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Reservoir structure and properties from geomechanical modeling and microseismicity analyses associated with an enhanced geothermal system at The Geysers, California
Geothermics 51 (2014) 460–469
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Geothermics journal homepage: www.elsevier.com/locate/geothermics
Reservoir structure and properties from geomechanical modeling and microseismicity analyses associated with an enhanced geothermal system at The Geysers, California Pierre Jeanne a,∗ , Jonny Rutqvist a , Craig Hartline b , Julio Garcia b , Patrick F. Dobson a , Mark Walters b a b
Lawrence Berkeley National Laboratory, Earth Sciences Division, Berkeley, CA 94720, USA Calpine Corporation, 10350 Socrates Mine Road, Middletown, CA 95461, USA
a r t i c l e
i n f o
Article history: Received 17 July 2013 Accepted 9 February 2014 Available online 9 April 2014 Keywords: Enhanced geothermal system Micro-earthquake analysis Fault zone network Inverse fluid flow modeling Thermo-hydromechanical simulation Micro-earthquake predicted
a b s t r a c t This work contributes to modeling studies associated with an enhanced geothermal system demonstration project in the northwestern region of The Geysers, California. We first attempt to determine the structural configuration and reservoir properties of the steam-bearing reservoir, based on microseismicity recorded during a one-year water injection operation. This is particularly challenging because errors in hypocenter determination (due primarily to errors in the velocity model and first-arrival picks) tend to “defocus” any microseismic events related to a distributed network of fractures, resulting in a “cloud” of microseismic events. This work includes a dynamic analysis of the observed alignments in daily microseismicity hypocenters during water injection, along with the constraints provided by geological data (surface mapping and drill cuttings) to determine the location and orientation of shear zones. We then evaluate the viability of the resulting network of proposed shear zones, using a 2D fluid flow and geomechanical model simulation of the injection and comparing it to the evolution of observed (1) pressure in nearby monitoring wells and (2) microseismicity hypocenters. The shear-zone hydraulic properties were estimated using inverse analysis of the pressure evolution in the surrounding wells, while mechanical properties were estimated by comparing the calculated stress changes and associated microseismic potential with the observed microseismicity. The results indicate that a model including the network of proposed shear zones does calculate reservoir hydraulic and mechanical responses similar to those observed during water injection. Finally, the results confirm previous studies at The Geysers indicating that the injection-induced microseismicity is caused by thermal contraction near the injection wells where strong cooling prevails, whereas away from the injection wells, small increases in steam pressure are the primary trigger of microseismicity. Published by Elsevier Ltd.
1. Introduction Enhanced geothermal systems (EGS) have the potential to extract geothermal energy under circumstances and in locations where conventional production is not economically feasible (Tester et al., 2006). Generally, EGS would be applied to extract geothermal energy at sites where the reservoir rock is hot (has sufficient heat content) but has insufficient permeability for economic production. The successful application of EGS technology could allow large, untapped resources to be utilized, greatly increasing
∗ Corresponding author. Tel.: +1 510 486 6261. E-mail addresses: [email protected], [email protected] (P. Jeanne). http://dx.doi.org/10.1016/j.geothermics.2014.02.003 0375-6505/Published by Elsevier Ltd.
geothermal electrical production worldwide. Reservoir permeability is increased by utilizing water injection to reactivate fractures or possibly create new fractures. Such fracture reactivation or creation can be accompanied, however, by microseismic events. The monitoring of such microseismic activity can provide valuable information concerning the response of the reservoir to water injection. Microseismicity monitoring and analysis has been a component of EGS experiments for over 30 years. Early programs simply used microseismicity to estimate the extent of the stimulation zone (Pearson, 1981). With the development of higher-precision hypocenter determination, it has been possible to extract more information concerning geological structures and fluid paths associated with shear zones and faults (Phillips et al., 1997). However, it
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is still a challenge to characterize the structural configuration and reservoir properties at several kilometers depth, owing to significant errors in hypocenter determination. At The Geysers geothermal area in California, induced seismicity has been the subject of numerous studies (e.g., Allis, 1992; Eberhart-Phillips and Oppenheimer, 1984; Stark, 1992; Ross et al., 1999; Smith et al., 2000; Mossop, 2001; Majer and Peterson, 2007; Rutqvist and Oldenburg, 2008; Boyle and Zoback, 2013; Rutqvist et al., 2013). Several mechanisms are responsible for The Geysers’ induced seismicity development, mechanisms linked to water injection or steam extraction. The first mechanism linked to water injection is the thermoelastic stress perturbation associated with cold-water injection into a hot reservoir. This causes a contraction of the rock volume near fracture surfaces, reducing the static friction and allowing shear movement in the vicinity of the injection well (Rutqvist and Oldenburg, 2008). Away from the well, injectioninduced changes in the steam pressure appear to be the dominant cause for triggering shear reactivation (Rutqvist et al., 2013). The last mechanism is linked to steam extraction. Poroelastic stress variations are associated with a nearly reservoir-wide reduction in steam pressure (Oppenheimer, 1986) and has led to subsidence in many parts of the geothermal field (e.g., Mossop and Segall, 1997). More recently, microseismicity was used to provide constraints on the conceptual geomechanical model (Rutqvist et al., 2013) of an EGS system. Accurate hydrological and geomechanical models allow an investigation of injection strategies for EGS development and to predict the extent of the stimulation zone. Here we present a modeling study conducted in support of an EGS demonstration project at the northwestern Geysers geothermal field, California (Fig. 1). The project aims to develop an EGS by directly and systematically injecting cool water at relatively low pressure into a known high temperature (280–400 ◦ C) zone (HTZ) located beneath the conventional (240 ◦ C) geothermal steam reservoir (Garcia et al., 2012; Rutqvist et al., 2013). Injection began on October 6, 2011, and microseismic events were monitored from September 1, 2011, to August 10, 2012. During this period, 2919 microseismic events were recorded in the study area (Fig. 2). Rutqvist et al. (2013) presented a comparison of pre-stimulation model predictions with observed microseismicity over the first few months of injection. The authors used a model in which the rock mass was represented by an equivalent continuum. Although the extent of the calculated stimulation zone matched the field observations very well over the first few months of injection, the long-term results suggest more complex geology. For example, it was found that the observed stimulation zone extends much deeper than that predicted by the numerical modeling, indicating greater vertical permeability perhaps associated with permeable vertical fractures. The existence of such discrete largescale permeable fractures is evident from logging of boreholes drilled into the reservoir, which frequently encounters distinct “steam entries” separated by large intervals of unproductive rock. Previous studies suggest that The Geysers’ reservoir transports steam via a relatively small number of fracture zones (e.g., Sammis et al., 1992). It is likely that such fracture zones connected to the injection well will also channel most of the injected water, which thereby could also migrate downwards by gravity. In this paper, we utilize microseismicity analysis coupled with hydro-thermo-mechanical modeling to characterize the structural, hydraulic, and mechanical reservoir properties. We present an accurate description of the spatial and temporal distribution of observed microseismicity during 10 months of water injection, and a description of proposed shear zones associated with that microseismicity. This can be challenging, since the errors in event location are often large. To compensate for event-location errors (from 120 to 600 m) we performed a geomechanical simulation of the observed injection and constrained the results with known
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Fig. 1. Geological setting with: (a) the location of the studied area (white star) in the San Andreas fault zone system, (b) the structural map of the enhanced geothermal system area with the well locations P25, PS31, P32 and P38, and (c) seismic array deployed throughout The Geysers.
reservoir properties, along with geological information from wells and surface mapping. Hydromechanical data were calibrated by matching the observed pressure variations in three surrounding wells, and then fitting the calculated and observed microseismic events.
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Fig. 2. Evolution of the injection rate in P32 and of the microseismicity between September 2011 and August 2012. Gray areas represent six relevant periods, where the dynamic appearance of microseismic hypocenters is presented in detail in Fig. 4.
