophysical monitoring

6.2.2.2 The reservoir model of the Ketzin site The seismic data enabled the mapping of a reflector near the base of the Stuttgart (K3 horizon, Juhlin et al., 2007) and the expected Top Stuttgart horizon, which was picked based on the drilling results of the Ketzin boreholes. In addition, a fault zone at the top of the anticline structure, about 1.5 km north of the CO2 injection site, could be confirmed. The WSW-ENE trending fault zone with throws in the order of up to 30 m (the Central Graben Fault Zone; Juhlin et al., 2007) is controlled by a series of discrete normal faults. The faults seem to die out quickly in the Tertiary Rupelian clay (Yordkayhun et al., 2009a,b). Other small-scale faults with a throw of 1.5
3.0 m (Juhlin et al., 2007) may be present, but were not detected in the vicinity of the injection site. Thus, the depth and general geometry of the reservoir formation, which is about 72 m thick, could be derived by the available exploration data. However, the detailed internal geometry of the Stuttgart Formation is below the seismic resolution: seismic analysis gave some indications for possible pathways of sand-filled channels within the muddy siltstones and mudstones of the floodplain facies, but did not allow a deterministic specification of the effective geometry. Therefore, the reservoir model of Ketzin was established using a stochastic approach. First, the facies distribution was modeled taking into account the general observed regional trends within the Stuttgart Formation (i.e., the net-to-gross ratio of sandy channel facies to muddy floodplain facies, the mean channel direction, and data on the expected channel sinuosity) as well as the site-specific data, obtained by the Ketzin drillings and the interpretation of 3D and 4D seismic data (location and thickness of reservoir sandstones, etc.). Based on the realized facies geometry, the distribution of petrophysical properties within the facies was modeled using a sequential Gaussian simulation and allocated co-Kriging algorithms. More details on lithology and mineralogy, the depositional system, available data, and the setup of the primary geological model are presented in Fo¨rster et al. (2010) and Norden and Frykman (2013). The initial reservoir model was continuously updated when new exploration results, operational data, or reservoir simulation results were available, helping to understand the monitoring of the CO2 injection and the reservoir behavior (see later).

6.2.3 Geophysical monitoring 6.2.3.1 Introduction A comprehensive monitoring program is an important prerequisite to guarantee safe and reliable operation of a CO2 storage site for demonstrating the containment of the CO2 and the conformance of predicted and observed reservoir behavior (IPCC, 2005; Jenkins et al., 2015). Geochemical and geophysical tools were deployed to monitor the CO2 migration in the reservoir and detect potential CO2

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leakage. At the Ketzin pilot site, a comprehensive monitoring program was established involving geophysical, geochemical, and microbial investigations. In this chapter, we focus on the geophysical methods, which were deployed in order to provide a wide range of spatial and temporal observational scales. Well logging and seismic, as well as geoelectric, crosshole measurements (ERT) provide the highest spatial resolution for the detected physical parameters (cm-scale for well logging, dm- to m-scale from crosshole geophysics). Combined surface-downhole (SD) measurements [ERT and seismic VSP (vertical seismic profiling) and MSP (moving source profiling) surveys] imaged the reservoir section around the injection and monitoring boreholes, and 4D seismic surveys provided images of the complete storage complex. In the following, we will focus on those geophysical observations which provided stable signatures of the CO2 reservoir processes, namely well logging (pulsed neutron gamma, PNG), 4D seismic surveying, and crosshole and surface ERT.

6.2.3.2 Well logging and permanent monitoring A combination of different geophysical borehole measurement techniques were applied in order to monitor the well conditions and the migration of the injected CO2 in the formation close to the boreholes. These measurements enabled derivation of detailed information about in situ conditions at borehole-scale and provided reference points for the seismic and geoelectric methods applied at Ketzin at larger scales.

6.2.3.2.1 Permanent sensor cables All four wells at Ketzin were equipped with permanent fiber-optic downhole sensor cables, which were installed in the annulus outside the borehole casing (Prevedel et al., 2008, 2014). The innermost casing string extending over the injection interval was equipped with preinstalled filter screens, and a staged cementation program with a succession of cemented and fluid-filled annuli along the well profiles was applied (see well sketch, Fig. 6.2.3). Using the permanent sensor cables, the evolution of temperature was continuously monitored over the entire length of the boreholes with DTS. After filling up with CO2, significant temperature anomalies were observed in the observation wells. Over large parts, pressure and temperature in the wells were controlled by two-phase conditions and a heat-pipe process, including evaporation and condensation of CO2 (Henninges et al., 2011). Within the two-phase zones, characteristic temperature gradients were established and the borehole temperatures deviated up to several degrees centigrade from the undisturbed formation temperatures. These processes also controlled the phase distribution and the related pressure and density profiles within the fluid column along the boreholes (Loizzo et al., 2013), which had important implications on the operational monitoring, for example, of the reservoir pressure (Liebscher et al., 2013), or application of borehole corrections during the evaluation of wireline logging data.

6.2.3 Geophysical monitoring

As a new tool for monitoring of saturation changes, the heat-pulse method was tested at Ketzin. The formation thermal conductivity was determined based on temperature changes under the influence of a controlled heat source (Freifeld et al., 2008). For the required heating, electrical conductors were included in the downhole sensor cables and temperature changes along the boreholes were measured using the DTS. By evaluating early- and late-time data, thermal conductivities of both the completed well (Prevedel et al., 2014) and the surrounding rock (Freifeld et al., 2009) could be determined, respectively. The results nevertheless showed a high sensitivity against external thermal influences, implying that further development of the method is required for the application. This can be addressed in future pilot or demonstration projects.

6.2.3.2.2 Pulsed neutron-gamma wireline logging For monitoring of in situ saturation changes at borehole-scale the PNG logging technique was applied at Ketzin (Baumann et al., 2014). The CO2 saturations derived from the PNG logs were used as input for CO2 mass estimations from 3D seismic data (Huang et al., 2018; Ivandic et al., 2015; Ivanova et al., 2012) and ERT evaluation (Bergmann et al., 2012). Therefore the PNG method and results are described in more detail in the following sections. PNG logging is frequently used for saturation evaluation in oil and gas fields (Smolen, 1996), and has also been applied successfully for monitoring at other test sites for CO2 injection in saline aquifers (Dance and Paterson, 2016; Mu¨ller et al., 2007). PNG tools radiometrically measure the macroscopic capture crosssection Σ (Plasek et al., 1995). The formation Σ value is equal to the volumeweighted average of the Σ values of the matrix components and the fluids filling the pore space. In time-lapse mode, changes to saturation S can be calculated from the Σ change between baseline and repeat logging runs, and the Σ difference of the involved pore fluids alone (Ellis and Singer, 2007): Sw;base 2 Sw;log 5

Σbase 2 Σlog φðΣw 2 Σg Þ

(6.2.1)

where the subscripts log and base refer to the repeat and baseline logging runs, respectively, and φ is formation porosity. For the current application, the subscripts w and g correspond to the considered pore fluids brine and CO2, and the CO2 saturation is equal to the change in brine saturation between the baseline and repeat logging runs, Sw;base 2 Sw;log . Favorable conditions for the application of the PNG method existed at Ketzin because of the high formation water salinity of 220 g/L and the high contrast in Σ between this saline formation water and the injected CO2, as well as the high formation porosity of about 20%
30%. As pore-fluid parameters, a Σw value of 97.6 capture units (cu), calculated based on the chemical composition of the formation brine from the Ketzin site, and a Σg value of 0.014 cu, for CO2 under reservoir conditions of 35 C and 75 bar, were used. Total porosity was derived from available open-hole logging data (Norden et al., 2010).

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PNG logs were acquired for the baseline in 2008 before the start of injection and for up to 11 repeat logging runs acquired in the different wells at about yearly intervals during and after CO2 injection using the reservoir saturation tool (RST) from Schlumberger. As for other tools of this type, environmental corrections for the particular borehole conditions (e.g., casing size and weight, borehole diameter, and well fluid properties) have to be applied during computation of the Σ formation values (Plasek et al., 1995). Especially in time-lapse mode, it is essential to apply these corrections consistently, so that the results of different logging runs can be compared.

6.2.3.2.3 Evolution of saturation conditions PNG logs and calculated CO2 saturations for the four Ketzin wells are shown in Fig. 6.2.4. Here, selected repeat logging runs, which were performed close to

FIGURE 6.2.4 Measured PNG Σ formation (SIGM) log curves of the baseline (B) and selected repeat (R) logging runs, as well as calculated CO2 saturations (Sg, d: displacement, e: extended PNG saturation models). The displayed repeat runs are representative for the times during which the three 3D seismic repeats were performed (see text). Numbers of depth intervals for calculation of average CO2 volumes and saturations (see Table 6.2.1) are indicated with black bars and bold numerals. PNG, Pulsed neutron gamma. Lithology after Fo¨rster, A., Scho¨ner, R., Fo¨rster, H.-J., Norden, B., Blaschke, A.-W., Luckert, J., et al., 2010. Reservoir characterization of a CO2 storage aquifer: the Upper Triassic Stuttgart Formation in the Northeast German Basin. Mar. Petrol. Geol. 27, 10, 2156
2172.

