Research on Migration and Surface Deformation during Goaf CO₂ Geological StorageTaking Mindong No.1 Mine in...

Goaf CO₂ geological storage is a key technology for addressing the dual issues of the utilization of large-scale coal mine goaf and the space for CO₂ storage. Exploring the migration range of CO₂ and the surface deformation during the storage process is the core challenge in ensuring the safety and stability. Based on the geological conditions of Mengdong Energy's Mindong No.1 mine in Hulunbuir, a real geological structure model of the goaf, overburden of goaf, caprock, and overburden was established, and numerical simulation experiments on CO₂ migration and surface deformation were conducted. The research results indicate that a higher storage pressure led to a larger CO₂ migration range and a faster migration rate. After 20 years of storage, the amount of CO₂ diffused from the overburden to the surface was consistently below 0.02% under all pressures investigated, and no significant leakage was observed. The amount of CO₂ diffused in the first five years exceeded 50% of the total migration over the 20 year period. In contrast, the amount diffused in the last five years was less than 7.4% of the total. In the first year of storage, pressure was identified as the dominant force driving CO₂ migration. By the twentieth year, the pressure difference had diminished, and the various forces mutually counteracted each other. Consequently, CO₂ migration nearly ceased. During the process of CO₂ storage in the goaf, the surface deformation is characterized by an initial uplift followed by a progressive decline from the peak-uplift position, while the surface remains above its preinjection level. Under the storage pressures of 10, 15, and 20 MPa, the maximum uplift amounts are 0.060, 0.0917, and 0.123 m, respectively. The evolution can be divided into three stages: rapid uplift-decline period, slow adjustment period, and stabilization period. The decline in uplift is mainly governed by changes in effective stress: at the initial stage of storage, the pore pressure is the highest and the effective stress is the lowest, and the surface uplift reaches the peak; when the pore pressure decreases and the effective stress increases, compression of the strata causes the surface displacement to decline from its peak-uplift position. After 20 years of storage, the Surface-uplift recovery ratios under the storage pressures of 10, 15, and 20 MPa are 27.48%, 14.99%, and 9.76%, respectively. The higher the storage pressure, the smaller the reduction from the peak uplift and the lower the recovery rate. These findings confirm the target strata's suitability for long-term CO₂ storage.