孔隙率的应力-剪胀依存性对CO_2地层注入热-气-应力耦合影响的数值分析

NUMERICAL ANALYSIS OF INFLUENCE OF STRESS-DILATANCY DEPENDENCE OF POROSITY ON THERMO-GAS-MECHANICAL COUPLING IN CO_2 INJECTION IN STRATUM

在所建立的热–气–应力耦合弹塑性模型中,引入已有的经验关系式,将岩体的孔隙率和渗透性作为平均有效应力和剪胀体积应变的函数,并编制相应的二维有限元计算程序。以一个假定的由注入层和冠石层组成的地质结构为考察对象,就岩体不同孔隙率和渗透系数的3种工况,在CO2注入过程中,对岩体中的热–气–应力耦合现象进行数值模拟。计算结果表明:在相同的注入速率和时间内,3种工况所对应地层的孔隙率、孔隙气体压力、岩体位移、应力状态和受拉、剪切破坏范围均有明显不同。因此,将岩体的孔隙率和渗透性作为应力的函数、特别是计入剪胀体积应变的影响,应该更为接近实际情况。

Introducing the existing empirical relationships into a coupled thermo-gas-mechanical model,the porosity and the permeability of rock mass are regarded as functions of mean effective stress and volumetric strain resulted from shear dilation;and a corresponding two-dimensional finite element code is developed.Taking an assumed geological storage system consisted of an under-part reservoir and an upper-part caprock as an analytic object,the numerical simulation of the coupled thermo-gas-mechanical process in the rock mass during CO2 injection is conducted under three cases of different porosities and permeabilities.The calculation results show that the distributions and changes in porosities,pore air pressures,displacements,stresses,zones of tensile and shear failure in the rock mass are obviously different in three cases.Therefore,it is more accurate to consider porosity and permeability of rock as functions of stress,especially reckoning in the effect of volumetric strain resulted from shear dilation than to consider them as constants.