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Rock Mechanical Approaches for Predicting Fault Behaviour During CO2 Injection
Available online at www.sciencedirect.com
ScienceDirect Energy Procedia 114 (2017) 3273 – 3281
13th International Conference on Greenhouse Gas Control Technologies, GHGT-13, 14-18 November 2016, Lausanne, Switzerland
Rock mechanical approaches for predicting fault behaviour during CO2 injection Eric Tenthoreya,b*, Thomas Richardc, David Dewhurstb,d a
Geoscience Australia / CO2CRC, Geoscience Australia, GPO Box 378, Canberra ACT 2601 Australia b CO2CRC, 700 Swanston St, Melbourne, VIC 3010, Australia c Epslog S.A., Rue Hocheporte 76, 4000 Liège, Belgium d CSIRO Energy, 26 Dick Perry Avenue, Kensington WA 6152 Australia
Abstract This paper presents the pre-injection fault characterisation that has been conducted at the CO2CRC Otway Project in Victoria, Australia, aimed at understanding the response of faults to pressure increases in the formation, fault transmissibility horizontally and vertically, and also presents a methodology to extract key rheological fault properties using novel rock mechanical techniques. The initial modelling results show that the likelihood of injection induced fault reactivation is exceedingly small, due to the very small pressure increases associated with injection (~ 0.05 MPa after about 100 days of injection during the Stage 2C experiment). The potential for CO2 flow across the main reservoir-cutting faults was quantified using the shale gouge ratio algorithm. Results indicate that the faults should be sealing to some degree and should therefore restrict the lateral movement of CO2. Despite these positive results with regard to fault behaviour during current operations, there remains significant uncertainty with regard to certain fault properties such as friction and cohesion that have a large effect on fault stability. Here we develop a workflow which involves several rock mechanical testing techniques to gain information regarding a key fault at the Otway site. One relatively novel technique that is used is the scratch test, which measures strength heterogeneity of the host rock at the centimetre scale. The scratch data can be correlated to certain well logs via a multivariate analysis in order to develop proxies for rock strength that can be applied to other wells and also to wells that traverse fault zones, thereby providing some estimate of fault strength. This scratch analysis is complemented by a series of triaxial experiments designed to measure poroelastic properties and also assess the rate and state frictional parameters of potential fault gouge materials. While the work presented here is aimed specifically at understanding the properties of key faults at the Otway Project, it is hoped that the knowledge gained and the transforms developed here can be used as a guide to constrain fault properties at other CCS sites. © 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license © 2017 The Authors. Published by Elsevier Ltd. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of GHGT-13. Peer-review under responsibility of the organizing committee of GHGT-13. Keywords: Otway Project; faults; rock mechanics
1876-6102 © 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of GHGT-13. doi:10.1016/j.egypro.2017.03.1459
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1. Introduction The CO2CRC Otway Project in Victoria, Australia (Figure 1) has successfully demonstrated CO2 storage projects involving transport, injection, and monitoring of CO2-rich gas into a geological storage site [1]. The current experiment being conducted at the Otway site is the Stage 2C experiment [2], in which 15,000 tonnes of CO2 were injected into a deep saline formation (Paaratte Formation), with the main goal being to seismically image the CO2 plume reaching stabilisation. In the future, a new Otway Stage 3 operation will be undertaken [3], with various experiments in the planning stage, some of which will monitor CO2 behaviour in and around fault zones. This paper presents the pre-injection fault characterisation that has been conducted at the Otway site, aimed at understanding the response of faults to pressure increases in the formation, fault transmissibility horizontally and vertically, and also presents a methodology to extract key rheological fault properties using novel rock mechanical techniques. The results presented, as well as being relevant for the Otway Project, provide a knowledge base that can be applied to CO2 storage sites around the world.
Figure 1. Location map of the Otway Project in southwestern Victoria, Australia.
2. Geologic Setting The CO2CRC Otway Project is located in south-western Victoria, Australia approximately 300 km southwest of Melbourne (Figure 1). Operations for the Project are situated on grazing farmland, above the Naylor structure, which was a gas field, depleted between 2002 and 2003. In most cases, CO2-rich gas (~80% CO2, ~20% CH4) from the nearby Buttress field is extracted and piped to the Otway site for use as a source gas in the various injection tests. The current stage of the experiment, Stage 2C, is focused on using geophysical methods for imaging CO2 storage in a saline aquifer. The experiment involves injection of ~15,000 tonnes of CO2-rich Buttress gas into the Paaratte Formation through the CRC-2 well (Figure 2) at a depth of ~1453 m-1464 m (TVDSS), with the primary aim being to detect and follow the CO2 plume using seismic time lapse techniques until plume stabilisation has been confirmed within seismic resolution limits. The Paaratte Formation represents a typical saline formation target for CCS, in that it does not have any obvious structural closure, and therefore relies on solubility and residual trapping to immobilize CO2. Lithologically, it is a
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complex formation with intercalations of medium to high permeability sands and carbonaceous mudstones, overprinted by diagenetic dolomitic cements. Across the study area shown in Figure 1, the Paaratte Formation varies in thickness from about 180 m at the Buttress-1 well to 507 m in Naylor South-1. The Paaratte Formation was deposited during the later stage of Cretaceous rifting in the Otway Basin, and is dominated by NW-SE trending normal faults, dipping to the south [4]. In the study area there are a number of faults that cross-cut the Paaratte Formation and which are relevant to plume migration during Stage 2C (Figure 2). In particular, the Naylor South splay fault (located several hundred metres north of the main Naylor South fault) may have an effect on plume migration depending on its hydraulic properties, and is therefore the focus of much of this paper. However, this fault does not extend vertically above the Paaratte Formation, implying that any along fault flow would result in CO2 entering an upper layer of the Paaratte formation and still being contained within the storage system.
