Fluid-solid Coupling Numerical Simulation On Natural Gas Production From Hydrate Reservoirs By Depressurization

By reason of extensive distributing and vast amount of quantity, natural gas hydrate is considered to be the most prospective energy in the 21st century. Considering the costs and feasibility, depressurization is the most economical and effective method for future large-scale natural gas hydrate production.During natural gas production from hydrate by depressurization, the petrophysical and mechanical parameters, the pore pressure and the temperature of hydrate reservoirs have been changing dynamically due to hydrate decomposition. These phenomena are named as gas hydrate decomposition effect in this paper. Furthermore, there is an intensive interaction between porous fluid flow and rock deformation during natural gas hydrate production. So it is of great theoretical and application importance to study on gas hydrate production with fluid-solid coupling numerical simulation.This paper focuses on the related problems during natural gas hydrate production by depressurization with the method of fluid-solid coupling numerical simulation.Firstly, a new phase change porous flow model for gas hydrate production by depressurization has been developed. The model includes mass and energy conservation theory, thermodynamics and kinetic of hydrate decomposition, gas-water two-phase flow and the gas hydrate decomposition effect. Based on this new model, a nonisothermal fluid-solid coupling numerical model has been developed for permeability anisotropy hydrate reservoirs. Moreover, several comprehensive dynamic models for hydrate reservoirs physical parameters have been developed. These models consider the gas hydrate decomposition effect based on former experimental research on stress sensitivity of unconsolidated sandstone reservoirs. After the discretization of the governing equations of the fluid-solid coupling model, a gas and water two-phase nonisothermal fluid-solid coupling numerical simulation software for gas hydrate production has been developed. By introducing the stability analysis module, the paper has developed a near well formation stability analysis software for hydrate reservoirs production by depressurization.Secondly, the dynamic variation and influence factors for the stress state and the physical and mechanical parameters of hydrate reservoirs have been analyzed with the self-developed fluid-solid coupling simulation software. The results show the gas hydrate decomposition effect, fluid-solid coupling and borehole effect are three main factors which have different mechanisms and influence level for stress state and parameters. The fluid-solid coupling has an inhibiting effect for variation of the physical and mechanical parameters due to gas hydrate decomposition effect, and more attention should be paid to the phenomena.Thirdly, the stability of near well formation of hydrate reservoirs has been analyzed using the self-developed stability analysis software, and this area has the features of weak consolidation, low strength and high poroperm due to hydrate decomposition. A method for critical drawdown pressure determination has been developed. The results show that the stability of the decomposition area is relatively poor, and borehole wall in the direction of the minimum horizontal principal stress is the preferred position for sand production. The influence factors analysis reveal that the gas hydrates decomposition effect, drawdown pressure; fluid-solid coupling and the nonhomogeneity degree of the principal stress are the most sensitive factors affecting the stability of the near well formation, and the influence from the pressure release velocity is relatively small.Finally, a fluid-solid coupling numerical simulation on the productivity of natural gas hydrate by depressurization has been performed. The results show that the decrease of the poroperm ability due to fluid-solid coupling is the dominant factor which affects the productivity of the natural gas hydrate, and the increase of the elastic drive energy by fluid-solid coupling is relatively small, so the predictive value of the productivity from fluid-solid coupling modeling is lower than that of the non-coupling modeling. The influence factors analysis reveals that the gas production rate and the cumulative gas increase with the increase of reservoir absolute permeability, with the decrease of bottom pressure, with the increase of reservoir temperature.