Dynamic fluid interactions during CO₂-ECBM and CO₂ sequestration in coal seams. Part 2: CO₂-H₂O wettability

In addition to CO₂-CH₄ interactions (Part 1), the success of CO₂ enhanced coalbed methane (CO₂-ECBM) and geological sequestration are significantly affected by the CO₂-H₂O wettability. Wettability controls both gas desorption and transport and is influenced by injection pressure, reservoir temperature and the state of water that is present – as either adsorbed- or free-water. Dynamic changes in wettability remains poorly constrained – due to the innate difficulty and invasive nature of conventional measurements (e.g., captive gas bubble and pendent drop tilted plate methods). In part 2, we use nuclear magnetic resonance (NMR) as a non-invasive method to explore the mechanisms of these factors (pressure, temperature, water-state) on CO₂-H₂O wettability during CO₂-ECBM. Results for contrasting subbituminous coal and anthracite show that the CO₂ wettability of coals significantly increases with increasing CO₂ injection pressure up to 5 MPa before stabilizing to a limiting value. This suggests that the most economically-suitable injection pressure is ~5 MPa. CO₂ wettability also increases with a decrease in temperature suggesting that shallower reservoirs may be marginally improved in this trend. Additionally, the presence of non-adsorbed water in coals significantly reduces both the sensitivity of CO₂ wettability to pressure and the absolute magnitude of wettability relative to the case where free-water is absent. Thus, draining free-water from the reservoir will serve the dual purposes of both increasing gas transport and the potential for desorption from the perspective of CO₂-H₂O wettability. The far-reaching results in this study, together with the companion paper (Part 1) are significant for evaluating CO₂-ECBM improvement both in enhancing methane recovery and CO₂ utilization in coals.