Figure 2. 3D PETREL model image of the Paaratte Formation depth surface at the level of the perforations for the CO2CRC Stage 2C experiment (near base Paaratte Fm). Contour intervals are every 2 m, depth is below mean sea level, warm colours shallow, cool colours deeper. The various faults cross-cutting the Paaratte Fm are shown. The southern boundary of the model is coincident with the Naylor South fault, which is not shown as a surface. Injection during that Stage 2C experiment will be via the CRC-2 well. Green arrow points north.
3. Initial Fault Modelling The potential response of the key Otway faults to injection-related pressure increase was initially modelled using a Mohr-Coulomb approach. To characterise fault stability in the vicinity of the Otway Project, it is essential to first determine the in situ stress tensor, as this provides strongest indication on whether or not a given fault is physically stable. A significant body of work was developed during Stage 1 geomechanical studies [6] and important additional information was added following the CRC-2 mini-frac testing program [5,6]. The conclusion from this work is that the Otway Project site is most likely in a normal faulting regime with the maximum horizontal stress direction oriented at 142˚, which is consistent with previous regional work [7]. To determine fault stability, the cohesion and friction of the fault are the two important parameters that must be input into the model. However, due to the uncertainty of these parameters, two scenarios were run; a weak fault and a strong fault scenario. Results from this modelling indicate that anywhere from 1.5 to 7 MPa of excess pressure might be needed to induce fault reactivation [2]. It is important to note that, while 1.5 MPa pressure increase might be exceeded in a commercial CO2 operation,
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the pressures during the Otway Stage 2C, through modelling then via continuous monitoring, were shown to be an order of magnitude lower [2]. The horizontal sealing properties of the splay fault will determine whether or not the CO2 plume migrates across the fault, where doing so would change the plume geospatial distribution, impacting the seismic monitoring objectives. The fault-parallel (vertical) sealing properties are important to the effective containment of CO2 during and following the experiment. However, unlike for across-fault flow situations, there are no well-accepted methodologies available to determine fault-parallel permeability, and instead, worse-case (ie. high fault permeability) scenarios were made for input into dynamic simulations. Across-fault and fault-parallel hydraulic properties of the splay fault were also modelled using the shale gouge ratio algorithm [8], together with other documented hydraulic characteristics related to fault heterogeneity. When incorporated into dynamic simulations for CO2 plume migration, it was clear that very limited vertical migration of CO2 could occur through the fault, even 100 years post-injection. These modelling results showed that, if migration occurred, the vertical CO2 plume migration is limited to 10s of metres and that any vertical migration is accompanied by bleeding of gas into overlying, normally pressured horizons safely within the storage complex, rather than far-reaching migration through the fault zone [2]. 4. Rock Mechanical Workflow for Fault Prediction One of the key learnings from the modelling described above is that there are significant uncertainties with some key parameters such as fault friction and cohesion, as well as internal fault structure which will affect hydraulic properties. These uncertainties present a key technical gap, and hence carry a risk that must be addressed in many CCS projects for dynamic mechanical models to increase in accuracy. To constrain these risks, we have undertaken a novel rock mechanics workflow in which detailed rock mechanical information of intact rock core is used in conjunction with image logs, conventional well logs and drilling information through faults, in order to more tightly constrain the key fault variables involved. One relatively novel technique that is used is the scratch test, which measures strength heterogeneity of the host rock at the centimeter scale [9,10]. This technique involves tracing a groove along the surface of core, with the resistive forces being directly related to the unconfined compressive strength (UCS). This high resolution technique allows one to understand how factors such as fracture density and mineralogy affect the compressive strength and, therefore, cohesion. In addition, the scratch technique also provides a measure of the coefficient of internal friction which is an important property when considering faulted rock. To complement the scratch testing, a series of triaxial rock mechanical tests were conducted, which allow characterisation of frictional properties both pre- and post-failure, and also provide key poroelastic parameters such as Young’s modulus and Poisson’s ratio. Correlations between measured mechanical properties and various logs can then be developed to produce some prediction of fault behaviour during injection, as shown in Figure 3. Although carbon storage sites and their accompanying faults will vary site to site, it is hoped that the knowledge gained and the transforms that are developed here can be used as a guide to constrain fault properties at other CCS sites.