2. The Northwest Geysers area The Geysers geothermal field is the largest geothermal electricity-generating geothermal operation in the world, and has been in commercial production since 1960. The vapor-dominated geothermal reservoir consists of Franciscan metagraywacke that has been intruded into by a series of granitic (felsite) plutons (Schmitt et al., 2003). This reservoir is highly fractured, with extensive hydrothermal alteration and re-crystallization. Lockner et al. (1982) suggested that fracturing has weakened the rock significantly, and geothermal field models should assume that only a frictional sliding load can be supported by the rock. The authors also suggested that shear stress in the region is probably near the rockmass frictional strength; therefore, very small perturbations of the stress field could induce seismicity. Two previously active rightlateral strike-slip fault zones (Mercuryville and Collayomi) related to the San Andreas fault system created a pull-apart basin and now represent the southwest and northeast boundaries, respectively, of The Geysers geothermal field (McLaughlin, 1981) (Fig. 1a). Boreholes drilled into the reservoir encounter distinct steambearing fractures or “steam entries” separated by large intervals of unproductive rock (Thomas, 1981). Sammis et al. (1992) observed that well paths that deviate to the northeast and southwest tend to intersect more steam entries, indicating a dominant near-vertical northwest-striking fracture orientation. The steam seems to be contained within and transported by a relatively small number of significant fracture zones several tens of meters wide, with a separation of several hundred meters (Sammis et al., 1992). Fault zones are represented by clusters of steam entries as well as relatively rapid drilling rates. However, fracturing in The Geysers is complex, and different areas of the steam field may be dominated by different fracture orientations associated with different origins (Nielson et al., 1991). The EGS Demonstration Project is located in the Northwest Geysers area, where two main fault orientations trend to the northeast and northwest (Fig. 1b, Nielson et al., 1991). These two fault zone orientations, N050 and N130, belong to the Riedel system, respectively, R and P shear fractures, and form within the regional strike-slip fault zone system of the North Coast Ranges. This study uses the term shear zone as a generalized term to include discontinuous faults of limited extent, transtensional faults, shear zones, and Riedel shears. Within the EGS area the high temperature zone (HTZ) is directly below the normal temperature reservoir (NTR) at ∼2.6 km below ground surface. For the EGS demonstration, two previously abandoned exploratory wells, Prati 32 (P32) and Prati State 31 (PS31), were re-opened and deepened as an injection/production pair (Garcia et al., 2012) (Fig. 1b). P32 is sufficiently deep to penetrate the upper portion of the HTZ, and was dedicated as the injection well. Three other wells, Prati State 31 (PS31), Prati 38 (P38) and Prati 25 (P25), were used to monitor wellhead pressure variations during the initial injection in P32, which began on October 6, 2011. Note that P25 was converted to a steam production well part way through the injection period (Fig. 2). The injection was conducted
within the open-hole portion of P32, at depths between 2590 and 3400 m. Injection began with a high initial rate of 1200 gallons per minute (gpm) during the first 24 h to collapse the steam bubble in the well bore and surrounding reservoir rock. Thereafter, the injection rate was maintained at rates of 400, 1000, 700, and 400 gpm for intervals of 55, 100, 105, and 35 days, respectively (Fig. 2). 3. Microseismicity analysis Microseismic data are recorded by a dedicated seismic array deployed throughout The Geysers (Fig. 1c). The seismic array consists of 31 three-component short-period stations with a sampling frequency of 500 Hz (Majer and Peterson, 2007). Additionally, 15 temporary stations were located around the EGS demonstration area. Microseismic events are located using methods derived from conventional earthquake seismology; the events were located with SimulPS using a minimum of 22 P or S event picks. Events were required to have a maximum RMS travel-time residual of 0.1 s, and horizontal and vertical errors between 200 m and 600 m (Boyle and Zoback, 2013). Microseismic data presented in this paper were recorded from September 1, 2011, through August 10, 2012, i.e., almost 1 year of monitoring. This study focuses on an area of about 2 km2 around the P32 injection well. No microseismic events were detected in this area from September 1, 2011, to October 6, 2011, whereas 2919 microseismic events were detected during the injection from October 6, 2011, through August 10, 2012. The seismic events ranged in magnitude from 0.4 to nearly 3.0, but only 41 of these events had a magnitude higher than 2.0 (Fig. 2). 3.1. Microseismicity evolution The mapped study area has been divided into 900 cells of 2.3 × 10−3 km2 . In each cell, the number of seismic events occurring during the 10-month injection period has been determined (Fig. 3a), along with the summed seismic moment (‘Mo’, Fig. 3b). Approximately 70% of the seismic activity and energy release was located within a 0.38 km2 area. This area is not at the center of the injection interval; rather, it is slightly shifted toward the north. To better understand this microseismicity distribution and the associated fluid flow paths, the spatial distribution of the microseismic events has been carefully analyzed over specific time intervals. Although large horizontal positioning errors are anticipated, a visual inspection of hypocenters calculated utilizing the same velocity control strongly suggests two preferred orientations for the seismic events. Six relevant time intervals selected for the study are highlighted within Fig. 2. The distribution of these microseismic events is presented in Fig. 4 (1) without the interpreted faults, and (2) with interpreted faults resulting from this study. Fig. 4a and b shows the microseismicity distribution from October 6, 2011 (the beginning of injection) to October 20, 2011. During the first two days, only 5 microseismic events occurred. Then, from October 8 to October 10, 2011, 45 microseismic events occurred, with a third of these events identified by large black
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shear zone oriented N130 appears to have been initiated (F7). For simplicity, Fig. 5c shows only the microseismicity occurring along the fault Fa during this period, and the vertical event alignment suggests a near-vertical fault. Finally, four additional shear zones have been proposed, two oriented N050 (Fb and Fc) and two oriented N130 (F6 and F8), based on microseismic event alignments from mid-December 2011 through early April 2012 (Fig. 4g and h). Seismicity alignments Fb and Fc correspond to two N050 previously identified surface faults. Since injection began, eight events with magnitudes ranging between 2.2 and 2.9 have occurred along Fc. F6, which seems to be a continuation of the Squaw Creek Fault into the southeast part of the study area, correlates well with the steam entries in P31 and P32. F8 was also added, based on the location of P32 steam entries. This approach must contend with the uncertainty of large horizontal location errors, and shear-zone separations smaller than these horizontal location errors. However, the structural setting, constrained by several lines of evidence, was integrated within a geomechanical model designed to reproduce the reservoir response to P32 water injection. The result was a good match between (1) the proposed shear zones and mapped surface faults, and (2), the locations of steam entries along the well trajectories and the N130 oriented shear zones. Most of the steam entries correspond to proposed near-vertical northwest-striking fractures, separated by 150–200 m, which is consistent with Sammis et al. (1992).
4. Thermo-hydromechanical simulation
Fig. 3. Map view of (a) the number of seismic events occurring during the injection, and (b) their summed seismic moment by cell size of 2.3 × 10−3 km2 .
points aligned along a line trending N130 and indicated as “F5” on subsequent figures. F5 appears to be a splay of the Squaw Creek Fault (Fig. 4b). From October 11–17, 2011, 35 microseismic events were detected; a third of these events are aligned along ‘F5’ (small red triangles), and another third are aligned along a new N130 shear zone (called ‘F4’, red triangles). From the 18th to 20th of October the microseismic events kept spreading toward the NE (green stars). These two shear zones (‘F4’ and ‘F5’) appear to be near vertical (Fig. 5a). Figs. 4c, d, and 5b show the microseismicity distribution for November 1–6, 2011 (6 days), November 14–19, 2001 (6 days), and November 28–30, 2011 (3 days). Within each of these periods, microseismic events begin to occur in areas that previously had no seismicity (or had very few events). For each of these periods, microseismic events are aligned along a N130 azimuth and are believed to indicate the presence of three N130 shear zones (green stars: F3, believed to be a splay of the Squaw Creek Fault; yellow triangles: F2; and blue squares: F1). Elsewhere, the microseismic events seem to occur randomly within the reservoir (gray points), or along the aforementioned N130 shear zones identified as F4 and F5 (red and black points). The faults F1, F2, and F3 also seem to be nearly vertical. Fig. 4e and f shows the microseismicity distribution during the first six days following the injection rate increase in December 2011 from 400 to 1000 gpm. Two new fluid flow paths seem to be developing. The first is a N050 shear zone (Fa) reactivation with apparent fluid migration to the southeast of the study area, where a second
In recent years, coupled thermo-hydro-mechanical simulation has been widely used to characterize the mechanical behavior of geological formations in many engineering disciplines: oil and gas production (Verdon et al., 2011), geological CO2 storage (Cappa and Rutqvist, 2011), radioactive waste repositories (Jing et al., 1995), and geothermal energy extraction (Rutqvist et al., 2010). Here, we use the TOUGH-FLAC simulator (Rutqvist et al., 2002; Rutqvist, 2011), which links a multiphase flow code (TOUGH2—Pruess et al., 2011) and a geomechanical code (FLAC3D —Itasca, 2009). This simulator has the required capabilities for modeling of nonisothermal, multiphase flow processes coupled with stress changes induced by temperature variations and fluid-pressure changes. In this simulation we apply TOUGH-FLAC to simulate The Geysers reservoir steam production and water injection with an approach previously used in Rutqvist and Oldenburg (2008), Rutqvist et al. (2010), and Rutqvist et al. (2013). According to this approach, the modeling and analysis is conducted in two steps: the first step is to calculate elastic stress changes and deformations induced by changes in fluid pressure and temperature; the second step is to look at these stress changes and evaluate the potential for induced seismicity. As described in Rutqvist et al. (2013), one advantage of the adopted approach is that it is not a requirement to know the exact absolute magnitude and direction of the in situ stress field, but it is assumed that the rock mass is near-critically stressed for shear failure (see Section 4.2). We simulate the rock mass as an equivalent continuum with an implicit representation of fractures. With this approach, the required hydraulic properties are fracture permeability and fracture porosity. In Rutqvist et al. (2013), the permeability of the reservoir rock was on the order of 1 × 10−14 m2 , and fracture porosity was back-calculated to 0.4%. Initially, the fracture system is filled with steam, having a very low liquid saturation, and the steam pressure is only a few MPa. In this study, we add the explicit representation of major hydraulic structures such as significant steam-bearing fracture zones or faults. These fracture zones or faults are represented by equivalent continuum properties,
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Fig. 4. Dynamic appearance of microseismic hypocenters presented in detail for four relevant periods. For the same period two maps are shown, one with all the seismic events detected during this period, and another map with our interpretation and the main faults mapped in the field. The circles represent the average location uncertainty and the small black squares denote steam entries in the wells. (For interpretation of the references to color in text, the reader is referred to the web version of the article.)
