A hydromechanically-based risk framework for CO b2 s storage coupled to underground coal gasification

Abstract

Most of the energy produced in the world comes from fossil fuels: coal, oil and gas. Amongst them, coal is the most abundant and widespread fossil fuel in the world. Underground Coal Gasi cation (UCG), an in situ method to extract the calori c value of the coal, has been known for a century but has had very limited implementation throughout the world, mainly due to the availability of cheap oil over that period. It is now gaining relevance in order to unlock vast resources of coal currently not exploitable by conventional mining. However, growing concern on increased levels of carbon dioxide concentration in the atmosphere is pointing out the necessity to reduce the use of fossil fuels. Since alternative sources of energy (e.g. nuclear and renewables) are not in a position to meet the constantly increasing demand in a short term, carbon capture and its geological sequestration (CCS) is considered the best remedial option. An environmental risk assessment framework has been developed for coupling UCG to CCS accounting for bene ts and cost from both global and local perspectives. A UCG site presents signi cant di erences from other typical CCS projected scenarios, most notably the injection of CO2 into a heavily fractured zone. A model which accounts for ow in fractures represented by dual-porosity ow (TOUGH2) is coupled to a geomechanical model (FLAC3D). The impact of this fractured zone in the CO2 injection pressure buildup and stress eld is evaluated. Furthermore the effect of stress-dependent fracture permeability is assessed with the hydro-mechanically i coupled compositional simulator GEM. Simulation results suggest that in such a scenario, CO2 injectivity and dissolution improve though con nement is compromised and commercial injection rates seem unattainable. The e ects of miscibility and relative permeability on pressure buildup implemented in semianalytical solutions are also evaluated. Albeit further research is required, a UCG operation may, therefore, not be able to accommodate the produced CO2 in the gasi ed cavity and its surroundings in a safe and economical fashion. Rigorous studies and management practices are needed to establish the requirements for secure long-term con nement of the carbon dioxide in such scenario.