Mapping permeability tensors in fractured geothermal reservoirs using MEQ data

In the stimulation of fractured geothermal reservoirs, injection wellhead pressure, flow rate and microearthquake (MEQ) data are crucial feedbacks recorded in order to characterize the evolution of subsurface fluid flow. However, one of the hurdles to successful EGS development and operation is the lack of reliable evaluation for the initial and evolving hydraulic properties of the fractured reservoir. Specific spatial conditions (e.g., location and direction) of fracture permeability in the field are vital in defining reservoir response during stimulation and then production. To constrain the evolving permeability, we propose a model that maps the in-situ permeability based onto the Oda crack tensor using the moment magnitude of individual MEQs, assuming that the induced seismicity is controlled by the Mohr-Coulomb shear criterion. The MEQ catalog of locations, fault plane solutions, and moment magnitudes are used to estimate fracture apertures of individual events/fractures that are a dynamic function of in-situ stress, fluid pressure, shear displacement and fracture size. The corresponding in-situ 2D permeability tensors are computed and mapped at various scales within the reservoir. Results suggest that the permeability magnitude largely depends on MEQ moment magnitude and fracture frictional properties while permeability direction is dominantly controlled by fracture orientation. However, uncertainty remains within the results, which need improvements in constraint from laboratory and in-situ fracture characterization, the quality of seismic monitoring and reliability of appropriate assumptions.