Figure 3. Example of how a fault property can be extracted from intact rock properties and novel rock mechanical experimentation. Scratch test data and slide-hold-slide (SHS) experiments can be used to link intact properties to those of faulted rock.
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5. Scratch Testing During this test campaign, about 80 m of core was scratched from the CRC-2 well. Well logs were used to identify regions possessing some heterogeneity, then results of the scratch tests were compared on the fly with the various logs (density, porosity, sonic) to adjust any wireline stretch offset. Two tests were systematically run on each core: x x
The scratch test, which provides a log of (UCS-like) strength; The ultrasonic test, which provides a log of pressure wave velocity ܸ and ܸݏ. The ultrasonic test was run in the groove left by the scratch test.
In terms of scratch intervals, a significant emphasis was placed on scratching the lower portion of the Paaratte Fm, as that is the interval of interest for both the Stage 2C experiment and also for future injection experiments (Figure 4). The UCS values measured by the scratch data vary greatly, from under 5 MPa to about 150 MPa. This is a very wide range that is very favourable for developing proxies for UCS over different lithological facies. The scratch measurements were carefully aligned with the wireline log data so that any wireline stretch offset was accounted for and then the scratch data was cross correlated against various logs to see what parameters were controlling strength, if any. This process showed that three main well logs correlated well with the UCS: CMR porosity, high resolution density and the photoelectric effect. These three cross correlations are shown in Figure 5, and show that each parameter exhibits a good correlation to UCS albeit with some data scatter. Such scatter is expected, as rock strength is a function of different parameters. To reduce the scatter, we have conducted a multivariate analysis which has resulted in the development of a proxy for UCS based on the three petrophysical logs described above:
ܷ ܵܥൌ െ͵ʹͲ െ ሺͳǤͶʹܲܨܧሻ ሺͲǤͳͷߩሻ െ ሺͳ͵߶ሻ where PEF is the photoelectric factor log, ߩ is density, and ߶ is the porosity as determined by the combinable magnetic resonance tool. It should be stated that this definition encompasses all lithologies, the low and medium density lithologies, as well as the high density carbonate cements which make up a small proportion of the overall core volumetrically. Figure 6 plots the above proxy (proxy 1) together with the actual scratch data, and shows how that the proxy is very effective in predicting rock strength. The proxy can in fact be improved further if two separate definitions are used, one for low density and one for high density (proxy 2 and proxy 3, respectively). However, for the sake of universality, we use the one above for the interpretation of the CRC-1 core below.
Figure 4. Intrinsic specific energy (UCS) as determined by scratch testing on CRC-2 core. UCS values up to about 150 MPa were observed using the scratch technique. A significant focus was placed on the lower Paaratte Formation, where current injection experiments are being conducted.
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Figure 5. Cross plots of scratch UCS data (50 cm resolution) against a) CMR porosity, b) density and c) photoelectric factor as determined from wireline logs. All three logs exhibit some correlation to strength, which is used in the multivariate analysis described in the text.
Figure 6. Plot of rock strength (UCS) as determined by scratch testing of the lower Paaratte Formation (blue curve). The other 3 curves are proxies for UCS based on CRC-2 well logs. Proxy 1 is a continuous curve applicable to all rocks. Proxy 2 is applicable to densities > 2450 kg/m3, and proxy 3 is applicable to densities < 2450 kg/m3.
The proxy for strength was then applied to the CRC-1 well, in which wireline data was collected through the splay fault. The result, shown in Figure 7 show that the splay fault at 1420 m MD is predicted to have a strength of approximately 20 MPa. This indicates that the fault likely has a significant degree of cohesion and tensile strength [11,12]. This is also supported by the fact that formation microresistivity images of the fault zone indicate significant cementation of fracture sets. It is also interesting to note that when a facies interpretation conducted years previously [13] is superimposed on the UCS proxy for CRC-1 the strength prediction very much aligns with the various facies. For example, the peaks in strength predicted by the proxy align with the carbonate cemented zones. Conversely, the weakest part of the curve predicted by the proxy appear to align with the coarsest lithologies, associated with sandstones or argillaceous sandstones. These early results, using scratch testing to develop proxies for strength and then applying such proxies to fault or fracture zones hold much promise for resolving some of the uncertainties related to fault zones.
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Figure 7. UCS predicted in CRC-1 well after proxy 1 was applied to the CRC-1 well data. The Naylor South splay fault domain is indicated by the shaded region. Proxy 1 indicates that the splay fault possesses a UCS of approximately 20 MPa. CRC-1 facies interpretation of [13] are shown above the graph. Curve height is grain size, yellow domains are sandstone, white are carbonate cements, green are mudstone, and brown are argillaceous sandstones.
