Molecular Dynamics Simulation Insights into CO2 Wettability of Low-Maturity Shales Organic Matters Under Various Reservoir Conditions

Abstract

Abstract Geological CO2 sequestration (GCS) stands as a principal global strategy to mitigate greenhouse gas emissions and address climate change. Injecting CO2 into shale not only reduces carbon emissions but also enhances oil recovery. Rock wettability significantly determines CO2 trapping efficiency. Shale consists both hydrophilic inorganic and organic matter like kerogen. Although traditionally viewed as hydrophobic, kerogen has been underscored in recent studies for its potential weak water-wettability. Consequently, understanding this behavior is crucial for GCS. Utilizing molecular dynamics simulations, we selected the quintessential kerogen type-Ⅰ A from low-maturity shale organic matter. Initially, we constructed a shale organic matter plate model via high-temperature annealing and subsequently amalgamated it with water and CO2, establishing a foundational model for wettability studies. Subsequently, we determined the contact angle of the kerogen plate with air. Then, based on NIST's relationships among density, temperature, and pressure from NIST handbooks, we adjusted the quantity of CO2 to mirror actual reservoir conditions. Ultimately, we assessed the contact angle between CO2 and low-maturity shale within a temperature range of 298.15k-358.15k and pressures of 5-65MPa. The study reveals that low maturity kerogen in shale organic matter is predominantly weakly water-wet. Under reservoir conditions, variations in temperature and pressure significantly influence the wettability of shale organic matter. Specifically, at a constant pressure, an increase in temperature results in a decreased contact angle, whereas at a consistent temperature, an elevated pressure leads to an increased contact angle. Consequently, the wettability of shale organic matter transitions from being weakly water-wet to predominantly CO2-wet. Enhanced accumulation of CO2 molecules at the H2O-kerogen interface and the emergence of apparent CO2 films indicate that kerogen exhibits substantial CO2 adsorption. Low maturity shale organic matter is thus conducive to the capillary trapping of CO2, facilitating its geological sequestration. In this study, a low-maturity shale organic matter model was established using molecular dynamics simulation to investigate the effect of CO2 on the wettability of shale organic matter. By examining the water contact angle in various reservoir conditions saturated with CO2, we addressed challenges in assessing shale wettability in such environments. Furthermore, we discerned that low-maturity kerogen exhibits enhanced CO2 capture capability, underscoring its significance in geological carbon sequestration.