Investigation Into Hydraulic Fracture Propagation Mechanism In Enhanced Geothermal System

Hot dry rock(HDR)reservoirs generally feature high temperature,ultralow permeability,and deficiency in water.Vertical or horizontal wells are drilled into HDR reservoirs to effectively extract heat in deep geothermal reservoirs.Hydraulic fracturing is widely used to create complex fracture networks(artificial geothermal reservoir)with large heat-exchange areas and high conductivity for fluid circulation between injection and production wells.This exploitation mode is known as an enhanced geothermal system(EGS).The spatial distribution of fracture network has a great influence on the heat-extraction performance and lifespan of EGS.During hydraulic fracturing and heat extraction,the injection of massive fluid causes the decrease of rock temperature,which results in variation in stress condition and rock properties.This thermal shock effect further influences the propagation mechanism and conductivity of hydraulic fracture in HDR.In this study,the thermal shock effect was simulated by slow heating and rapid water-cooling treatment.Afterthat,a series of laboratory tests,including SEM,P-wave velocity tests,porosity and permeability tests,tensile strength tests and compression tests,was performed to investigate the influence of thermal shock effect on physical and mechanical properties of granite.Based on the energy evolution theory and complete stress-strain curve analysis,a new model for brittle evaluation was proposed by considering the effect of confinement.The essence of brittle was illustrated from thress aspect,i.e.,the energy conversion efficiency during pre-peak stage,and the rate and extent of energy release during post-peak stage.Further,the influence of thermal shock effect on deformation and failure behavior and brittle variation of granite was discussed.By combining laboratory fracturing simulation experiments,SEM,CT scanning and Acoustic emission(AE)monitoring,the initiation and propagation mechanism,injection pressure response,and AE activity under thermal shock effect were analyzed.Additionally,3D profile tests were conduct to reveal the mesoscopic characteristics of fracture surface.The new design concept named “cyclic thermal shock stimulation” was proposed in terms of the demand in reducing breakdown pressure and induced seismicity during hydraulic fracturing in HDR.Considering the results of physical and mechanical parameters,a thermal-hydro-mechanical-damage coupled model for hydraulic fracture propagation in HDR was established based on the finite element method.The influence of thermal shock effect on activation criterion of natural fractures and fracture network geometry was numerically studied.In combination of micro-indentation test and conductivity test of propped fracture,fracture surface softing effect and its influence on fracture conductivity was illustrated from the view of micromechanics.Through laboratory experiments and numerical simulation study,the roles of thermal shock effect on physical and mechanical properties of granite,propagation mechanism of hydraulic fractures and evaluation of fracture conductivity were systematically revealed.The investigation method and conclusion can provide some guidances and references for the development of HDR geothermal energy.