Methane and Carbon Dioxide in Dual-Porosity Organic Matter: Molecular Simulations of Adsorption and Diffusion

Abstract

The manuscript describes a computational study that provides molecular-level insight into shale gas adsorption and transport in shale rocks, which are composed of organic and inorganic matter. Atomistic simulations were used to generate realistic models of the organic matter structures with both micro-and mesoporosity, and correspond to mature and overmature type-II kerogens. These porous material models are unique to most other previous kerogen models since they contain other components (asphaltene/resin, hydrocarbons and carbon dioxide/water fractions) that are typically not modeled. The inclusion of these additional components significantly influences the resulting porous structure characteristics. The adsorption and diffusion behavior of methane (as a shale gas proxy) and methane/carbon dioxide mixtures were simulated in the model structures.Several key industrial-relevant findings are described in the manuscript.

Dynamics adsorption of the enhanced CH4 recovery by CO2 injection

“…On the contrary, they may be close to the supercritical point of CO 2 (pressure of 7.38 MPa, temperature of 31.26°C). Therefore, CH 4 adsorption isotherm curves show a linear increase followed by a plateau, 1,3,5,8–12 which can be described well by Langmuir equation. A typical CO 2 adsorption isotherm consists of a linear increase in gas adsorption with pressure followed by a plateau or saturation region and finally decreases in a nonlinear way on various rank coals, shales, and activated carbons 1,3,8–12 .…”

“…Therefore, CH 4 adsorption isotherm curves show a linear increase followed by a plateau, 1,3,5,8–12 which can be described well by Langmuir equation. A typical CO 2 adsorption isotherm consists of a linear increase in gas adsorption with pressure followed by a plateau or saturation region and finally decreases in a nonlinear way on various rank coals, shales, and activated carbons 1,3,8–12 . The adsorption saturation of CO 2 appears close to its supercritical point.…”

“…For example, CO 2 injection could be used as a displacement step of a pressure swing adsorption (PSA) cycle in the separation of coalbed gas extracted (CH 4 /N 2 mixture gas) using grain activated carbon (GAC) as adsorbent 1 . And CO 2 injection has the possibility to enhance shale gas recovery (CO 2 ‐ESGR) 2,3 and coalbed methane recovery(CO 2 ‐ECBM) 4 . In fact, the adsorption principle of all these CO 2 ‐enhanced gas recovery (CO 2 ‐EGR) processes is the same, that is, CO 2 can displace the adsorbed CH 4 in the adsorbent and reservoir.…”

“…There are relatively few studies on adsorption kinetics of CO 2 and CH 4 . The study of adsorption kinetics mainly focuses on the adsorption rate equation and the diffusion coefficient of CO 2 and CH 4 3–15 …”

“…CO 2 ‐EGR is a dynamic process in which CH 4 /CO 2 competitive adsorption always changes with CO 2 injection. The core flooding test (fixed‐bed experiment) is a direct experimental method to simulate the dynamic process, 1–4,16 whose results are breakthrough curves of CO 2 and CH 4 . By using numerical models composed of material balance equation, adsorption equation, and mass transfer equation to fit the breakthrough curve, the CH 4 recovery factor, the cumulative CH 4 production, and CO 2 storage capacity are obtained 1,2,16,17 .…”