Abstract
A coupled thermo-mechanical model has been developed to assess permeability changes in the vicinity of an underground coal gasification (UCG) reactor resulting from excavation and thermo-mechanical effects. Thereto, we consider a stepwise UCG reactor excavation based on a pre-defined coal consumption rate and dynamic thermal boundary conditions. Simulation results demonstrate that thermo-mechanical rock behavior is mainly driven by the thermal expansion coefficient, thermal conductivity, tensile strength and elastic modulus of the surrounding rock. A comparison between temperature-dependent and temperature-independent parameters applied in the simulations indicates notable variations in the distribution of total displacements in the UCG reactor vicinity related to thermal stress, but only negligible differences in permeability changes. Hence, temperature-dependent thermo-mechanical parameters have to be considered in the assessment of near-field UCG impacts only, while far-field models can achieve a higher computational efficiency by using temperature-independent thermo-mechanical parameters. Considering the findings of the present study in the large-scale assessment of potential environmental impacts of underground coal gasification, representative coupled simulations based on complex 3D large-scale models become computationally feasible.
Highlights
Underground coal gasification (UCG) has the potential to increase the worldwide coal reserves by utilization of coal deposits that are currently not mineable by conventional methods
As summarized by Tian et al [47,48,49,50,51], significant experimental research on mechanical and thermal rock properties during and after high temperature treatment has been carried out in the scope of underground coal gasification research, whereby the results indicate that temperature-dependent parameters have to be considered in numerical simulations on high temperature rock mechanical processes (e.g., [54,55,56,57,63,64])
Mechanical processes in the UCG reactor vicinity are strongly influenced by mechanical and thermal properties, which are in turn controlled by the temperature distribution in the surrounding coal and its adjoining rock, provided that properties are considered to be temperature-dependent
Summary
Underground coal gasification (UCG) has the potential to increase the worldwide coal reserves by utilization of coal deposits that are currently not mineable by conventional methods. The original idea of UCG is not new, but rather has a long history. Apart from its high energetic and economic potential [10,11,12,13,14], UCG may cause environmental impacts such as ground subsidence and groundwater pollution [8,9,15,16,17]. In order to completely avoid or significantly mitigate these potential environmental concerns, the UCG reactor is generally operated below hydrostatic pressure to hinder the outflow of UCG process fluids into adjacent aquifers [8,18,19]
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