Experimental and theoretical characterization of the natural gas migration and accumulation mechanism in low-permeability (tight) sandstone cores

Abstract

The natural gas migration and accumulation properties of low-permeability (tight) sandstone are among the most important issues in the study of natural gas resource evaluation and exploitation. Understanding the internal mechanisms of these phenomena is critical to guiding the exploitation of natural gas resources and enhancing gas recovery. In this work, physical simulation experiments on gas migration and accumulation in natural tight sandstone cores are designed and implemented. The results show that the flow curve of steady gas migration is non-linear at a low pressure gradient and then becomes linear with the increase of the squared pressure gradient. The apparent permeability can be used to judge the ease of gas migration in tight sandstone, and the growth of apparent permeability can be divided into three stages (fast growth stage, slow growth stage, and stable stage) according to the level of the pressure gradient. The minimum migration pressure gradient (MMPG) is also determined from the gas displacement experiments. The terminal gas saturation of tight cores is distributed across a wide range of 18.73%–90.03%, and it correlates well with the permeability and MMPG: terminal gas saturation increases as the permeability increases or the MMPG decreases. Gas saturation also increases as the permeability and pressure gradient increase, but the rate of increase varies.