ALBERT

Description: This paper presents an investigation of the transport and fate of hydrogen gas through compacted bentonite buffer. Various geochemical reactions that may occur in the multiphase and multicomponent system of the unsaturated bentonite buffer are considered. A reactive gas transport model, developed within an established coupled thermal, hydraulic, chemical and mechanical (THCM) framework, is presented. The reactive transport module of the model considers the transport of multicomponent chemicals both in liquid and gas phases, together with an advanced geochemical reaction model. The results of a series of numerical simulations of the reactive transport of hydrogen in unsaturated bentonite are presented in which hydrogen gas, because of the corrosion of a steel canister, has been injected at a realistic rate into a partially saturated bentonite buffer. Gas pressure development and the fate of the hydrogen gas with respect to the geochemical reactions are studied. The results show the high buffering capacity of unsaturated bentonite, when considering a steel canister, over a period of 10 000 years. The presence of accessory minerals is shown to have an important role in mitigating excess hydrogen ions, thus increasing the dissolution capacity of the system to gas. The development of various forms of aqueous complexations between the inorganic components and the hydrogen ions were also found to be important in buffering the excess hydrogen that evolved. Based on the results obtained, it is postulated that the presence of various chemical components in the clay buffer may influence the transport and fate of the hydrogen gas.

Description: At the post-closure stage of a geological disposal facility for higher activity radioactive waste several species of gas are likely to be generated in the near-field environment. These could alter the sealing and chemical properties of the bentonite buffer and the local geochemical environment significantly. The authors' attempt to simulate multicomponent gas flow through variably saturated porous media is presented. Governing equations have been developed for a reactive gas-flow model to simulate the thermo-hydro-gas-chemical-mechanical behaviour, with specific reference to the performance of highly compacted bentonite buffer subjected to repository gas generation and migration. The developed equations have been included in the bespoke numerical model COMPASS and some generic simulations are also presented. The model presented extends current capability to assess buffer performance.

Description: At the post-closure stage of a geological disposal facility for higher activity radioactive waste several species of gas are likely to be generated in the near-field environment. These could alter the sealing and chemical properties of the bentonite buffer and the local geochemical environment significantly. The authors' attempt to simulate multicomponent gas flow through variably saturated porous media is presented. Governing equations have been developed for a reactive gas-flow model to simulate the thermo-hydro-gas-chemical-mechanical behaviour, with specific reference to the performance of highly compacted bentonite buffer subjected to repository gas generation and migration. The developed equations have been included in the bespoke numerical model COMPASS and some generic simulations are also presented. The model presented extends current capability to assess buffer performance.

Description: This paper presents a series of surface experimental simulations of methane-oriented underground coal gasification using hydrogen as gasification medium. The main aim of the experiments conducted was to evaluate the feasibility of methane-rich gas production through the in situ coal hydrogasification process. Two multi-day trials were carried out using large scale gasification facilities designed for ex situ experimental simulations of the underground coal gasification (UCG) process. Two different coals were investigated: the “Six Feet” semi-anthracite (Wales) and the “Wesoła" hard coal (Poland). The coal samples were extracted directly from the respective coal seams in the form of large blocks. The gasification tests were conducted in the artificial coal seams (0.41 × 0.41 × 3.05 m) under two distinct pressure regimes - 20 and 40 bar. The series of experiments conducted demonstrated that the physicochemical properties of coal (coal rank) considerably affect the hydrogasification process. For both gasification pressures applied, gas from “Six Feet” semi-anthracite was characterized by a higher content of methane. The average CH4 concentration for “Six Feet” experiment during the H2 stage was 24.12% at 20 bar and 27.03% at 40 bar. During the hydrogasification of “Wesoła" coal, CH4 concentration was 19.28% and 21.71% at 20 and 40 bar, respectively. The process was characterized by high stability and reproducibility of conditions favorable for methane formation in the whole sequence of gasification cycles. Although the feasibility of methane-rich gas production by underground hydrogasification was initially demonstrated, further techno-economic studies are necessary to assess the economic feasibility of methane production using this process.

Description: 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 CO2 storage in coal seams.

Description: The yield and composition of tar depending on coal rank and pressure during underground coal gasification (UCG) were studied. Two coals were used in a series of ex-situ UCG experiments: a Welsh semi-anthracite (Six Feet) and a Polish bituminous coal (Wesoła). Four high-pressure gasification trials under two distinct pressure regimes (20 and 40 bar) were conducted. The tar samples were collected directly from the reactor outlet. The following groups of compounds were analysed by use of gas chromatography (GC-MS): light monoaromatic hydrocarbons (BTEX – benzene, toluene, ethylbenzene and xylenes), polycyclic aromatic hydrocarbons (PAHs) and phenols. A series of gasification experiments revealed significant differences in tar yields and composition depending on the coal rank and gasification pressure. Significant decreases in tar contents were observed with the increase in gasification pressure from 20 to 40 bar for both coals. The total yields of the analysed tar components per kg of gasified coal were 2.58 g and 0.41 g for the experiments conducted on the Six Feet samples at 20 bar and 40 bar, respectively. The corresponding values for the Wesoła coal amounted to 5.48 g and 0.95 g. In all experiments, BTEX was a dominant group of tar components, constituting 69–86 % of the total tar yield within the tested range of compounds. The present study further proves that gasification pressure has a significant effect on the chemical composition of the produced UCG tars for both coal samples under study.

Description: This study, conducted as part of the ROCCS project, investigates the potential of coal seams for CO2 sequestration through in situ tests. The in situ tests, performed at Experimental Mine Barbara in Mikołów, Poland, involved injecting CO2 through a horizontal well into a coal seam, with variable well lengths and injection parameters. The experiments included monitoring for CO2 leakage and migration within the coal seam. The objective was to examine the correlation between the CO2 injection rate and the coal–CO2 contact area, monitoring for any potential leakage. The total mass of CO2 injected was about 7700 kg. Significant leakage, probably due to the formation of preferential pathways, prevented pressure buildup in the injection well. The results provide insights into challenges regarding CO2 injection into coal seams, with implications for the design of commercial-scale CO2 storage installations.