CO_(2)-水作用下页岩微观孔隙结构变化特征

Variation characteristics of micro⁃pore structure of shale after CO_(2)⁃water interaction

为明确注CO_(2)提高页岩气开采过程中CO_(2)与水作用后对页岩微观孔隙结构的影响,采用X射线衍射、低温N_(2)吸附和低场核磁共振等实验方法,研究了原始页岩及纯盐水、注入压力为6和12 MPa的CO_(2)水溶液浸泡后页岩的矿物组成、孔径分布和微观孔隙结构变化特征,并结合分形理论,获取了孔表面和孔体积的分形特征,明确了页岩中吸附孔隙和渗流孔隙的变化特征。结果表明:CO_(2)水溶液浸泡后,页岩中石英含量增大,黏土矿物和碳酸盐含量降低;随着CO_(2)注入压力的增加,微孔比表面积和总比表面积减小,平均孔径、核磁孔隙度和大孔孔体积比例增大,表明在CO_(2)水溶液的溶解作用下微孔和介孔逐渐向大孔转化;低温N2吸附和核磁共振(NMR)获得的分形维数均随CO_(2)注入压力的增加逐渐降低,表明页岩在CO_(2)水溶液浸泡后孔隙结构的非均质性和复杂性降低;孔表面分形维数DL1、DN1和孔体积分形维数DL2、DN2与总比表面积和总孔质量体积呈正相关性,这2个指标可分别用于表征页岩中的吸附孔隙和渗流孔隙特征,能够有效评价页岩气体吸附与渗流能力。研究成果为注CO_(2)高效开发页岩气提供了参考和依据。

To clarify the effect of CO_(2) injection on micro-pore structure of shale gas after interaction of CO_(2) with wa⁃ter during the process of shale gas production,experimental methods of XRD,low-temperature N2 adsorption and low-field NMR are used to study mineral composition,pore size distribution and micro-pore structure variation of shale after immersing in original shale and pure saline water,6 and 12 MPa(injection pressures)CO_(2) aqueous solu⁃tions.Combined with fractal theory,the fractal characteristics of pore surface and pore volume are obtained,deter⁃mining variation characteristics of adsorption pores and flow pores in shale.The results show that after immersing in CO_(2) injection aqueous solution,quartz content in shale increases,while clay mineral and carbonate contents de⁃crease.As CO_(2) pressure increases,specific surface area of micro-pores and total specific surface area decrease,while average pore size,nuclear magnetic porosity and proportion of macro-pore volume increase,indicating a trend of gradual transformation from micro-pores and meso-pores to macro-pores effected by dissolution of CO_(2) aque⁃ous solution.Fractal dimension obtained by low-temperature N2 adsorption and NMR decreases gradually with CO_(2)injection pressure increase,indicating that heterogeneity and complexity of pore structure of shale are gradually re⁃duced after immersion in CO_(2) aqueous solution.Pore surface fractal dimension DL1 and DN1 and pore volume fractal dimension DL2 and DN2 are positively correlated with total specific surface area and total pore volume.These 2 indica⁃tors can be used to characterize the adsorption pores and flow pores in shale,respectively,and can effectively eval⁃uate gas adsorption and flow capacity of shale,providing reference and basis for efficient development of shale gas by CO_(2) injection.