Coupled Thermal-Hydro-Mechanical Mechanism And Its Application For Fracturing Reform Of Low Permeability Shale Reservoirs

Shale gas,as an unconventional energy source with abundant reserves,clean and environmental friendly,has become an important alternative to traditional fossil fuels.The widely used hydraulic fracturing is difficult to conduct in the southwest China with abundant shale gas reserves since the complicated geological structure,distribution of faults,and lack of water resources.Thus,it is urgent to establish reservoir transformation theory and mining technology suitable for China’s geological conditions.In this work,the research is based on the background of fracturing shale gas reservoirs.The damage evolution of shale rock and fracturing mechanism during low permeability shale reservoir reformed by injection fluids are studied combining theoretical modeling,numerical simulations and laboratory experiments.The innovative achievements are obtained as follows.(1)Considering the characteristics of temperature field in deep shale gas reservoirs and the density and viscosity of compressible fluids varying with temperature and pressure,fracturing reconstruction of shale gas reservoirs is considered as a fully coupled process of stress field,seepage field and temperature field.And a fully coupled Thermal-Hydro-Mechanical model for shale media is established,it can more effectively reflect the geological conditions of deep shale gas reservoirs compared with the existing Hydro-Mechanical coupling model.Besides,a finite element solution combining Matlab and Comsol based on incremental iteration method is proposed for the coupled model.The validity of the model is verified by comparing the simulation results with theoretical solutions and experimental results.(2)A Thermal-Hydro-Mechanical coupling numerical model considering damage evolution of surrounding rock for fracturing reconstruction of shale borehole surrounding rock is established,and damage theory is adopted to describe the weakening effect of damage on elastic modulus,permeability and thermal conductivity of rock.The effects of in-situ stress conditions,surrounding rock temperature,fracturing fluid temperature and injection rate on the fracture initiation and propagation of borehole surrounding rock are obtained.Based on the maximum tensile stress criterion and Mohr-Coulomb criterion,the different fracture propagation mechanism of surrounding rock under different conditions is obtained.And it can provide theoretical basis for optimum design of drilling parameters in shale fracturing reconstruction.(3)Considering the characteristics of a large number of joints in shale gas reservoirs,the influence of joint distribution on fracture initiation and propagation of boreholes surrounding rocks in shale reservoirs fracturing reformation is analyzed.Four kinds of intersection relations between fracture propagation path and joint structure in borehole surrounding rock are obtained,inserting,direct crossing,L-shaped crossing and T-shaped crossing.The evolution mechanism of fracture network in fractured borehole surrounding rock under different in-situ stress and joint structure conditions is revealed.(4)With the standard triaxial hydraulic fracturing testing system at Pennsylvania State University,fracturing tests and micro-fracture morphology observation are carried out on Green River Shale Formation in Utah,USA,using water,helium,liquid carbon dioxide,nitrogen and supercritical carbon dioxide as driving fracturing fluids.Three parameters are introduced to characterize the fracturing effect: fracture tortuosity,fluid propagation distance and wedge aperture.The research results indicate that nitrogen and supercritical carbon dioxide fracturing are better than other fracturing fluids.