6. Triaxial Testing A series of triaxial tests were conducted with more planned going forward. The goal of these experiments is to add the poroelastic properties to the brittle UCS measurements conducted above. Furthermore the behavior of the specimens post failure can reveal important properties of the fractures surfaces, specifically the frictional behavior under different sliding velocities [14] Rock mechanics tests were performed on an autonomous triaxial cell capable of imposing 70 MPa confining and pore pressures and up to 400 MPa axial load on a 38 mm diameter sample. For each test specimen, the sample was loaded axially at a strain rate of 10-6 s-1 until failure. At each depth, four tests were conducted at different confining pressures, so that a Mohr Coulomb failure envelope could be generated [15]. From the stresses and strains measured during triaxial compression tests, the elastic properties such as Young’s modulus (E), Poisson’s ratio (σ) and compressive strength were determined for each test specimen. Young’s modulus, a measure of rock stiffness, was determined from the loading curves and is defined as the change of axial stress per change in axial strain and has the units in GPa. Poisson’s ratio is the negative ratio of transverse to axial strain and is dimensionless. In the mechanical tests, the transverse strain was measured using radial strain gauges. Finally, the strength of the specimens under confined conditions was taken as the point where sample failure occurred, and is manifested by a sudden drop in axial load. It should be noted that the peak strength under confined conditions is highly dependent on the confining pressure and is therefore less useful than UCS as an indicator of intrinsic strength.
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Figure 8. Mohr diagram showing the results of 4 different triaxial tests conducted at different confining pressures. Samples were taken from the carbonate cement lithology in CRC-2 at around 1475 m MD. The test indicated with the dashed red line has not been used to define the failure envelope as it proved to have an anomalously high strength. The remaining three tests together indicate that the Paaratte Formation cements possess high cohesion and high frictional properties
Figure 8 shows the results of the first series of triaxial experiments conducted on specimens from the carbonate cemented lithology at about 1475 m MD. The results confirm the scratch testing in that the UCS is around 100 MPa or higher. The results also show that the coefficient of internal, as shown by the slope of the red line on Figure 8 is quite high. The high strength of the carbonate cemented layer is further evidence that the Otway splay fault is likely to possess some significant strength that will counteract elevated fluid pressures that might push the fault toward reactivation. Lawrence [16] documented that the fractures within the splay fault domain at ~1420 m MD were resistive, most likely indicating some fracture filling cements that would serve to increase the integrity and cohesion of the fault zone. In the future, we will examine whether the fracture filling cements within the fault zone are related to the strong carbonate lithologies throughout the Paaratte Formation.
7. Summary We have used a novel rock mechanical testing approach to constrain some of the uncertainties related to fault properties, both at the Otway Project but also applicable to other sites internationally. Using scratch testing methodology, it was possible to develop a transform (proxy) for rock strength based on several different well logs. When the transform was applied to the CRC-1 well data, which traversed the Naylor South splay fault, it was found that that the fault is predicted to have an appreciable UCS. This is an important result with regard to potential fault reactivation pressures, suggesting that the fault should be more resistant to pressure fluctuations than would a fault with low friction and devoid of cohesion. Complementary triaxial tests confirm the high UCS values measured using the scratch technique and indicate that probable carbonate cements within the fault zones are likely to help strengthen the Naylor South splay fault. Acknowledgements We thank the CO2CRC for jointly conceptualizing the project, technical oversight, providing data and also funding the project. Many thanks to Adam Bailey, Tess Dance, John Kaldi, Robert Langford and Max Watson for constructive reviews of the paper. ET publishes with the approval of the CEO of Geoscience Australia.
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References [1] Jenkins C, Cook P, Ennis-King J, Underschultz J, Boreham C, de Caritat P, Dance T, Etheridge D, Hortle A, Freifeld B, Kirste D, Paterson L, Pevzner R, Schacht U, Sharma S, Stalker L and Urosevic M. Safe storage and effective monitoring of CO2 in depleted gas fields. Proceedings of the National Academy of Science of the USA 2012; 109 (2), E35-E41. [2] Tenthorey E, Dance T, Cinar Y, Ennis-King J, Strand J. Fault modelling and CO2 migration potential associated with the CO2CRC Otway 2C injection experiment. International Journal of Greenhouse Gas Control 2014; 30, 72-85. [3] Jenkins, C, Dance, T, Ennis-King, J, Glubokovskikh, S, Gurevich, B, La Force, T, Marshall, S, Paterson, L, Pevzner, R, Tenthorey, E and Watson, M, 2016. Validating subsurface monitoring as an alternative option to surface M. In: GHGT-13, Lausanne, 114-18 November 2015. [4] Krassay AA, Cathro DL, Ryan DJ. A regional tectonostratigraphic frame-work for the Otway basin. In: PESA Eastern Australasian Basins Symposium II 2004; 97–116. [5] Tenthorey E, John Z, Nguyen D. CRC-2 Extended Leak-Off and Mini-Frac Tests: Results and Implications. Cooperative Research Centre for Greenhouse Gas Technologies, Canberra, Australia, 2010 CO2CRC Publication Number RPT10-2228. 23pp. [6] Tenthorey E. Geomechanical investigations. In: P J Cook, Geologically Storing Carbon: Learning from the Otway Project Experience. 