Evaluation of flow paths during stimulation in an EGS reservoir using microseismic information

Abstract

Evaluation of fluid flow behavior is important for understanding various phenomena in a geothermal reservoir. Microseismic monitoring and location can be used to track pore pressure migration to some extent; however, microseismic information cannot directly provide the detail of flow behavior, such as how fluid flow occurs in the reservoir. In this study, we propose a new method to evaluate the entire flow-path system in a fractured reservoir. We first estimate the pore-pressure increase for each microseismic event based on microseismic and in-situ stress information. We extend the discrete pore pressure data to a continuous distribution based on the idea of a main flow pathway and sub flow pathways. We also introduce a honeycomb-shaped flow pathway model, which consists of flow pathways and nodes. Hydraulic parameters are different between different flow pathways. To match observed pore pressures at the nodes, an optimization process is used to adjust the hydraulic parameters for each flow pathway. We get a proxy of the hydraulic conductivity distribution based on the flow pathway model, that can evaluate flow pathways and flow behavior. We apply this method to hydraulic stimulation and microseismic data from the Basel enhanced geothermal system (EGS) project. Our method successfully delineated the main permeable zone and caught the tendency of fluid flow behavior that is also indicated by other microseismic analysis. Our method can catch the qualitative behavior of fluid flow in the geothermal reservoir and can contribute to designing geothermal energy extraction systems and forecasting.