This study focuses on improving fundamental understanding of low, subcritical CO₂ adsorption-desorption behavior of bituminous coals with the aim to evaluate the utility of shallow-depth coal seams for safe and effective CO₂ storage. Comprehensive data and a detailed description of coal-CO₂ interactions, e.g., adsorption, desorption, and hysteresis behavior of intact bituminous coals at CO₂ pressures <0.5 MPa, are limited. Manometric sorption experiments were performed on coal cores (50 mm dia. and 30- or 60-mm length) obtained from a 30 m deep coal seam located at the Upper Silesian Basin in Poland. Experimental results revealed that the adsorption capacities were correlated to void volume and equilibrium time under low-pressure injection (0.5 MPa). The positive deviation, observed in the hysteresis of adsorption-desorption isotherm patterns, and the increased sample mass at the end of the tests suggested CO₂ pore diffusion and condensation. This behavior is vital for assessing low-pressure CO₂ injection and storage capabilities of shallow coal seams where confining pressure is much lower than that of the deeper seams. Overall, CO₂ adsorption depicts a type II adsorption isotherm and a type H3 hysteresis pattern of the IUPAC classification. Experimental results fitted better to the Brunauer-Emmett-Teller model than the Langmuir isotherm model. CO₂ adsorption behavior of intact cores was also evaluated by characteristic curves. It was found that Curve I favored physical forces, i.e., the presence of van der Waals/London dispersion forces to describe the coal-CO₂ interactions. However, analysis of Curve II indicated that the changing pressure-volume behavior of CO₂ in the adsorbed phase, under low equilibrium pressures, cannot be ignored.

Deep un-mineable coal deposits are viable reservoirs for permanent and safe storage of carbon dioxide (CO₂) due to their ability to adsorb large amounts of CO₂ in the microporous coal structure. A reduced amount of CO₂ released into the atmosphere contributes in turn to the mitigation of climate change. However, there are a number of geomechanical risks associated with the commercial-scale storage of CO₂, such as potential fault or fracture reactivation, microseismic events, cap rock integrity or ground surface uplift. The present study assesses potential site-specific hydromechanical impacts for a coal deposit of the Upper Silesian Coal Basin by means of numerical simulations. For that purpose, a near-field model is developed to simulate the injection and migration of CO₂, as well as the coal-CO₂ interactions in the vicinity of horizontal wells along with the corresponding changes in permeability and stresses. The resulting effective stress changes are then integrated as boundary condition into a far-field numerical model to study the geomechanical response at site-scale. An extensive scenario analysis is carried out, consisting of 52 simulation runs, whereby the impacts of injection pressures, well arrangement within two target coal seams as well as the effect of different geological uncertainties (e.g., regional stress regime and rock properties) is examined for operational and post-operational scenarios. The injection-induced vertical displacements amount in maximum to 3.59 cm and 1.07 cm directly above the coal seam and at the ground surface, respectively. The results further demonstrate that neither fault slip nor dilation, as a potential consequence of slip, are to be expected during the investigated scenarios. Nevertheless, even if fault integrity is not compromised, dilation tendencies indicate that faults may be hydraulically conductive and could represent local pathways for upward fluid migration. Therefore, the site-specific stress regime has to be determined as accurately as possible by in-situ stress measurements, and also fault properties need to be accounted for an extensive risk assessment. The present study obtained a quantitative understanding of the geomechanical processes taking place at the operational and post-operational states, supporting the assessment and mitigation of environmental risks associated with CO${}_2$ storage in coal seams.