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Fig. 6. Structural map built from the microseismicity analyses, with the location of the largest earthquakes (magnitude > 2.0) and the map of the seismic event density. The circles represent the average location uncertainty.
presence of high-permeability near-vertical structures as an explanation for microseismicity far below the P32 injection well just a few days after injection start-up. However, our approach does not consider the vertical extent of the stimulation zone, and shear-zone permeability may be underestimated. The model explicitly represents eight shear zones oriented N130 (F1–F8), which are intersected perpendicularly by three N050 shear zones (Fa–Fc) (Fig. 7). These modeled shear zones are 15 m thick as suggested by Sammis et al. (1992). The model extends 12 km × 12 km laterally, far enough from the injection point to simulate an infinite acting boundary. The injection point (P32) and the monitoring point (PS31) are located along the same shear zone, F5,
Fig. 5. View in 3D of the dynamic appearance of microseismic hypocenters along the shear zones (a) F4 and F5, (b) ‘F1’–‘F3’ and (c) along ‘Fa’. (For interpretation of the references to color in text, the reader is referred to the web version of the article.)
consisting of fractured rocks with an identified fracture permeability, fracture porosity, and elastic moduli different from the surrounding rock mass. 4.1. Model setup The model geometry and parameters allow validation of the proposed geological structural setting (Fig. 6) by comparing the lateral extent of the stimulation zone to the actual microseismicity patterns. A two-dimensional horizontal model is used to represent the study area around the NW Geysers P32 injection well. The results from previous 3D simulations (Rutqvist et al., 2013) suggest the
Fig. 7. Model geometry for the fluid flow simulation.
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with a separation of 360 m. Additional monitoring points P25 and P38 are located 750 m and 1400 m away from the injection point, respectively. Injection was simulated at a depth of 2450 m below sea level. At this depth, the initial steam pressure and temperature are assumed to be 1.8 MPa and 271 ◦ C (Rutqvist and Oldenburg, 2008).
4.2. Properties calibration We first used the iTOUGH2 inverse modeling code (Finsterle, 1999) in combination with the EOS3 module of TOUGH2 (Pruess et al., 2011) to estimate the hydraulic reservoir properties. iTOUGH2 (http://www-esd.lbl.gov/iTOUGH2) is a computer program designed for parameter estimation, sensitivity analysis, and uncertainty propagation analysis (Finsterle, 1999). The inverse problem program is designed for automatic model calibration, and the best-fit is determined by minimizing the weighted differences between measured and calculated hydrological data. For this study, the permeability and porosity values assigned to the host rock and the shear zones were estimated by matching the monitored pressure variations in wells PS31 and P25 (Fig. 7). The pressure variation in P38 after 300 days of injection is considered to be negligible (less than 1.5 × 105 Pa). The injection rate, derived from field data, was reduced by a factor of 45 due to the geometric relationship between the model and the actual system. This factor was calculated during the iTOUGH2 simulation. Then, we conducted a geomechanical analysis to estimate Young’s moduli for the rock mass and the N050 and N130 shear zones. The best match was found by fitting the map of the predicted cumulative microseismicity potential (Rutqvist et al., 2013) with observed locations of microseismic events. The microseismicity potential was calculated following the approach used by Rutqvist et al. (2010), which evaluates the potential for shear slip under the conservative assumption that fractures of any orientation could exist anywhere. At The Geysers, such an assumption is consistent with the findings of Oppenheimer (1986), who indicated that seismic sources occur from essentially randomly oriented fracture planes. One key parameter in estimating the likelihood of shear reactivation along a fracture is the coefficient of static friction, , entering the Coulomb shear failure criterion. Cohesionless faults are usually assumed to have a friction coefficient of 0.6–0.85 (e.g., Barton et al., 1995). Moreover, a frictional coefficient of = 0.6 is a lower-limit value observed in fractured rock masses (Barton et al., 1995). Thus, using = 0.6 in the Coulomb
criterion would most likely give a conservative estimate of likely seismicity. For = 0.6, the Coulomb criterion for the onset of shear failure can be written in the following form: = 33 1c
(1)
is the critical maximum principal stress for the onset where 1c of shear failure. Thus, shear reactivation of a fracture would be induced whenever the maximum principal effective stress is three times higher than the minimum principal stress. Based on the concept of a critically stressed rock mass, the initial condition will be a state of incipient failure. Studying stress-state deviation from the near-critical condition allows us to better understand whether these variations tend to move the system toward or away from a state of failure. Of course, it is possible to model any initial condition and consider if a change in the stress state increases or decreases the likelihood of shear failure. The likelihood of shear reactivation increases if the change in the maximum principal compressive effective stress is more than three times the change in minimum principal effective stress (i.e., if 1 ≥ 3 × 3 ). Conversely, the likelihood of shear reactivation would decrease if the change in maximum principal compressive effective stress were less than three times the change in minimum principal effective stress (i.e., if 1 < 3 × 3 ). Considering that the initial stress might not be exactly at the point of critical stress, we may quantify how much the 1 has to exceed 3 × 3 to induce additional shear reactivation. We = therefore define a stress-to-strength change margin as 1m 1 − 3 × 3 . 1m represents the potential to induce shear reactivation and induced seismicity, in which a higher value would imply a higher potential for triggering induced seismicity. We therefore denote 1m as the microseismicity potential.
4.3. Results The best-fit solution to match (1) the pressure evolution in the surrounding wells (Fig. 8) and (2) the map of the cumulative microseismicity potential with observed locations of microseismic events (Fig. 9) was obtained for highly contrasting hydraulic (Table 1) and mechanical values between the host rock and the two shear zone families. The estimated host-rock permeability (Khr ) is equal to 1.3 × 10−14 m2 , while N130 shear-zone permeability values are estimated to be one to two orders higher than Khr , ranging between
Fig. 8. Comparison between the pressure evolutions measured in situ in wells PS31 and P25 with the calculated pressure evolution.
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Fig. 9. Comparisons, at two different scales (a) and (c) of the maps of the observed seismic event density (see Fig. 3, for the legend of the seismic event density) and the modeled cumulative microseismicity potential (b and d). Table 1 Reservoir hydraulic properties estimated from iTough2.
Matrix f1 f2 f3 f4 f5 f6 f7 f8 Fa Fb Fc
Permeability (m2 )
Porosity
1.69E−14 3.80E−13 3.80E−13 3.72E−13 7.12E−13 3.80E−13 4.80E−13 2.34E−12 2.34E−12 1.91E−14 1.91E−14 1.00E−22
0.028 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.01 0.01 0.01
2.3 × 10−12 m2 to 3.8 × 10−13 m2 . The permeabilities estimated for the N050 shear zones (1.9 × 10−14 m2 ) are slightly lower than Khr , and the shear zone Fc is estimated to be eight orders of magnitude lower than that of the host rock (1.0 × 10−22 m2 ). The very low permeability in Fc is necessary to match the lack of pressure response in P38; this is also consistent with the abrupt decrease in microseismicity on the SE side of this structure. Estimated porosities are very low: 0.2% in the host rock, and 0.1% and 0.5% in the N050 and N130 shear zones, respectively. Fig. 9 shows comparisons between the maps of cumulative microseismicity potential and the map of the seismic density. Three areas of different seismicity characteristics were identified: (1) Area 1, around the injection point; 1m is close to 20 MPa, with (during the injection) 25 to 15 events detected over 2.3 × 10−3 km2 ; (2) Area 2, with 1m ranging from 20 to 10 MPa, with 15 to 7 events varies detected over 2.3 × 10−3 km2 ; and (3) Area 3, where 1m from 10 to 3 MPa, with 7 to 3 events monitored per 2.3 × 10−3 km2 .
