页岩应力敏感实验与机理

为了研究页岩渗透率的应力敏感现象及其机理,从毛细管、平板裂缝和双重孔隙介质的渗透率应力敏感机理出发,得到了多孔介质渗透率应力敏感系数的通用表达式,并结合下志留统龙马溪组、下寒武统牛蹄塘组页岩岩心的覆压孔渗联测实验,分析了页岩应力敏感的机理。实验结果表明:渗透率应力敏感系数为孔渗幂指数与孔隙压缩系数的乘积,孔渗幂指数表示孔隙的几何特征,孔隙压缩系数反映岩石力学参数和孔隙形状的影响。页岩岩心微裂缝较发育,微裂缝尺度与孔隙尺度相当的岩心孔渗幂指数小于3,微裂缝尺度远大于孔隙尺度岩心的孔渗幂指数大于3;与砂岩相比,实验所用页岩具有较低的孔渗幂指数,但孔隙压缩系数较高,因此页岩的应力敏感较强。

Shale stress sensitivity experiment and mechanism

To study the stress sensitivity phenomenon and mechanism of shale permeability, a common expression of the stress sensitivity coefficient of porous medium permeability was obtained based on the permeability stress sensitivity mechanism of capillary, planar crack and dual porosity media. In combination with overburden-pressure porosity-permeability experiments of shale cores in Longmaxi Formation and Niutitang Formation, the mechanism of shale stress sensitivity was analyzed in this study. The experimental results indicate that the permeability stress sensitivity coefficient is the product of porosity-permeability power exponent and pore compressibility, of which porosity-permeability power exponent represents the geometric characteristics of pores, and pore compressibility reflects the influence of rock mechanics parameters and pore shapes. Micro-cracks are relatively developed in shale cores, and the porosity-permeability power exponent of core with micro-crack size equivalent to pore size is less than 3, while that with micro-crack size much larger than pore size is greater than 3. Compared with sandstone, the shale for experiments has a lower porosity-permeability power exponent, but higher pore compressibility, leading to stronger shale stress sensitivity.