2014 CSIRO Publishing, Melbourne, pp. 113-128. [7] Nelson, E, Hillis, R, Sandiford, M, Reynolds, S, Mildren, S. Present –day state-of-stress of southeast Australia. APPEA Journal, 2006; 283306. [8] Yielding G, Freeman B, Needham DT. Quantitative fault seal prediction. AAPG Bulletin 1997; 81, 897-917. [9] Richard T, Dagrain F, Poyol E. Rock strength determination from scratch tests. Eng. Geology 2012; 147, 91-100. [10]Germay C, Richard T, Mappanyompa E, Lindsay C, Kitching D, Khaksar A. The continuous-scratch profile: A high-resolution strength log for Geomechanical and petrophysical characterization of rocks. SPE Paper 174086, 2015. [11]Altindag R, Guney A. Predicting the relationships between brittleness and mechanical properties (UCS, TS and SH) of rocks. Scientific Research and Essays, 2010; 5 (16), 2107-2118. [12]Nazir R., Momeni E., Armaghani DJ, Amin MFM. Correlation between unconfined compressive strength and indirect tensile strength of limestone rock samples. The Electronic Journal of Geotechnical Engineering, 2013; 18, 1737-1746. [13]Lawrence M, Arnot M, Browne G, Bunch M, Dance T. Geological Interpretation of Core and Wireline Data from Otway Project Wells CRC1 and CRC-2. Cooperative Research Centre for Greenhouse Gas Technologies, Canberra, Australia, 2013 CO2CRC Publication Number RPT12-3928. [14]Marone C. 1998. Laboratory-derived friction laws and their application to seismic faulting. Annual Review of Earth and Planetary Science, 1998; 26, 643-696. [15]Scholz CH. The Mechanics of Earthquakes and Faulting. 1990 Cambridge University Press, 439 pp. [16]Lawrence M. Structural Analysis of Borehole Images from Otway Wells CRC-1 and CRC-2. Cooperative Research Centre for Greenhouse Gas Technologies, Canberra, Australia, 2011 CO2CRC Publication Number RPT11-2833.
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13th International Conference on Greenhouse Gas Control Technologies, GHGT-13, 14-18 November 2016, Lausanne, Switzerland
Rock mechanical approaches for predicting fault behaviour during CO2 injection Eric Tenthoreya,b*, Thomas Richardc, David Dewhurstb,d a
Geoscience Australia / CO2CRC, Geoscience Australia, GPO Box 378, Canberra ACT 2601 Australia b CO2CRC, 700 Swanston St, Melbourne, VIC 3010, Australia c Epslog S.A., Rue Hocheporte 76, 4000 Liège, Belgium d CSIRO Energy, 26 Dick Perry Avenue, Kensington WA 6152 Australia
Abstract This paper presents the pre-injection fault characterisation that has been conducted at the CO2CRC Otway Project in Victoria, Australia, aimed at understanding the response of faults to pressure increases in the formation, fault transmissibility horizontally and vertically, and also presents a methodology to extract key rheological fault properties using novel rock mechanical techniques. The initial modelling results show that the likelihood of injection induced fault reactivation is exceedingly small, due to the very small pressure increases associated with injection (~ 0.05 MPa after about 100 days of injection during the Stage 2C experiment). The potential for CO2 flow across the main reservoir-cutting faults was quantified using the shale gouge ratio algorithm. Results indicate that the faults should be sealing to some degree and should therefore restrict the lateral movement of CO2. Despite these positive results with regard to fault behaviour during current operations, there remains significant uncertainty with regard to certain fault properties such as friction and cohesion that have a large effect on fault stability. Here we develop a workflow which involves several rock mechanical testing techniques to gain information regarding a key fault at the Otway site. One relatively novel technique that is used is the scratch test, which measures strength heterogeneity of the host rock at the centimetre scale. The scratch data can be correlated to certain well logs via a multivariate analysis in order to develop proxies for rock strength that can be applied to other wells and also to wells that traverse fault zones, thereby providing some estimate of fault strength. This scratch analysis is complemented by a series of triaxial experiments designed to measure poroelastic properties and also assess the rate and state frictional parameters of potential fault gouge materials. While the work presented here is aimed specifically at understanding the properties of key faults at the Otway Project, it is hoped that the knowledge gained and the transforms developed here can be used as a guide to constrain fault properties at other CCS sites. © 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license © 2017 The Authors. Published by Elsevier Ltd. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of GHGT-13. Peer-review under responsibility of the organizing committee of GHGT-13. Keywords: Otway Project; faults; rock mechanics
1876-6102 © 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of GHGT-13. doi:10.1016/j.egypro.2017.03.1459
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1. Introduction The CO2CRC Otway Project in Victoria, Australia (Figure 1) has successfully demonstrated CO2 storage projects involving transport, injection, and monitoring of CO2-rich gas into a geological storage site [1]. The current experiment being conducted at the Otway site is the Stage 2C experiment [2], in which 15,000 tonnes of CO2 were injected into a deep saline formation (Paaratte Formation), with the main goal being to seismically image the CO2 plume reaching stabilisation. In the future, a new Otway Stage 3 operation will be undertaken [3], with various experiments in the planning stage, some of which will monitor CO2 behaviour in and around fault zones. This paper presents the pre-injection fault characterisation that has been conducted at the Otway site, aimed at understanding the response of faults to pressure increases in the formation, fault transmissibility horizontally and vertically, and also presents a methodology to extract key rheological fault properties using novel rock mechanical techniques. The results presented, as well as being relevant for the Otway Project, provide a knowledge base that can be applied to CO2 storage sites around the world.
