Coupled geomechanics and InSAR inversion of CO2 injection parameters

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1 Scrutinizing CO2 Sequestration - A Case Study Coupling InSAR and Geomechanical Modelling to Monitor Spatial and Temporal Characteristics of CO2 Injection at In Salah, Algeria N. Gourmelen (University of Leeds), D. Angus* (University of Leeds), A. Shepherd (University of Leeds), Q. Fisher (University of Leeds) & A. Gouldson (University of Leeds) Main objectives We analyze the surface deformation resulting from CO2 storage at In Salah in order to provide constraints on the temporal and spatial evolution of CO2 within the reservoir. Specifically, we process InSAR from the pre-injection period and the injection period and combine the InSAR observations with geomechanical modeling of reservoir deformation to determine the volume of CO2 stored. New aspects covered Coupled geomechanics and InSAR inversion of CO2 injection parameters Summary We analyze the surface deformation resulting from CO2 storage at In Salah in order to provide constraints on the temporal/spatial evolution of CO2 within the reservoir. Specifically, we process InSAR from the pre-injection period and the injection period and combine the InSAR observations with geomechanical modeling of reservoir deformation to determine the volume of CO2 stored. The results using a simple yet fast geomechanical model (Geertsma, 1973) indicate that, between 2004 and 2008, 97 ± 9 % of the 1157 x 106 m3 of CO2 injected into the reservoir has been stored. This value is close to the stringent 99 % target for CO2 storage permanence set by the Intergovernmental Panel on Climate Change. Our study provides the first practical assessment of CO2 storage and demonstrates that it is possible to develop independent, fast and cost-effective assessments of future schemes in the absence of ground-based surveys. The strength of this approach is that little field data is needed to provide a sufficiently accurate initial assessment of CO2 storage at relatively little cost. The ability to scrutinize CCS sites globally based on limited data and cost effectively will be crucial for implementation of international monitoring of CO2 sequestration agreements. Topic(s) Monitoring (CO2 Storage) Induced Seismicity, uplift & subsidence (CO2 Storage) Risk assessment (CO2 Storage)

2 Introduction Options for monitoring CO2 storage are varied and range from discrete chemical well sampling programs to full field time-lapse seismic surveys. Crucial for any monitoring program is that it be as cost effective as possible yet yielding sufficiently accurate measurement. Time-lapse seismics has generally proven to be a sufficiently accurate means of monitoring CO2 in the subsurface. However, there is debate as to whether seismics is the most cost effective approach in the quantitative measurement of CO2 flow and containment. The cost of monitoring is compounded potentially further by the various international regulations related to CO2 sequestration, where, for example, it can be argued that CO2 storage monitoring requirements are much stricter than those for natural gas storage. For on-shore sequestration, there has been a significant drive to integrate satellite interferometric synthetic aperture radar (InSAR) with geomechanical modelling to link surface deformation with the movement and storage of injected CO2. At the In Salah CO2 storage project, Algeria, export gas specifications require the removal of CO2 from the produced natural gas with strict long term monitoring requirements to ensure that CO2 is contained indefinitely. Thus there has been significant research into linking geomechanical modelling with InSAR observations (e.g., Rutqvist et al., 2010; Vasco et al., 2010). In principle, the geo-engineering solution of carbon capture and storage (CCS) has been demonstrated in recent pilot studies (e.g., Weyburn, Canada and Sleipner, off-shore Norway). However, quantitatively accounting of stored CO2 has not been adequately demonstrated in part due to lacking accounting protocols. Given that the extent to which CCS will be successful is both commercially and politically sensitive, it is imperative that independent assessment of stored CO2 be achievable and economically viable. In this paper, we present a simple yet fast assessment of the volume of injected CO2 that requires very little input data to yield sufficiently accurate estimates of injected and stored CO2. Nonlinear inversion approach: integrated InSAR and geomechanics 1st Sustainable Earth Sciences Conference & Exhibition

3 Satellite observations of surface deformation are integrated with geo-mechanical modelling to scrutinize the CCS pilot site at In Salah with very limited geological and engineering data. The surface deformation inversion for injection and reservoir properties involves three steps: Processing ground surface deformation (amplitude, extent and timing) over the three CO2 injection wells using the Small Baseline Subset InSAR technique (e.g., Gabriel et al., 1989; Lanari et al., 2007). Modelling surface displacement using the nucleus-of-strain approach (Geertsma, 1973) and incorporating transient pressure evolution (Dake, 2001). Nonlinear inversion of surface deformation to retrieve injection parameters (e.g., stored CO2 volume) using Monte Carlo sampling approach (Cervelli et al., 2001) and analysis of joint distribution of model parameters and statistics using Gibbs sampler algorithm (Geman & Geman, 1984). Results Figures 1-3 show the results of the coupled InSAR and geomechanical nonlinear inversion approach. Figure 1 displays the processed surface deformation at In Salah over the injection period between 2004 and The total ground displacement is mapped across an area of approximately 200 km2 to 300 km2 with uplift of up to 2 cm for all three injection sites and up to 1 cm of subsidence along the edge of the production zone. The observed displacements are consistent with previous findings (e.g., Onuma & Ohkawa, 2009). Figure 2 compares the observed and predicted surface deformation pattern for two cross sections (roughly NE-SW and NW-SE) at injection well KB-501. The time evolution of the surface deformation is shown in both cross sections. Figure 3 presents the results of the nonlinear inversion for well KB-501. The parameters inverted for are the stored CO2, the reservoir height, the porosity, and the average rock bulk compressibility. Also shown is the trade-off between porosity and injected volume of CO2. Table 1 summarizes the results of the nonlinear inversion with comparison to published values in the literature. 1st Sustainable Earth Sciences Conference & Exhibition

