Imaging of Reservoirs and Fracture Systems Using Microearthquakes Induced by Hydraulic Injections

Abstract

Predicting the future performance of a geothermal reservoir and planning a strategy for increasing productivity from the reservoir require an intimate knowledge of the fracture system through which geothermal fluids permeate. Indirect methods, such as the use of tracers and modeling of thermal drawdown, can be used to infer bulk fracture system properties. These methods, however, provide no means for determining the locations of the fractures that comprise the system. Wellbore geophysical logs can be used to infer the locations of fractures intersecting the wellbores but provide not information about the fracture system away from the wellbore. Microearthquakes often accompany hydraulic fracturing as well as normal production activities in geothermal fields. The waveforms from these microearthquake provide valuable information that can be used to infer the three dimensional structure of the fracture system in the reservoir. The locations of the microearthquakes can be used to infer the presence of large fractures along which shear slip has occurred. Tomographic imaging using arrival times of the seismic waves, provides a three dimensional image of the P and S wave velocity structure of the reservoir. These velocities yield information about the presence of micro fractures in the rock. Waveform stacking methods can be used to both corroborate seismic velocities and image seismic scatterers in the reservoir. The most prominent seismic scatterers are likely to be fluid filled fractures Thus, seismic data provide information about a fractures over a large scale range which can be of use in reservoir engineering.