| Abstract |
We demonstrate an analysis of micro-earthquake data in a geothermal environment. The goal is to identify fractures, the state of fluids, and permeable zones. The approach is to extract as much information as possible from micro-earthquake recordings. 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. We then utilize rock physics in interpretation to identify reservoir properties. We also test an approach where reservoir properties are determined rapidly, with high resolution, and cheaply. To this end, we have developed an inexpensive, automated micro-earthquake data collection and processing system and computational capability necessary to record and process large numbers of micro-earthquake recordings and obtain tomographic images at lower costs and in a shorter time frame than has been previously possible. Such information provides the basis for reservoir analysis. The easily deployable nature of our system allows for potential large-scale assessment of resources in regions throughout the world. Further, the system’s low hardware cost, simple operation, and automated data processing make it attractive to small companies and developing countries with little money and few trained personnel to process and analyze data. We apply the system to the Enhanced Geothermal System (EGS) demonstration project at the northwest part of The Geysers, California. We found that one month after injection at Prati-32 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 the second month of injection we found that 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. Qs shows no change for either one or two months after. 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. |