Figure 1. Location map of the Otway Project in southwestern Victoria, Australia.
2. Geologic Setting The CO2CRC Otway Project is located in south-western Victoria, Australia approximately 300 km southwest of Melbourne (Figure 1). Operations for the Project are situated on grazing farmland, above the Naylor structure, which was a gas field, depleted between 2002 and 2003. In most cases, CO2-rich gas (~80% CO2, ~20% CH4) from the nearby Buttress field is extracted and piped to the Otway site for use as a source gas in the various injection tests. The current stage of the experiment, Stage 2C, is focused on using geophysical methods for imaging CO2 storage in a saline aquifer. The experiment involves injection of ~15,000 tonnes of CO2-rich Buttress gas into the Paaratte Formation through the CRC-2 well (Figure 2) at a depth of ~1453 m-1464 m (TVDSS), with the primary aim being to detect and follow the CO2 plume using seismic time lapse techniques until plume stabilisation has been confirmed within seismic resolution limits. The Paaratte Formation represents a typical saline formation target for CCS, in that it does not have any obvious structural closure, and therefore relies on solubility and residual trapping to immobilize CO2. Lithologically, it is a
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complex formation with intercalations of medium to high permeability sands and carbonaceous mudstones, overprinted by diagenetic dolomitic cements. Across the study area shown in Figure 1, the Paaratte Formation varies in thickness from about 180 m at the Buttress-1 well to 507 m in Naylor South-1. The Paaratte Formation was deposited during the later stage of Cretaceous rifting in the Otway Basin, and is dominated by NW-SE trending normal faults, dipping to the south [4]. In the study area there are a number of faults that cross-cut the Paaratte Formation and which are relevant to plume migration during Stage 2C (Figure 2). In particular, the Naylor South splay fault (located several hundred metres north of the main Naylor South fault) may have an effect on plume migration depending on its hydraulic properties, and is therefore the focus of much of this paper. However, this fault does not extend vertically above the Paaratte Formation, implying that any along fault flow would result in CO2 entering an upper layer of the Paaratte formation and still being contained within the storage system.
Figure 2. 3D PETREL model image of the Paaratte Formation depth surface at the level of the perforations for the CO2CRC Stage 2C experiment (near base Paaratte Fm). Contour intervals are every 2 m, depth is below mean sea level, warm colours shallow, cool colours deeper. The various faults cross-cutting the Paaratte Fm are shown. The southern boundary of the model is coincident with the Naylor South fault, which is not shown as a surface. Injection during that Stage 2C experiment will be via the CRC-2 well. Green arrow points north.
3. Initial Fault Modelling The potential response of the key Otway faults to injection-related pressure increase was initially modelled using a Mohr-Coulomb approach. To characterise fault stability in the vicinity of the Otway Project, it is essential to first determine the in situ stress tensor, as this provides strongest indication on whether or not a given fault is physically stable. A significant body of work was developed during Stage 1 geomechanical studies [6] and important additional information was added following the CRC-2 mini-frac testing program [5,6]. The conclusion from this work is that the Otway Project site is most likely in a normal faulting regime with the maximum horizontal stress direction oriented at 142˚, which is consistent with previous regional work [7]. To determine fault stability, the cohesion and friction of the fault are the two important parameters that must be input into the model. However, due to the uncertainty of these parameters, two scenarios were run; a weak fault and a strong fault scenario. Results from this modelling indicate that anywhere from 1.5 to 7 MPa of excess pressure might be needed to induce fault reactivation [2]. It is important to note that, while 1.5 MPa pressure increase might be exceeded in a commercial CO2 operation,
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the pressures during the Otway Stage 2C, through modelling then via continuous monitoring, were shown to be an order of magnitude lower [2]. The horizontal sealing properties of the splay fault will determine whether or not the CO2 plume migrates across the fault, where doing so would change the plume geospatial distribution, impacting the seismic monitoring objectives. The fault-parallel (vertical) sealing properties are important to the effective containment of CO2 during and following the experiment. However, unlike for across-fault flow situations, there are no well-accepted methodologies available to determine fault-parallel permeability, and instead, worse-case (ie. high fault permeability) scenarios were made for input into dynamic simulations. Across-fault and fault-parallel hydraulic properties of the splay fault were also modelled using the shale gouge ratio algorithm [8], together with other documented hydraulic characteristics related to fault heterogeneity. When incorporated into dynamic simulations for CO2 plume migration, it was clear that very limited vertical migration of CO2 could occur through the fault, even 100 years post-injection. These modelling results showed that, if migration occurred, the vertical CO2 plume migration is limited to 10s of metres and that any vertical migration is accompanied by bleeding of gas into overlying, normally pressured horizons safely within the storage complex, rather than far-reaching migration through the fault zone [2]. 