Insights into CH4/CO2 Adsorption and Mechanisms Enhanced CH4 Recovery in Kerogen Slit Nanopores by Innovative Molecular Dynamics

Abstract

Abstract CO2 injection into shale reservoirs to enhance methane (CH4) production is treated as a better way to promote gas recovery efficiency as well as easing carbon emission by CO2 sequestration and storage. Most CH4 is adsorbed inside the organic micropores and mesopores (≤ 50 nm), which hold large surface areas and enormous adsorption sites. And another key structure characteristic in shale reservoir is the slit, which is usually treated as a significant sequestration medium. To better understand the microscopic mechanism of enhanced CH4 recovery by CO2 in nanopores and slit becomes necessary in shale reservoirs owing to the geological and physical properties. Laboratory experiments show challenges in observation of molecular adsorption and displacement processes inside the nanoscale pores, and also present restrains in achieving high temperature and high pressure simultaneously. Thereby, Molecular Dynamics simulation (MD) method supports a solid foundation for constructing the nanosized kerogen frameworks to investigate the gas adsorption behavior on the kerogen-accessible surface. This study innovatively introduced a new method of constructing kerogen slit nanopores, making the model more practical and approaching real underground environments. The grand canonical Monte Carlo (GCMC) method is employed to reveal the gas adsorption and sequestration in the kerogen nanopores and slit at various subsurface conditions. According to our results, the slit overlooked by previous studies particularly impacts gas adsorption process and CH4 recovery efficiency, owing to the space facilities the free gas resides. This study also examines the widespread water encroachment phenomenon, which includes various water contents and saline concentrations to mimic the real subsurface environment. Pure water has an overall negative influence on gas adsorption and sequestration, whereas, promoting the recovery efficiency of CH4 by CO2 injection. Moreover, saline has a further enhanced negative influence on gas adsorption due to the presence of ions, whereas, encourages the displacement process by CO2 injection. Ethane (C2H6) impacts the CH4 adsorption negatively but favors the recovery process. This work shows significant importance in underlining the kerogen slit nanopores structure and develops the knowledge of the comprehensive underground conditions effects on gas adsorption and recovery mechanisms at a thorough level to enhance CH4 extraction and CO2 utilization and sequestration, and provides a solid background for gas industrial production.