A Coupled Thermal-hydraulic-mechanical Rock Damage Model With Multiphase Multicomponent Fluid Inside And Its Associated Applications

Coupled processes are vital in reservoir engineering during both reservoir stimulation and subsequent energy production(e.g.,geothermal,oil,and gas reservoirs).Complex thermalhydraulic-mechanical(THM)processes,including heat transfer,fluid flow,and rock deformations occurring simultaneously and are affected by many non-linear processes.Due to the uncertainty of the stress state of deep reservoirs and the high cost of traditional experimental methods,numerical modeling is recognized as an expedite and low-cost approach to simulate and analyze the multiphysics processes.In this thesis,numerical simulation is the basic research method.Coupled multi-component THM rock failure process analysis model is the theoretical foundation.The fracture propagation induced by pore pressure or temperature gradient during hydraulic fracturing is the main research content.The research object is to investigate the controlling factors of fracture propagation.The main work and research results are as follows:(1)Coupled multiphase multicomponent THM rock damage model was proposed to simulate coupled rock failure processes by considering the heterogeneity of rock material and the damage evolution and permeability evolution of the element.The THM model is implemented in the solver module that couples the multiphase,multicomponent fluid flow simulator TOUGH(Transport of Unsaturated Groundwater and Heat)with the geomechanical simulator RFPA(Rock Failure Process Analysis).The coupled simulator is called TOUGH-RFPA.The capability of the TOUGH-RFPA simulator was verified by two benchmark problems related to hydro-mechanical and thermal-mechanical response in a hollow cylinder and a one-dimensional consolidation benchmark problem.Moreover,simulation of a hydraulic fracturing experiment was developed in TOUGH-RFPA,demonstrating the ability of TOUGH-RFPA in modelling fracture propagation.TOUGH-RFPA and TOUGH-FLAC were compared by a CO2 disposal model.The advantages and disadvantages of TOUGH-RFPA and other simulators were discussed briefly.The comparisons show that the TOUGH-RFPA simulator takes into account the heterogeneity of materials and the spontaneous initiation and propagation of fractures,and has high computational efficiency,which is better than other software in simulating the deformation and fracture process of heterogeneous rocks.(2)The fracture propagation and stress shadow effect in layered heterogeneous rock mass model under simultaneous fracturing were investigated based on numerical simulation.The influence of interlayer properties(elastic modulus,Poisson’s ratio,strength,and permeability)and fracturing fluid properties(flow rate,viscosity)on the intersection of dual hydraulic fractures and geological interlayer were discussed.The results show that:due to the heterogeneity of rock and the stress shadow effect induced by the propagation of hydraulic fracture,the propagation path of multiple fractures will be asynchronous,asymmetric,and the fracture deflection angles will not be equal.During hydraulic fracturing,large fracture spacing and horizontal stress differences are conducive to the propagation of each fracture in a multi-fracture system.For layered heterogeneous rock models,the difference of mechanical parameters between the interlayer and oil-bearing layer affects the propagation path and breakdown pressure of hydraulic fractures,while the difference of permeability only affects the propagation paths of fractures.Increasing the fracturing fluid flux is beneficial in the dual fractures crossing the interlayer,while increasing the fracturing fluid viscosity makes it easier for the primary fracture to cross the interlayer.(3)The fracture propagation from a horizontal well during hydraulic fracturing in unconventional reservoirs was investigated based on numerical simulation.The effects of perforation height and spacing,in-situ stress and lithofacies differences between layers,and reservoir saturation on multi-fracture propagation path were discussed.The results show that large perforation height and perforation spacing will make the fracture farther away from the stress shadow area induced by adjacent fractures and form longer and higher fractures.High minimum horizontal principal stress and elastic modulus of the interlayer inhibit the growth of hydraulic fracture.The breakdown pressure of unsaturated reservoirs is higher than that of saturated reservoirs.The hydraulic fracture length and height are smaller than that of saturated reservoirs.The saturation of reservoirs cannot be ignored in the numerical simulation.(4)Based on numerical simulation,the propagation of fracture near the borehole during cold water injection and the propagation of secondary fractures during the cold water injection and heat extraction of main fractures in the enhanced geothermal system were studied.The effects of in-situ stresses and injection pressure on fracture propagation near borehole were discussed.The influence of the propagation of secondary fractures on heat extraction was discussed.The influence of the mass flow rate,injection temperature and horizontal stress on the propagation of secondary fractures and the heat extraction were discussed.The results show that in-situ stress affects the number of damages,the number of main fractures,and the propagation direction of main fractures.The increase of the injection pressure will lead to the enlargement of the tensile stress zone near the borehole and around the crack,and many elements will be destroyed simultaneously,which will affect the safety of the borehole.Higher injection pressure produces longer and wider fractures resulting in larger leakage,whereas lower injection pressure induces thin and complex fractures with less leakage.Secondary fractures propagate in the direction perpendicular to the main fracture,and micro damages occur around the main fracture and secondary fractures.Larger injection mass flux induces a larger heat extraction rate.The lower injection temperature produces more,denser and longer secondary fractures,and increases the heat extraction rate.There will be micro-fractures around the secondary fractures.The propagation of secondary fractures accelerates the heat convection,increases the area of heat transfer between water and rock,and significantly promotes heat extraction.However,in order to ensure an effective heat exchange,more micro damages should generate near the fractures,and the heat exchange zone should closely follow the development of the damaged area.