6.2.3 Geophysical monitoring

three 3D seismic repeats, are displayed. Average CO2 saturations, calculated as weighted arithmetic mean values for individual lithological units, are listed in Table 6.2.1 (see Fig. 6.2.4 for position of units). Saturation changes are indicated by offsets between the baseline and repeat formation Σ log curves, which mainly can be identified within a limited number of continuous intervals with a clear correlation to lithology. The highest changes occur within the porous and permeable sandstone intervals. The changes observed between the baseline and repeat Σ formation log curves are in part also influenced by the imbibition of CO2 into the uncemented sections of the well annuli (Baumann et al., 2014). Affected intervals are located, for example, above the reservoir intervals in Ktzi 201 and 202, and to a lesser extent also in Ktzi 200 (633
630 m), as well as in the larger section below the injection interval in Ktzi 202, between about 650 and 632 m. For several PNG logging runs recorded in the injection well Ktzi 201 during the injection phase, clear indications for salt precipitation were observed. For these runs (R2/3 and R6, see below), saturations were computed using both the standard saturation model, considering displacement of brine by CO2 only, as well as a newly developed extended saturation model, which also accounts for evaporation and salt precipitation (Baumann et al., 2014).

6.2.3.2.4 Pulsed neutron gamma results for the first 3D seismic repeat survey (2009) For the first 3D seismic repeat, results of PNG repeats R2 and R3 were averaged (denoted as R2/3), due to the temporal offset between the acquisition of the PNG logs and the 3D seismic survey. The highest CO2 saturations occurred in the upper sandstone layer of the storage interval (denoted as Unit 1, Fig. 6.2.4), with average values of up to 65% for the injection well Ktzi 201 (Table 6.2.1). Here, saturations of up to 100% occur locally, indicating complete displacement of the mobile and dry-out of the immobile pore water fractions. As a result of the extended PNG saturation model, more variable CO2 saturation profiles with a general tendency toward higher saturation values compared to the standard displacement model were calculated. Toward the observation wells, a lateral decrease of CO2 saturation can be observed, with average values of 56% (Ktzi 200) and 40% (Ktzi 202).

6.2.3.2.5 Pulsed neutron gamma results for the second 3D seismic repeat survey (2012) For the second 3D seismic repeat, PNG repeats R6 (Ktzi 200 and 201) and R2 (Ktzi 203) are displayed (Fig. 6.2.4). For the Ktzi 203 well, the baseline logging run from the adjacent Ktzi 201 well was used (applying a depth shift of 2.1 m upwards), because Ktzi 203 was only drilled about 4 years after the start of injection. A decreased thickness of the interval containing CO2 can be observed, with increasing saturations within the upper reservoir intervals. This is consistent with

535

Table 6.2.1 Average CO2 saturations (Sg) from results of pulsed neutron gamma logging for times of the 3D seismic repeat surveys, together with averaging interval parameters in bold letters (for locations of intervals see Fig. 6.2.4). In wells 202 and 203 there is only one interval, respectively (bold letters). 3D repeat one Well 201

200

202 203

3D repeat two

3D repeat three

Interval Nr.

Top (m)

Bottom (m)

Thickness (m)

Phi (%)

Sg min. (%)

Sg max. (%)

Sg min. (%)

Sg max. (%)

Sg (%)

1 2 3 4 Eff. 1 2 Eff. 1 1

633.75 642.87 657.89 661.85

642.09 650.99 661.85 664.11

8.34 8.12 3.96 2.26

23.5 25.9 26.3 27.2

634.58 643.66

642.24 649.49

7.66 5.83

27.5 29.6

627.55 631.12

631.60 639.17

4.05 8.05

28.2 25.9

62.0 43.5 17.5 15.0 42.9 56.5 14.0 38.1 40.5


65.0 53.0 17.5 15.0 47.4 56.5 14.0 38.1 40.5


50.0 15.0

32.7 58.0
58.0 56.0 80

56.0 21.0

38.7 58.0
58.0 56.0 80

44.0
19.0 1.0 31.0 36.0
36.0 (n.a.) 64.0

For 3D repeats one and two, both minimum and maximum Sg values are listed, resulting from application of the standard displacement and the extended saturation model, also considering evaporation and salt precipitation (see text for further details).

6.2.3 Geophysical monitoring

the injection regime, as injection rates after 2009 were for most of the time about 50% lower than previously, and PNG repeat R6 was recorded during a shut-in period of several months duration.

6.2.3.2.6 Pulsed neutron gamma results for the third 3D seismic repeat survey (2015) This repeat survey was recorded almost 2 years after the stop of injection in August 2013. For the third 3D seismic repeat, PNG repeats R9, R8, and R5 are displayed, for Ktzi 200, Ktzi 201, and Ktzi 203, respectively (Fig. 6.2.4). The trend observed in the previous survey, a general tendency toward lower saturations at the bottom, and increased saturations at the top of the CO2 plume, is continued. This is interpreted as a rising of the CO2 plume due to buoyancy forces after stop of injection. The highest CO2 saturations with an average of 64% now occur at Ktzi 203, whereas lower CO2 saturations are observed in Ktzi 201 and 200 compared to 2012. This indicates that the CO2 plume is moving away from the injection point in the up-dip direction. In contrast to this general tendency toward an upward movement of the CO2 plume, there is also evidence for the presence of CO2 within a thinner sand and silt layer some meters below the main injection interval in the Ktzi 201 well (units 3 and 4). Here, CO2 had been detected during earlier times of injection including the first 3D seismic repeat, but not during 2012 (see Fig. 6.2.4). With respect to the lateral extent of this deeper CO2 interval it should be noted that Ktzi 203 is only accessible to about 640 m depth, due to a blockage of the well with cement, and no PNG data are available from the intervals below. There are nevertheless indications from gas measurements on core samples for occurrence of CO2 below this depth in Ktzi 203 (Barth et al., 2015).

6.2.3.3 Seismic monitoring 6.2.3.3.1 Introduction In deep saline aquifer formations, seismic monitoring has proven to be highly successful in imaging the injected CO2 (e.g., Chadwick et al., 2009). For Ketzin, a cascaded combination of seismic surveys covering different spatial and temporal scales was applied, comprising crosshole tomography between two monitoring wells in the early phase of CO2 injection (Zhang et al., 2012), VSP, and MSP surveys to image the reservoir around the injection and monitoring wells (Go¨tz et al., 2014), and 2D as well as 3D time-lapse surface seismic surveys (4D and “star” profiles). An additional VSP survey with distributed acoustic sensing was performed by making use of fiber-optic cables deployed in the injection and monitoring wells in order to demonstrate the applicability of this technology on pilot-scale CO2 reservoirs. The focus of this survey was on a comparison of the technology with conventional wireline VSP surveying (Daley et al., 2013) and on generating a 3D-VSP image of the reservoir close to injection and monitoring wells rather than imaging the CO2 plume as this survey was performed only once,

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not providing time-lapse images of reservoir processes (Go¨tz et al., 2018). In the following, the 4D seismic surveys will be described in more detail since they have provided the most comprehensive image of the CO2 injected into the storage formation.

6.2.3.3.2 4D seismic The first 3D seismic survey acquired in the fall of 2005 prior to CO2 injection provided information about the geometry of the reservoir and its overburden (Juhlin et al., 2007) and served as a baseline for later surveys conducted to monitor key reservoir changes caused by the subsequent injection of CO2. During the ongoing injection, two repeat surveys were acquired, the first repeat in the Fall of 2009, after about 23 kt of CO2 had been injected into the storage formation (Ivanova et al., 2012), and the second repeat 3 years later, in the Summer/Fall of 2012, when the cumulative amount of injected CO2 was 61 kt (Ivandic et al., 2015). In Fall 2015, 2 years after the end of the injection, the third repeat was acquired, serving as the first postinjection survey. The acquisition followed the same scheme and used identical recording equipment (Table 6.2.2) as in the baseline survey to ensure maximum repeatability. The sizes of the baseline and repeat surveys were adjusted according to their acquisition target: full size for the baseline survey, reduced size for the repeat surveys according to the expected maximum extent of the CO2 plume (Fig. 6.2.5). To maximize repeatability in fold and azimuthal coverage for the time-lapse analysis, the data sets were limited to the baseline and repeat data subsets that contain common traces. The same data-processing workflow and parameters used in the baseline survey (Juhlin et al., 2007) were applied in the repeat surveys. However, in spite of all the surveys being acquired in the same season of the Table 6.2.2 Acquisition parameters for repeated 3D seismic surveys. Parameter

Value

Receiver line spacing/number per template Receiver station spacing/channels per template Source line spacing/number per template Source point spacing CDP bin size Nominal fold Geophones Sampling rate Record length Source

96 m/5 24 m/48

Acquisition unit

48 m/12 24 and 72 m 12 m 3 12 m 25 28 Hz, vertical 1 ms 3s 240 kg accel. weight drop, 8 hits per shot point Sercel 428 XL

