三维页岩储层多重压力流固耦合模型研究

Study on the 3D fluid-solid coupling model with multi-pressure system of shale

研究裂缝页岩储层在生产过程中的流固耦合力学过程是准确认识气藏生产动态的关键科学问题.针对页岩储渗机制复杂的难题,本文建立有机质-基质孔-天然裂缝和离散人工裂缝的多尺度流动介质与多重压力系统;考虑吸附气的非平衡解吸附和表面扩散,自由气的黏性流和Knudsen扩散,以及基质孔、天然裂缝和人工裂缝压力系统对页岩变形的影响,建立了页岩双重有效应力流固耦合数学模型,分析了压裂井生产过程中岩石变形对气体流动的影响规律.研究发现:天然裂缝压力系统是引起页岩气生产过程中应力敏感性的主要因素;固体变形对基质孔渗透率的影响小于Knudsen扩散,基质孔的表观渗透率并不会发生应力敏感;随着天然裂缝密度的增大,天然裂缝压力系统的有效应力系数迅速增大,页岩气产量应力敏感效应增强.

The study of fluid solid coupling process in fractured shale reservoir, with massive hydraulic and natural fracture networks, during production is a key scientific issue for accurately understanding the production performance of multistage fractured wells. In view of the complicated flow mechanism of shale gas flow, multi-scale media and multiple pressure systems of organic matter/matrix pore/natural fractures and discrete artificial fractures are established to accurately describe the gas flow in shale reservoir. The mathematic model considers mechanisms of non-equilibrium desorption and surface diffusion of the adsorbed gas, the viscous flow and Knudsen diffusion of the free gas, and the influence of matrix pore, natural fracture and artificial fracture pressure system on shale deformation. A fully coupled dual effective stress fluid-solid coupling is established, which considers different pressure system respectively. The influence of rock deformation on shale gas flow in the fractured well production is analyzed. It is found that the natural fracture pressure system is the main influence factor that causes stress sensitivity during the shale gas production. The impact of solid deformation on the permeability of matrix pores continuum is smaller than that of Knudsen diffusion. The apparent permeability of matrix pores continuum is still larger than its original permeability, thus there is no stress sensitivity of matrix pore permeability. With the increase of natural fracture density, the effective stress coefficient of natural fracture pressure system increases rapidly, and the stress sensitivity effect during shale gas production process is stronger.