页岩气表观渗透率与多流动机理及有效应力演化关系

由于多尺度孔隙存在纳米尺度到宏观尺度的过渡,气体在页岩中流动的主导机制会在达西流和努森扩散等运移机理之间发生转换,该现象结合页岩基质内的气体吸附作用使得页岩气渗透率的测算变得更为复杂.因此,在实验室尺度确定有效应力及气体运移方式对渗透率的综合影响力对实际页岩气开采中的渗透率及产量评估准确性有着不可忽视的影响.采用脉冲衰减渗透率测算法,在不同围压条件下,对氦气和二氧化碳在富含有机质的页岩薄片中的气体渗透率进行测量.结果表明:努森扩散作用在低孔压时对页岩气渗透率有显著的正面影响,其强度与孔压成反比.在较大围压作用下的高孔压下的超临界二氧化碳可导致页岩基质最大吸附量降低,进而增加表观渗透率.表观渗透率随着有效应力的变化在不同孔压区间有着截然相反的趋势,有效应力系数此时出现非单一值,其主要是由于孔压变化带来的气体流动机理变化而引起的.

The Relationship between Shale Gas Apparent Permeability, Multi-flow Mechanisms and Effective Stress Evolution

Because of the transition from nanoscale to macro-scale in multi-scale pores, the dominant mechanism of gas flow in shale would be transformed between Darcy flow, Knudsen diffusion and other flow mechanisms. This phenomenon combined with gas adsorption in shale matrix makes the calculation of shale gas permeability more complicated. Therefore, to determine the comprehensive influence of effective stress and gas flow mechanisms on permeability at the laboratory scale has a significant impact on the accuracy of permeability and production assessment in real-life shale gas recovery. In this study, the pulse-decay method was used to measure the gas permeability of helium and carbon dioxide in organic-rich shale slices under different confining pressures. The results show that Knudsen diffusion has a significant positive effect on shale gas permeability at low pore pressure, and its strength is inversely proportional to pore pressure. Supercritical carbon dioxide under high pore pressure under high confining pressure can reduce the maximum adsorption capacity of shale matrix and increase its apparent permeability. The apparent permeability has a completely opposite trend with the change of effective stress at different pore pressure region, and the effective stress coefficient appears non-single values at this time, which is mainly caused by the change of gas flow mechanism due to the change of pore pressure.