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Coalbeds are subject to diverse load conditions stemming from hydraulic fracturing, mining activities, and geological tectonic forces. Understanding how coalbed permeability evolves under various stress conditions—such as effective stress, peak stress, axial deviatoric stress, and stress cycling—is crucial for optimizing coalbed methane flow dynamics. In this study, coal sample permeability evolution was assessed using the steady-state method under various loading paths. The study revealed insights into the impact of irreversible deformation induced by different axial deviatoric stresses on coal permeability. Our results indicate that confining pressure has a greater impact on axial permeability than axial stress does. Initial stress cycles involving confining pressure notably reduce coal permeability, an effect that is less pronounced in subsequent cycles. Different levels of axial deviatoric stress have varied consequences for coal fractures. Specifically, high axial deviatoric stress conditions promote fracture propagation, thereby enhancing coal seam permeability. Conversely, under low axial deviatoric stress, the cyclical application of axial and confining pressures results in coal compaction and fracture closure, leading to a decrease in permeability after unloading. To visualize microcrack development and propagation in coal under differing axial deviatoric stress conditions, we integrated the discrete element method with the Mohr–Coulomb model in a particle flow program. The findings from our triaxial seepage experiments corroborate well with this computational model, providing a robust validation and deeper insight into the observed permeability changes. [ABSTRACT FROM AUTHOR]

Cleat and permeability altered by CO2 injection are key issues for enhanced coalbed methane production and CO2 geo-sequestration into unminable coal seams. As the main channel for CO2 flowing through, pre-existing macro- and micro-cleats in coal directly determine permeability. Both permeability and cleats are pressure-dependent. However, few researches have been made to observe directly changes in micro-cleats as a function of injection pressure during CO2 flooding. In this study, using a novel in situ micro-CT core flooding apparatus, we observed the gradual closure of micro-cleats in small anthracite coal at CO2 injection pressures from 1 to 4 MPa. Quantitative micro- aperture size variations were analyzed further. An increasing-then-decreasing trend of the mean aperture size of the micro-cleat with increasing injection pressures was obtained, and the rebound in mean micro-cleat aperture size measured parallel to cleat directions was higher than that perpendicular to bedding plane direction. The permeability decreased drastically first and then slowly with increase in saturation time. Relatively weak coal permeability anisotropy (< 50%) was observed in this test and the permeability measured in the bedding plane direction was not necessarily smaller than that in the cleat directions for a small coal body. [ABSTRACT FROM AUTHOR]

Characterization of coal micro-structure and the associated rock mechanical properties are of key importance for coal seam exploration, coal bed methane development, enhanced coal bed methane production and CO2 storage in deep coal seams. Considerable knowledge exists about coal chemical properties, but less is known about the nanoscale to the micro-scale structure of coals and how they change with coal strength across coal ranks. Thus, in this study, 3D X-ray micro-computed tomography (with a voxel size of 3.43 µm) and nano-indentation tests were conducted on coal samples of different ranks from peat to anthracite. The micro-structure of peats showed a well-developed pore system with meso- and micro-pores. The meso-pores essentially disappear with increasing rank, whereas the micro-pores persist and then increase past the bituminous rank. The micro-fracture system develops past the peat stage and by sub-bituminous ranks and changes into larger and mature fracture systems at higher ranks. The nano-indentation modulus showed the increasing trend from low- to high-rank coal with a perfect linear relationship with vitrinite reflectance and is highly correlated with carbon content as expected. [ABSTRACT FROM AUTHOR]

Abstract Carbon dioxide geosequestration into deep unmineable coal seams is a technique which can mitigate anthropogenic greenhouse gas emissions. However, coal composition is always complex, and some minerals such as calcite chemically react when exposed to the acidic environment (which is created by scCO 2 mixing with formation water). These reactive transport processes are still poorly understood. We thus imaged a water-bearing heterogeneous coal (calcite rich) core before and after scCO 2 injection in-situ at high resolutions (3.43 µm) in 3D via X-ray in-situ microCT flooding system. Indeed, the calcite-coal mixed layer was partially dissolved, and absolute porosity and connectivity significantly increased. We thus suggested that such process could be used as an acidizing method in CO 2 ECBM. However, such dissolved damage also can significantly affect the rock mechanical properties and potentially induce geohazards. [ABSTRACT FROM AUTHOR]

Rock mechanical properties are of key importance in coal mining exploration, coal bed methane production and CO 2 storage in deep unmineable coal seams; accurate data is required so that geohazards (e.g. layer collapse or methane/CO 2 leakage) can be avoided. In this context it is well established that coal matrix swelling due to water adsorption significantly changes the coal microstructure. However, how water adsorption and the associated with microstructural changes affect the mechanical properties is only poorly understood, despite the fact that micro-scale mechanical properties determine the overall geo-mechanical response as failure initiates at the weakest point. Thus, we measured nanoscale rock mechanical properties via nanoindentation tests and compared the results with traditional acoustic methods on heterogeneous medium rank coal samples in both dry and brine saturated conditions. The microscale heterogeneity of the rock mechanical properties was mapped and compared with the morphology of the sample (measured by SEM and microCT). While the nanoindentation tests measured decreasing indentation moduli after water adsorption (−60% to −66%), the traditional acoustic tests measured an increase (+17%). We concluded that acoustic tests failed to capture the accurate rock mechanical properties changes for the heterogeneous coal during water adsorption. It is thus necessary to measure the coal rock mechanical properties at the microscale to obtain more accurate data and reduce the risk of geohazards. [ABSTRACT FROM AUTHOR]

