Anisotropic deformation and stress-dependent permeability of fractured porous rock interacting with adsorptive fluids

Naturally fractured reservoirs like coalbeds are considered as potential storage formations for sequestering carbon dioxide (CO₂) – the major anthropogenic greenhouse gas. Coalbeds are also exploited for methane recovery. Although there has been a significant effort to store CO₂ in coal seams and enhance coalbed methane recovery, inherent challenges like loss of gas injectivity due to adsorption-induced coal swelling are still encountered, which leads to difficulties in achieving a sustainable gas injection rate. As a result, bulk of the potential coal seam remains unused, raising questions on the validity and practicability of carbon sequestration in coal. Thus, a comprehensive understanding of coal permeability and swelling behaviors is significant in addressing the gas injection and storage challenges.

Structure of coal is usually characterized by two distinctly porous systems, namely, the porous matrix and the cleats, e.g., face cleats and butt cleats. Face cleats provide more continuous flow pathways than the butt cleats; and they are mutually perpendicular and in turn perpendicular to the bedding plane of a coal seam. Permeability of coal is strongly influenced by its cleat network properties. Cleat distribution in coalbeds results in anisotropic permeability with higher permeability in the direction of the face cleats. This has been reported in previous studies and has raised questions on the notion of isotropic material behaviors often considered in numerical modeling studies.

Generally, permeability changes in coal are predominantly attributed to the net effects of the effective stress and the adsorption/desorption-induced swelling or shrinkage. Many models have been developed to study the permeability behaviors of coal. Nevertheless, most of these models neglected some critical anisotropic properties of coal. Therefore, an effective model for accurate characterization of anisotropic permeability and directional swelling of coal is crucial.

Herein, Dr Min Chen, Dr Shakil Masum, Dr. Sivachidambaram Sadasivam, and Professor Hywel Thomas from Cardiff University developed a new comprehensive model for investigating permeability behaviors and anisotropy of coal swelling. An effective stress model for saturated, adsorptive, fractured porous media is derived following the principles of thermodynamics. By considering the direction-dependent fracture compressibility factor, the anisotropy of coal permeability was represented using a newly proposed stress-dependent directional permeability model. The effect of gas adsorption on fracture compressibility was estimated by introducing a weakening coefficient. Model predicted results were compared against relevant experimental data to validate its accuracy and reliability. In all validation exercises, the model predictions agreed well with the experimental data. The work has been published in the International Journal of Rock Mechanics and Mining Sciences.

The research team demonstrated the model’s capability to capture the relationship between anisotropic swelling, anisotropic mechanical properties, and anisotropic permeability behavior of coal. The model predicted that the coal swelling in the parallel direction was lower than that of the perpendicular to the bedding plane. Fracture compressibility increased with CO₂ adsorption. While the permeability parallel to the bending plane was more sensitive to stress variation than that of the perpendicular direction, due to higher fracture compressibility, stress changes in any direction could still affect coal permeability in both directions.

Coal permeability reduced significantly under constant volume conditions without a rebound. However, it could rebound with increased pressure under plane strain and uniaxial strain conditions. Furthermore, permeability loss in the direction parallel to the bedding plane was greater than that in the perpendicular direction. The presented model is a promising tool for studying the mechanisms controlling deformation and anisotropic permeability evolutions in coal and other fractured rocks that interact with adsorptive gases.

In summary, Cardiff University researchers developed new adsorption-induced anisotropic coal permeability and swelling model. The coal permeability anisotropy was affected by both mechanical properties and anisotropic swelling of the coal. Finally, in comparison to the existing models, the proposed model requires fewer parameters avoiding the effects of strain/stress superposition that are often considered in existing models. In a joint statement to Advances in Engineering, the authors explained that their findings would contribute to developing more effective analytical tools for evaluating subsurface carbon sequestration techniques.