These results were obtained for Young’s moduli equal to 2.5 GPa in the N130 shear zones, and 8 GPa in the host rock and in the N050 shear zones. 5. Discussion 5.1. Influence of the geological setting on the EGS The two shear-zone families highlighted in his study (N130 and N050) belong to the Riedel system forming, within the San Andreas fault system, respectively R and P shear fractures. Their locations correspond to the faults mapped in the field. This suggests that fluid flow is strongly influenced by the shear-zone network inherited from the regional tectonic activity, and the tectonic structures visible on the surface have a strong impact on the fluid flow at several kilometers depth. The low permeability back-calculated for some shear zones can be explained by the presence of a thick fault gouge, which may allow for high permeability along its strike, but lower permeability across its strike. 5.2. Mechanisms of inducing microseismicity Fig. 10 a shows predicted changes in fluid pressure greater than 2.7 × 105 Pa at the end of the injection after 294 days. The figure shows that the injection has led to a pressurization of the reservoir at a kilometer scale. The shape of the pressurized area is strongly influenced by (i) the N050 shear zone (Fc), which acts as an impermeable boundary; (ii) the more permeable N130 shear zones, which favor pressure diffusion; and (iii) slight permeability contrasts among the N130-trending shear zones. The comparison between the map of the pressure variation and that of the seismic event density recorded during the injection (Fig. 9a and d) shows
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Fig. 10. Contour of changes in (a) pressure higher than 2.7 × 105 Pa, and (b) in temperature replaced and oriented on the map.
good agreement. The contour areas where 70% and 100% of the seismic activity occurred correspond to the areas where the fluid pressure increases at least 4.2 × 105 Pa and 2.7 × 105 Pa, respectively. Cooling is observed exclusively around the injection point in P32 (−180 ◦ C), due to the presence of cold liquid water (Fig. 10b). This suggests that injection-induced changes in the steam pressure are the dominant cause for triggering shear reactivation far from the injection. These results are consistent with Rutqvist et al. (2013). 5.3. Larger microseismic events We have located (on the structural map) the positions of the 41 microseismic events with a magnitude higher than 2.0 (Fig. 6). It appears that most of them are located either (i) just around the injection zone, with a high concentration of microseismic events of magnitudes ranging from 2.0 to 2.2; or (ii) along the shear zones previously identified, with magnitudes ranging from 2.2 to 2.9. Interestingly, by comparing the location of these 41 microseismic events with the map of the microseismicity density, it appears that the largest magnitude events occur outside the high density microseismicity area. Moreover it seems that the events with largest magnitude occur at intersections between the N130 and N050 shear zones. The magnitude of a seismic event depends on several factors: the stress accumulated before slipping, how fast it fails, the depth where it occurs (Brune and Thatcher, 2002), and the size of the slipping surface (the bigger the fault, the larger the seismic magnitude). Here, the biggest microseismic events were located between 2.5 and 3.5 km (below sea level), outside the area of high seismic density and close to the intersections of shear zones N130 and N050 (Fig. 6). The geomechanical simulation has highlighted a strong permeability contrast between these two shear zone families, with one acting as a drain and favoring fluid pressure diffusion, and the other potentially acting as an impermeable boundary and allowing slight steam-pressure buildup. These results seem to indicate that the area in which the largest microseismic events could occur are: (i) far from the injection, because close to the injection, the high-density low-magnitude micro-earthquake activity gradually relieves stresses and thereby prevents a significant stress drop needed for the development of larger events; and (ii) at the intersections of permeable and semi-permeable faults, where slight steam overpressure can quickly develop. 5.4. Geomechanical simulation Characterizing the structural and hydromechanical properties of the EGS reservoir under exploitation can greatly benefit its design
and management. If microseismicity monitoring is used to identify the main fluid paths, the large errors in event locations may be a limiting factor, preventing an accurate description of the geological structures. In this study, we show that this limitation can be reduced with a combination of geological studies and geomechanical modeling. In this study, structural reservoir properties were characterized by matching the monitored pressure variations in the surrounding wells, and the shape and size of the observed seismicity with the map of the calculated microseismicity potential. This approach allows testing hypotheses made about the shear-zone properties: orientations, number, and spacing. Here, we used geological data to build our structural model, but if these kinds of data are not available, a sensitivity analyses could be done on the structural reservoir properties to create the map of the microseismicity potential. Identifying the existence of shear zones and their hydromechanical properties can facilitate reservoir management. Such activity can help to predict where permeability increases would be critical. Indeed, the existence of a highly permeable shear zone could limit the effect of hydraulic stimulation, because fast fluid flow would prevent higher overpressure. Inversely, field observations at the Soultz site have shown that important injectivity gains due to hydraulic stimulation are usually observed in the initially less productive shear zone (Gentier et al., 2013). Hence, geomechanical modeling can help to identify which shear zone is most susceptible to reactivation, and to investigate its future performance.
6. Conclusion In this study, we utilized microseismicity analysis coupled with thermo-hydro-mechanical modeling to characterize the structural, hydraulic, and mechanical reservoir properties at the Northwest Geysers EGS project, in California. We presented a detailed description of the spatial and temporal distribution of observed microseismicity during 10 months of water injection, and delineated proposed shear zones associated with that microseismicity. The results of this study show that (1) the hydraulic and mechanical behavior of the studied EGS reservoir is controlled by a network of intersecting shear zones and their hydraulic properties, (2) detailed microseismic monitoring and analysis can be useful in delineating such shear zones, and (3) coupled fluid flow and geomechanical analysis, corroborated with field data of pressure and microseismicity, can be used to back-calculate their hydraulic and mechanical properties. In addition, our study—which for the first time includes the detailed discrete modeling of preferential flow along shear zone—confirms previous findings regarding the cause
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and mechanisms of injection-induced seismicity at The Geysers. That is, over a one-year injection operation, the microseismicity is primarily caused by thermal contraction near the injection well where strong cooling prevails; whereas away from the injection well, small increases in steam pressure are the primary trigger of microseismicity. Such increases in steam pressure seem to be particularly important for triggering microseismicity at reservoirbounding semi-permeable shear zones. Acknowledgments This work was conducted with funding by the Assistant Secretary for Energy Efficiency and Renewable Energy, Geothermal Technologies Program, of the U.S. Department under the U.S. Department of Energy Contract No. DE-AC02-05CH11231, and by Calpine Corporation. We are grateful to Katie Boyle and Lawrence Hutchings from Lawrence Berkeley National Laboratory (LBNL) for making seismic data available to us, and for the constructive comments and recommendations of the reviewers.
References Allis, R.G., 1992. Mechanism of induced seismicity at The Geysers geothermal reservoir, California. Geophys. Res. Lett. 9, 629–632. Barton, C.A., Zoback, M.D., Moos, D., 1995. Fluid flow along potentially active faults in crystalline rock. Geology 23, 683–686. Boyle, K., Zoback, M., 2013. Stress and fracture orientation in the northwest Geysers geothermal field. In: Proceedings of the Thirty-Eighth Workshop on Geothermal Reservoir Engineering, Stanford University, Stanford, California, February 11–13. Brune, J., Thatcher, W., 2002. International Handbook of Earthquake and Engineering Seismology. International Association of Seismology and Physics of Earth’s Interior, Committee on Education, vol. 81A., pp. 569–588. Cappa, F., Rutqvist, J., 2011. Modeling of coupled deformation and permeability evolution during fault reactivation induced by deep underground injection of CO2 . Int. J. Greenhouse Gas Control 5, 336–346, http://dx.doi.org/10.1016/j.ijggc.2010.08.005. Eberhart-Phillips, D., Oppenheimer, D.H., 1984. Induced seismicity in The Geysers geothermal area, California. J. Geophys. Res. 89, 1191–1207. Finsterle, S., 1999. iTOUGH2 Sample Problems. Lawrence Berkeley National Laboratory LBNL-40042, Berkeley. Garcia, J., Walters, M., Beall, J., Hartline, C., Pingol, A., Pistone, S., Wright, M., 2012. Overview of the Northwest Geysers EGS demonstration project. In: Proceedings of the 37th Workshop on Geothermal Reservoir Engineering, Stanford University, Stanford, California, January 30–February 1, 2012. Gentier, S., Rachez, X., Blaionneau, A., Guillon, T., 2013. Role of the 3D network of faults on the response to hydraulic stimulation of deep geothermal wells. In: Proceedings of the 47th U.S. Rock Mechanics Symposium, San Francisco, CA, USA, June 26, 2013. American Rock Mechanics Association Paper No. 13-278. Itasca, 2009. FLAC3D V4.0, Fast Lagrangian Analysis of Continua in 3 Dimensions, User’s Guide. Itasca Consulting Group, Minneapolis, MN. Jing, L., Tsang, C.-F., Stephansson, O., 1995. DECOVALEX—an international cooperative research project on mathematical models of coupled THM processes for safety analysis of radioactive waste repositories. Int. J. Rock Mech. Min. Pi. Geomech. Abstr. 32 (5), 389–398. Lockner, D.A., Summer, R., Moore, D., Byerlee, J.D., 1982. Laboratory measurements of reservoir rock from the Geysers Geothermal Field, California. Int. J. Rock Mech. Min. Sci. 19, 65–80.