Gas permeability (k) and porosity (φ) are the most important parameters in CBM/ECBM and CCS in deep unmineable coal seams. k and φ depend on the coal microstructure, and k and φ significantly change with varying effective stress. However, how the coal microstructure is related to such permeability and porosity changes is only poorly understood. We thus imaged sub-bituminous coal samples at two resolutions (medium - 33.7 μm and high - 3.43 μm voxel size) in 3D with an x-ray micro-computed tomography as a function of applied effective stress; and investigated how cleat morphology, k and φ are influenced by the changes in effective stress and how these parameters are interrelated. In the images, three phases were identified: microcleats (void), a mineral phase (carbonate) and the coal matrix. When effective stress increased, the cleats became narrow and closed or disconnected. This resulted in a dramatic permeability drop with increasing effective stress, while porosity decreased only linearly. [ABSTRACT FROM AUTHOR]

The gas adsorption/desorption-induced coal deformation effect is a significant factor governing the evolution of coalbed permeability. Current theoretical investigations typically coal bulk and fracture deformation induced by gas are equivalent, neglecting the matrix-fracture interactions. Based on internal adsorption stress, this paper proposes Internal Differential Strain Coefficient (IDSC) to quantitatively characterize the relationship between coal bulk and fracture strain under equilibrium conditions. Coupling this coefficient constructs a binary gas permeability evolution model considering matrix-fracture interactions. Through numerical simulations of CO 2 -ECBM processes under various internal differential strain circumstances using this model, dynamic evolution patterns of diverse parameters are obtained. The research findings indicate that along the direction of CO 2 injection, matrix-fracture interactions exhibit a complex trend of initially increasing, then decreasing and then increasing, and the increase in internal differential strain levels results in a downward trend in permeability peak. Additionally, the evolutionary characteristics of CH 4 recovery and cumulative CO 2 storage rising with increasing internal differential strain levels were obtained on time scales using a fixed-point monitoring methodology. Inspired by the aforementioned laws, this paper discusses the macroscopic influence of burial depth on the effects of internal differential strain, providing new theoretical support for CO 2 sequestration injection methods in deep coal seams. • A coefficient capable of quantitatively analyzing the internal differential strain in the CO 2 -ECBM process is proposed. • Based on IDSC, a model describing the evolution of coal seam permeability in CO 2 -ECBM process is established. • Analyzed the influence of internal differential strain on the permeability of coal beds in the CO 2 -ECBM process. • Analyzed the influence of internal differential strain characteristics on CO 2 injection capacity. [ABSTRACT FROM AUTHOR]

CO 2 geo-sequestration is a practical approach to achieve net-zero carbon target. However, one of the main challenges for successful CO 2 geo-sequestration is the reduced coal permeability and injectivity that are caused by coal swelling. Coal has complex and heterogenous internal pore and fracture structure. The processes of gases adsorbing, desorbing, and transporting within multiscale heterogeneous coal structures are more complicated compared with conventional rocks. This paper aims to gain insights about the gas transport behaviours in coal by developing a coupled model to simulate gas flow multiphysics as well as dynamic coal deformation. This work develops an image-based 3D fracture network model, called Fracture Box Model (FBNM). In this model, each fracture is described by arrays of box elements such that the regional change of fracture opening widths can be preserved. Compared with other fracture models (e.g. discrete fracture network), FBNM can simulate complicated multiphysical gas transport more efficiently, but also be able to simulate corresponding coal matrix deformation. By comparing permeability results between direct simulation method with FBNM, it is found that FBNM can effectively estimate the permeability of original fracture networks, but requiring significantly less computational cost. To study the implications of gas types, effective stress, gas adsorption, and thermal expansion on coal permeability, gas injection pressures, gas types, coal seam temperatures are varied and investigated in the simulations. In addition to the advantage of computational efficiency, FBNM is more preferable for complicated flow transport simulations where direct simulation methods are still challenging. This work provides a promising framework which could be further developed for multiphase and multicomponent flow simulations for CO 2 geo-sequestration projects. • A novel fracture network model is directly extracted from micro-CT images of a real coal sample. • The developed fracture network has preserved both fracture connectivity as well as surface heterogenicities. • This model has coupled multiphysics gas transport with sorption-induced deformation of coal. • This work provides a promising framework for the application in CO 2 geo-sequestration modelling. [ABSTRACT FROM AUTHOR]