Reservoir permeability mapping using microearthquake data

Evaluating hydraulic properties of fractured reservoirs both during and after stimulation is vital for the development of Enhanced Geothermal System (EGS). To constrain the evolution of fracture permeability at sufficiently fine resolution to define reservoir response, we propose a model that couples the moment magnitude to fracture aperture and then estimates the reservoir permeability at relatively high resolution. The critical parameters controlling fracture aperture and permeability evolution are stress-drop, the bulk modulus of the fracture embedded matrix, and the dilation angle of fractures. We employ Oda's crack tensor theory and a cubic-law based analog to estimate the permeability of a synthetic fractured reservoir at various scales, demonstrating that the resolution of permeability is largely determined by the cellular grid size. These methods are applied to map the in-situ permeability of the Newberry EGS reservoir using observed microearthquakes (MEQs) induced during two rounds of reservoir stimulations in 2014. The equivalent mean permeability evaluated by each method is consistent and unlimited by representative elementary volume (REV) size. With identical parameters, Oda's crack tensor theory produces a more accurate estimation of permeability than that of the cubic law method, but estimates are within one order of magnitude. The permeability maps show that the most permeable zone is located within the zone of most dense seismicity, providing a reference for the siting of the production well. This model has the potential for mapping permeability evolution from MEQ data in conventional and unconventional resources and at various scales.