不同应力条件下页岩表观渗透率模型试验研究

Experimental study on shale apparent permeability model under different stress conditions

为探究页岩气抽采过程中气体传输和应力耦合作用对页岩表观渗透率的影响,提出了一种含加权因子耦合多种传输机理、吸附变形及应力应变关系的页岩表观渗透率模型,用来描述气体流动,通过试验数据验证其合理性,并对模型相关参数对页岩表观渗透率的影响进行讨论。研究结果表明:新建考虑流体流态和应力耦合作用的页岩表观渗透率模型能合理地表征页岩气体流动,包括考虑应力变化下气体滑脱效应、孔隙结构和吸附变形因素,以及体相气体传输的黏性流动(滑脱流动)和努森扩散因素。在不同围压、孔隙压力2种应力条件下,新建模型计算的曲线均与实测值吻合较好;且随孔隙压力、围压的升高,页岩表观渗透率呈指数函数形式降低,同时裂隙压缩系数Cf的绝对值整体呈降低的趋势。孔隙压力越低努森扩散所占比重越高,相同孔隙压力状态下,孔隙尺度越小,努森扩散的贡献率越大;页岩表观渗透率对弹性模量较为敏感,弹性模量越大吸附引起的基质变形量越小,相应渗透率升高量也将随之降低。不同页岩孔径所对应的主控传输机理不同,随孔径的减小,此时孔径与气体平均分子自由程具有可比性,发生滑脱流动,渗透率升高;考虑滑脱效应页岩表观渗透率计算值均大于不考虑滑脱效应计算值,且更接近试验测量值,随孔隙压力、围压的升高,滑脱效应引起的渗透率变化量逐渐降低。

To explore the influence of gas transport and stress coupling on shale apparent permeability in the process of shale gas extrac⁃tion,this paper proposes a contains a variety of weighted factors coupling transmission mechanism,the adsorption of deformation and stress strain relationship of shale model to describe the gas flow,apparent permeability,the rationality of the model is verified by experimental da⁃ta,and the influence of model-related parameters on shale apparent permeability were discussed.The results show that a new shale appar⁃ent permeability model considering fluid flow and stress coupling can reasonably describe shale gas flow,including considering the gas slip⁃page effect under stress,pore structure and adsorption deformation,viscous flow of gas transmission and body phase flow(slip)and Knud⁃sen diffusion.Under different confining pressures and pore pressures,the curves calculated by the new model are in good agreement with the measured values.With the increase of pore pressure and confining pressure,the apparent permeability of shale decreases exponentially,and the absolute value of the cleat compressibility Cf decreases as a whole.The lower the pore pressure,the higher the proportion of Knud⁃sen diffusion.Under the same pore pressure,the smaller the pore size,the greater the contribution rate of Knudsen diffusion.The apparent permeability of shale is sensitive to the elastic modulus,and the larger the elastic modulus is,the smaller the matrix deformation caused by adsorption will be,and the corresponding increase of permeability will also decrease.The main control transmission mechanism of different shale pore sizes is different.With the decrease of pore sizes,pore sizes are comparable to the average molecular free path of gas,and slip flow occurs and permeability increases.The calculated values of apparent shale permeability with slippage effect taken into account are all greater than those without slippage effect,and are closer to the measured values.With the increase of pore pressure and confining pressure,the permeability change caused by slippage effect gradually decreases.