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Majer, E.L., Peterson, J.E., 2007. The impact of injection on seismicity at The Geysers, California Geothermal Field. Int. J. Rock Mech. Min. Sci. 44, 1079–1090. McLaughlin, R.J., 1981. Tectonic Setting of Pre-Tertiary Rocks and Its Relation to Geothermal Resources in The Geysers-Clear Lake Area. U.S. Geological Survey Professional Paper 1141, Northern California, pp. 3–23. Mossop, A., Segall, P., 1997. Subsidence at The Geysers geothermal field, N. California from a comparison of GPS and leveling surveys. Geophys. Res. Lett. 24, 1839–1842. Mossop, A.P., (Ph.D. Dissertation) 2001. Seismicity, Subsidence and Strain at The Geysers Geothermal Field. Stanford University. Nielson, D.L., Walters, M.A., Hulen, J.B., 1991. Fracturing in the Northwest Geysers, Sonoma county, California. Geotherm. Resourc. Counc. Trans. 15, 27–33. Oppenheimer, D.C., 1986. Extensional tectonics at the Geysers Geothermal Area, California. J. Geophys. Res. 91, 11463–11476. Pearson, C., 1981. The relationship between microseismicity and high pore pressures during hydraulic stimulation experiments in low permeability granitic rock. J. Geophys. Res. 86, 7855–7864. Phillips, W.S., House, L.S., Fehler, M.C., 1997. Detailed joint structure in a geothermal reservoir from studies of induced microearthquake clusters. J. Geophys. Res. 102, 11745–11763. Pruess, K., Oldenburg, C., Moridis, G., 2011. TOUGH2 User’s Guide, Version 2.1, LBNL43134 (Revised). Lawrence Berkeley National Laboratory, Berkeley, CA. Ross, A., Foulger, G.R., Julian, B.R., 1999. Source processes of industrially-induced earthquakes at The Geysers geothermal area, California. Geophysics 64, 1877–1889. Rutqvist, J., 2011. Status of the TOUGH-FLAC simulator and recent applications related to coupled fluid flow and crustal deformations. Comput. Geosci. 37, 739–750. Rutqvist, J., Wu, Y.-S., Tsang, C.-F., Bodvarsson, G., 2002. A modeling approach for analysis of coupled multiphase fluid flow, heat transfer, and deformation in fractured porous rock. Int. J. Rock Mech. Min. Sci. 39, 429–442. Rutqvist, J., Oldenburg, C.M., 2008. Analysis of injection-induced micro-earthquakes in a geothermal steam reservoir, Geysers Geothermal Field, California. In: Proceedings of the 42nd U.S. Rock Mechanics Symposium, San Francisco, CA, USA, June 29–July 2, 2008. American Rock Mechanics Association, Paper No. 151. Rutqvist, J., Oldenburg, C.M., Dobson, P.F., Garcia, J., Walters, M., 2010. Predicting the spatial extent of injection-induced zones of enhanced permeability at the Northwest Geysers EGS demonstration project. In: Proceedings of the 44th U.S. Rock Mechanics Symposium, Salt Lake City, UT, USA, June 27–June 30, 2010. American Rock Mechanics Association ARMA, Paper No. 502. Rutqvist, J., Dobson, P.F., Garcia, J., Hartline, C., Jeanne, P., Oldenburg, C.M., Vasco, D.W., Walters, M., 2013. The Northwest Geysers EGS demonstration project, California: pre-stimulation modeling and interpretation of the stimulation. Math. Geosci., http://dx.doi.org/10.1007/s11004-013-9493-y. Sammis, C.G., An, L., Ershaghi, I., 1992. Determining the 3-D fracture structure in the geysers geothermal reservoirs. In: Proceedings of 21st Workshop on Geothermal Reservoir Engineering, Stanford, CA. Schmitt, A.K., Grove, M., Harrison, T.M., Lovera, O., Hulen, J., Walters, M., 2003. The Geysers–Cobb Mountain magma system, California (Part 2): timescales of pluton emplacement and implications for its thermal history. Geochim. Cosmochim. Acta 67, 3443–3458. Smith, J.L.B., Beall, J.J., Stark, M.A., 2000. Induced seismicity in the SE Geysers field. Geotherm. Resourc. Counc. Trans. 24, 24–27. Stark, M.A., 1992. Microearthquakes – a tool to track injected water in The Geysers reservoir. In: Monograph on The Geysers Geothermal Field, Geothermal Resources Council, Special Report No. 17, pp. 111–117. Tester, J.W., et al., 2006. The Future of Geothermal Energy. Part 1—Summary and Part 2—Full Report. Massachusetts Institute of Technology, Cambridge, MA. Thomas, R.P., 1981. Subsurface geology. In: A Reservoir Assessment of The Geysers: California Department of Conservation, Division of Oil and Gas Report TR-27, pp. 9–20. Verdon, P.P., Kendall, J.M., White, D.J., Angus, D.A., 2011. Linking microseismic event observations with geomechanical models to minimize the risks of storing CO2 in geological formations. Earth Planet. Sci. Lett. 305, 143–152.
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Geothermics journal homepage: www.elsevier.com/locate/geothermics
Reservoir structure and properties from geomechanical modeling and microseismicity analyses associated with an enhanced geothermal system at The Geysers, California Pierre Jeanne a,∗ , Jonny Rutqvist a , Craig Hartline b , Julio Garcia b , Patrick F. Dobson a , Mark Walters b a b
Lawrence Berkeley National Laboratory, Earth Sciences Division, Berkeley, CA 94720, USA Calpine Corporation, 10350 Socrates Mine Road, Middletown, CA 95461, USA
a r t i c l e
i n f o
Article history: Received 17 July 2013 Accepted 9 February 2014 Available online 9 April 2014 Keywords: Enhanced geothermal system Micro-earthquake analysis Fault zone network Inverse fluid flow modeling Thermo-hydromechanical simulation Micro-earthquake predicted
a b s t r a c t This work contributes to modeling studies associated with an enhanced geothermal system demonstration project in the northwestern region of The Geysers, California. We first attempt to determine the structural configuration and reservoir properties of the steam-bearing reservoir, based on microseismicity recorded during a one-year water injection operation. This is particularly challenging because errors in hypocenter determination (due primarily to errors in the velocity model and first-arrival picks) tend to “defocus” any microseismic events related to a distributed network of fractures, resulting in a “cloud” of microseismic events. This work includes a dynamic analysis of the observed alignments in daily microseismicity hypocenters during water injection, along with the constraints provided by geological data (surface mapping and drill cuttings) to determine the location and orientation of shear zones. We then evaluate the viability of the resulting network of proposed shear zones, using a 2D fluid flow and geomechanical model simulation of the injection and comparing it to the evolution of observed (1) pressure in nearby monitoring wells and (2) microseismicity hypocenters. The shear-zone hydraulic properties were estimated using inverse analysis of the pressure evolution in the surrounding wells, while mechanical properties were estimated by comparing the calculated stress changes and associated microseismic potential with the observed microseismicity. The results indicate that a model including the network of proposed shear zones does calculate reservoir hydraulic and mechanical responses similar to those observed during water injection. Finally, the results confirm previous studies at The Geysers indicating that the injection-induced microseismicity is caused by thermal contraction near the injection wells where strong cooling prevails, whereas away from the injection wells, small increases in steam pressure are the primary trigger of microseismicity. Published by Elsevier Ltd.
1. Introduction Enhanced geothermal systems (EGS) have the potential to extract geothermal energy under circumstances and in locations where conventional production is not economically feasible (Tester et al., 2006). Generally, EGS would be applied to extract geothermal energy at sites where the reservoir rock is hot (has sufficient heat content) but has insufficient permeability for economic production. The successful application of EGS technology could allow large, untapped resources to be utilized, greatly increasing
∗ Corresponding author. Tel.: +1 510 486 6261. E-mail addresses: [email protected], [email protected] (P. Jeanne). http://dx.doi.org/10.1016/j.geothermics.2014.02.003 0375-6505/Published by Elsevier Ltd.
geothermal electrical production worldwide. Reservoir permeability is increased by utilizing water injection to reactivate fractures or possibly create new fractures. Such fracture reactivation or creation can be accompanied, however, by microseismic events. The monitoring of such microseismic activity can provide valuable information concerning the response of the reservoir to water injection. Microseismicity monitoring and analysis has been a component of EGS experiments for over 30 years. Early programs simply used microseismicity to estimate the extent of the stimulation zone (Pearson, 1981). With the development of higher-precision hypocenter determination, it has been possible to extract more information concerning geological structures and fluid paths associated with shear zones and faults (Phillips et al., 1997). However, it
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is still a challenge to characterize the structural configuration and reservoir properties at several kilometers depth, owing to significant errors in hypocenter determination. At The Geysers geothermal area in California, induced seismicity has been the subject of numerous studies (e.g., Allis, 1992; Eberhart-Phillips and Oppenheimer, 1984; Stark, 1992; Ross et al., 1999; Smith et al., 2000; Mossop, 2001; Majer and Peterson, 2007; Rutqvist and Oldenburg, 2008; Boyle and Zoback, 2013; Rutqvist et al., 2013). Several mechanisms are responsible for The Geysers’ induced seismicity development, mechanisms linked to water injection or steam extraction. The first mechanism linked to water injection is the thermoelastic stress perturbation associated with cold-water injection into a hot reservoir. This causes a contraction of the rock volume near fracture surfaces, reducing the static friction and allowing shear movement in the vicinity of the injection well (Rutqvist and Oldenburg, 2008). Away from the well, injectioninduced changes in the steam pressure appear to be the dominant cause for triggering shear reactivation (Rutqvist et al., 2013). The last mechanism is linked to steam extraction. Poroelastic stress variations are associated with a nearly reservoir-wide reduction in steam pressure (Oppenheimer, 1986) and has led to subsidence in many parts of the geothermal field (e.g., Mossop and Segall, 1997). More recently, microseismicity was used to provide constraints on the conceptual geomechanical model (Rutqvist et al., 2013) of an EGS system. Accurate hydrological and geomechanical models allow an investigation of injection strategies for EGS development and to predict the extent of the stimulation zone. Here we present a modeling study conducted in support of an EGS demonstration project at the northwestern Geysers geothermal field, California (Fig. 1). The project aims to develop an EGS by directly and systematically injecting cool water at relatively low pressure into a known high temperature (280–400 ◦ C) zone (HTZ) located beneath the conventional (240 ◦ C) geothermal steam reservoir (Garcia et al., 2012; Rutqvist et al., 2013). Injection began on October 6, 2011, and microseismic events were monitored from September 1, 2011, to August 10, 2012. During this period, 2919 microseismic events were recorded in the study area (Fig. 2). Rutqvist et al. (2013) presented a comparison of pre-stimulation model predictions with observed microseismicity over the first few months of injection. The authors used a model in which the rock mass was represented by an equivalent continuum. Although the extent of the calculated stimulation zone matched the field observations very well over the first few months of injection, the long-term results suggest more complex geology. For example, it was found that the observed stimulation zone extends much deeper than that predicted by the numerical modeling, indicating greater vertical permeability perhaps associated with permeable vertical fractures. The existence of such discrete largescale permeable fractures is evident from logging of boreholes drilled into the reservoir, which frequently encounters distinct “steam entries” separated by large intervals of unproductive rock. Previous studies suggest that The Geysers’ reservoir transports steam via a relatively small number of fracture zones (e.g., Sammis et al., 1992). It is likely that such fracture zones connected to the injection well will also channel most of the injected water, which thereby could also migrate downwards by gravity. In this paper, we utilize microseismicity analysis coupled with hydro-thermo-mechanical modeling to characterize the structural, hydraulic, and mechanical reservoir properties. We present an accurate description of the spatial and temporal distribution of observed microseismicity during 10 months of water injection, and a description of proposed shear zones associated with that microseismicity. This can be challenging, since the errors in event location are often large. To compensate for event-location errors (from 120 to 600 m) we performed a geomechanical simulation of the observed injection and constrained the results with known
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Fig. 1. Geological setting with: (a) the location of the studied area (white star) in the San Andreas fault zone system, (b) the structural map of the enhanced geothermal system area with the well locations P25, PS31, P32 and P38, and (c) seismic array deployed throughout The Geysers.
