Molecular mechanisms of shale wettability alteration by multicomponent gases in carbon capture, utilization, and storage

The wetting behavior of multiphase fluids in reservoir rocks significantly affects the recovery efficiency of shale reservoirs. This study systematically investigated the mechanisms by which multiple gases influence shale wettability. The results showed that HF and HCl dissolve in water to form an acidic environment, which erodes the rock surface and exposes polar hydrophilic sites, thereby significantly enhancing hydrophilicity. CH4 and H2S reduce interfacial tension through adsorption onto the rock surface, thereby improving shale hydrophilicity. However, H2S also reacts with iron-bearing minerals in the rock to form hydrophobic FeS, which weakens hydrophilicity. CO2 generates a hydrophilic surface via hydrolysis; whereas, in the oil–water coexisting systems, its miscibility increases hydrocarbon activity, raises the water–phase contact angle, and consequently reduces hydrophilicity. All five gases increase the wetting angle of the oil phase significantly. In a single oil–phase system, the degree of influence from largest to smallest is HF, HCl, CH4, CO2, and H2S. In the oil–water coexisting system, due to the difference in water solubility, the degree of influence decreases from largest to smallest, which is HF, H2S, HCl, CH4, and CO2. Elevated temperatures generally increase the oil–phase contact angle. Increased pressures strengthen the gas–phase impact on contact angles primarily by enhancing gas molecule density and promoting dissolution and adsorption processes.