Multi-scale Simulation Theory And Method Of Gas Transport In Shale Gas Reservoirs

With the rapid decline in conventional reserves, shale gas reservoir with huge reserve and extensive distribution represents a significant portion of unconventional natural gas resources, is becoming more important to the global energy supply in years to come. Due to the complex shale pore types with multi-scale pore size, various gas storage types and “Horizontal well+ Massive hydraulic fracturing” development treatments, the existing traditional flow theory is not suitable to model the fluid flow in fractured shale gas reservoirs. Thus, developing a corresponding fluid flow model and its numerical simulation method is urgent. The multi-scale simulation theory and method of gas transport in shale gas reservoirs is introduced in this dissertation. Firstly, we investigate gas transport mechanisms in porous media and develop three coupled models using different parameters(Knudsen number, equivalent hydraulic radius and transport properties(intrinsic permeability, tortuosity and porosity)) to describe the combined mechanisms of gas transport in tight porous media with only free gas and porous media with both free gas and adsorbed gas, In porous media with both free gas and adsorbed gas, surface diffusion through adsorbed layer and the effect of volume reduction by adsorption are considered in the transport model. Secondly, micro/nano-scale flow simulation theory based on the digital rock is constructed. A computing method of transport properties of porous media based on pore structure image and a determination of gas transport pattern in porous media based on pore structure image are introduced, we can obtain the transport properties and the gas transport pattern from the pore structure image of porous media. Thirdly, a single porosity and a double porosity finite element model with consideration of the combined gas transport mechanisms are developed to evaluate the influence of gas transport mechanisms, the matrixfracture transfer function, the gas adsorption and fracture parameters on the shale gas production respectively. Fourthly, the macroscopic scale flow simulation theory of the multistaged fractured horizontal well shale gas reservoirs based on discrete fractured model(DFM) is introduced. The multi-staged fractured horizontal well models of three types medium including single matrix medium, dual matrix-natural fracture medium and composite medium(dual matrix-natural fracture medium in SRV region and single matrix medium in outside of SRV region) are built, hydraulic fracturing fractures are dealt with discrete fracture model. Mathematical model is built and solved by finite element method, dynamic characteristics during shale gas reservoir development is analyzed. The influence of parameters of horizontal well, hydraulic fracturing, shale matrix, natural fractures and SRV on shale gas productivity is analyzed, which provides effective development guidance in shale gas reservoirs. Finally, the numerical simulation method of CO2 sequestration with enhanced natural gas recovery(CSEGR) in shale gas reservoirs is introduced. A new dual-porosity mathematical model incorporating the multiple gas transport mechanisms including viscous flow, Knudsen diffusion and ordinary diffusion is established; meanwhile, the transfer function and well flow model with considering the multiple mechanisms are developed. The mathematical model is implemented by the finite element simulation method, while the accuracy of the numerical solution is verified by comparison with the classic analytical solution. he results show that CSEGR is feasible to achieve CO2 sequestration and enhance CH4 recovery in shale gas reservoirs, the CO2 storage and natural gas production are improved as injection pressure increased. Summing up, an efficient multi-scale simulation theory and method of gas transport of shale gas reservoirs is developed in this dissertation, which provides a stable theory and technology guidance for the study of the effective development of shale gas reservoirs.