4. Rock Mechanical Workflow for Fault Prediction One of the key learnings from the modelling described above is that there are significant uncertainties with some key parameters such as fault friction and cohesion, as well as internal fault structure which will affect hydraulic properties. These uncertainties present a key technical gap, and hence carry a risk that must be addressed in many CCS projects for dynamic mechanical models to increase in accuracy. To constrain these risks, we have undertaken a novel rock mechanics workflow in which detailed rock mechanical information of intact rock core is used in conjunction with image logs, conventional well logs and drilling information through faults, in order to more tightly constrain the key fault variables involved. One relatively novel technique that is used is the scratch test, which measures strength heterogeneity of the host rock at the centimeter scale [9,10]. This technique involves tracing a groove along the surface of core, with the resistive forces being directly related to the unconfined compressive strength (UCS). This high resolution technique allows one to understand how factors such as fracture density and mineralogy affect the compressive strength and, therefore, cohesion. In addition, the scratch technique also provides a measure of the coefficient of internal friction which is an important property when considering faulted rock. To complement the scratch testing, a series of triaxial rock mechanical tests were conducted, which allow characterisation of frictional properties both pre- and post-failure, and also provide key poroelastic parameters such as Young’s modulus and Poisson’s ratio. Correlations between measured mechanical properties and various logs can then be developed to produce some prediction of fault behaviour during injection, as shown in Figure 3. Although carbon storage sites and their accompanying faults will vary site to site, it is hoped that the knowledge gained and the transforms that are developed here can be used as a guide to constrain fault properties at other CCS sites.
Figure 3. Example of how a fault property can be extracted from intact rock properties and novel rock mechanical experimentation. Scratch test data and slide-hold-slide (SHS) experiments can be used to link intact properties to those of faulted rock.
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5. Scratch Testing During this test campaign, about 80 m of core was scratched from the CRC-2 well. Well logs were used to identify regions possessing some heterogeneity, then results of the scratch tests were compared on the fly with the various logs (density, porosity, sonic) to adjust any wireline stretch offset. Two tests were systematically run on each core: x x
The scratch test, which provides a log of (UCS-like) strength; The ultrasonic test, which provides a log of pressure wave velocity ܸ and ܸݏ. The ultrasonic test was run in the groove left by the scratch test.
In terms of scratch intervals, a significant emphasis was placed on scratching the lower portion of the Paaratte Fm, as that is the interval of interest for both the Stage 2C experiment and also for future injection experiments (Figure 4). The UCS values measured by the scratch data vary greatly, from under 5 MPa to about 150 MPa. This is a very wide range that is very favourable for developing proxies for UCS over different lithological facies. The scratch measurements were carefully aligned with the wireline log data so that any wireline stretch offset was accounted for and then the scratch data was cross correlated against various logs to see what parameters were controlling strength, if any. This process showed that three main well logs correlated well with the UCS: CMR porosity, high resolution density and the photoelectric effect. These three cross correlations are shown in Figure 5, and show that each parameter exhibits a good correlation to UCS albeit with some data scatter. Such scatter is expected, as rock strength is a function of different parameters. To reduce the scatter, we have conducted a multivariate analysis which has resulted in the development of a proxy for UCS based on the three petrophysical logs described above:
ܷ ܵܥൌ െ͵ʹͲ െ ሺͳǤͶʹܲܨܧሻ ሺͲǤͳͷߩሻ െ ሺͳ͵߶ሻ where PEF is the photoelectric factor log, ߩ is density, and ߶ is the porosity as determined by the combinable magnetic resonance tool. It should be stated that this definition encompasses all lithologies, the low and medium density lithologies, as well as the high density carbonate cements which make up a small proportion of the overall core volumetrically. Figure 6 plots the above proxy (proxy 1) together with the actual scratch data, and shows how that the proxy is very effective in predicting rock strength. The proxy can in fact be improved further if two separate definitions are used, one for low density and one for high density (proxy 2 and proxy 3, respectively). However, for the sake of universality, we use the one above for the interpretation of the CRC-1 core below.
Figure 4. Intrinsic specific energy (UCS) as determined by scratch testing on CRC-2 core. UCS values up to about 150 MPa were observed using the scratch technique. A significant focus was placed on the lower Paaratte Formation, where current injection experiments are being conducted.
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Figure 5. Cross plots of scratch UCS data (50 cm resolution) against a) CMR porosity, b) density and c) photoelectric factor as determined from wireline logs. All three logs exhibit some correlation to strength, which is used in the multivariate analysis described in the text.
Figure 6. Plot of rock strength (UCS) as determined by scratch testing of the lower Paaratte Formation (blue curve). The other 3 curves are proxies for UCS based on CRC-2 well logs. Proxy 1 is a continuous curve applicable to all rocks. Proxy 2 is applicable to densities > 2450 kg/m3, and proxy 3 is applicable to densities < 2450 kg/m3.
