Methane (CH₄) Wettability of Clay-Coated Quartz at Reservoir Conditions

Methane (CH₄) wettability of shale is a key parameter which determines pore and reservoir-scale fluid distributions, CH₄ reserves estimation, and ultimate recovery efficiency from shale gas reservoirs. Clay minerals usually fill the pore spaces or are adsorbed on the surface of shale rock, thus influencing CH₄ wettability. However, a systematic investigation of the influence of clay on CH₄ shale wettability is lacking. Herein, we investigated the role of clay, pressure, temperature, and salinity on CH₄ wettability of clay-coated quartz (i.e., a well-defined model system for shales). Results indicated that the advancing and receding water contact angles for clean, kaolinite-coated, and montmorillonite-coated quartz increased with pressure. However, the effect of temperature on wettability is complex, and thus the advancing water contact angle for clean quartz increased with temperature while an opposite trend was found for clay-coated quartz. At low temperature (i.e., 300 K), clay coating dewetted the quartz surface, while at elevated temperature (i.e., 323 K), clay coating increased the hydrophilicity of the quartz surface. Furthermore, kaolinite clay particles demonstrated a stronger influence on quartz wettability than montmorillonite particles, both at high and low temperatures. In addition, higher NaCl salinity led to higher advancing water contact angles for the aforementioned three solid surfaces. The effect of salinity on CH₄ wettability is thus intensified in the presence of clays. These insights will thus improve the accuracy of CH₄ reserve estimates and aid methane recovery schemes.