Wetting Behavior of CO2–H2O–coal with Different Metamorphic Degrees under In Situ Thermophysical Conditions

Wettability in CO2–H2O–coal directly determines pore-scale configuration and further affects practice projects on the hectometer scale; however, coal rank and thermophysical condition dependences of wettability and their underlying mechanisms are inadequate. Potential candidate coal beds (hvBb, hvAb, mvb, sa) were considered, and wettability reflected by static/dynamic contact angles was evaluated under in situ reservoir conditions (temperature and pressure) by the captive bubble method. Physicochemical properties with potential effects on wetting behavior were characterized via SEM-EDS, FTIR, and XRD. Static/dynamic contact angles increased with pressure and coal rank, while they decreased with temperature, typically showing intermediate- or CO2-wetting in deep reservoirs. Advancing contact angles depended less on temperature and coal rank than static/receding contact angles. At a low temperature (T = 25 °C), CO2-wetting was stronger and was more significantly affected by coal rank and pressure. The curves of contact angles and CO2 density as a function of pressure almost coincided and increased steeply at pressures between 60 and 90 bar (∼64.3 bar). With increasing metamorphic degrees, defects, oxygen content, and polar oxygen-containing functional groups (hydroxyl, etc.) on the surface decreased, while C/O (the ratio of carbon to oxygen), C, and aromatic hydrocarbons increased. The results of the gray relational model indicate that C/O and hydroxyl are the primary factors affecting wettability in chemical properties. With coal rank, the degree of aromatic ring condensation increased and inorganic minerals (e.g., clay minerals and calcite) reduced; the crystallite structure tended to grow and become graphite-like. Measurement data and an understanding of wetting behavior have implications for project site selection and provide input parameters for field-scale reservoir modeling.