Micro-earthquake Analysis for Reservoir Properties at the Prati-32 Injection Test, The Geysers, California

Abstract:

The Prati-32 injection test offers a particular opportunity to test rock physics theories and tomography results as it occurred in a previously undisturbed portion of The Geysers, California. Within the northwest Geysers, there is a high temperature zone (HTZ) directly below the normal temperature reservoir (NTR) at ∼2.6 km below ground surface. We demonstrate an analysis of micro-earthquake data with rock physics theory to identify fractures, state of fluids, and permeable zones. We obtain earthquake source properties (hypocenters, magnitudes, stress drops, and moment tensors), 3D isotropic velocity (Vp and Vs) and attenuation (Qp and Qs seismic quality factors), derived elastic moduli (Lambda, Bulk and Young's moduli), and Poisson's ratio. After one month of injection changes in these parameters occur right at the point where injection occured, which confirms the accuracy of the tomography. Bulk modulus, Poisson's ratio, and Lambda increased. Vs decreased. Qp and Vp increased slightly and Qs did not change. We interpret this observation to indicate that there is fluid saturation along with fracturing around the well bottom. Fracturing would decrease Vs, while saturation would not affect Vs. Whereas, saturation would increase Vp, even with fracturing. Saturation and fracturing should have competing effect of intrinsic and extrinsic Q. Saturation should increase intrinsic Qp, but not affect extrinsic Qp. We can't explain the unchanged Qs, unless the effect of increasing intrinsic Qs is offset by a decrease in extrinsic Qs. Poisson's ratio, and Lambda increased, which is another indication of saturation. After two months of injection, as compared to one month before injection. Bulk modulus and Vp have returned to values comparable to before injection for the volume around the well bottom. A new anomaly in Vp has moved below the well. Vs continues to be low and Lambda and Poisson's ratio continue to be high compared to before injection. These changes have not moved, but increased in size. We interpret these observations to indicate continued saturation, but with increased fracturing. Only Vp and bulk modulus have changed significantly and this is due to the increased fracturing offsetting the saturation.