Alterations in pore and fracture structure of coal matrix and its surrounding rocks due to long-term CO2-H2O-rock interaction: Implications for CO2-ECBM

To comprehend the impacts of coal reservoir physical property change because of exposure to acidic environ-ment formed by CO2 on validity, safety, and stability of CO2- ECBM, the CO2-H2O-rock interaction was con-ducted on a high-rank coal and surrounding rocks, i.e., sandstone roof and mudstone floor, at 318.15 K and 12 MPa for a period of 240 days. Multiple characterizations were also conducted to reveal the alterations in inor-ganic minerals as well as pores and fractures of coal matrix and surrounding rocks. Results indicate that the long-term CO2-H2O-rock interaction leads to both dissolution and precipitation of inorganic minerals of coal matrix, sandstone roof, and mudstone floor. Despite coal matrix swelling found during the interaction, mineral disso-lution acts dominant role toward rising full-scale pores and fractures of coal matrix, thereby remarkably increasing its porosity and absolute permeability. Such changing trends are also noticed in sandstone roof. As for mudstone floor, mineral dissolution effect also upgrades its micro-and mesopores, but mineral precipitation dramatically reduces its macropores. Additionally, the interaction increases absolute permeability of mudstone floor by three orders of magnitude because of newly-formed fractures. The aforementioned alterations in pores and fractures could further strengthen storage potential of coal seams but weaken sealing performance of sur-rounding rocks to injected CO2. Overall, metamorphic degree of coal matrix and inorganic mineral compositions of surrounding rocks are crucial for choosing candidate coal reservoirs for implementing CO2-ECBM.