Confinement Effects and CO₂/CH₄ Competitive Adsorption in Realistic Shale Kerogen Nanopores

Abstract

The knowledge of adsorption behaviors and mechanism of CO2/CH4 in organic matter is of great importance for CO2 geological sequestration with enhanced gas recovery in shale reservoirs. In this study, the adsorption behaviors and confinement effects of CO2/CH4 in realistic kerogen nanopores have been investigated by using the grand canonical Monte Carlo method. To represent realistic nanopores in the kerogen matrix, the inkbottle-shaped and slit-like nanopores were developed. The effects of temperature, pressure, and pore size on competitive adsorption behaviors and adsorption mechanism of CO2/CH4 were explored. Simulation results indicate that the adsorption capacity of CH4 is lower than that of CO2 in the kerogen matrix with/without kerogen nanopores. A higher pressure and lower temperature are favorable for the adsorption capacities of CO2 and CH4. The gas adsorption capacities have been enhanced in both the inkbottle-shaped and slit-like nanopores. Meanwhile, the existence of inkbottle-shaped micropores is favorable for improving the selectivity of CO2/CH4 in shale organic matter. A higher CO2 injection pressure could improve its adsorption capacity but lower the adsorption selectivity of CO2 over CH4. Furthermore, confinement effects were observed in inkbottle-shaped and slit-like kerogen micropores and small mesopores. Two major factors, including the supercritical state of gas and microscale pores, could enhance the confinement effects. In addition to monolayer adsorption, micropore filling was observed in inkbottle-shaped and slit-like kerogen nanopores because of the confinement effects. It is expected that these results could help in understanding the microscopic adsorption mechanism and provide fundamental information for shale gas exploitation and CO2 sequestration.

Transport of Shale Gas in Microporous/Nanoporous Media: Molecular to Pore-Scale Simulations

“…Fortunately, molecular dynamics (MD) simulations, originating from Newtonian mechanics to define the force and capture the corresponding motion trajectory of all the particles, make it possible to directly characterize and observe the dynamics behaviors of each atom or molecule at the nanoscale. − Typically for shale nanopores flow, the multiple phases (e.g., water and methane) and radical ambient conditions (e.g., high pressure and temperature) can be easily taken account into MD simulations system, − and, hence, a great deal of MD simulation studies have been performed to reveal the transport characteristics of shale gas within nanopores, providing important cognitions and fundamental frameworks for gas transport through shale nanopores. In this work, the authors attempt to present a comprehensive review on current advances of shale gas transport through microporous/nanoporous media from molecular perspectives, including molecular models, flow simulation strategies, and significant results.…”

Methane and carbon dioxide in dual‐porosity organic matter: Molecular simulations of adsorption and diffusion

“…In another study, Ho et al 33 showed that methane release in nanoporous kerogen matrices is characterized by a fast release of pressurized free gas (accounting for less than 50% recovery), followed by a slow release of adsorbed gas as the gas pressure decreases. There are still other related molecular‐level simulation studies on enhanced shale gas recovery by carbon dioxide and CO 2 sequestration in kerogen matrices of different maturity and porosity, see, for example 34‐40 …”

“…There are still other related molecular-level simulation studies on enhanced shale gas recovery by carbon dioxide and CO 2 sequestration in kerogen matrices of different maturity and porosity, see, for example. [34][35][36][37][38][39][40] Most molecular models treat the shale OM as a dense porous kerogen matrix. 9,10 Although kerogen is the abundant species in the OM, the OM also contains other compounds such as asphaltenes, resins, and dissolved species (typically hydrocarbons, water and carbon dioxide), where their absence may be mimicked by introducing dummy particles into the kerogen matrix.…”

Molecular Modeling of Subsurface Phenomena Related to Petroleum Engineering

“…They attributed this behavior to the fluid curvature which is higher in the cylindrical pore than in slit pores. In the same vein, 141 studied the adsorption selectivity of CH 4 and CO 2 in realistic pore structures considering the surface roughness. They found that inkbottle-shaped pores enhance the selectivity of CO 2 / CH 4 compared to slit pores.…”