reservoir properties, along with geological information from wells and surface mapping. Hydromechanical data were calibrated by matching the observed pressure variations in three surrounding wells, and then fitting the calculated and observed microseismic events.
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Fig. 2. Evolution of the injection rate in P32 and of the microseismicity between September 2011 and August 2012. Gray areas represent six relevant periods, where the dynamic appearance of microseismic hypocenters is presented in detail in Fig. 4.
2. The Northwest Geysers area The Geysers geothermal field is the largest geothermal electricity-generating geothermal operation in the world, and has been in commercial production since 1960. The vapor-dominated geothermal reservoir consists of Franciscan metagraywacke that has been intruded into by a series of granitic (felsite) plutons (Schmitt et al., 2003). This reservoir is highly fractured, with extensive hydrothermal alteration and re-crystallization. Lockner et al. (1982) suggested that fracturing has weakened the rock significantly, and geothermal field models should assume that only a frictional sliding load can be supported by the rock. The authors also suggested that shear stress in the region is probably near the rockmass frictional strength; therefore, very small perturbations of the stress field could induce seismicity. Two previously active rightlateral strike-slip fault zones (Mercuryville and Collayomi) related to the San Andreas fault system created a pull-apart basin and now represent the southwest and northeast boundaries, respectively, of The Geysers geothermal field (McLaughlin, 1981) (Fig. 1a). Boreholes drilled into the reservoir encounter distinct steambearing fractures or “steam entries” separated by large intervals of unproductive rock (Thomas, 1981). Sammis et al. (1992) observed that well paths that deviate to the northeast and southwest tend to intersect more steam entries, indicating a dominant near-vertical northwest-striking fracture orientation. The steam seems to be contained within and transported by a relatively small number of significant fracture zones several tens of meters wide, with a separation of several hundred meters (Sammis et al., 1992). Fault zones are represented by clusters of steam entries as well as relatively rapid drilling rates. However, fracturing in The Geysers is complex, and different areas of the steam field may be dominated by different fracture orientations associated with different origins (Nielson et al., 1991). The EGS Demonstration Project is located in the Northwest Geysers area, where two main fault orientations trend to the northeast and northwest (Fig. 1b, Nielson et al., 1991). These two fault zone orientations, N050 and N130, belong to the Riedel system, respectively, R and P shear fractures, and form within the regional strike-slip fault zone system of the North Coast Ranges. This study uses the term shear zone as a generalized term to include discontinuous faults of limited extent, transtensional faults, shear zones, and Riedel shears. Within the EGS area the high temperature zone (HTZ) is directly below the normal temperature reservoir (NTR) at ∼2.6 km below ground surface. For the EGS demonstration, two previously abandoned exploratory wells, Prati 32 (P32) and Prati State 31 (PS31), were re-opened and deepened as an injection/production pair (Garcia et al., 2012) (Fig. 1b). P32 is sufficiently deep to penetrate the upper portion of the HTZ, and was dedicated as the injection well. Three other wells, Prati State 31 (PS31), Prati 38 (P38) and Prati 25 (P25), were used to monitor wellhead pressure variations during the initial injection in P32, which began on October 6, 2011. Note that P25 was converted to a steam production well part way through the injection period (Fig. 2). The injection was conducted
within the open-hole portion of P32, at depths between 2590 and 3400 m. Injection began with a high initial rate of 1200 gallons per minute (gpm) during the first 24 h to collapse the steam bubble in the well bore and surrounding reservoir rock. Thereafter, the injection rate was maintained at rates of 400, 1000, 700, and 400 gpm for intervals of 55, 100, 105, and 35 days, respectively (Fig. 2). 3. Microseismicity analysis Microseismic data are recorded by a dedicated seismic array deployed throughout The Geysers (Fig. 1c). The seismic array consists of 31 three-component short-period stations with a sampling frequency of 500 Hz (Majer and Peterson, 2007). Additionally, 15 temporary stations were located around the EGS demonstration area. Microseismic events are located using methods derived from conventional earthquake seismology; the events were located with SimulPS using a minimum of 22 P or S event picks. Events were required to have a maximum RMS travel-time residual of 0.1 s, and horizontal and vertical errors between 200 m and 600 m (Boyle and Zoback, 2013). Microseismic data presented in this paper were recorded from September 1, 2011, through August 10, 2012, i.e., almost 1 year of monitoring. This study focuses on an area of about 2 km2 around the P32 injection well. No microseismic events were detected in this area from September 1, 2011, to October 6, 2011, whereas 2919 microseismic events were detected during the injection from October 6, 2011, through August 10, 2012. The seismic events ranged in magnitude from 0.4 to nearly 3.0, but only 41 of these events had a magnitude higher than 2.0 (Fig. 2). 3.1. Microseismicity evolution The mapped study area has been divided into 900 cells of 2.3 × 10−3 km2 . In each cell, the number of seismic events occurring during the 10-month injection period has been determined (Fig. 3a), along with the summed seismic moment (‘Mo’, Fig. 3b). Approximately 70% of the seismic activity and energy release was located within a 0.38 km2 area. This area is not at the center of the injection interval; rather, it is slightly shifted toward the north. To better understand this microseismicity distribution and the associated fluid flow paths, the spatial distribution of the microseismic events has been carefully analyzed over specific time intervals. Although large horizontal positioning errors are anticipated, a visual inspection of hypocenters calculated utilizing the same velocity control strongly suggests two preferred orientations for the seismic events. Six relevant time intervals selected for the study are highlighted within Fig. 2. The distribution of these microseismic events is presented in Fig. 4 (1) without the interpreted faults, and (2) with interpreted faults resulting from this study. Fig. 4a and b shows the microseismicity distribution from October 6, 2011 (the beginning of injection) to October 20, 2011. During the first two days, only 5 microseismic events occurred. Then, from October 8 to October 10, 2011, 45 microseismic events occurred, with a third of these events identified by large black
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shear zone oriented N130 appears to have been initiated (F7). For simplicity, Fig. 5c shows only the microseismicity occurring along the fault Fa during this period, and the vertical event alignment suggests a near-vertical fault. Finally, four additional shear zones have been proposed, two oriented N050 (Fb and Fc) and two oriented N130 (F6 and F8), based on microseismic event alignments from mid-December 2011 through early April 2012 (Fig. 4g and h). Seismicity alignments Fb and Fc correspond to two N050 previously identified surface faults. Since injection began, eight events with magnitudes ranging between 2.2 and 2.9 have occurred along Fc. F6, which seems to be a continuation of the Squaw Creek Fault into the southeast part of the study area, correlates well with the steam entries in P31 and P32. F8 was also added, based on the location of P32 steam entries. This approach must contend with the uncertainty of large horizontal location errors, and shear-zone separations smaller than these horizontal location errors. However, the structural setting, constrained by several lines of evidence, was integrated within a geomechanical model designed to reproduce the reservoir response to P32 water injection. The result was a good match between (1) the proposed shear zones and mapped surface faults, and (2), the locations of steam entries along the well trajectories and the N130 oriented shear zones. Most of the steam entries correspond to proposed near-vertical northwest-striking fractures, separated by 150–200 m, which is consistent with Sammis et al. (1992).
