Fault Reactivation Can Generate Hydraulic Short Circuits in Underground Coal Gasification—New Insights from Regional-Scale Thermo-Mechanical 3D Modeling

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.

RETRACTED: Techno-Economic Comparison of Onshore and Offshore Underground Coal Gasification End-Product Competitiveness

“…Table 2), the total daily coal consumption (1.1 kt/day for electricity, 538 t/day for methanol and 234 t/day for ammonia production) and the geometrical gasification channel data (cf . Tables 1 and 2), an average daily horizontal gasification front progress of up to one meter can be achieved [18,25]. The overall coal yield of the chosen UCG well layout is about 25%.…”

“…Referring to thermomechanical modelling results on channel stability in the absence of subseismic faults, 60 m (cf. Figure 4) represents a sufficient pillar width to avoid inter-channel hydraulic short circuits [18]. The achievable gasification channel width in the Wieczorek target coal seam amounts to 20 m (cf.…”

“…Table 1. Geological data of target coal seam, adapted from Otto et al [18]. The suggested optimised vertical well layout for injection and production as well as the network of lined deviated injection boreholes, drilled horizontally into the coal seam are presented in Figure 4.…”

“…The achievable gasification channel width in the Wieczorek target coal seam amounts to 20 m (cf. Table 2) according to Otto et al [18]. In order to ensure the daily required synthesis gas feed in the reference scenario for an overall operational lifetime of 20 years, 1.2 UCG panels (15 gasification channels) are required for electricity generation.…”

“…Table 1) and coal seam extraction (cf. Table 2) were adapted from the coupled thermo-mechanical 3D UCG model set-up introduced by Otto et al [18] and data elaborated by Otto et al [19,20]. Table 1.…”