Estimating In-Situ Permeability of Stimulated EGS Reservoirs Using MEQ Moment Magnitude: an Analysis of Newberry MEQ Data

摘要

Evaluating both initial and evolving hydraulic properties of fractured reservoirs is vital for the development of Enhanced Geothermal System (EGS) reservoirs. During stimulation the virgin hydraulic diffusivity can be estimated through the analysis of spatio-temporal growth of abundant induced microearthquakes (MEQs). This method is effective to estimate the permeability at reservoir scale. However it has limitations in constraining the subsequent evolution of fracture permeability at relatively smaller length scales that are important in defining reservoir response during stimulation and then production. To constrain regional initial and evolving permeability at relatively smaller length scales we explore the linkage between moment magnitudes of observed MEQs and the measured evolution of fracture-network permeability. We use observations from the Newberry EGS stimulation assuming that the induced seismicity is controlled by the Mohr-Coulomb shear criterion. Catalog data for in-situ location and moment magnitude of MEQs are used to estimate fracture apertures of individual events/fractures that are a dynamic function of in-situ stresses, fluid pressure, shear displacement and fracture size. Assuming the veracity of the cubic law, results show that the equivalent virgin permeability at reservoir scale may be enhanced by about one order of magnitude while some local fracture permeability can be enhanced by ～2 to ～3 orders of magnitude. This method is of importance as it allows abundant observations of MEQs to constrain the structure and distribution of in-situ permeability evolution. Since in-situ geomechanical conditions (e.g., fracture stiffness, dilation and friction) also play a crucial role in affecting the accuracy of the permeability estimation, in-situ fracture characterization is an important component in constraining these observations.