页岩气渗流过程中的多场耦合机理

The coupling of multi-physics for gas flow in shale reservoirs

本文简要概述了目前模拟页岩气渗流机理数学模型的不足之处,基于线弹性多孔介质力学,建立了自由气渗流、吸附气表面扩散、吸附气非平衡解吸附、岩石变形的全耦合页岩气微观渗流数学模型,推导了纯孔隙介质与包含裂隙介质的主要物性参数的应力敏感模型,并推导了解吸附过程非平衡效应的理论模型,形成了一套较全面的描述页岩气在线弹性微纳多孔介质中的微观流动数学模型.通过数值模拟分析了应力敏感、表面扩散、非平衡效应对页岩气传输的影响,结果表明:页岩的应力敏感效应由基质中的天然裂缝决定,页岩孔隙本身的应力敏感性可以忽略;吸附气的表面扩散十分缓慢,可以忽略;平衡常数与孔隙度对非平衡效应影响显著,吸附速率对页岩气流动的影响十分明显,吸附速率越大,页岩气流速越大.本文建立的数学模型为深入理解页岩气渗流微观机理,科学开发页岩气藏提供了一定的理论基础.

The deficiencies in present mathematic models of shale gas flow are outlined briefly. Based on the linear poroelasticity, the transfer of free gas in nanopores, surface diffusion of adsorbed gas, nonlinear and non-equilibrium gas adsorption-desorption kinetics as well as the rock deformation are all coupled to formulate a new mathematic model to unlock the true potential of shale-gas development. The stress-and-pressure dependent permeability and porosity models of matrix containing random fractures, together with the non-equilibrium adsorption-desorption kinetics, are also deduced in this paper. Numerical method is applied to solve the model and analyze the impacts of stress-and-pressure sensitivity, surface diffusion and non-equilibrium adsorption on shale gas flow. The results suggest： （1） The fracture density in the shale matrix plays an important role in the permeability and porosity, the stress-and-pressure sensitivity of shale rock itself can be ignored; （2） compared with the free gas flow in pores, the surface diffusion of adsorbed gas can be ignored; （3） the recovery time during non-equilibrium sorption process relies heavily on equilibrium coefficient and rock porosity, and the desorption rate significantly impacts the gas flow rate.