致密岩石气体渗流滑脱效应试验研究

由于致密结构和低的渗透率,气体在孔隙喉道小的致密岩石中流动会受到滑脱效应的影响。以湖南某试验场地致密砂岩为研究对象,对岩样进行了微观结构的SEM分析,通过一系列围压和孔隙压力作用下的砂岩气体流量和渗透率测试,研究气体在致密岩石中渗流特征,证明了致密砂岩气体流动存在滑脱效应现象,其渗流不符合Darcy定律。分析了孔压对滑脱效应的影响、滑脱效应对气测渗透率的影响以及滑脱因子与绝对渗透率的函数关系。研究结果表明,滑脱效应对气测渗透率的影响随着围压和气体孔隙压力的变化有所不同。同等围压下,孔隙压力越小,滑脱效应越明显,导致气测渗透率大于砂岩绝对渗透率。同等孔压下,当围压达到某一值后,其对滑脱效应的影响有限,同时也说明围压对岩石的压密是有限的。砂岩的平均气体孔隙压力与气测渗透率关系更加符合二次项曲线方程。计算获得的克努森数Kn说明了在相对高的围压和低的孔隙压力条件下,气体渗流过程位于滑脱流和过渡流之间,传统的N-S方程可能不再适用,应用Knudsen扩散方程更加合理,特别是当克努森数Kn比较高时。

Experimental study on slippage effects of gas flow in compact rock

Due to low porosity structure and low permeability, the gas flow in small pore throats in compact rock is usually affected by the gas slippage effects. In this study, the compact sandstone at a test site in Hunan Province is taken as an example, the micro-structure is studied using the scanning electron microscopy (SEM), and the permeability and flow rate of the sandstone under different pore pressures and confining pressures are measured. It is verified by the experiment that the gas flow in the compact rock does not meet the Darcy’s law due to the effect of gas slippage, and that the measured permeability should be corrected by the gas slippage effects. The results show that the impact of slippage on the gas permeability is different due to the change of the confining pressure and pore pressure. Under the same confining pressure, when the pore pressure is smaller, the gas slippage effects are more obvious, leading to that the measured permeability is greater than the absolute permeability. Under the same pore pressure, after the confining pressure reaches a certain value, it has a limited impact on the slippage, and the confining pressure has a limited impact on the rock compaction. The relationship between the average pore pressure and the gas permeability obeys the quadratic term equation of Knudsen’s permeability. The calculated Knudsen number (K_n) states that under relatively high confining pressure and low pore pressure, the gas flow is between slip flow and transitional flow, the traditional fluid dynamics N-S equation starts to fail, and it is safer to use Knudsen’s diffusion equation.