A laboratory observation for gas-rock mechanics and gas transport mechanism in low-permeability rocks

Abstract

The fluid migration (including gases) in sedimentary rocks affects fluid overpressurization, resource enrichment, and other geologic processes in the basin, while fluid mobility is mainly controlled by low-permeability rocks. This study evaluated the gas transport in low-permeability rocks in various confining and pore pressures conditions by experimental methods. Firstly, we measured the gas permeability (apparent permeability) of the low-permeability rocks of the Yanchang Formation in the Ordos Basin, China. Secondly, we defined the intrinsic permeability (liquid permeability) and the effective stress coefficient of the permeability of the core samples by the Klinkenberg correction method, and then we determined the stress sensitivity of porosity and permeability. Finally, we analyzed the gas transport mechanism in the low-permeability rocks. This study found that the stress sensitivity of the porosity of the studied samples showed minor variations and was mainly influenced by the clay minerals; however, the stress sensitivity of the permeability was considerably varied and was not significantly related to the clay minerals, and the effective stress coefficient for the Klinkenberg correction is not equal to 1 as conventional studies have suggested. We also found that the gas permeability is affected by the molecular slippage effect, the Klinkenberg curve conforms to a second-order slippage model (quadratic curve) rather than the classical linear model. The gas transport flow regimes are slippage flow and transition flow, and the contribution of the slippage effect to the apparent permeability is negatively correlated with the pore size, gas pressure, and roughness of the channel surface.