Thermo-poro-elastic stress induced aperture alteration of fractured geothermal reservoir and its effect on geothermal energy production

Abstract

 Deep geothermal reservoirs are often naturally or hydraulically fractured media which consist of a rock matrix and a network of randomly oriented discrete fractures of different sizes. In the present study, a three-dimensional model of coupled flow, heat transfer and deformation of fractured geothermal reservoir is developed. Because of high thermal expansibility and high mobility, supercritical C02 is under consideration as an alternative fluid to water/brine for extracting energy from geothermal reservoir. However, for simulation of CO2 based EGS, two phase flow model should be included and this makes the coupled model far more nonlinear and complex. FEHM which is one of the most robust code developed at LANL is capable to simulate the multi-physics problem of geosciences. We have found that the computational cost for CO2 based EGS a few times higher than that of water based EGS. Due to temperature drawdown and pressure difference between injection and production wells thermo-poro-elastic stresses are induced within the reservoir. This can influence larger shear dislocation, and normal opening /closing of fracture causing changes in the fracture aperture and permeability during the extraction of the geothermal energy from a reservoir. The correlation of the variation of fracture permeability with the variation of the local stress tensor has been taken into account in this study. To study the permeability alteration on geothermal energy extraction for water and CO2  based EGS different fracture networks, different values of injection temperature and injection pressure are considered. Three-dimensional fracture networks of randomly oriented rectangular fractures with different sizes, and dip angles are created using ADFNE Matlab code. A structured computational mesh is created for simulating the multiphase flow, heat transfer and geomechanics. The nodes belonging to the fractures are assigned appropriate permeability, thermal conductivity and mechanical properties depending on the fracture aperture. The values of these properties are subjected to alteration based on local stress values.