Study on Gas Production and Heat Transfer Characteristics of Underground Coal Gasification with Groundwater-Related Operating and Reservoir Factor Influence

Abstract

Underground coal gasification (UCG) is the core technology for in situ clean conversion of deep coal resources, and one of its important differences from surface coal gasification is that the reaction process is significantly affected by groundwater. To address the insufficient consideration of groundwater effects in existing studies, the controlled retraction injection point technology-based multiphysics coupled numerical simulation model driven by partitioned reaction zone was established using COMSOL, and the reliability of the model was verified by pilot-scale tests. Then, the influences of three groundwater-related parameters, i.e., water injection flow velocity, coal seam permeability, and gasification pressure, on coal gas composition, temperature distribution, and heat-affected zone evolution were systematically investigated. The results show that the high-temperature zone and heat penetration depth are expanded by enhancing the water–gas reaction when the flow velocity is below the best-performing value of 0.55 m/min within the investigated range, and excessive heat absorption causes the maximum heat-affected depth to shrink sharply from 0.22 to 0.09 m when exceeding the threshold. Proper permeability improves gas–solid contact conditions, and the heat-affected thickness increases by over 175% at an optimal permeability of 6 mD among the tested points in this study, while excessively high permeability intensifies convective heat dissipation and reduces heat accumulation efficiency. The increase in gasification pressure significantly promotes CH4 enrichment, with the content increasing by more than 18 times as the pressure rises from atmospheric pressure to 8 MPa, and leads to the high-temperature zone and heat-affected zone being characterized by axial shrinkage and vertical widening. This study reveals the characteristics and mechanisms of heat transfer and gas production in the UCG process under the influence of groundwater-related factors, which can provide a scientific basis for the stable operation and thermal field regulation of UCG in deep coal seams.