Abstract
Underground coal gasification (UCG) has the potential to increase worldwide coal reserves by utilization of coal deposits not mineable by conventional methods. This involves combusting coal in situ to produce a synthesis gas, applicable for electricity generation and chemical feedstock production. Three-dimensional (3D) thermo-mechanical models already significantly contribute to UCG design by process optimization and mitigation of the environmental footprint. We developed the first 3D UCG model based on real structural geological data to investigate the impacts of using isothermal and non-isothermal simulations, two different pillar widths and four varying regional stress regimes on the spatial changes in temperature and permeability, ground surface subsidence and fault reactivation. Our simulation results demonstrate that non-isothermal processes have to be considered in these assessments due to thermally-induced stresses. Furthermore, we demonstrate that permeability increase is limited to the close reactor vicinity, although the presence of previously undetected faults can introduce formation of hydraulic short circuits between single UCG channels over large distances. This requires particular consideration of potentially present sub-seismic faults in the exploration and site selection stages, since the required pillar widths may be easily underestimated in presence of faults with different orientations with respect to the regional stress regime.
Highlights
Underground coal gasification aims at in situ conversion of coal deposits, currently not mineable by conventional methods and production of a high-calorific synthesis gas, applicable for power generation, natural gas substitution and chemical feedstock production
In Underground coal gasification (UCG), target coal deposits are developed by directional drilling and converted into a synthesis gas by sub-stoichiometric combustion, using gasification agents based on oxygen-enriched air and steam
We compare isothermal with non-isothermal simulation results for the scenarios with a 60 m pillar width to assess the thermal impact on induced stresses, resulting in rock expansion occurring in the surrounding lithological layers
Summary
Underground coal gasification aims at in situ conversion of coal deposits, currently not mineable by conventional methods and production of a high-calorific synthesis gas, applicable for power generation, natural gas substitution and chemical feedstock production. In UCG, target coal deposits are developed by directional drilling and converted into a synthesis gas by sub-stoichiometric combustion, using gasification agents based on oxygen-enriched air and steam. Synthesis gas constituents are mainly hydrogen, carbon monoxide, carbon dioxide and methane in addition to nitrogen and minor components such as sulfuric acid [1,2,3,4,5,6,7,8]. Apart from the high economic potentials, UCG may effect site-specific environmental impacts, including fault reactivation, induced seismicity and ground surface subsidence. Thermo-mechanical effects and/or fault reactivation may introduce potential
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