6.2.3 Geophysical monitoring

FIGURE 6.2.5 Map showing the outlines of acquisition areas for 4D seismic monitoring. Colors indicate outlines of baseline and repeat surveys, respectively: baseline 2005—thick black line, repeat 2009—red line, repeat 2012—green line, repeat 2015—blue line. Red and yellow dots indicate locations of CO2 injection and monitoring wells.

year, variable weather and ground conditions during the periods of the 3D seismic acquisitions required a revision of the static shifts for each survey (Bergmann et al., 2011). In order to enable time-lapse analysis of the 4D data, the baseline and repeat data sets had to be cross-calibrated. This cross-calibration process consisted of phase and time matching, phase and frequency shaping by filtering, crosscorrelation statics and time-variant shifting, and cross-normalization, and resulted in an enhancement of the time-lapse reservoir signal and generally good repeatability (normalized-root-mean-square levels of 15%
25%), and suppression of differences caused by other factors. Fig. 6.2.6 shows vertical sections of the time-lapse amplitude difference (repeat minus baseline) for the three repeat data sets. Presented are inline 1167

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FIGURE 6.2.6 Vertical sections of amplitude difference data. Inline 1166 and crossline 1100 of the stacked and migrated 3D cube are shown, crossing the injection well (Ktzi 201). (A) Inline 1166, first repeat survey, (B) crossline 1100, first repeat survey, (C and D) inline 1166 and crossline 1100, second repeat survey, (E and F) inline 1166 and crossline 1100, third repeat survey.

and crossline 1098, both located in the vicinity of the injection well. The observed increase in the reflection amplitudes at approximately 530 ms, near the injection borehole in both the inline and crossline images, is interpreted to be due to the presence of injected CO2 as the CO2 should enhance the impedance contrast of the internal layers in the aquifer (Kazemeini et al., 2010). Amplitude difference maps at the reservoir level (Fig. 6.2.7) show that the lateral extent of the observable CO2 plume at the time of the first 3D repeat survey was approximately 300
400 m (Ivanova et al., 2012). The predominantly westward

6.2.3 Geophysical monitoring

FIGURE 6.2.7 Amplitude difference maps from the first, second, and third repeat surveys. The normalized difference amplitudes were extracted from the depth level of the top Stuttgart Formation, indicating the lateral extent of the seismically detected CO2 plume in the reservoir. White contour lines show the depth below ground level of the top Stuttgart Formation.

propagation and irregular pattern of the CO2-induced anomaly was seen as an indicator of the variable permeability and strong lateral heterogeneity of the reservoir. The second 3D repeat data set revealed that the CO2 plume had expanded since the time of the first repeat survey. The amplitude differences at the reservoir level indicated that the observable CO2 plume had grown by 150 m in the N
S direction and 200 m in the W
E direction (Ivandic et al., 2015). The pronounced westerly spreading tendency of the plume confirmed the heterogeneity of the reservoir sandstones. The first postinjection survey, however, revealed a decrease in the intensity and in the extent of the amplitude anomaly in both the horizontal and vertical directions (Huang et al., 2016). Assessment of the observable CO2 plume distribution suggested that the decrease in the size of the anomaly may have been due to multiple factors, such as limited vertical resolution, CO2 dissolution, CO2 migration into thin layers due to ongoing pressure relaxation in the reservoir, in addition to the effects of ambient noise. Changes in the seismic signature, petrophysical measurements on core samples, and geophysical logging of CO2 saturation levels (PNG logging, see above), allowed the amount of CO2 imaged by the seismic data to be estimated and compared to the actual injected quantity. Although the success of quantitative assessment of the injected CO2 mass is difficult, it still is an important component in monitoring the CO2 plume for possible leakage. The estimates performed on the first 3D seismic repeat data set showed consistency between the calculated CO2 mass and the actual amount injected at the time of the survey with the discrepancy being within 3%
5% (Fig. 6.2.8).

541

FIGURE 6.2.8 Mass estimations based on the results of 4D seismic measurements.

6.2.3 Geophysical monitoring

However, the estimates obtained for the 2012 data show a difference of about 15% between the two values (Ivandic et al., 2015). Although this shortfall can, in principle, be explained by the ongoing dissolution processes, significant uncertainties, attributed mainly to the limited vertical resolution of the seismic data, the heterogeneous reservoir composition at the site, and a limited knowledge about the distribution of its properties still play a significant role. The quantitative interpretation of the first postinjection 3D seismic survey demonstrated considerable postinjection changes in the CO2 plume behavior at Ketzin. That is, using the same approach and input parameters as for the previous surveys, the estimated percentage of detected CO2 was dramatically lower for the third repeat (Huang et al., 2016). Nevertheless, the data demonstrate the ongoing stabilization of the CO2 plume.

6.2.3.3.3 Star profiles Time-lapse seismic surveys in a star-like configuration, that is, with a radial distribution of acquisition profiles directed toward the approximate location of the injection well, were acquired in order to link downhole surveys with the 3D surface seismic surveys (Fig. 6.2.9). The baseline survey was acquired in 2005 along with the regular 3D survey. Two repeat surveys were conducted in September 2009 and February 2011, after about 23 and 45 kt of CO2 had been injected, respectively. The “star” survey consisted of seven surface seismic shot/receiver lines (lines V1
V7) and two shorter receiver lines (lines 19 and 20) near the injection site. Since the source points were activated along the lines V1
V7, and data were recorded simultaneously on all the lines, it was possible to generate a “sparse” 3D image of the subsurface around the injection well (Ivandic et al., 2012). Processing was performed, following the processing scheme applied in the regular 3D surveys (Juhlin et al., 2007; Ivanova et al., 2012). Since the data sets had to be time-lapse analyzed, nearly the same processing parameters were applied to each data set, except for refraction and residual statics, as the surveys were carried out during different weather and ground conditions. Time-lapse analyses of the repeat data sets show less well constrained images than those obtained with the regular 3D setup due to a very variable fold and azimuthal offset distribution (Fig. 6.2.10). Changes in amplitudes are found within the Stuttgart Formation near the injection borehole at approximately 530 ms depth in both the inline and crossline directions, where the time-lapse effects are expected to be most prominent. Time-lapse difference images at the reservoir level (Fig. 6.2.10) show that the observed CO2-induced amplitude anomaly at the time of the first repeat survey extends for B120 m in the inline direction (crosslines 1095
1105) and B75 m in the crossline direction (inlines 1162
1168). The amplitude anomaly observed in the second repeat seismic cube extends for some 290 m in the inline direction (crosslines 1086
1111) and about 190 m in the crossline direction (inlines 1159
1172). In both data sets the anomaly is visible within a time window of

543

FIGURE 6.2.9 Location map of star profiles.

FIGURE 6.2.10 Time-lapse sections of the repeated star profile surveys. Modified from Bergmann, P., Diersch, M., Go¨tz, J., Ivandic, M., Ivanova, A., Juhlin, C., et al., 2016. Review on geophysical monitoring of CO2 injection at Ketzin, Germany. J. Petrol. Sci. Eng. 139, 112
136 (Bergmann et al., 2016).

6.2.3 Geophysical monitoring

less than 15 ms (525
538 ms). There are no time-lapse anomalies observed above the K2 reflection (a strong reflection caused by an anhydritic layer at the top of the caprock) in the vicinity of the injection site, indicating that there is no CO2 leakage from the reservoir level (Ivandic et al., 2012). Although the apparent size of the anomaly is smaller than that observed in the regular 3D surveys, a similar migration pattern is observed in both data sets. Uneven fold and higher noise levels in the sparse 3D seismic data do not allow a quantitative comparison to be made, but can provide an insight into how the injected CO2 is spreading within the reservoir. Furthermore, an irregular 3D geometry, like the “star” configuration, can be useful in obtaining better coverage near the injection site. Another advantage of the sparse 3D survey compared to the regular 3D is the substantially lower cost and less time needed for seismic acquisition and processing. Furthermore, the “star” geometry utilizes to a higher degree the existing roads and, therefore, minimizes damage to agricultural fields. However, the resulting uneven fold and azimuth distributions over the survey area may lead to obscured actual amplitude anomalies and, thus, to unreliable data interpretation. Moreover, introduction of amplitude artifacts may occur and, thus, data-processing steps and parameters should be carefully chosen.