The proxy for strength was then applied to the CRC-1 well, in which wireline data was collected through the splay fault. The result, shown in Figure 7 show that the splay fault at 1420 m MD is predicted to have a strength of approximately 20 MPa. This indicates that the fault likely has a significant degree of cohesion and tensile strength [11,12]. This is also supported by the fact that formation microresistivity images of the fault zone indicate significant cementation of fracture sets. It is also interesting to note that when a facies interpretation conducted years previously [13] is superimposed on the UCS proxy for CRC-1 the strength prediction very much aligns with the various facies. For example, the peaks in strength predicted by the proxy align with the carbonate cemented zones. Conversely, the weakest part of the curve predicted by the proxy appear to align with the coarsest lithologies, associated with sandstones or argillaceous sandstones. These early results, using scratch testing to develop proxies for strength and then applying such proxies to fault or fracture zones hold much promise for resolving some of the uncertainties related to fault zones.
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Figure 7. UCS predicted in CRC-1 well after proxy 1 was applied to the CRC-1 well data. The Naylor South splay fault domain is indicated by the shaded region. Proxy 1 indicates that the splay fault possesses a UCS of approximately 20 MPa. CRC-1 facies interpretation of [13] are shown above the graph. Curve height is grain size, yellow domains are sandstone, white are carbonate cements, green are mudstone, and brown are argillaceous sandstones.
6. Triaxial Testing A series of triaxial tests were conducted with more planned going forward. The goal of these experiments is to add the poroelastic properties to the brittle UCS measurements conducted above. Furthermore the behavior of the specimens post failure can reveal important properties of the fractures surfaces, specifically the frictional behavior under different sliding velocities [14] Rock mechanics tests were performed on an autonomous triaxial cell capable of imposing 70 MPa confining and pore pressures and up to 400 MPa axial load on a 38 mm diameter sample. For each test specimen, the sample was loaded axially at a strain rate of 10-6 s-1 until failure. At each depth, four tests were conducted at different confining pressures, so that a Mohr Coulomb failure envelope could be generated [15]. From the stresses and strains measured during triaxial compression tests, the elastic properties such as Young’s modulus (E), Poisson’s ratio (σ) and compressive strength were determined for each test specimen. Young’s modulus, a measure of rock stiffness, was determined from the loading curves and is defined as the change of axial stress per change in axial strain and has the units in GPa. Poisson’s ratio is the negative ratio of transverse to axial strain and is dimensionless. In the mechanical tests, the transverse strain was measured using radial strain gauges. Finally, the strength of the specimens under confined conditions was taken as the point where sample failure occurred, and is manifested by a sudden drop in axial load. It should be noted that the peak strength under confined conditions is highly dependent on the confining pressure and is therefore less useful than UCS as an indicator of intrinsic strength.
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Figure 8. Mohr diagram showing the results of 4 different triaxial tests conducted at different confining pressures. Samples were taken from the carbonate cement lithology in CRC-2 at around 1475 m MD. The test indicated with the dashed red line has not been used to define the failure envelope as it proved to have an anomalously high strength. The remaining three tests together indicate that the Paaratte Formation cements possess high cohesion and high frictional properties
Figure 8 shows the results of the first series of triaxial experiments conducted on specimens from the carbonate cemented lithology at about 1475 m MD. The results confirm the scratch testing in that the UCS is around 100 MPa or higher. The results also show that the coefficient of internal, as shown by the slope of the red line on Figure 8 is quite high. The high strength of the carbonate cemented layer is further evidence that the Otway splay fault is likely to possess some significant strength that will counteract elevated fluid pressures that might push the fault toward reactivation. Lawrence [16] documented that the fractures within the splay fault domain at ~1420 m MD were resistive, most likely indicating some fracture filling cements that would serve to increase the integrity and cohesion of the fault zone. In the future, we will examine whether the fracture filling cements within the fault zone are related to the strong carbonate lithologies throughout the Paaratte Formation.
7. Summary We have used a novel rock mechanical testing approach to constrain some of the uncertainties related to fault properties, both at the Otway Project but also applicable to other sites internationally. Using scratch testing methodology, it was possible to develop a transform (proxy) for rock strength based on several different well logs. When the transform was applied to the CRC-1 well data, which traversed the Naylor South splay fault, it was found that that the fault is predicted to have an appreciable UCS. This is an important result with regard to potential fault reactivation pressures, suggesting that the fault should be more resistant to pressure fluctuations than would a fault with low friction and devoid of cohesion. Complementary triaxial tests confirm the high UCS values measured using the scratch technique and indicate that probable carbonate cements within the fault zones are likely to help strengthen the Naylor South splay fault. Acknowledgements We thank the CO2CRC for jointly conceptualizing the project, technical oversight, providing data and also funding the project. Many thanks to Adam Bailey, Tess Dance, John Kaldi, Robert Langford and Max Watson for constructive reviews of the paper. ET publishes with the approval of the CEO of Geoscience Australia.
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