Mechanistic and Kinetic Study of Gas Storage and Transport in Unconventional Reservoirs Using Molecular Simulation

In the past decades, exploration and production activities gradually shift to unconventional resources such as tight gas reservoirs. However, the techniques for reservoir simulation and production forecast fail in shale reservoirs due to the distinct behavior of oil and gas in the micropores of the shale formations. Molecular simulation is a strong tool in simulating fluid flow and gas storage in the micropores and can provide us with valuable information for better understanding rock/fluid interactions in those unconventional resources and improve hydrocarbon recovery. Our research mainly focuses on two aspects: first, we study the microscopic mechanisms and kinetics of fluid flow in oil and gas shales and the process of CH4-CO2 exchange method in natural gas hydrates recovery. The fluid transport in shale nanopores shows distinct behavior from that in conventional reservoirs as adsorption plays an important role in nano-scale pore systems. The simulation in natural gas hydrates validates the great potential of using CO2 to replace the CH4 trapped in hydrate cages and temperature can significantly impact the exchange rate. Second, to improve shale hydrocarbon recovery as well as to sequester CO2 for carbon neutrality, we propose to use microporous material as a multi-functional proppant to carry adsorbed CO2 into shale formation and replace the sorbed CH4, such that CH4 recovery can be enhanced and large amount of CO2 can be geologically sequestered. To prove the thermodynamic feasibility of this process, we investigate gas adsorption capacity and the CO2/CH4 selectivity in silicalite and kerogen organic matter. It is shown that both materials have a preference to adsorb CO2 over CH4 and kerogen has a stronger CO2/CH4 selectivity with or without water presence. These findings validate the thermodynamic feasibility of this exchange process. Simulation at microscopic level is very critical in unconventional reservoirs as the transport and storage properties of reservoir fluid in nanopores deviates from those at macroscopic scale. Not only can molecular simulations reveal fundamental information about the behavior of oil and gas molecules in confined spaces, but also can they help study the potential approaches for enhanced hydrocarbon recovery from the unconventional resources and lay the foundation for numerical simulation and prediction at larger scales.