6.2.3.4 Geoelectric monitoring 6.2.3.4.1 Motivation and background of geoelectric monitoring Due to its sensitivity to pore-fluid changes, geoelectric monitoring is a promising technique for detecting and imaging CO2 in the underground (Ramirez et al., 2003; Christensen et al., 2006; Kiessling et al., 2010; Carrigan et al., 2013). The property of gaseous CO2 to act as an insulator forms a significant contrast compared to the very conductive saline formation water (brine) in the rock pores of a deep aquifer. At the Ketzin site, a geoelectric monitoring concept has been developed for periodic and permanent data acquisition to provide time-lapse results of the injection process (Bergmann et al., 2017). Based on a petrophysical relation derived from flow-through experiments on Ketzin core samples (Kummerow and Spangenberg, 2011), the transformation of resistivity images into saturation profiles offers the opportunity of continuous saturation monitoring in the nearwellbore area for the whole life-cycle of the storage site (Schmidt-Hattenberger et al., 2016). For the Ketzin reservoir at medium depth it has been proven that geoelectric monitoring can be realized by reasonable operational and computational costs and efforts. The electrical resistivity ρ in Ω m (reciprocal of conductivity) of a fluidsaturated rock describes its ability to impede the flow of electric current through the rock under investigation. Dry rocks exhibit infinite resistivity, that is, they act as an insulator similar to gases. The resistivity of reservoir rocks is a function of the salinity of the formation water, effective porosity, and quantity of fluid/gas trapped in the pore space (Tiab and Donaldson, 2015). The relationships between

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the rock physical parameters indicate the feasibility of electrical resistivity measurements for evaluation of fluid contrasts, as, for example, saline water, oil, or gas, in a formation. ERT is an established geophysical imaging technique for deducing subsurface structures from electrical resistivity measurements conducted either at the surface, and/or by using electrodes in one or more boreholes (e.g., Daily and Owen, 1991; Bing and Greenhalgh, 2000; Tsourlos et al., 2003; Loke et al., 2013). In addition, geoelectric inversion schemes have to be applied for estimating the resistivity distribution of the underground through an iterative procedure. Here, predicted and measured data are compared under the assumption to minimize the data misfit and approach toward a model which best fits the observed field data (Sasaki, 1994; LaBrecque and Yang, 2001; Gu¨nther et al., 2006; Hayley et al., 2011).

6.2.3.5 Ketzin monitoring system design and deployment The Ketzin ERT crosshole monitoring setup is based on a total of 45 ringshaped stainless-steel electrodes permanently installed in the three wells Ktzi 200, Ktzi 201, and Ktzi 202, at a depth-range of 590
735 m (Fig. 6.2.11, left side), crossing the sandstone storage horizon of the Stuttgart Formation (Norden and Frykman, 2013). The horizontal borehole distances are 50 m between Ktzi 200 and Ktzi 201, and 100 m between Ktzi 200 and Ktzi 202. The behind-casing installation of the permanent downhole electrodes was an integrated part of the drilling and well completion program and, therefore, did not pose an additional operational risk at any time. After installing the permanent downhole electrodes in 2007, the system was in standby mode for nearly 1 year under high-saline formation water conditions. As of the start of the CO2 injection in June 2008, crosshole ERT measurements have been performed at first in a daily mode until the first breakthrough at the observation well Ktzi 200 (July 15, 2008; Zimmer et al., 2011). From this time until the second breakthrough at the observation well Ktzi 202 (March 21, 2009; Zimmer et al., 2011), the CO2 migration in the reservoir zone was imaged at least twice per week, and afterwards, the measurements were conducted weekly. As shown schematically in Fig. 6.2.11 (right side), the electrode array has also been operated in combination with 16 surface dipoles arranged in concentric circles around the site for periodic SD surveys (Bergmann et al., 2012).

6.2.3.6 Data acquisition, processing, and inversion Before ERT data acquisition starts, synthetic modeling studies can provide a first indication about the expected resistivity signatures in the underground and their temporal and spatial resolution by the given acquisition geometry (Kiessling et al., 2010). Various electrode configurations can be measured to achieve an optimal coverage of the target reservoir zone. Optimized electrode arrays can achieve the highest possible spatial resolution with as few

FIGURE 6.2.11 Schematic illustration of key technical components of the Ketzin ERT concept: The permanent downhole array and its major components, together with the sandstone target zone of the injection well Ktzi 201 (left-hand side). Principal setup of a surface-downhole acquisition (righthand side), where the schematic surface dipole stands as an example for the 16 surface dipoles used in periodic surveys. ERT, Electrical resistivity tomography. Modified after Bergmann, P., Schmidt-Hattenberger, C., Labitzke, T., Wagner, F.M., Just, A., Flechsig, C., et al., 2017. Fluid injection monitoring using electrical resistivity tomography - five years of CO2 injection at Ketzin, Germany. Geophys. Prospect. 65 (3), 859
875.

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measurements as possible (Stummer et al., 2004; Wilkinson et al., 2006; al Hagrey, 2012; Wagner et al., 2015). In addition to the crosshole functionality, the SD concept enabled to enlarge the area of observation around the nearwellbore zone (Bergmann et al., 2014). There are several approaches for data quality assurance. A very common practice for noise estimation is based on normal and reciprocal measurements, recorded by switching the role of transmitting and receiving electrodes (Slater et al., 2000). Repeat measurements with the same transmitting and receiving electrode configuration give a good measure of equipment stability and allow stacking of the signal. Full waveform acquisition conducted in the SD surveys provides a complete picture of noise distribution of an individual measurement (Bergmann et al., 2012). The field data sets are inverted using the open source finite element code BERT (Gu¨nther et al., 2006), which offers inversion on blocky grids as well as tetrahedral grids of arbitrary geometry. The program facilitates appropriate handling of field data even with significant noise by means of an error-weighting procedure. This enables data with proper signal-to-noise ratio to contribute with stronger weight than erratic data. An absolute prerequisite for any survey is the presence of a thorough and reliable baseline. The frequency of monitor data collection depends on the timing of CO2 breakthrough and the rate of CO2 plume growth. Higher temporal resolution, such as hourly or daily data collection, might be necessary during the early period due to rapid plume growth. First reservoir simulations can help to determine a suitable ERT monitoring frequency.

6.2.3.7 Key results from crosshole and surface-downhole measurements The Ketzin active monitoring concept has demonstrated the successful application of geoelectrical measurements during the whole injection history. In Fig. 6.2.12A, the Ketzin injection history, starting in June 2008 until the end of the injection phase in August 2013 (67,000 t of stored CO2), together with the first year of postinjection is shown. Along this injection history, a succession of imaged resistivity ratios ρt/ρ0 of the observation plane Ktzi 200
Ktzi 201 for selected operational stages is presented (Fig. 6.2.12B). In Fig. 6.2.12C, a representative section from the middle of this observation plane has been extracted from the tomographical results and tracked for all time steps along the injection history. The inverted resistivity ρ is displayed across the reservoir depth and provides a continuous data basis for saturation monitoring, together with a corresponding petrophysical relation. As a first approach, Archie’s law, the resistivity index (Nakatsuka et al., 2010), gives a reasonable estimate on the CO2 saturation in the reservoir zone. Within the first month of CO2 injection, the ERT system has rapidly detected a

FIGURE 6.2.12 (A) Injection rates and downhole pressure of Ktzi 201 at depth 550 m, (B) time-lapse sequence with selected cases of 2D results from different operational stages (1, 2: high injection rates, 3, 4: reduced injection rates, 5: injection shut-in, 6: restart, 7: injection of two-phase CO2, 8: postinjection), (C) resistivities of a vertical profile from 2625 to 2665 m depth (indicated by the black box in Fig. 6.2.12B/1) through the observation plane Ktzi 201
Ktzi 200, plotted on the same time-scale as the injection data above. (A) Modified after Mo¨ller, F., Liebscher, A., Martens, S., Schmidt-Hattenberger, C., Ku¨hn, M., 2013. Yearly operational datasets of the CO2 storage pilot site Ketzin, Germany. In: Scientific Technical Report, Data. doi:10.2312/GFZ.b103-12066 (Mo¨ller et al., 2013).

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resistivity response due to the presence of CO2. A typical CO2-related resistivity signature is visible in the sandstone layer between the injector (Ktzi 201) and the first observation well (Ktzi 200). In this shorter observation plane of the triangular wellbore arrangement, nearly all electrodes are without degradation effects and contribute to good imaging coverage of the highly resistive CO2 signature. Further time steps in Fig. 6.2.12B reveal various operational phases of the CO2 injection, as, for example, regular injection at different rates, shut-in, and restart phases. The tomographic images display blurred resistivity signatures beyond the boundaries of the sandstone compartment. This inherent feature of geoelectrics as a potential field method is caused by the inversion algorithm and its smoothnessconstrained least-squares technique, which searches for models having minimal contrast between adjacent cells, and therefore, tends to “smear” the resistivity value into adjacent cells (Ramirez et al., 2003).

6.2.3.8 Lessons learned from geoelectric monitoring Experiences drawn from the Ketzin site yield a set of pro and con arguments for the application of the ERT method as part of a multidisciplinary monitoring concept: (1) an ERT array is capable of detecting even small amounts of CO2, for example, at the Ketzin site, a clear signature at about 600 t after the first breakthrough has been imaged; (2) an ERT array can serve as a permanent RST for the near wellbore area; (3) although inverted ERT images typically do not achieve a similar sharp spatial resolution as seismic reflection images, they constitute a complementary means of CO2 monitoring, being directly affected by the pore-fluid changes, and therefore, providing valuable results. It forms a system in place, which provides a complementary data set for CO2 quantification, constraint, or joint inversion with seismic data (Bergmann et al., 2014; Jordan et al., 2017) as well as fully coupled hydrogeophysical inversion (Wiese et al., 2017). As potential drawbacks, it is expected that (4) a higher technical installation risk for reservoir depth .1000 m will occur, (5) costs for basic components and installation effort will increase with wellbore depth, and, (6) adequate knowledge about the system’s lifetime is not available yet, this can only be deduced from a larger number of in situ tested systems.

