29 / 2023-05-14 23:24:26

Numerical Simulation Study on Gas Displacement by Injecting Liquid CO2 Based on CT Image Technology

Coal seam gas; Meso scale fissures; Displacement by injecting liquid carbon dioxide; CT image; Two-phase flow

Current research shows that liquid CO₂ injection technology not only helps to improve gas extraction efficiency, but also can store greenhouse gases such as CO₂. In response to the current unclear understanding of the meso scale water-gas two-phase flow mode, this paper proposes a research method based on CT image processing to numerically simulate the liquid CO₂-gas displacement mechanism in meso scale fissures of coal and rock. This article used CT scanning technology to reconstruct the geometric digital model of meso scale fissures in coal and rock, and combines the level set theory to construct a displacement model in a two-phase fluid environment. Explored the distribution characteristics of meso scale fissures in coal and rock with different structural characteristics and injection pressures on gas-liquid two-phase flow, thereby revealed the gas-liquid two-phase displacement mechanism within meso scale fissures in coal and rock. The research results indicated that the process of liquid CO₂ displacing gas in meso scale fissures of coal and rock is as follows: liquid CO₂ migrates from the entrance to the export, and its area continuously increases. Affected by displacement effect, gas migrates towards the export, and its area continuously decreases. The two-phase displacement process in meso scale fissures can be mainly divided into three phases: rapid fluid change phase, slow fluid change phase, and almost constant fluid phase. Liquid CO₂-gas displacement first occurs in the main fissures section, and this process takes a relatively short time; Later, it occurs in the branching fissures and end fissures, and this process takes a long time. The two-phase displacement efficiency in the direction of liquid CO₂ migration is higher, while the two-phase displacement efficiency perpendicular to the migration direction is lower. The different structural characteristics of meso scale fissures in coal and rock have a significant impact on the two-phase displacement effect. The displacement efficiency of meso scale fissures structures with larger widths is higher, while pore throat structures with smaller widths will reduce the displacement efficiency. The tortuosity of fissures is the influencing factor of displacement effect. The two-phase displacement effect is better in the fissures structure with smaller tortuosity, and worse in the fissures structure with larger tortuosity. The connectivity of fissures has a decisive impact on displacement efficiency. The higher the connectivity between fissures, the weaker the water-lock effect, and the higher the two-phase displacement efficiency. The lower the connectivity between fissures, the higher the water-lock effect at the end of the fissures, and the lower the two-phase displacement efficiency. There is a linear relationship between injection pressure and displacement efficiency. As the injection pressure increases, the curvature of the interface between liquid CO₂ and gas increases, and the displacement rate and efficiency within the fissures increase. The injection pressure decreases, the curvature of the interface between liquid CO₂ and gas decreases, and the displacement rate and efficiency within the fissures decrease. The research results of this article will provide a theoretical basis for the study of two-phase displacement and have significant implications for improving coalbed methane production.