4 Table 1: Inverted parameters and published values (values shown are the total for all three wells with the exception of injected volume which is for well KB-501 only). Injected volume Reservoir height Porosity η K (10 6 m3) (m) (%) (Pa.s/mD) (10 10 m 2 /N) Inversion 1129± ±2 17±7 2±0.7x ±0.1 Literature to 20 2x10-7 to 1x Conclusions The results of our study suggest that the storage of CO 2 at the In Salah CCS site has been to date successful, with no significant leakage distal from the reservoir. This is supported by the general homogeneous trend of the surface displacement field at each injection well and the agreement between the published injection rates and the stored volume determined from inversion of the surface deformation during the first 42 months of the pilot project. The precision of the determined volume of stored CO 2 is constrained to some degree by information about the reservoir properties. For example, the trade-off between the stored volume and the reservoir porosity (see Figure 3 f) limits the absolute resolution of stored CO 2. However, it is generally possible to estimate parameters such as porosity with limited knowledge of the geological formation. The complex surface deformation patterns resulting from reservoir heterogeneity also impact upon the accuracy of the retrieved stored volume of CO 2 ; this is particularly the case for well KB-502. The likely scenario for this surface complexity is related to fluid and/or pressure migration into the cap rock via fractures (Vasco et al., 2010). However, 4D seismic imaging of potentially migrating CO 2

5 within this region will likely ambiguous given the drilling mud loses experience within 100 m of the reservoir near KB-502 (Devex Conference, 2008). Due to the simplicity of the geomechanical model, the inversion results do not allow us to rule out the hypothesis of fractures as the cause of fluid or pressure migration above the reservoir. However, the results indicate that the quantity of fluid migration is at most 12% of the total injected CO 2 and still within at most 100 m to 200 m from the top of the reservoir. Although technologically proven, CCS technologies are still too expensive for CCS technologies to be commercially viable (McKinsey, 2008). A key source of financial support could come through the generation of certified emissions reduction credits (United Nations, 1998). Although our inversion approach is simple and lacks the fine-scale accuracy of more complex approaches (e.g., Rutqvist et al., 2010; Vasco et al., 2010), its simplicity is also its key strength. Our inversion approach requires very little starting information to provide sufficiently accurate estimates of injected CO 2. Given that verification of emissions reductions can be expensive, time consuming and politically sensitive, the simplicity of the approach provides a framework to scrutinize international agreements. References Cervelli, P., M.H. Murray, P. Segall, Y. Aoki and T. Kato, 2001, Estimating source parameters from deformation data, with an application to the March 1997 earthquake swarm off the Izu Peninsula, Japan, Journal of Geophysical Research, 106(B6), 11,217-11,237. Dake, L.P., 2001, The Practice of Reservoir Engineering, Elsevier, London. Devex Conference, 2008, Data acquisition to support the In Salah gas CO2 injection monitoring and verification programme, Gabriel, A. K., R. M. Goldstein, and H. A. Zebker, 1989, Mapping small elevation changes over large areas, differential radar interferometry, Journal of Geophysical Research, 94(B7), Geertsma, J., 1973, Land subsidence above compacting oil and gas reservoirs, Journal of Petroleum Technology, 25, Geman, S. and D. Geman, 1984, Stochastic relaxation, Gibbs distribution and the Bayesian restoration of images, IEEE Transactions on Pattern Analysis and Machine Intelligence, 6(6), Lanari, R., F. Casu, M. Manzo, G. Zeni, P. Berardino, M. Manunta and A. Pepe, 2007, An Overview of the Small BAseline Subset Algorithm: a DInSAR Technique for Surface Deformation Analysis, Pure and Applied Geophysics, 164(4), McKinsey & Company, 2008, Carbon Capture & Storage: Assessing the Economics, London, Metz, B., O. Davidson, H. de Coninck, M. Loos and L. Meyer (eds.), 2005, IPCC Special Report: Carbon Dioxide Capture and Storage, Cambridge University Press, Cambridge. Onuma, T. and S. Ohkawa, 2009, Detection of surface deformation related with CO 2 injection by DInSAR at In Salah, Algeria, Energy Procedia, 1(1), Rutqvist, J., D.W. Vasco, and L. Myer, 2010, Coupled reservoir-geomechanical analysis of CO2 injection and ground deformations at In Salah, Algeria, International Journal of Greenhouse Gas Control, 4, United Nations, 1998, Kyoto Protocol to the United Nations Framework Convention on Climate Change. Vasco, D. W., A. Ferretti, and F. Novali, 2008, Estimating permeability from quasi-static deformation: Temporal variations and arrival-time inversion, Geophysics, 73(6), O37-O52. Vasco, D.W., A. Rucci, A. Ferretti, F. Novali, R.C. Bissell, P.S. Ringrose, A.S. Mathieson and I.W. Wright, 2010, Satellitebased measurements of surface deformation reveal fluid flow associated with the geological storage of carbon dioxide, Geophysical Research Letters, 37, L03303.