On the mitigating environmental aspects of a vertical well in underground coal gasification method

Underground coal gasification (UCG) is an energy production pathway in underground coal deposits with the potential advantage of decreasing the greenhouse gas emissions during the energy extraction process. The environmental benefits of UCG are mainly due to eliminating (i) conventional mining operations, (ii) the presence of coal miners in the underground, (iii) coal washing and fines disposal, and (iv) coal stockpiling and coal transportation activities. Furthermore, UCG has a capacity of great potential to provide a clean coal energy source by the implementing carbon capture and storage techniques as part of the energy extraction process. In this method, coal seams in the underground were converted into syngas including hydrogen (H₂), carbon monoxide (CO), carbon dioxide (CO₂), and methane (CH₄) gasses with an advanced thermochemical process. UCG operation effected significant geomechanical changes to the overburden strata. In this process, the vertical well (especially the production well) was mainly affected by the mechanical stresses and the thermal stress, induced by the high syngas temperature. This high temperature changed the mechanical, thermal, and physical properties of the coal seam and its surrounding rocks (the host rocks), finally causing instability of the vertical well, while leading to serious production and environmental problems. One of these environmental issues is the possibility of syngas leakage in to the environment, resulting in water pollution and acidification. In addition, the released syngas could trigger global warming and air pollution. This research evaluated environmental aspects of UCG vertical well (production well) based on the stability analysis. Therefore, a flow sheet form was developed for three-dimensional thermomechanical numerical modeling of an UCG vertical well by explicit Lagrangian finite difference method. In this model, a criterion was established based on normalized yielded zone area to assess the stability conditions. The methodology was able to capture all factors that influence the instability of UCG well while selecting suitable mud pressure and lining system during the well drilling process. Hence, when wellbore integrity issues arose during drilling, the mitigation strategies were applied to rectify these problems. The results demonstrated that the vertical well should be drilled at a constant mud pressure of 9 MPa (megapascal), thus causing minimum environmental problems. The thermomechanical modeling provided an opportunity to assess the potential environmental impacts and identify reliable global climate change mitigation strategies.