Research on calculation methods for CO2 storage control pressure in goafs of abandoned coal mines

Abstract

[Objective] Storing CO₂ in goafs is a feasible technical approach for addressing the issue of reusing underground spaces in abandoned coal mines in China and achieving the “dual carbon” goals, with promising application prospects. However, it currently faces numerous challenges. To enhance the safety and efficiency of CO₂ storage in goafs, it is necessary to establish control pressure indicators and their calculation method for the entire CO₂ storage process. [Methods] Taking the optimization of CO₂ storage control pressure in the goaf of Changcun Coal Mine in Shanxi Province as a case study, the physical properties of CO₂ were first analyzed under different temperature and pressure conditions. Based on the climatic temperatures in Shanxi and the burial depth conditions of abandoned coal mines, the CO₂ injection process was determined, and its storage characteristics in the goaf were analyzed. Subsequently, based on the storage principles of CO₂ in the caprock and the requirement for preventing groundwater infiltration, the concepts of upper and lower pressure limits for CO₂ storage in goafs were proposed, along with corresponding calculation method and analyses of major influencing factors. Finally, integrating the specific geological conditions of the goaf in Changcun Coal Mine, the approximate range of CO₂ storage control pressure in this goaf was derived. [Results] The annual average temperature in Shanxi was 15 ℃, and the burial depths of abandoned coal mines generally ranged from 100 to 300 m. Under these temperature and pressure conditions, CO₂ exhibited liquid-gas two-phase storage characteristics in the goaf. The available goaf space capacity in Changcun Coal Mine was 1.509 2 million m³, with maximum control pressure for CO₂ storage recommended to be 8~10 MPa and the minimum control pressure recommended to be 4~6 MPa, indicating significant storage potential in the region. The goafs of abandoned coal mines in Shanxi had large capacities, moderate temperature and pressure conditions, and low hydraulic head of groundwater, indicating their potential for large-scale CO₂ storage. [Conclusion] The lower limit of storage pressure is relatively small, while the upper limit of control pressure primarily depends on the breakthrough pressure and fracture pressure of the caprock. Based on the sealing capacity of the caprock, if sealing the goaf is required to utilize the enclosed space for CO₂ storage, it is necessary to comprehensively consider sealing costs, implementation method, and the re-establishment of mechanical models. The investigation of calculation method for CO₂ storage control pressure in goafs in Shanxi provides valuable references for the application and development of large-scale CO₂ storage technology in abandoned coal mines.

Keywords

abandoned coal mines / goaf / CO_2 storage / control pressure / influencing factors