6.2.4 Numerical simulations of multiphase flow Numerical modeling accompanied the scientific and operational activities at the Ketzin pilot site already in the early planning stage to determine optimum CO2 injection regimes in view of feasible flow rates and pore pressure propagation (Bielinski, 2007; Probst, 2008). As the project proceeded, the static reservoir model of the Stuttgart Formation was elaborated (Norden and Frykman, 2013)

6.2.4 Numerical simulations of multiphase flow

and continuously further developed according to the findings from monitoring and modeling (e.g., Kempka et al., 2013) to enable undertaking reliable predictions of short- to long-term CO2 behavior in the storage formation (Kempka et al., 2013, 2014a,b; Klein et al., 2013; De Lucia et al., 2015). Process understanding was substantially improved by integrating monitoring and experimental data with numerical modeling, specifically demonstrated by the progress in analyzing the deviation by a factor of three between the monitored and modeled CO2 arrival times at the second observation well Ktzi 202 (Kempka et al., 2010). Here, Kempka and Ku¨hn (2013) achieved a good agreement between observed and simulated CO2 arrival times and bottom hole pressures for 3 years of site operation with two different numerical simulators. However, further numerical studies (e.g., Class et al., 2015) demonstrated that structural features of the graben zone at the top of the Ketzin anticline, and moreover a permeability reduction between the three wells are of paramount importance to achieve a short- and long-term bottom hole pressure match that is in good agreement with the observations. Aiming at contributing to the EU regulation process on geological CO2 storage, Lu¨th et al. (2015) undertook a conformance assessment to quantify the agreement of the data observed and modeled at the Ketzin pilot site. In summary, the study shows that a good agreement can be achieved and demonstrates an approach for its quantitative assessment. Kempka et al. (2017) integrated two forward models, considering the hydraulic well tests undertaken after drilling the wells Ktzi 200, Ktzi 201, and Ktzi 202 in 2007, as well as the 5-year period (2008
13) of CO2 injection with gaseous CO2 thickness maps derived from 4D seismic data (2005, 2009, and 2012 3D seismic campaigns) in an inverse modeling approach to improve the latest geological model. For that purpose, hydraulic permeabilities have been fitted using pilot points in the inverse optimization approach. Fig. 6.2.13 shows the discretization of the numerical multiphase flow reservoir model of the Stuttgart Formation. Implementation of nested local grid refinements allowed the reduction of model elements by more than a factor of six to about 102,000, compared to the Kempka and Ku¨hn (2013) model grid. Lateral model extensions are 5 km 3 5 km, while its thickness amounts to about 74 m. An integrated modeling scheme has been employed by implementation of two forward models, whereby the first one accounts for the hydraulic testing phase (about 135 days of simulation) and the second one for almost 5 years of CO2 injection (June 2008 to January 2013). Both forward models have been integrated into the PEST11 inverse modeling framework (Welter et al., 2015), whereby the numerical simulator MUFITS (Afanasyev, 2015; Afanasyev et al., 2016) has been applied to simulate the multiphase flow behavior in the Stuttgart Formation. Model integration and implementation are discussed in detail by Kempka and Norden (2017) and Kempka et al. (2017). In total, 736 integrated forward model runs (hydraulic testing and CO2 injection) were required to meet the predetermined convergence criteria of the inverse modeling procedure, whereby 157 parameters have been matched to observations, including bottom hole pressures

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FIGURE 6.2.13 (A) Revised reservoir model grid with 102,366 elements and local grid refinements (LGR). (B) Close-up view of near-well area, showing well locations and nested LGR structure. Distance between Ktzi 201 and Ktzi 200 is 50 m. Copyright Kempka, T., Norden, B., 2017. Inverse modelling of hydraulic testing to revise the static reservoir model of the Stuttgart Formation at the Ketzin pilot site. Energy Procedia 125, 640
649 and Kempka, T., Norden, B., Ivanova, A., Lu¨th, S., 2017. Revising the static geological reservoir model of the upper Triassic Stuttgart formation at the Ketzin pilot site for CO2 storage by integrated inverse modelling. Energies 10 (10). doi:10.3390/en10101559, licensed under CC BY-NC-ND 4.0.

(Chen et al., 2014), CO2 arrival times, and the outer contours of the gaseous CO2, which has been detected by 4D seismic data. Fig. 6.2.14 shows the applied flow rates as well as the observed and simulated bottom hole pressures during the hydraulic testing phase. The simulated pressure drawdown shows good agreement with the observations, whereby deviations during the pumping phases in the Ktzi 201 well for the Ktzi 200 well and vice versa indicate that the resulting permeability distribution only partially represents the hydraulic regime between both wells. On the other hand, pressure data for the crosslinks to the Ktzi 202 well are in excellent agreement, indicating a sufficiently good representation of the hydraulic properties between wells Ktzi 200 and Ktzi 202, as well as Ktzi 201 and Ktzi 202. For the CO2 injection phase, a very good agreement between simulated and observed bottom hole pressures in the Ktzi 201 and Ktzi 200 wells is achieved (Fig. 6.2.15). Deviations in bottom hole pressure are all below 1 bar, whereby the time period from day 790 to day 842 especially emphasizes the high quality of the results of the integrated inverse modeling procedure. Bottom hole pressure variations triggered by the well interference test undertaken in this period are perfectly represented by the numerical simulations. Simulated CO2 arrival times exhibit a deviation of 12 days for the Ktzi 200 well, emphasizing the complex hydraulic property distribution between the Ktzi 201 and Ktzi 200 wells, whereas a perfect match is achieved for the arrival time at the Ktzi 202 well (about 271 days after start of injection). Since an isothermal black-oil model has been applied

Flow rate (sm3/day)

6.2.4 Numerical simulations of multiphase flow

50 40

K tzi 201 K tzi 200 K tzi 202

30 20

Pressure drawdown (bar)

10 0 0 −1 −2 −3 −4 −5 −10 −15

0

5

10

15

20

25 110 115 Time (days)

120

125

130

135

FIGURE 6.2.14 Flow rates as well as observed and simulated bottom hole pressure drawdown in the wells Ktzi 200, Ktzi 201, and Ktzi 202 in the hydraulic testing phase. Solid lines indicate applied flow rates and dashed lines the observed pressure drawdown, while circles represent the flow rates applied in the reservoir simulator and diamonds the simulated pressure response of the calibrated static reservoir model. Copyright Kempka, T., Norden, B., Ivanova, A., Lu¨th, S., 2017. Revising the static geological reservoir model of the upper Triassic Stuttgart formation at the Ketzin pilot site for CO2 storage by integrated inverse modelling. Energies 10 (10). doi:10.3390/en10101559, licensed under CC BY-NC-ND 4.0.

in the inverse modeling procedure, temperature changes in the well during the shut-in phase (day 1442 to day 1660) cannot be matched by the simulations. This is mainly due to temperature-driven density changes in the well, resulting in higher bottom hole pressures. However, restart of CO2 injection after this time period is again very well represented by the numerical model, emphasizing the validity of the applied modeling approach. A good agreement has also been achieved in view of geophysical and dynamic multiphase flow modeling integration. Fig. 6.2.16 shows a close-up view of the near-well area with a comparison of the gaseous CO2 contour determined by the 4D seismic data and that simulated for an equivalent CO2 thickness for 2009 and 2012. Considering the seismic detection thresholds of about 5 m in 2009 and about 7.5 m in 2012, CO2 migration may have been not detected by geophysics, supporting the understanding of the deviations between modeling and observations northwest from the wells. On the other hand, seismic interpretation suggests CO2 migration east of the wells, which is not accounted for in the flow

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Bottomhole pressure (bar) Bottomhole pressure (bar)

75 70

K tzi 201—Simulated K tzi 202—Simulated K tzi 201—Observed

65 60

0

200

K tzi 202—Observed K tzi 201—Observation points K tzi 202—Observation points

400

600

800

75 70 65 60

1000

1200 1400 1600 Time since start of injection (days)

1800

FIGURE 6.2.15 Comparison of observed and simulated bottom hole pressures in the injection well Ktzi 201 and the second observation well Ktzi 202 in the CO2 injection phase. Circles indicate the observations used in the integrated inverse modeling approach. Copyright Kempka, T., Norden, B., Ivanova, A., Lu¨th, S., 2017. Revising the static geological reservoir model of the upper Triassic Stuttgart formation at the Ketzin pilot site for CO2 storage by integrated inverse modelling. Energies 10 (10). doi:10.3390/en10101559, licensed under CC BY-NC-ND 4.0.

