Space–Time Evolution of Water and the Response to CO ₂ Phase with CO ₂ Injection into Coal Seams

Coal seam CO2 sequestration is a promising approach for geological carbon storage, where CO2 injection inherently displaces pore water. The migration and retention of CO2 are strongly affected by water evolution within coal, which, in turn, is governed by the CO2 behavior. However, the redistribution of water under different CO2 phases (gaseous, liquid, and supercritical) remains poorly understood. In this study, nuclear magnetic resonance (NMR) was used to monitor water saturation in adsorption and seepage pores during displacement experiments involving three CO2 phases. Results revealed a two-stage displacement process: an initial seepage-dominated stage characterized by free water removal, followed by an adsorption-driven stage involving the desorption of adsorbed water. An adsorption kinetic model was fitted to the data, confirming that the reduction in adsorbed water is primarily attributed to competitive CO2 adsorption. Among the experimental conditions, supercritical CO2 exhibited the highest displacement efficiency in adsorption pores, whereas liquid CO2 achieved stronger displacement in seepage pores. The overall displacement efficiency was governed by the interfacial tension, density difference, viscosity ratio, and related factors. Gaseous CO2, with the highest interfacial tension against water, generated the greatest capillary resistance and thus lower efficiency, while its large density difference with water further impeded displacement. By contrast, liquid CO2, characterized by a smaller viscosity ratio to water, displaced water more effectively and demonstrated higher efficiency in seepage pores. Extrapolation to the reservoir scale indicates that supercritical CO2 offers superior displacement potential for long-term injection. These findings suggest that selecting supercritical CO2 and ensuring an adequate injection duration are essential for maximizing storage efficiency in coal seams.