A comprehensive analysis of the structural alterations occur in CO2- interacted coal: An insight into CO2 sequestration in coal

© 2020 Suresh Madushan Sampath Kadinappuli HewageHuman activities from overpopulation to pollution are drastically increasing the global temperature and causing fundamental changes in the Earth. The primary reason for this phenomenon is the greenhouse gas effect that primarily comes from the destruction of the forest cover and conventional energy production. This would cause extreme weather conditions, consequently imperilling the natural habitats, ecosystems and all the Earth’s activities. Emission of an enormous amount of anthropogenic carbon dioxide (CO2) has become the primary reason for global warming; so, it is vital to advance towards novel techniques to alleviate the issue. Within a portfolio of options, CO2 sequestration has gained growing attention – aimed at carbon capture and storage in geological formations. In this context, CO2 sequestration in coal reservoirs is identified as an effective method, because the process is associated with enhanced coalbed methane (ECBM) extraction – that further offsets the cost of CO2 storage in coal reservoirs. The successful implementation of the process would certainly lead to a pollution-free world and ultimately a better future, through the mitigation of the greenhouse gas effect. Although the fundamental physics of CO2 sequestration have been evolved over several decades, there are some grey areas and unexplored aspects, that need further improvement through investigations. For instance, the injection of CO2 into coal affects its structural properties, and consequently, the efficiency of the sequestration process as well as the long-term integrity of the reservoir. The coal-CO2 interaction is a complicated physico-chemical and thermodynamic process that depends on multiple parameters, which in turn makes the evaluations quite challenging. A deep understanding of the structural alterations, in terms of coal hydro-mechanical properties, is thus essential to optimize the process, while minimizing the reservoir damage. Considering the above aspects, this research aims to comprehensively characterize the alterations in hydraulic and mechanical parameters of CO2-interacted coal, through a combination of systematic experimental, numerical and statistical analyses, integrated with advanced image analysis techniques and chemical analyses. The study is specifically focused on analysing the fully-coupled CO2 flow–adsorption–deformation process in coal and investigating the consequent hydro-mechanical modifications occur in coal structure, due to the complex coal-CO2 interaction. Initially, the fully-coupled process in a single coal matrix block was modelled, in which the multiple diffusion mechanisms in matrix pore network were simulated using a theoretically-extended diffusion modelling approach, and the consequent CO2 adsorption-induced matrix swelling was modelled and validated with experimental results. The analysis was then extended on a fractured coal geometry at the coal constituent-scale, through the inclusion of a 3D – discrete fracture matrix (DFM) network, in which the alterations in local fracture aperture and the subsequent fracture permeability were investigated. In general, the results conclude that the CO2 interaction-induced hydro-mechanical alterations of a fractured coal seam are largely geometry-dependent and highly-localized; thus, should be resolved accurately through a fully-coupled DFM modelling approach. Next, a series of experimental, numerical and statistical studies was conducted to further assess the CO2-interaction-induced complex mechanical property alterations in coal. The alterations in compressive strength, Young’s modulus, brittleness index and dynamic moduli were experimentally investigated, in which the temporal and localized mechanical alterations were specifically analysed in detail. A numerical model was developed to evaluate the free and adsorbed CO2-induced compressive strength alterations in coal, under both confined and unconfined conditions. The compressive strength was determined with the Mohr-Coulomb failure criterion, where a theoretically-modified cohesion model was derived, by integrating the effective stress change and the surface energy change – caused by CO2 interaction. Further, a statistical analysis was conducted using advanced soft computing techniques to predict the mechanical property alterations in coal, under different CO2 interaction conditions. The studies combinedly conclude that CO2 interaction results in adverse mechanical alterations in coal and the developed numerical and statistical models are capable of accurately simulating/capturing this behaviour. Finally, the conclusions, recommendations for future research and an insight into the field-application of CO2 sequestration technique were provided based on the results of the combined analyses. Overall, the outcome of this research is intended to provide knowledge and insight into the productivity and safety of the CO2 sequestration in coal reservoirs, by conclusively evaluating the CO2 interaction-induced structural alterations in coal, in terms of its hydro-mechanical properties. The information, results and conclusions given herein can be utilised by researchers to build upon and will be beneficial on reducing the global warming and climate change, through effectively minimising the atmospheric CO2 level