Characterization of Fracture Patterns in The Geysers Geothermal Reservoir by Shear-wave Splitting J.A. Rial (P.I.) M. Elkibbi, M. Yang, G. Vlahovic, A. Gero Wave Propagation Lab UNC- Chapel Hill Introductory Remarks Characteristics of split shear-waves • Anisotropy-inducing stress-aligned microcracks cause shear-waves to split into a fast and a slow component; a phenomenon analogous to optical birefringence. • The leading shear-wave is polarized parallel to the strike of the predominant fracture system, while the slow shear-wave is polarized perpendicular to it. • The time delay between the arrivals of the fast and slow S-waves is proportional to crack density (number of cracks per unit volume). The Geysers seismicity Project Objectives • Test the applicability and limitations (observability) of shear- wave splitting (SWS) in imaging anisotropic fractured reservoirs. Application to The Geysers and Coso. • Detect potential directions of subsurface fluid-flow through stress-aligned cracks and regions of high crack density. • Develop software for the processing, modeling, inversion, and interpretation of SWS. Methodology • Data processing • Forward Modeling • Inversion Anisot X, a software package for SWS PROCESSING Anisot X is a multi-objective program to image fractured FORWARD reservoirs using shear-wave splitting data. INVERSION HELP Anisot X is Matlab-based CONVENTIONS with three operation modes: EXIT • PROCESSING • FORWARD MODELING • INVERSION Data Processing Modeling assumption • Observed SWS is solely induced by crack-anisotropy in an otherwise isotropic medium, since the fabric of the reservoir rocks in both The Geysers and Coso lacks significant lithology-induced anisotropy. The forward problem Theoretical Theoretical Theoretical polarizations polarizations time delays Rose diagram Equal-area plot Equal-area plot The forward problem is solved by simulating propagation of a plane S-wave through a transversely isotropic (TI) model. Inversion Scheme • Both polarization and time delay observations are used to invert for the most likely crack geometry and fracture density in the subsurface. • The method includes optimization-based and trial-and-error inversion schemes. Residual functions (RMS of observed – calculated) for both polarizations and time delays are minimized in the least-squares sense. • The starting model to simulate crack-induced anisotropy is HTI (horizontal transverse isotropy). Deviations from HTI are modeled as non-vertically dipping cracks or biplanar intersecting crack systems. Inversion Scheme Residual functions strike=55, dip=81 complement crack dip crack strike Polarizations Time delays Inversion Examples : SE Geysers NW Geysers NW Geysers Models of Fracture Geometry and Crack Densities SE Geysers Concluding remarks • The observability test has been very successful. The SWS method has proven to be effective in imaging subsurface crack geometry and density in geothermal reservoirs. The inversion of SWS data provides a 3D view of the crack structure not obtainable by any other imaging method. • Modeled crack geometries are in good to excellent agreement with drill core data, tracer tests, locally mapped fractures, and the regional tectonic setting, both in The Geysers and Coso. • Models of fracture geometry and crack density can potentially reduce the risk of exploratory drilling and help increase geothermal productivity. Impact of work • Coso: Results dramatically corroborated by production engineers. A strong and important variation in crack orientation detected by measurements of SWS at station CE6 was confirmed by site drilling (Sheridan et al.,2003; S. Petty, personal comm., 2003). CE6 Impact of work (cont’d) “ The UNC shear-wave splitting analysis resulting from the 2002 injection studies at the Coso east flank has added significant value to the DOE-funded EGS program at Coso.The analysis identified sets of fractures that became conductive upon injection of cold condensate at modest injection rates and pressures… … With these results in hand, it would be very interesting to conduct a similar shear-wave splitting analysis in conjunction with the massive-hydraulic-fracturing experiments planned for the near future at Coso.” Peter Rose, PI, Coso EGS Project Geothermal Program Coordinator Energy & Geoscience Institute, University of Utah Impact of work (cont’d) • Upon a detailed presentation of our results, Iceland’s National Energy Authority (Orkustofnun) enthusiastically invited our research group to submit a proposal to map subsurface crack geometry and density in Hengill (Nesjavellir) and Krafla geothermal reservoirs. Krafla Nesjavellir Work supported by the U.S. Department of Energy, Assistant Secretary Energy Efficiency and Renewable Energy under DOE Idaho Operations Office, Financial Assistance Award DE--FG07-00ID13956. DISCLAIMER This information was prepared as an account of work sponsored by an agency of the U.S. Government. Neither the U.S. Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. References herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the U.S. Government or any agency thereof. 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