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

Abstract

Shale gas, which predominantly consists of methane, is an important unconventional energy resource that has had a potential game-changing effect on natural gas supplies worldwide in recent years. Shale is comprised of two distinct components: organic material and clay minerals, the former providing storage for hydrocarbons and the latter minimizing hydrocarbon transport. The injection of carbon dioxide in the exchange of methane within shale formations improves the shale gas recovery, and simultaneously sequesters carbon dioxide to reduce greenhouse gas emissions. Understanding the properties of fluids such as methane and methane/carbon dioxide mixtures in narrow pores found within shale formations is critical for identifying ways to deploy shale gas technology with reduced environmental impact. In this work, we apply molecularlevel simulations to explore adsorption and diffusion behavior of methane, as a proxy of shale gas, and methane/carbon dioxide mixtures in realistic models of organic materials. We first use molecular dynamics simulations to generate the porous structures of mature and overmature type-II organic matter with both micro-and mesoporosity, and systematically characterize the resulting dual-porosity organic-matter structures. We then employ the grand canonical Monte Carlo technique to study the adsorption of methane and the competing adsorption of methane/carbon dioxide mixtures in the organic-matter porous structures. We complement the adsorption studies by simulating the diffusion of adsorbed methane, and adsorbed methane/carbon dioxide mixtures in the organic-matter structures using molecular dynamics.

Investigation into adsorption equilibrium and thermodynamics for water vapor on montmorillonite clay

“…With the growing demand for natural gas resources, shale gas has been identified as a promising and significant alternative for conventional natural gas. [1][2][3][4] Given the inherent nature of shale with extremely low porosity and permeability, the economic exploitation of shale gas depends on water-based hydraulic fracturing approach. After the fracturing process, the majority of injected water is trapped in the shale formation.…”

A new approach for modeling adsorption kinetics and transport of methane and carbon dioxide in shale

“…The behavior of adsorption 2–6 and transport 16–19 of a gas in pores depend on the pore size. For a media with wide PSD, the adsorption and transport behavior of a gas in small pores differ significantly from that in large pores.…”

“…CO 2 is vertically adsorbed on the surface of small pores 22 . CO 2 adsorption decreases faster than the CH 4 adsorption upon increasing temperature, 5,6 because the adsorption heat of CO 2 is larger than CH 4. 7 At pore sizes below ~50 nm, CH 4 adsorption on pore walls becomes significant 3 .…”

“…This possibility of the enhanced CH 4 recovery by CO 2 injection has been understood by extensive studies of the adsorption thermodynamics of CO 2 and CH 4. 4–10 The desorbed CH 4 from pores needs to be transported out of the shale, which decides the shale gas production rate. However, the studies of the adsorption kinetics and transport properties of CO 2 and CH 4 , which related to shale production rate, are still very lack 9–18 …”

“…Shale usually has a wide range of pore size distribution (PSD) from nanometers to micrometers, 3–11 which determines that the adsorption kinetics and transport mechanism of gas flow in the shale are very complicated. Most of the adsorbed gas is stored in the nanopores of shale due to their large specific surface areas and strong adsorption potential 2–9 . The adsorption rate in small pores is different from that in large pores.…”