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.