Numerical Simulation of Hydraulic Fracturing In Geothermal Reservoirs With Natural Fractures Considering Thermal Stress

Enhanced geothermal system (EGS) is the key technology for efficient development of geothermal energy. The formation of interconnected complex fracture networks in geothermal reservoirs by hydraulic fracturing is particularly important for the establishment of high quality EGS. Hydraulic fracturing of geothermal reservoir is a complex process, which is affected by in-situ stress, injection water pressure and the cryogenic induced thermal stress. And the existence of natural fractures will also greatly affect the expansion of hydraulic fractures. Therefore, the numerical simulation of hydraulic fracturing in geothermal reservoirs with natural fractures is of positive significance for clarifying the mechanism of hydraulic fracture propagation and establishing a high-quality EGS. In this paper, a two-dimensional geothermal reservoir model with randomly distributed natural fractures is constructed, and the heterogeneity of reservoir physical properties and rock mechanical properties is considered by using Weibull distribution. Then, a thermo-hydro-mechanical-damage (THMD) coupling mathematical model is established based on meso-damage mechanics, elastic thermodynamics and Biot’s classical seepage mechanics theory. The coupling calculation of thermal field, seepage field, solid stress field and damage variable is realized by using COMSOL and MATLAB software. And the propagation process of hydraulic fracture is reflected according to the geothermal reservoir’s damage evolution. Finally, the influences of reservoir temperature, rock thermal expansion coefficient, natural fracture distribution density, in-situ stress difference, reservoir permeability, fracturing fluid viscosity and injection displacement on the fracture propagation of hydraulic fracturing in geothermal reservoirs are analyzed. The results indicate that the thermal stress induced by temperature difference will reduce the reservoir crack pressure and fracture extension pressure. The higher the reservoir temperature and the greater the thermal expansion coefficient, the greater the thermal stress generated, resulting in a lot of damage near the wellbore. Natural fractures will guide hydraulic fractures to turn and bifurcate along natural fractures to form complex fractures. The larger the distribution density of natural fractures, the smaller the in-situ stress difference and the viscosity of fracturing fluid, the more conducive it is to build complex fracture networks. The decrease of permeability and the increase of injection displacement will reduce the crack pressure and the fracture extension pressure, thus forming longer hydraulic fracture, which is beneficial to expand the range of geothermal reservoir reconstruction.