FIGURE 6.2.16 (A) Close-up plan view of gaseous CO2 extent detected by 4D seismic data in 2009 (detection threshold 5 m, red dashed line) and 2012 (detection threshold 7.5 m, yellow dashed line), and simulated gaseous CO2 thickness contours for 2009 (5 m thickness, solid red line) and 2012 (7.5 m thickness, solid yellow line). Empty circles represent pilot points, white-filled circles with crosses well locations. Blue-to-red contours show the permeability multipliers determined by inverse model calibration. (B) Close-up plan view of absolute thickness residuals. Circles are scaled to absolute residuals at their respective locations (UTM-WGS84 projection). Modified from Kempka, T., Norden, B., Ivanova, A., Lu¨th, S., 2017. Revising the static geological reservoir model of the upper Triassic Stuttgart formation at the Ketzin pilot site for CO2 storage by integrated inverse modelling. Energies 10 (10). doi:10.3390/en10101559.

References

simulations. Nevertheless, residuals (Fig. 6.2.16B) show that the gaseous CO2 shape is matched well by the simulations, what is supported by the findings of Lu¨th et al. (2015). The integrated inverse modeling approach, taking into account bottom hole pressures recorded during two different campaigns at the Ketzin pilot site (about 4 months of hydraulic testing and almost 5 years of CO2 injection), CO2 arrival times, and 4D seismic data proves to be an efficient method to optimize the hydraulic permeability distribution in the reservoir model of the Stuttgart Formation. Kempka et al. (2017) demonstrate that uncertainty in the parameter data can be substantially reduced by up to a factor of 30 in the near-well area (radius of about 150 m) by the presented approach. Nevertheless, further work is required to resolve the detailed hydraulic parameter distribution between the Ktzi 201 and Ktzi 200 wells to understand the deviations between the observed and monitored bottom hole pressures during hydraulic testing and arrival times in the CO2 injection period.

6.2.5 Conclusion The 5 years of active CO2 injection and the subsequent postinjection phase provide an excellent basis for the application of numerous geophysical monitoring approaches which have been presented in this review. Seismic and geoelectric monitoring imaged CO2 time-lapse signatures, demonstrating its detectability in saline aquifers already at relatively low quantitative thresholds. In addition to these methods, PNG borehole logging and permanent pressure and temperature monitoring provided crucial data for a quantitative characterization of the storage reservoir behavior. The geophysical characterization and monitoring results were fed into the iterative process of building and optimizing the reservoir model as a base for the numerical modeling of the reservoir behavior in a wide range of temporal scales. The closed life-cycle of the Ketzin pilot site has demonstrated the feasibility of saline aquifer CO2 storage and the applicability of geophysical methods to detect accumulations of small quantities in aquifers at intermediate depths which may be a relevant leakage scenario for large industrial-scale storage projects.

References Afanasyev, A., 2015. Hydrodynamic modelling of petroleum reservoirs using simulator MUFITS. Energy Procedia 76, 427
435. Afanasyev, A., Kempka, T., Ku¨hn, M., Melnik, O., 2016. Validation of the MUFITS reservoir simulator against standard CO2 storage benchmarks and history-matched models of the Ketzin pilot site. Energy Procedia 97, 395
402.

555

556

CHAPTER 6.2 Geophysical monitoring

Aigner, T., Bachmann, G.H., 1992. Sequence-stratigraphic framework of the German Triassic. Sedimentary Geol. 80, 115
135. al Hagrey, S.A., 2012. 2D optimized electrode arrays for borehole resistivity tomography and CO2 sequestration modelling. Pure Appl. Geophys. 169 (7), 1283
1292. Barth, J.A.C., Nowak, M.E., Zimmer, M., Norden, B., van Geldern, R., 2015. Monitoring of cap-rock integrity during CCS from field data at the Ketzin pilot site (Germany): evidence from gas composition and stable carbon isotopes. Int. J. Greenhouse Gas Control 43, 133
140. Baumann, G., Henninges, J., De Lucia, M., 2014. Monitoring of saturation changes and salt precipitation during CO2 injection using pulsed neutron-gamma logging at the Ketzin site. Int. J. Greenhouse Gas Control 28, 134
146. Bergmann, P., Yang, C., Lu¨th, S., Juhlin, C., Cosma, C., 2011. Time-lapse processing of 2D seismic profiles with testing of static correction methods at the CO2 injection site Ketzin (Germany). J. Appl. Geophys. 75, 124
139. Bergmann, P., Schmidt-Hattenberger, C., Kiessling, D., Ru¨cker, C., Labitzke, T., Henninges, J., et al., 2012. Surface-downhole electrical resistivity tomography applied to monitoring of CO2 storage at Ketzin, Germany. Geophysics 77, B253
B267. Bergmann, P., Ivandic, M., Norden, B., Ru¨cker, C., Kiessling, D., Lu¨th, S., et al., 2014. Combination of seismic reflection and constrained resistivity inversion with an application to 4D imaging of the CO2 storage site, Ketzin, Germany. Geophysics 79 (2), B37
B50. Bergmann, P., Diersch, M., Go¨tz, J., Ivandic, M., Ivanova, A., Juhlin, C., et al., 2016. Review on geophysical monitoring of CO2 injection at Ketzin, Germany. J. Petrol. Sci. Eng. 139, 112
136. Bergmann, P., Schmidt-Hattenberger, C., Labitzke, T., Wagner, F.M., Just, A., Flechsig, C., et al., 2017. Fluid injection monitoring using electrical resistivity tomography - five years of CO2 injection at Ketzin, Germany. Geophys. Prospect. 65 (3), 859
875. Beutler, G., Tessin, R., 2005. Der Keuper im Norddeutschen Becken. In: Beutler, G. (Ed.), Stratigraphie von Deutschland IV, vol. 253. Keuper, Cour. Forsch.-Inst, Senckenberg, pp. 134
150. Beutler, G., Hauschke, N., Nitsch, E., 1999. Faziesentwicklung des Keupers im Germanischen Becken. In: Hauschke, N., Wilde, V. (Eds.), Trias
Eine ganz andere Welt. Mitteleuropa im fru¨hen Erdmittelalter, Pfeil, Mu¨nchen, pp. 129
174. Bielinski, A., 2007. Numerical Simulation of CO2 Sequestration in Geological Formations. Mitteilungsheft Nr. 155 (Ph.D. thesis). Department of Hydraulic Engineering, University of Stuttgart, Stuttgart. Bing, Z., Greenhalgh, S.A., 2000. Cross-hole resistivity tomography using different electrode configurations. Geophys. Prospect. 48 (5), 887
912. Carrigan, C.R., Yang, X., LaBrecque, D.J., Larsen, D., Freeman, D., Ramirez, A.L., et al., 2013. Electrical resistance tomographic monitoring of CO2 movement in deep geologic reservoirs. Int. J. Greenhouse Gas Control 18, 401
408. Chadwick, R.A., Noy, D., Arts, R., Eiken, O., 2009. Latest time-lapse seismic data from Sleipner yield new insights into CO2 plume development. Energy Procedia 1, 2103
2110. Chen, F., Wiese, B., Zhou, Q., Kowalsky, M.B., Norden, B., Kempka, T., et al., 2014. Numerical modelling of the pumping tests at the Ketzin pilot site for CO2 injection: model calibration and heterogeneity effects. Int. J. Greenhouse Gas Control 22, 200
212.

References

Christensen, N.B., Sherlock, D., Dodds, K., 2006. Monitoring CO2 injection with crosshole electrical resistivity tomography. Explor. Geophys. 37 (1), 44
49. Class, H., Mahl, L., Ahmed, W., Norden, B., Ku¨hn, M., Kempka, T., 2015. Matching pressure measurements and observed CO2 arrival times with static and dynamic modelling at the Ketzin storage site. Energy Procedia 76, 623
632. Daily, W., Owen, E., 1991. Cross-borehole resistivity tomography. Geophysics 56 (8), 1228
1235. Daley, T.M., Freifeld, B.M., Ajo-Franklin, J., Dou, S., Pevzner, R., Shulakova, V., et al., 2013. Field testing of fiber-optic distributed acoustic sensing (DAS) for subsurface seismic monitoring. Leading Edge 32, 699
706. Dance, T., Paterson, L., 2016. Observations of carbon dioxide saturation distribution and residual trapping using core analysis and repeat pulsed-neutron logging at the CO2CRC Otway site. Int. J. Greenhouse Gas Control 47, 210
220. De Lucia, M., Kempka, T., Ku¨hn, M., 2015. A coupling alternative to reactive transport simulations for long-term prediction of chemical reactions in heterogeneous CO2 storage systems. Geosci. Model Dev. 2015 (8), 279
294. Ellis, D.V., Singer, J.M., 2007. Well Logging for Earth Scientists, second ed. Springer, Dordrecht. Fo¨rster, A., Norden, B., Zinck-Jørgensen, K., Frykman, P., Kulenkampff, J., Spangenberg, E., et al., 2006. Baseline characterization of the CO2SINK geological storage site at Ketzin, Germany. Environ. Geosci. 13 (3), 145
161. Fo¨rster, A., Scho¨ner, R., Fo¨rster, H.-J., Norden, B., Blaschke, A.-W., Luckert, J., et al., 2010. Reservoir characterization of a CO2 storage aquifer: the Upper Triassic Stuttgart Formation in the Northeast German Basin. Mar. Petrol. Geol. 27 (10), 2156
2172. Freifeld, B.M., Finsterle, S., Onstott, T.C., Toole, P., Pratt, L.M., 2008. Ground surface temperature reconstructions: using in situ estimates for thermal conductivity acquired with a fiber-optic distributed thermal perturbation sensor. Geophys. Res. Lett. 35, L14309. Freifeld, B.M., Daley, T., Hovorka, S., Henninges, J., Underschultz, J., Sharma, S., 2009. Recent advances in well-based monitoring of CO2 sequestration. Energy Procedia 1, 2277
2284. Go¨tz, J., Lu¨th, S., Krawczyk, C.M., Cosma, C., 2014. Zero-offset VSP monitoring of CO2 storage: impedance inversion and wedge modelling at the Ketzin pilot site. Int. J. Geophys. 2014, . Available from: https://doi.org/10.1155/2014/294717Article ID 294717, 15 pages. Go¨tz, J., Lu¨th, S., Henninges, J., Reinsch, T., 2018. Vertical seismic profiling using a daisy-chained deployment of fibre-optic cables in four wells simultaneously
case study at the Ketzin CO2 storage site. Geophys. Prospect. Available from: https://doi. org/10.1111/1365-2478.12638. Gu¨nther, T., Ru¨cker, C., Spitzer, K., 2006. Three-dimensional modelling and inversion of dc resistivity data incorporating topography - II. Inversion. Geophys. J. Int. 166 (2), 506
517. Hayley, K., Pidlisecky, A., Bentley, L.R., 2011. Simultaneous time-lapse electrical resistivity inversion. J. Appl. Geophys. 75 (2), 401
411. Henninges, J., Liebscher, A., Bannach, A., Brandt, W., Hurter, S., Ko¨hler, S., et al., 2011. P-T-rho and two-phase fluid conditions with inverted density profile in observation wells at the CO2 storage site at Ketzin (Germany). Energy Procedia 4, 6085
6090. Available from: https://doi.org/10.1016/j.egypro.2011.02.614.

