Uncertainties exist on the efficiency of CO2 injection and storage in deep unminable coal seems due to potential reduction in the permeability of coal that is induced by CO2 adsorption into the coal matrix. In addition, there is a limited knowledge about the stability of CO2 stored in coal due to changes in gas partial pressure caused by potential leakage. This paper presents an experimental study on permeability evolution in a high rank coal from South Wales coalfield due to interaction with different types of gases. The reversibility of the processes and stability of the stored CO2 in coal are investigated via a series of core flooding experiments in a bespoke triaxial flooding setup. A comprehensive and new set of high-resolution data on the permeability evolution of anthracite coal is presented. The results show a considerable reduction of permeability above 1.5 MPa CO2 pressure that is correlated with the coal matrix swelling induced by CO2 adsorption. Notably studied in this work, the chemically-induced strain due to gas sorption into coal, that has been isolated and quantified from the mechanically-induced strain as a result of changes in effective stress conditions. The results of post-CO2 core flooding tests using helium (He), nitrogen (N2) and methane (CH4) demonstrated a degree of restoration of the initial permeability. The injection of N2 showed no significant changes in the coal permeability and reversibility of matrix swelling. The initial permeability of the coal sample was partially restored after replacing N2 by CH4. Observation of permeability evolution indicates that the stored CO2 has remained stable in coal under the conditions of the experiments.@en

This paper presents an investigation of coupled thermal, hydraulic, and chemical behavior of a compacted bentonite buffer under the heating and hydration conditions of geological disposal of high-level nuclear waste. The study presented provides further insight into the evolution of hydro-geochemistry of the compacted bentonite and the clay microstructure effects through a numerical modelling development of the reactive transport of multicomponent chemicals. The application/validation case study is based on a series of laboratory tests on heating and hydration of compacted bentonite for a period of 0.5-7.6 years reported in the literature. The effects of microstructure evolution during hydration and dehydration on the transport phenomena are included via a new approach that links the geochemistry of clay hydration/dehydration with the transport properties. The analysis results related to the moisture flow and chloride transport demonstrate close correlation with the experimental results by the inclusion of the effects of microstructure evolution in the transport phenomena. The results of numerical analysis of reactive transport of chemicals highlight the importance of accessory minerals present in bentonite on the distribution of some anionic species. The behavior of major cationic species is shown to be mainly governed by the transport processes. Further insights into the chemically driven processes in clay buffer due to coupled hydraulic and thermal effects are presented and discussed that are captured from the results of modeling the clay-water-chemical system.@en

The high affinity and adsorption capacity of coal to carbon dioxide provides an alternative approach for the enhanced recovery of methane from unminable coalfields (CO2-ECBM) by which a potential solution for long-term CO2 sequestration in deep geological formations can also be achieved. However, due to chemomechanical effects induced by the interactions between CO2 and coal, the effective methane production and carbon dioxide storage can be degraded which has caused uncertainties about the techno-economic feasibility of the CO2-ECBM process. This study presents an experimental investigation that aims to address key knowledge gaps related to the efficiency of CO2 storage and CH4 recovery in high rank coals for which a comprehensive experimental data set and analysis are largely missing. Competitive displacements of CH4 with N2 or CO2 in an anthracite coal sample from a South Wales coalfield have been studies, based on a series of core flooding experiments. The results show that the N2 breakthrough time (the time at which 1% of the total gas injected was recovered) was almost spontaneous whereas a considerably delayed breakthrough time was observed for the case of the CO2-ECBM experiment. In addition it was observed that for the CO2-ECBM experiment, the ratios of CH4 recovery with respect to the total amount of gas injected and gas stored were higher by factors of 10 and 2.4, respectively. The results also show that 90% of the total N2 injected was produced in the outflow gas, whereas for the case of the CO2 experiment, only 63% of the total injected CO2 was produced. The presence of a high amount of N2 in the outflow may lead to additional challenges in order to separate N2 from CH4 and thus affect the efficiency of the N2-ECBM method. Under the conditions of the experiments, the total CH4 displacement ratio and breakthrough for the case CO2-ECBM were found to be more favorable compared to those obtained from N2-ECBM. This study provides new insights into the efficiency of the CO2-ECBM process and offers a comprehensive experimental data set that can be used for testing the accuracy of predictive models.@en