To reduce the emissions of carbon dioxide (CO2) into the atmosphere, it is proposed to inject anthropogenic CO2 into deep geological formations. Deep un-mineable coalbeds are considered to be possible CO2 repositories because coal is able to adsorb a large amount of CO2 inside its microporous structure. However, the response of coalbeds is complex because of coupled flow and mechanical processes. Injection of CO2 causes coal to swell, which leads to reductions in permeability and hence makes injection more difficult, and at the same time leads to changes in the mechanical properties which can affect the stress state in the coal and overlying strata. The mechanical properties of coal under storage conditions are of importance when assessing the integrity and safety of the storage scheme. On the other hand, the geomechanical response of coalbed will also influence the reservoir performance of coalbed. This paper provides an overview of processes associated with coalbed geosequestration of CO2 while the importance of geomechanical characteristics of coalbeds is highlighted. The most recent findings about the interactions between gas transport and geomechanical characteristics of coal will be discussed and the essence will be delivered. The author suggests areas for future research efforts to further improve the understanding of enhanced coalbed methane (ECBM) and coalbed geosequestration of CO2

To reduce the emissions of carbon dioxide into the atmosphere, it is proposed to inject anthropogenic carbon dioxide into deep geological formations. Deep, unmineable coalbeds are considered to be a possible carbon dioxide repository because coal is able to adsorb a large amount of carbon dioxide inside its microporous structure. However, the response of coalbeds is complex because of coupling between the flow and mechanical responses. Injection of carbon dioxide causes coal to swell, which leads to reductions in permeability and hence makes injection more difficult, and at the same time leads to changes in the mechanical properties that can affect the stress state in the coal and overlying strata. Apart from the influence of swelling on permeability, the chemo-mechanical aspects of carbon dioxide sequestration in coalbeds have been little studied. In this paper, the results of a series of triaxial tests on a bituminous coal are presented that show a reduction in elastic modulus as a result of carbon dioxide adsorption. Using these experimental results, a linear relationship between the reduction in elastic modulus and the amount of adsorbed gas was found. This relationship, in conjunction with a dual-porosity model, has been employed to simulate the hydromechanical response of a coal bed to carbon dioxide injection. The results of the numerical simulation show that the softening of the coal can change the permeability and stresses in the coal, and suggest that considering the softening effect is crucial for accurate assessment of the performance and safety of carbon dioxide storage in coalbeds.

Shale gas has recently gained significant attention as one of the most important unconventional gas resources. Shales are fine-grained rocks formed from the compaction of silt- and clay-sized particles and are characterised by their fissured texture and very low permeability. Gas exists in an adsorbed state on the surface of the organic content of the rock and is freely available within the primary and secondary porosity. Geomechanical studies have indicated that, depending on the clay content of the rock, shales can exhibit a brittle failure mechanism. Brittle failure leads to the reduced strength of the plastic zone around a wellbore, which can potentially result in wellbore instability problems. Desorption of gas during production can cause shrinkage of the organic content of the rock. This becomes more important when considering the use of shales for CO₂ sequestration purposes, where CO₂ adsorption-induced swelling can play an important role. These phenomena lead to changes in the stress state within the rock mass, which then influence the permeability of the reservoir. Thus, rigorous simulation of material failure within coupled hydro-mechanical analyses is needed to achieve a more systematic and accurate representation of the wellbore. Despite numerous modelling efforts related to permeability, an adequate representation of the geomechanical behaviour of shale and its impact on permeability and gas production has not been achieved. In order to achieve this aim, novel coupled poro-elastoplastic analytical solutions are developed in this paper which take into account the sorption-induced swelling and the brittle failure mechanism. These models employ linear elasticity and a Mohr–Coulomb failure criterion in a plane-strain condition with boundary conditions corresponding to both open-hole and cased-hole completions. The post-failure brittle behaviour of the rock is defined using residual strength parameters and a non-associated flow rule. Swelling and shrinkage are considered to be elastic and are defined using a Langmuir-like curve, which is directly related to the reservoir pressure. The models are used to evaluate the stress distribution and the induced change in permeability within a reservoir. Results show that development of a plastic zone near the wellbore can significantly impact fracture permeability and gas production. The capabilities and limitations of the models are discussed and potential future developments related to modelling of permeability in brittle shales under elastoplastic deformations are identified.