Pore pressure dependent gas flow in tight porous media

Gas flow in micro-porous media is more complex than liquid flow, partially because of the gas compressibility storage effect, and because of that, permeability as a function describing the flow efficiency for liquid might not apply to gas, but gas pressure diffusivity concept taking into account the effect of gas compressibility might do. However, verifying this supposition in conventional reservoirs at the core scale is challenging because the gas flow process finishes almost instantly. In comparison, gas flow time in unconventional tight cores is longer, and pulse-decay experiments are frequently used to estimate permeability, which requires a period to be finished. Therefore, theoretically, the supposition could be tested. In this study, pulse-decay experiments were performed on one shale core plug under different pore pressures with different pulse magnitudes to explore the relationship between gas permeability/diffusivity with experiment durations. We put forward a novel approach to determining the intrinsic permeability and gas slippage factor through a single pulse-decay experiment, which requires running multiple experiments by the conventional approach. We verified that gas pressure diffusivity instead of the permeability is the appropriate measure of the efficiency of gas propagating through the porous media, given the fact that the ratio of pulse size over diffusivity is roughly proportional to the experiment duration. Consistent results were obtained using different empirical models to obtain the intrinsic permeability, gas slippage factor, and pressure-dependent apparent gas permeability simultaneously. Results in this study also verified that pulse size affects the permeability determination more under a lower pore pressure in a pulse-decay experiment.

Automated summary

Gas flow in micro-porous media is more complex than liquid flow, and the efficiency of gas propagation through porous media can be appropriately measured by gas pressure diffusivity instead of permeability. Pulse-decay experiments were conducted in this study to explore the relationship between gas permeability/diffusivity and experiment durations, providing consistent results in determining intrinsic permeability and gas slippage factor through a single experiment. The study also showed that pulse size affects the permeability determination more under lower pore pressure in a pulse-decay experiment.