4. Thermo-hydromechanical simulation
Fig. 3. Map view of (a) the number of seismic events occurring during the injection, and (b) their summed seismic moment by cell size of 2.3 × 10−3 km2 .
points aligned along a line trending N130 and indicated as “F5” on subsequent figures. F5 appears to be a splay of the Squaw Creek Fault (Fig. 4b). From October 11–17, 2011, 35 microseismic events were detected; a third of these events are aligned along ‘F5’ (small red triangles), and another third are aligned along a new N130 shear zone (called ‘F4’, red triangles). From the 18th to 20th of October the microseismic events kept spreading toward the NE (green stars). These two shear zones (‘F4’ and ‘F5’) appear to be near vertical (Fig. 5a). Figs. 4c, d, and 5b show the microseismicity distribution for November 1–6, 2011 (6 days), November 14–19, 2001 (6 days), and November 28–30, 2011 (3 days). Within each of these periods, microseismic events begin to occur in areas that previously had no seismicity (or had very few events). For each of these periods, microseismic events are aligned along a N130 azimuth and are believed to indicate the presence of three N130 shear zones (green stars: F3, believed to be a splay of the Squaw Creek Fault; yellow triangles: F2; and blue squares: F1). Elsewhere, the microseismic events seem to occur randomly within the reservoir (gray points), or along the aforementioned N130 shear zones identified as F4 and F5 (red and black points). The faults F1, F2, and F3 also seem to be nearly vertical. Fig. 4e and f shows the microseismicity distribution during the first six days following the injection rate increase in December 2011 from 400 to 1000 gpm. Two new fluid flow paths seem to be developing. The first is a N050 shear zone (Fa) reactivation with apparent fluid migration to the southeast of the study area, where a second
In recent years, coupled thermo-hydro-mechanical simulation has been widely used to characterize the mechanical behavior of geological formations in many engineering disciplines: oil and gas production (Verdon et al., 2011), geological CO2 storage (Cappa and Rutqvist, 2011), radioactive waste repositories (Jing et al., 1995), and geothermal energy extraction (Rutqvist et al., 2010). Here, we use the TOUGH-FLAC simulator (Rutqvist et al., 2002; Rutqvist, 2011), which links a multiphase flow code (TOUGH2—Pruess et al., 2011) and a geomechanical code (FLAC3D —Itasca, 2009). This simulator has the required capabilities for modeling of nonisothermal, multiphase flow processes coupled with stress changes induced by temperature variations and fluid-pressure changes. In this simulation we apply TOUGH-FLAC to simulate The Geysers reservoir steam production and water injection with an approach previously used in Rutqvist and Oldenburg (2008), Rutqvist et al. (2010), and Rutqvist et al. (2013). According to this approach, the modeling and analysis is conducted in two steps: the first step is to calculate elastic stress changes and deformations induced by changes in fluid pressure and temperature; the second step is to look at these stress changes and evaluate the potential for induced seismicity. As described in Rutqvist et al. (2013), one advantage of the adopted approach is that it is not a requirement to know the exact absolute magnitude and direction of the in situ stress field, but it is assumed that the rock mass is near-critically stressed for shear failure (see Section 4.2). We simulate the rock mass as an equivalent continuum with an implicit representation of fractures. With this approach, the required hydraulic properties are fracture permeability and fracture porosity. In Rutqvist et al. (2013), the permeability of the reservoir rock was on the order of 1 × 10−14 m2 , and fracture porosity was back-calculated to 0.4%. Initially, the fracture system is filled with steam, having a very low liquid saturation, and the steam pressure is only a few MPa. In this study, we add the explicit representation of major hydraulic structures such as significant steam-bearing fracture zones or faults. These fracture zones or faults are represented by equivalent continuum properties,
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Fig. 4. Dynamic appearance of microseismic hypocenters presented in detail for four relevant periods. For the same period two maps are shown, one with all the seismic events detected during this period, and another map with our interpretation and the main faults mapped in the field. The circles represent the average location uncertainty and the small black squares denote steam entries in the wells. (For interpretation of the references to color in text, the reader is referred to the web version of the article.)
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Fig. 6. Structural map built from the microseismicity analyses, with the location of the largest earthquakes (magnitude > 2.0) and the map of the seismic event density. The circles represent the average location uncertainty.
presence of high-permeability near-vertical structures as an explanation for microseismicity far below the P32 injection well just a few days after injection start-up. However, our approach does not consider the vertical extent of the stimulation zone, and shear-zone permeability may be underestimated. The model explicitly represents eight shear zones oriented N130 (F1–F8), which are intersected perpendicularly by three N050 shear zones (Fa–Fc) (Fig. 7). These modeled shear zones are 15 m thick as suggested by Sammis et al. (1992). The model extends 12 km × 12 km laterally, far enough from the injection point to simulate an infinite acting boundary. The injection point (P32) and the monitoring point (PS31) are located along the same shear zone, F5,
Fig. 5. View in 3D of the dynamic appearance of microseismic hypocenters along the shear zones (a) F4 and F5, (b) ‘F1’–‘F3’ and (c) along ‘Fa’. (For interpretation of the references to color in text, the reader is referred to the web version of the article.)
consisting of fractured rocks with an identified fracture permeability, fracture porosity, and elastic moduli different from the surrounding rock mass. 4.1. Model setup The model geometry and parameters allow validation of the proposed geological structural setting (Fig. 6) by comparing the lateral extent of the stimulation zone to the actual microseismicity patterns. A two-dimensional horizontal model is used to represent the study area around the NW Geysers P32 injection well. The results from previous 3D simulations (Rutqvist et al., 2013) suggest the
Fig. 7. Model geometry for the fluid flow simulation.
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with a separation of 360 m. Additional monitoring points P25 and P38 are located 750 m and 1400 m away from the injection point, respectively. Injection was simulated at a depth of 2450 m below sea level. At this depth, the initial steam pressure and temperature are assumed to be 1.8 MPa and 271 ◦ C (Rutqvist and Oldenburg, 2008).
4.2. Properties calibration We first used the iTOUGH2 inverse modeling code (Finsterle, 1999) in combination with the EOS3 module of TOUGH2 (Pruess et al., 2011) to estimate the hydraulic reservoir properties. iTOUGH2 (http://www-esd.lbl.gov/iTOUGH2) is a computer program designed for parameter estimation, sensitivity analysis, and uncertainty propagation analysis (Finsterle, 1999). The inverse problem program is designed for automatic model calibration, and the best-fit is determined by minimizing the weighted differences between measured and calculated hydrological data. For this study, the permeability and porosity values assigned to the host rock and the shear zones were estimated by matching the monitored pressure variations in wells PS31 and P25 (Fig. 7). The pressure variation in P38 after 300 days of injection is considered to be negligible (less than 1.5 × 105 Pa). The injection rate, derived from field data, was reduced by a factor of 45 due to the geometric relationship between the model and the actual system. This factor was calculated during the iTOUGH2 simulation. Then, we conducted a geomechanical analysis to estimate Young’s moduli for the rock mass and the N050 and N130 shear zones. The best match was found by fitting the map of the predicted cumulative microseismicity potential (Rutqvist et al., 2013) with observed locations of microseismic events. The microseismicity potential was calculated following the approach used by Rutqvist et al. (2010), which evaluates the potential for shear slip under the conservative assumption that fractures of any orientation could exist anywhere. At The Geysers, such an assumption is consistent with the findings of Oppenheimer (1986), who indicated that seismic sources occur from essentially randomly oriented fracture planes. One key parameter in estimating the likelihood of shear reactivation along a fracture is the coefficient of static friction, , entering the Coulomb shear failure criterion. Cohesionless faults are usually assumed to have a friction coefficient of 0.6–0.85 (e.g., Barton et al., 1995). Moreover, a frictional coefficient of = 0.6 is a lower-limit value observed in fractured rock masses (Barton et al., 1995). Thus, using = 0.6 in the Coulomb
criterion would most likely give a conservative estimate of likely seismicity. For = 0.6, the Coulomb criterion for the onset of shear failure can be written in the following form: = 33 1c
(1)
is the critical maximum principal stress for the onset where 1c of shear failure. Thus, shear reactivation of a fracture would be induced whenever the maximum principal effective stress is three times higher than the minimum principal stress. Based on the concept of a critically stressed rock mass, the initial condition will be a state of incipient failure. Studying stress-state deviation from the near-critical condition allows us to better understand whether these variations tend to move the system toward or away from a state of failure. Of course, it is possible to model any initial condition and consider if a change in the stress state increases or decreases the likelihood of shear failure. The likelihood of shear reactivation increases if the change in the maximum principal compressive effective stress is more than three times the change in minimum principal effective stress (i.e., if 1 ≥ 3 × 3 ). Conversely, the likelihood of shear reactivation would decrease if the change in maximum principal compressive effective stress were less than three times the change in minimum principal effective stress (i.e., if 1 < 3 × 3 ). Considering that the initial stress might not be exactly at the point of critical stress, we may quantify how much the 1 has to exceed 3 × 3 to induce additional shear reactivation. We = therefore define a stress-to-strength change margin as 1m 1 − 3 × 3 . 1m represents the potential to induce shear reactivation and induced seismicity, in which a higher value would imply a higher potential for triggering induced seismicity. We therefore denote 1m as the microseismicity potential.
