Molecular simulation of kerogen-water interaction: Theoretical insights into maturity

Nanoscale Insights into CO2 Enhanced Shale Gas Recovery in Gas-Water Coexisting Kerogen Nanopores

The presence of water clusters in kerogen nanopores reduces the occurrence and migration of methane (CH4) and thus affects shale gas extraction. CO2 injection, as an effective approach to enhance shale gas recovery, still presents challenges in its ability to mitigate the impact of immobile water clusters within the kerogen. In this work, molecular dynamics simulations were employed to investigate the microscopic transport process of water clusters and CH4 following CO2 injection in the gas-water coexisting kerogen nanopores. The results demonstrate that CO2 can desorb irreducible water clusters to dredge the pores while extracting CH4, enhancing gas-water mobility, and shale gas recovery by transitioning the wettability of the kerogen nanopore surface from weakly water-wet to CO2-wet. The impact of CO2 on the wettability of kerogen surfaces is primarily manifested in two aspects: CO2 can intrude the interface between water clusters and kerogen to reduce the number of hydrogen bonds between them, resulting in the detachment of water clusters; and the surface of kerogen nanopores can form a layer of CO2 gas film, which prevents desorbed water clusters and CH4 from readsorbing onto the wall surface. This study provides important insights in enhancing the understanding of the microscopic mechanisms in nanoscale flow, as well as for the development of an unconventional gas reservoir.

Molecular Origin of Wettability Alteration of Subsurface Porous Media upon Gas Pressure Variations

Upon extraction/injection of a large quantity of gas from/into a subsurface system in shale gas production or carbon sequestration, the gas pressure varies remarkably, which may significantly change the wettability of porous media involved. Mechanistic understanding of such changes is critical for designing and optimizing a related subsurface engineering process. Using molecular dynamics simulations, we have calculated the contact angle of a water droplet on various solid surfaces (kerogen, pyrophyllite, calcite, gibbsite, and montmorillonite) as a function of CO₂ or CH₄ gas pressure up to 200 atm at a temperature of 300 K. The calculation reveals a complex behavior of surface wettability alteration by gas pressure variation depending on surface chemistry and structure, and molecular interactions of fluid molecules with surfaces. As the CO₂ gas pressure increases, a partially hydrophilic kerogen surface becomes highly hydrophobic, while a calcite surface becomes more hydrophilic. Considering kerogen and calcite being the major components of a shale formation, we postulate that the wettability alteration of a solid surface induced by a gas pressure change may play an important role in fluid flows in shale gas production and geological carbon sequestration.