Deformation of Coal Induced by Methane Adsorption at Geological Conditions

Abstract

The quenched solid density functional theory (QSDFT) is employed to study methane adsorption on coal at geological conditions. The main focus is made on coal deformation in the course of adsorption that may result in either expansion/swelling or contraction, depending upon the pressure, temperature, and pore size. Two qualitatively different types of deformation behavior were found depending upon the pore width. Type I shows a monotonic expansion in the whole pressure range. This behavior is characteristic for the smallest pores <1.3σff (0.5 nm) that cannot accommodate more than one layer of methane. Type II displays contraction at low pressures followed by expansion. Type II behavior was found for several groups of pores, which can accommodate dense packing with an integer number (from 2 to 6) of adsorbed layers. The results of the QSDFT model are compared to literature experimental data, and the model is employed to study the adsorption behavior of model coals at elevated pressures and temperatures. We established the relationships between the methane capacity and the solvation pressure that it exerts on the coal matrix and the depth of coal bed for pores of different sizes. We found that the coal deformation depends upon the bed depth, and at different depths, it either swells or contracts depending upon the pore size distribution. The implications of these findings for evaluating coal gas resources and coal mine recovery are discussed.

Adsorption-induced deformation of nanoporous materials—A review

“…A number of papers employed molecular modeling techniques for predictions of adsorption-induced strains in various nanoporous materials: Monte Carlo simulations 40,[81][82][83][84][85][86][87][88][89][90][91][92] or classical density functional theory (DFT). [93][94][95][96] At the same time, several purely analytical theories were also proposed. 80,[97][98][99][100][101] Most of these papers, however, were still limited to qualitative comparisons with experimental data.…”

“…Despite the number of strong assumptions, this "thermodynamic approach" made it possible to calculate strain isotherms for various other microporous systems, e.g., activated carbons, 108 synthetic carbon monoliths, 50 coal, 95,109 and even breathing MOFs. 101 The key part is that the grand potential of the system (and therefore its derivative) is calculated based on a theory of adsorption in a rigid pore.…”

“…23. Adsorption-induced deformation of coal measured experimentally 157 and predicted by three different models: poromechanical models by Vandamme et al 156 and by Pijaudier-Cabot et al, 160 and thermodynamic model by Yang et al 95 very promising sensing mechanism. More recently, the same team extended the application of these types of actuators to the liquid phase.…”

Supercritical Fluids in Confined Geometries

“…Understanding the adsorption-induced . The adsorption of fluid molecules may lead to development of stress (also known as solvation pressure [88]), which is responsible for modification of the nano-mechanical properties and shape of micropores. The origin of the stress is related to the tight packing of the fluid molecules in the micropores, and its magnitude may depend on the pressure, fluid-wall interactions and pore size distribution among other parameters.…”

Adsorption-induced permeability change of porous material: a micromechanical model and its applications

“…However, the pore pressure in the mesopores and micropores is much different from the fluid bulk pressure. For example, the Monte Carlo simulation indicated that the pore pressure in the micropores of coal is much larger than the bulk [44]. One possible reason for this is the presence of the so-called disjoining pressure in slit-like micropores, but to directly measure it by experiment is almost impossible.…”