Mechanisms for kerogen wettability transition from water-wet to CO2-wet: Implications for CO2 sequestration

Geological CO2 sequestration (GCS) is an essential building block of the global strategy to alleviate greenhouse gas emissions and mitigate the climate change. Injecting CO2 into the shale formations can not only reduce carbon emissions but also enhance oil recovery (EOR). Rock wettability is of great importance to CO2 storage as it determines the efficiency of structural and residual trapping of CO2 and plays a crucial role in CO2-EOR. In this work, molecular dynamics (MD) simulations are adopted to investigate the CO2-H2O-kerogen systems under various CO2 pressures. In a vacuum or under low CO2 pressures, kerogen surface is weakly water-wet thanks to the hydrogen bonding between H2O and kerogen. As CO2 pressure increases, kerogen wettability shifts from water-wet to CO2-wet, because more CO2 molecules accumulate at the H2O-kerogen interface and a distinct CO2 thin film emerges. Density functional theory (DFT) calculations reveal that the O-containing functional groups preferably adsorb H2O molecules over CO2 through hydrogen bonding, which is responsible for the weakly water-wet tendency at low CO2 pressures. In contrast, the carbon skeleton of kerogen exhibits a stronger affinity to CO2, leading to the formation of CO2 thin film on the kerogen surface. The CO2 crowding close to the kerogen surface at high CO2 pressures gives rise to the CO2-wet state. This study provides, for the first time, the fundamental mechanism for the kerogen wettability transition from water-wet to CO2-wet. The work also indicates that wettability of the mature kerogen is more likely to be CO2-wet during GCS, which is unfavorable for capillary trapping of CO2, but is favorable for CO2-EOR.

Automated summary

Geological CO2 sequestration is crucial for reducing greenhouse gas emissions and combating climate change. This study explores the wettability transition of kerogen from water-wet to CO2-wet under different CO2 pressures, finding that at high pressures, kerogen tends to be more CO2-wet, which is unfavorable for capillary trapping of CO2 but beneficial for CO2-EOR.