557

558

CHAPTER 6.2 Geophysical monitoring

Hoth, K., Rusbu¨lt, J. Zagora, K., Beer, H., Hartmann, O., 1993. Die tiefen Bohrungen im Zentralabschnitt der Mitteleuropa¨ischen Senke
Dokumentation fu¨r den Zeitabschnitt 1962
1990. In: Schriftenreihe fu¨r Geowissenschaften 2, Gesellschaft fu¨r Geowissenschaften e.V. (i.G.), Berlin, 145 p. Huang, F., Juhlin, C., Han, L., Kempka, T., Lueth, S., Zhang, F., 2016. Quantitative evaluation of thin-layer thickness and CO2 mass utilizing seismic complex decomposition at the Ketzin CO2 storage site, Germany. Geophys. J. Int. 207 (1), 160
173. Huang, F., Bergmann, P., Juhlin, C., Ivandic, M., Lu¨th, S., Ivanova, A., et al., 2018. The first post-injection seismic monitor survey at the Ketzin pilot CO2 storage site: results from time-lapse analysis. Geophys. Prospect. 66, 62
84. IPCC Special Report on CO2 Capture and Storage, 2005. In: Metz, B., Davidson, O., de Coninck, H., Loos, M., Meyer, L. (Eds.), Prepared by Working Group III of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge. Ivandic, M., Yang, C., Lu¨th, S., Cosma, C., Juhlin, Ch, 2012. Time-lapse analysis of sparse 3D seismic data from the CO2 storage pilot site at Ketzin, Germany. J. Appl. Geophys. 84, 14
28. Available from: https://doi.org/10.1016/j.jappgeo.2012.05.010. Ivandic, M., Juhlin, C., Lu¨th, S., Bergmann, P., Kashubin, A., Sopher, D., et al., 2015. Geophysical monitoring at the Ketzin pilot site for CO2 storage: new insights into the plume evolution. Int. J. Greenhouse Gas Control 32, 90
105. Ivanova, A., Kashubin, A., Juhojuntti, N., Kummerow, J., Henninges, J., Juhlin, C., et al., 2012. Monitoring and volumetric estimation of injected CO2 using 4D seismic, petrophysical data, core measurements and well logging: a case study at Ketzin, Germany. Geophys. Prospect. 60, 957
973. Jenkins, C., Chadwick, A., Hovorka, S.D., 2015. The state of the art in monitoring and verification — ten years on. Int. J. Greenhouse Gas Control 40, 312
349. Jordan, M., Rippe, D., Schmidt-Hattenberger, C., Romdhane, A., 2017. Joint Inversion for Improved CO2 Monitoring at the Ketzin Pilot Site, Germany. EAGE/SEG Research Workshop, Trondheim, p. 2017. Juhlin, C., Giese, R., Zinck-Jørgensen, K., Cosma, C., Kazemeini, H., Juhojuntti, N., et al., 2007. 3D baseline seismics at Ketzin, Germany: the CO2SINK project. Geophysics 72 (5), B121
B132. Kazemeini, H., Yang, C., Juhlin, Ch, Fomel, S., Cosma, C., 2010. Enhancing seismic data resolution using the prestack blueing technique: an example from the Ketzin CO2 injection site, Germany. Geophysics 75 (6), V101
V110. Available from: https://doi.org/ 10.1190/1.3483900. Kempka, T., Ku¨hn, M., Class, H., Frykman, P., Kopp, A., Nielsen, C.M., et al., 2010. Modelling of CO2 arrival time at Ketzin—Part I. Int. J. Greenhouse Gas Control 4, 1007
1015. Kempka, T., Ku¨hn, M., 2013. Numerical simulations of CO2 arrival times and reservoir pressure coincide with observations from the Ketzin pilot site, Germany. Environ. Earth Sci. 70, 3675
3685. Kempka, T., Class, H., Go¨rke, U.J., Norden, B., Kolditz, O., Ku¨hn, M., et al., 2013. A dynamic flow simulation code intercomparison based on the revised static model of the Ketzin pilot site. Energy Procedia 40, 418
427. Kempka, T., De Lucia, M., Ku¨hn, M., 2014a. Geomechanical integrity verification and mineral trapping quantification for the Ketzin CO2 storage pilot site by coupled numerical simulations. Energy Procedia 63, 3330
3338.

References

Kempka, T., Klapperer, S., Norden, B., 2014b. Coupled hydro-mechanical simulations demonstrate system integrity at the Ketzin pilot site for CO2 storage, Germany. In: In the Proceeding of ISRM Regional Symposium—EUROCK 2014, Vigo, Spain, 27
29 May 2014. Kempka, T., Norden, B., 2017. Inverse modelling of hydraulic testing to revise the static reservoir model of the Stuttgart Formation at the Ketzin pilot site. Energy Procedia 125, 640
649. Kempka, T., Norden, B., Ivanova, A., Lu¨th, S., 2017. Revising the static geological reservoir model of the Upper Triassic Stuttgart Formation at the Ketzin pilot site for CO2 storage by integrated inverse modelling. Energies 10 (10). Available from: https://doi. org/10.3390/en10101559. Kiessling, D., Schmidt-Hattenberger, C., Schu¨tt, H., Schilling, F., Kru¨ger, K., Scho¨bel, B., et al., 2010. Geoelectrical methods for monitoring geological CO2 storage: first results from cross-hole and surface-downhole measurements from the CO2SINK test site at Ketzin (Germany). Int. J. Greenhouse Gas Control 4 (5), 816
826. Klein, E., De Lucia, M., Kempka, T., Ku¨hn, M., 2013. Evaluation of long-term mineral trapping at the Ketzin pilot site for CO2 storage: an integrative approach using geochemical modelling and reservoir simulation. Int. J. Greenhouse Gas Control 19, 720
730. Kummerow, J., Spangenberg, E., 2011. Experimental evaluation of the impact of the interactions of CO2-SO2, brine, and reservoir rock on petrophysical properties: a case study from the Ketzin test site, Germany. Geochem. Geophy. Geosyst. 12. Available from: https://doi.org/10.1029/2010GC003469. LaBrecque, D.J., Yang, X., 2001. Difference inversion of ERT data: a fast inversion method for 3-D in situ monitoring. J. Environ. Eng. Geophys. 6 (2), 83
89. Liebscher, A., Mo¨ller, F., Bannach, A., Ko¨hler, S., Wiebach, J., Schmidt-Hattenberger, C., et al., 2013. Injection operation and operational pressure
temperature monitoring at the CO2 storage pilot site Ketzin, Germany—design, results, recommendations. Int. J. Greenhouse Gas Control 15, 163
173. Loizzo, M., Henninges, J., Zimmer, M., Liebscher, A., 2013. Multi-phase equilibrium in a CO2-filled observation well at the Ketzin pilot site. Energy Procedia 37, 3621
3629. Available from: https://doi.org/10.1016/j.egypro.2013.06.255. Loke, M.H., Chambers, J.E., Rucker, D.F., Kuras, O., Wilkinson, P.B., 2013. Recent developments in the direct-current geoelectrical imaging method. J. Appl. Geophys. 95, 135
156. Lu¨th, S., Ivanova, A., Kempka, T., 2015. Conformity assessment of monitoring and simulation of CO2 storage: a case study from the Ketzin pilot site. Int. J. Greenhouse Gas Control 2015 (42), 329
339. Martens, S., Liebscher, A., Mo¨ller, F., Henninges, J., Kempka, T., Lu¨th, S., et al., 2013. CO2 storage at the Ketzin pilot site, Germany: fourth year of injection, monitoring, modelling and verification. Energy Procedia 37, 6434
6443. Available from: https:// doi.org/10.1016/j.egypro.2013.06.573. Martens, S., Mo¨ller, F., Streibel, M., Liebscher, A., the Ketzin Group, 2014. Completion of five years of safe CO2 injection and transition to the post-closure phase at the Ketzin pilot site. Energy Procedia 59, 190
197. Martens, S., Kempka, T., Liebscher, A., Mo¨ller, F., Schmidt-Hattenberger, C., Streibel, M., et al., 2015. Field experiment on CO2 back-production at the Ketzin pilot site. Energy Procedia 76, 519
527.