4.3. Results The best-fit solution to match (1) the pressure evolution in the surrounding wells (Fig. 8) and (2) the map of the cumulative microseismicity potential with observed locations of microseismic events (Fig. 9) was obtained for highly contrasting hydraulic (Table 1) and mechanical values between the host rock and the two shear zone families. The estimated host-rock permeability (Khr ) is equal to 1.3 × 10−14 m2 , while N130 shear-zone permeability values are estimated to be one to two orders higher than Khr , ranging between
Fig. 8. Comparison between the pressure evolutions measured in situ in wells PS31 and P25 with the calculated pressure evolution.
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Fig. 9. Comparisons, at two different scales (a) and (c) of the maps of the observed seismic event density (see Fig. 3, for the legend of the seismic event density) and the modeled cumulative microseismicity potential (b and d). Table 1 Reservoir hydraulic properties estimated from iTough2.
Matrix f1 f2 f3 f4 f5 f6 f7 f8 Fa Fb Fc
Permeability (m2 )
Porosity
1.69E−14 3.80E−13 3.80E−13 3.72E−13 7.12E−13 3.80E−13 4.80E−13 2.34E−12 2.34E−12 1.91E−14 1.91E−14 1.00E−22
0.028 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.01 0.01 0.01
2.3 × 10−12 m2 to 3.8 × 10−13 m2 . The permeabilities estimated for the N050 shear zones (1.9 × 10−14 m2 ) are slightly lower than Khr , and the shear zone Fc is estimated to be eight orders of magnitude lower than that of the host rock (1.0 × 10−22 m2 ). The very low permeability in Fc is necessary to match the lack of pressure response in P38; this is also consistent with the abrupt decrease in microseismicity on the SE side of this structure. Estimated porosities are very low: 0.2% in the host rock, and 0.1% and 0.5% in the N050 and N130 shear zones, respectively. Fig. 9 shows comparisons between the maps of cumulative microseismicity potential and the map of the seismic density. Three areas of different seismicity characteristics were identified: (1) Area 1, around the injection point; 1m is close to 20 MPa, with (during the injection) 25 to 15 events detected over 2.3 × 10−3 km2 ; (2) Area 2, with 1m ranging from 20 to 10 MPa, with 15 to 7 events varies detected over 2.3 × 10−3 km2 ; and (3) Area 3, where 1m from 10 to 3 MPa, with 7 to 3 events monitored per 2.3 × 10−3 km2 .
These results were obtained for Young’s moduli equal to 2.5 GPa in the N130 shear zones, and 8 GPa in the host rock and in the N050 shear zones. 5. Discussion 5.1. Influence of the geological setting on the EGS The two shear-zone families highlighted in his study (N130 and N050) belong to the Riedel system forming, within the San Andreas fault system, respectively R and P shear fractures. Their locations correspond to the faults mapped in the field. This suggests that fluid flow is strongly influenced by the shear-zone network inherited from the regional tectonic activity, and the tectonic structures visible on the surface have a strong impact on the fluid flow at several kilometers depth. The low permeability back-calculated for some shear zones can be explained by the presence of a thick fault gouge, which may allow for high permeability along its strike, but lower permeability across its strike. 5.2. Mechanisms of inducing microseismicity Fig. 10 a shows predicted changes in fluid pressure greater than 2.7 × 105 Pa at the end of the injection after 294 days. The figure shows that the injection has led to a pressurization of the reservoir at a kilometer scale. The shape of the pressurized area is strongly influenced by (i) the N050 shear zone (Fc), which acts as an impermeable boundary; (ii) the more permeable N130 shear zones, which favor pressure diffusion; and (iii) slight permeability contrasts among the N130-trending shear zones. The comparison between the map of the pressure variation and that of the seismic event density recorded during the injection (Fig. 9a and d) shows
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Fig. 10. Contour of changes in (a) pressure higher than 2.7 × 105 Pa, and (b) in temperature replaced and oriented on the map.
good agreement. The contour areas where 70% and 100% of the seismic activity occurred correspond to the areas where the fluid pressure increases at least 4.2 × 105 Pa and 2.7 × 105 Pa, respectively. Cooling is observed exclusively around the injection point in P32 (−180 ◦ C), due to the presence of cold liquid water (Fig. 10b). This suggests that injection-induced changes in the steam pressure are the dominant cause for triggering shear reactivation far from the injection. These results are consistent with Rutqvist et al. (2013). 5.3. Larger microseismic events We have located (on the structural map) the positions of the 41 microseismic events with a magnitude higher than 2.0 (Fig. 6). It appears that most of them are located either (i) just around the injection zone, with a high concentration of microseismic events of magnitudes ranging from 2.0 to 2.2; or (ii) along the shear zones previously identified, with magnitudes ranging from 2.2 to 2.9. Interestingly, by comparing the location of these 41 microseismic events with the map of the microseismicity density, it appears that the largest magnitude events occur outside the high density microseismicity area. Moreover it seems that the events with largest magnitude occur at intersections between the N130 and N050 shear zones. The magnitude of a seismic event depends on several factors: the stress accumulated before slipping, how fast it fails, the depth where it occurs (Brune and Thatcher, 2002), and the size of the slipping surface (the bigger the fault, the larger the seismic magnitude). Here, the biggest microseismic events were located between 2.5 and 3.5 km (below sea level), outside the area of high seismic density and close to the intersections of shear zones N130 and N050 (Fig. 6). The geomechanical simulation has highlighted a strong permeability contrast between these two shear zone families, with one acting as a drain and favoring fluid pressure diffusion, and the other potentially acting as an impermeable boundary and allowing slight steam-pressure buildup. These results seem to indicate that the area in which the largest microseismic events could occur are: (i) far from the injection, because close to the injection, the high-density low-magnitude micro-earthquake activity gradually relieves stresses and thereby prevents a significant stress drop needed for the development of larger events; and (ii) at the intersections of permeable and semi-permeable faults, where slight steam overpressure can quickly develop. 5.4. Geomechanical simulation Characterizing the structural and hydromechanical properties of the EGS reservoir under exploitation can greatly benefit its design
and management. If microseismicity monitoring is used to identify the main fluid paths, the large errors in event locations may be a limiting factor, preventing an accurate description of the geological structures. In this study, we show that this limitation can be reduced with a combination of geological studies and geomechanical modeling. In this study, structural reservoir properties were characterized by matching the monitored pressure variations in the surrounding wells, and the shape and size of the observed seismicity with the map of the calculated microseismicity potential. This approach allows testing hypotheses made about the shear-zone properties: orientations, number, and spacing. Here, we used geological data to build our structural model, but if these kinds of data are not available, a sensitivity analyses could be done on the structural reservoir properties to create the map of the microseismicity potential. Identifying the existence of shear zones and their hydromechanical properties can facilitate reservoir management. Such activity can help to predict where permeability increases would be critical. Indeed, the existence of a highly permeable shear zone could limit the effect of hydraulic stimulation, because fast fluid flow would prevent higher overpressure. Inversely, field observations at the Soultz site have shown that important injectivity gains due to hydraulic stimulation are usually observed in the initially less productive shear zone (Gentier et al., 2013). Hence, geomechanical modeling can help to identify which shear zone is most susceptible to reactivation, and to investigate its future performance.
6. Conclusion In this study, we utilized microseismicity analysis coupled with thermo-hydro-mechanical modeling to characterize the structural, hydraulic, and mechanical reservoir properties at the Northwest Geysers EGS project, in California. We presented a detailed description of the spatial and temporal distribution of observed microseismicity during 10 months of water injection, and delineated proposed shear zones associated with that microseismicity. The results of this study show that (1) the hydraulic and mechanical behavior of the studied EGS reservoir is controlled by a network of intersecting shear zones and their hydraulic properties, (2) detailed microseismic monitoring and analysis can be useful in delineating such shear zones, and (3) coupled fluid flow and geomechanical analysis, corroborated with field data of pressure and microseismicity, can be used to back-calculate their hydraulic and mechanical properties. In addition, our study—which for the first time includes the detailed discrete modeling of preferential flow along shear zone—confirms previous findings regarding the cause
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and mechanisms of injection-induced seismicity at The Geysers. That is, over a one-year injection operation, the microseismicity is primarily caused by thermal contraction near the injection well where strong cooling prevails; whereas away from the injection well, small increases in steam pressure are the primary trigger of microseismicity. Such increases in steam pressure seem to be particularly important for triggering microseismicity at reservoirbounding semi-permeable shear zones. Acknowledgments This work was conducted with funding by the Assistant Secretary for Energy Efficiency and Renewable Energy, Geothermal Technologies Program, of the U.S. Department under the U.S. Department of Energy Contract No. DE-AC02-05CH11231, and by Calpine Corporation. We are grateful to Katie Boyle and Lawrence Hutchings from Lawrence Berkeley National Laboratory (LBNL) for making seismic data available to us, and for the constructive comments and recommendations of the reviewers.
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