Wellbore Cooling and Wellbore Temperature Field Simulation for Underground Coal Gasification

Abstract

ABSTRACT Underground coal gasification (UCG) technology integrates three processes: well construction, coal mining and gasification. It not only overcomes the safety problems of existing mine coal mining, but also avoids the pollution of waste gas, waste water and waste residue in the process of surface coal gasification of traditional coal mining. It has the advantages of low gas production cost, high safety and good environmental benefits. However, effective temperature control technology of production wellbore still needs to be developed. Based on the annulus spray method and the combination of insulated oil pipe and spray device, a set of high-temperature gas spray cooling scheme is developed by using NASA variable specific heat calculation formula, considering the variation of specific heat value of main components of high-temperature mixture with temperature, and the finite element model of production wellbore is established by using commercial software (CFX), The temperature field and pressure field of production wellbore are simulated and analyzed. The analysis results show that the developed spray cooling scheme can effectively control the production wellbore temperature, achieve the design goals of slightly lower than 350 °C when the high-temperature gas is output to the ground and no gasification of annulus cooling water. The research methods and results have important practical significance for promoting the development of underground coal gasification process and even engineering application. Underground Coal Gasification (UCG) is a chemical mining method, which conducts controlled in-situ combustion and gasification of underground coal (Gregg D W and Edgar T F, 1978; Shafirovich E et al., 2009; Khan et al., 2015). A series of chemical reactions produce combustible gases such as H2, CO and CH4 (Perkins and Sahajwalla, 2006; Takyi Shadrack Adjei et al., 2023). UCG technology integrates three processes of well construction, coal mining and gasification. It belongs to a special coal mining method and is an important supplement to traditional physical coal mining technology (Lanhe Yang, 2003; Karol Kostúr et al., 2015). It replaces the huge and bulky underground coal mining equipment and ground coal gasification equipment, changes the traditional physical coal mining to chemical coal mining, realizes underground unmanned production, and avoids personal injury and mine accidents. It can not only recover coal resources abandoned in the mine, but also be used to mine thin coal seams, deep coal seams, "three lower" coal seams, and high-sulfur, high-ash, and high-gas coal seams that are difficult for underground workers to mine or are less economical and safe to mine. Moreover, dust pollution caused by coal mining and transportation is avoided, and the gasified gangue and ash remain underground (Rajesh P. Barnwal et al., 2017; Kariznovi, M et al., 2013), which reduces the environmental impact caused by the accumulation of surface solid waste, and prevents surface subsidence to a certain extent (Peng Pei et al., 2016). In a word, it overcomes the safety problems of existing mine coal mining, and also avoids the pollution of waste gas, waste water, waste residue, etc. in the surface coal gasification process of traditional coal mining, and has the advantages of low gas production cost, high safety and environmental protection. Good benefits. UCG technology is suitable for in-situ mining and transformation of difficult-to-mine coal seams, low-grade coal seams, especially deep coal seams, which improves resource utilization and drives the development of traditional industries such as coal, electric power, and chemical industry (Hossein Nourozieh et al., 2010). Therefore, UCG is a new technology of high-carbon resources and low-carbon development of clean energy, and it is also a green coal mining technology that coordinates resources and environment.