559

560

CHAPTER 6.2 Geophysical monitoring

Mo¨ller, F., Liebscher, A., Martens, S., Schmidt-Hattenberger, C., Ku¨hn, M., 2013. Yearly operational datasets of the CO2 storage pilot site Ketzin, Germany. In: Scientific Technical Report, Data. https://doi.org/10.2312/GFZ.b103-12066. Mo¨ller, F., Liebscher, A., Martens, S., Schmidt-Hattenberger, C., Streibel, M., 2014. Injection of CO2 at ambient temperature conditions - pressure and temperature results of the “cold injection” experiment at the Ketzin pilot site. Energy Procedia 63, 6289
6297. Mo¨ller, F., Liebscher, A., Schmidt-Hattenberger, C., 2016. Report on the Dataset of the Brine Injection at the CO2 Storage Pilot Site Ketzin, Germany, Scientific Technical Report STR; 16/05. GFZ German Research Centre for Geosciences, Potsdam. Mu¨ller, N., Ramakrishnan, T.S., Boyd, A., Sakruai, S., 2007. Time-lapse carbon dioxide monitoring with pulsed neutron logging. Int. J. Greenhouse Gas Control 1, 456
472. Nakatsuka, Y., Xue, Z.Q., Garcia, H., Matsuoka, T., 2010. Experimental study on CO2 monitoring and quantification of stored CO2 in saline formations using resistivity measurements. Int. J. Greenhouse Gas Control 4 (2), 209
216. Norden, B., 2011. Modelling of the near-surface groundwater flow system at the CO2SINK site Ketzin, Germany. Z. Dtsch. Ges. Geowiss. 162 (1), 63
77. Norden, B., Frykman, P., 2013. Geological modelling of the Triassic Stuttgart Formation at the Ketzin CO2 storage site, Germany. Int. J. Greenhouse Gas Control 19, 756
774. Norden, B., Fo¨rster, A., Vu-Hoang, D., Marcelis, F., Springer, N., Le Nir, I., 2010. Lithological and petrophysical core-log interpretation in CO2SINK, the European CO2 onshore research storage and verification project. SPE Reserv. Eval. Eng. 13, 179
192. Norden, B., Kling, C., Frykman, P., Krawczyk, C., 2013. Structural-Geological Models of the Ketzin CO2 Storage Pilot Site Used for Site Evaluation, Dynamic Reservoir Simulations, and Monitoring Purposes, (Geophysical Research Abstracts Vol. 15, EGU2013-10764, 2013). General Assembly European Geosciences Union, Vienna. Plasek, R.E., Adolph, R.A., Stoller, C., Willis, D.J., Bordon, E.E., Portal, M.G., 1995. Improved pulsed neutron capture logging with slim carbon-oxygen tools: methodology, SPE 30598. In: SPE Annual Technical Conference and Exhibition. Society of Petroleum Engineers, Inc., Dallas, Texas, p. 15. Prevedel, B., Wohlgemuth, L., Henninges, J., Kru¨ger, K., Norden, B., Fo¨rster, A., et al., 2008. The CO2SINK boreholes for geological storage testing. Sci. Drilling 6, 32
37. Prevedel, B., Martens, S., Norden, B., Henninges, J., Freifeld, B.M., 2014. Drilling and abandonment preparation of CO2 storage wells
experience from the Ketzin pilot site. Energy Procedia 63, 6067
6078. Probst, P., 2008. Numerical Simulations of CO2 Injection into Saline Aquifers: Estimation of Storage Capacity and Arrival Times using Multiple Realizations of Heterogeneous Permeability Fields (Master’s thesis). Department of Hydraulic Engineering, University of Stuttgart, Stuttgart. Ramirez, A.L., Newmark, R.L., Daily, W.D., 2003. Monitoring carbon dioxide floods using electrical resistance tomography (ERT): sensitivity studies. J. Environ. Eng. Geophys. 8 (3), 187
208. Sasaki, Y., 1994. 3-D resistivity inversion using the finite-element method. Geophysics 59 (12), 1839
1848. Schmidt-Hattenberger, C., Bergmann, P., Labitzke, T., Wagner, F., 2014. CO2 migration monitoring by means of electrical resistivity tomography (ERT) - review on five years

References

of operation of a permanent ERT system at the Ketzin pilot site. Energy Procedia 6, 4366
4373. Available from: https://doi.org/10.1016/j.egypro.2014.11.471. Schmidt-Hattenberger, C., Bergmann, P., Labitzke, T., Wagner, F.M., Rippe, D., 2016. Permanent crosshole electrical resistivity tomography as an established method for longterm CO2 monitoring at the Ketzin pilot site. Int. J. Greenhouse Gas Control 52, 432
448. Slater, L., Binley, A.M., Daily, W., Johnson, R., 2000. Cross-hole electrical imaging of a controlled saline tracer injection. J. Appl. Geophys. 44 (2
3), 85
102. Smolen, J., 1996. Cased Hole and Production Log Evaluation. PenWell Publishing Company, Tulsa, OK. Stummer, P., Maurer, H., Green, A.G., 2004. Experimental design: electrical resistivity data sets that provide optimum subsurface information. Geophysics 69 (1), 120
139. Tiab, D., Donaldson, E.C., 2015. Petrophysics (Fourth Edition): Theory and Practice of Measuring Reservoir Rock and Fluid Transport Properties. Elsevier, ISBN: 978-0-12803188-9. Tsourlos, P., Ogilvy, R., Meldrum, P., Williams, G., 2003. Time-lapse monitoring in single boreholes using electrical resistivity tomography. J. Environ. Eng. Geophys. 8 (1), 1
14. Wagner, F., Gu¨nther, T., Schmidt-Hattenberger, C., Maurer, H., 2015. Constructive optimization of electrode locations for target-focused resistivity monitoring. Geophysics 80 (2), E29
E40. Welter, D.E., White, J.T., Hunt, R.J., Doherty, J.E., 2015. Approaches in Highly Parameterized Inversion: PEST11 Version 3, A Parameter ESTimation and Uncertainty Analysis Software Suite Optimized for Large Environmental Models. U.S. Geological Survey Techniques and Methods. Book 7. Section C12. 2015. Available: ,https://pubs.er.usgs.gov/publication/tm7C12. (accessed 22.09.17.). Wiese, B.U., Wagner, F.M., Norden, B., Schmidt-Hattenberger, C., 2017. Fully coupled hydrogeophysical inversion of hydraulics, gas pressure and geoelectrics. Energy Procedia 114, 3588
3596. Wilkinson, P.B., Meldrum, P.I., Chambers, J.C., Kuras, O., Ogilvy, R.D., 2006. Improved strategies for the automatic selection of optimized sets of electrical resistivity tomography measurement configurations. Geophys. J. Int. 167, 1119
1126. Wu¨rdemann, H., Mo¨ller, F., Ku¨hn, M., Heidug, W., Christensen, N.P., Borm, G., et al., 2010. CO2SINK
from site characterization and risk assessment to monitoring and verification: one year of operational experience with the field laboratory for CO2 storage at Ketzin, Germany. Int. J. Greenhouse Gas Control 4 (6), 938
951. Yordkayhun, S., Juhlin, C., Norden, B., 2009a. 3D seismic reflection surveying at the CO2SINK project site, Ketzin, Germany: a study for extracting shallow subsurface information. Near Surface Geophys. 7, 75
91. Yordkayhun, S., Tryggvason, A., Norden, B., Juhlin, C., Bergman, B., 2009b. 3D seismic traveltime tomography imaging of the shallow subsurface at the CO2SINK project site, Ketzin, Germany. Geophysics 74, G1
G15. Zhang, F., Juhlin, Ch, Cosma, C., Tryggvason, A., Pratt, G., 2012. Cross-well seismic waveform tomography for monitoring CO2 injection: a case study from the Ketzin Site, Germany. Geophys. J. Int. 189 (1), 629
646. Available from: https://doi.org/10.1111/ j.1365-246X.2012.05375.x. Zimmer, M., Erzinger, J., Kujawa, C., Grp, C.-S., 2011. The gas membrane sensor (GMS): a new method for gas measurements in deep boreholes applied at the CO2SINK site. Int. J. Greenhouse Gas. Control 5 (4), 995
1001.

561