Enhanced Geothermal Systems (EGS) use hydraulic fracturing to create a fracture network that facilitate the circulation of water between injection and extraction boreholes. Microseismic monitoring provides a unique method for the evaluation of the effectiveness and impact of the stimulation and reducing potential risks such as uncontrolled growth or induction of high magnitude seismic events. This study presents tools to interpret the geometry of the induced fracture network and quantify the changes in fluid conductivity induced in the reservoir during stimulations applied to a catalogue of over 8,000 events recorded at the Salton Sea Geothermal field over a 4-year period. The method integrates the temporal, spatial and size characteristics of the induced events to quantify the degree of interaction and connectivity between the fractures within the network, identify enhanced paths for fluid flow and highlight potential zones of induction of larger seismic events.
The objective of Enhanced Geothermal Systems (EGS) is the creation of a fracture network geometry that facilitates the maximum circulation of fluid between injection and production wells to maximize heat exchange avoiding interaction with in-situ faults that may potentially trigger damaging seismicity at the surface and neighbouring infrastructures. Ensuring that the suitable fracture network is stimulated and permeability is enhanced requires careful planning that typically involves modelling of the fracture growth based on local stress magnitude and orientation, reservoir rheological and fluid conductivity properties and characteristic of the in-situ Discrete Fracture Network (DFN). However, the impact of heterogeneity, local stress shadows and rotation and the fracturing process itself can result in unpredicted fracture growth deviating from the target objectives.
Microseismic monitoring of the microseismic activity associated to the induced fractures provides an essential feedback in EGS operations by imaging the fracturing process as it develops allowing the adoption of remedial actions that reduce potential risks. A valuable information extracted from the microseismic catalogue is the interpretation of the geometry of the induced fracture network, the connectivity within the network and its relation with mapped in-situ structures.
