Two-phase flow characteristics of high-temperature CO2 and water and dynamic wettability variations in nano-scale coal pores with different sizes and shapes

Abstract

Coal, a typical porous medium, contains numerous nano-scale pores with varying sizes and shapes within its matrix. Since wettability variations in these pores are crucial for gas–liquid flow, they considerably affect the efficiency of coalbed methane (CBM) extraction. In this study, the morphologies of coal pores subjected to high-temperature and high-pressure CO₂ treatment were observed by means of scanning electron microscopy (SEM) and atomic force microscopy (AFM). Based on these observations, molecular dynamics simulations were conducted to construct several typical water-bearing pore models. With these models, the characteristics of gas–liquid flow and wettability variations during CO₂ injection were explored. The results reveal that the wetting mode of droplets is closely related to the pore size, with droplets that exist in a Type I wetting mode boasting better wettability than those that exist in a Type II wetting mode. During the transition from water-wet to gas-wet states, maintaining the continuous movement of droplets proves to be more challenging than detaching them from their initial adsorbed configurations. CO₂ displaces water molecules in pores through two distinct mechanisms (competitive wetting and high-speed impact). In pores with weak water-wet properties, CO₂ advances in a crescent shape; otherwise, it progresses in a planar shape. Due to their different patterns of wettability variation, the displacement rates of water molecules in connected pores are remarkably higher than those in semi-closed pores. These findings, which elucidate the characteristics of gas–water two-phase flow in nano-scale pores and the transition from water-wet to gas-wet states, are beneficial for grasping the reasons for CBM production enhancement after CO₂ injection at the nanoscale.