Step-by-step CO2 injection pressure for enhanced coal seam gas recovery: A laboratory study

With the objective of achieving an 'emission peak and carbon neutrality', the injection of CO2 into deep unmineable coal seams or abandoned mines can enhance the methane recovery from coalbeds and can realise CO2 geological sequestration. Traditionally, Constant CO2 injection pressure method(Const-CO2) is expensive and low injection capacity of coal seam. Therefore, this study proposes an SBS-CO2 method that steps the CO2 injection pressure. The results revealed that compared with const-CO2 method, the total recovery rate of CH4 was increased by 9.148% by SBS-CO2 method, with the largest increase (7.933%) in the stage II. The injection -production ratio of SBS-CO2 injection method was always smaller than Const-CO2. At the stage I, CO2 injected amount was reduced by 9.54%. CO2 injection amount of unit mass coal was reduced by 1.211 mL/g. The lower the injection pressure was, the higher was the permeability. In the proposed method, each pressurisation was followed by a temporary recovery in permeability, and the critical CO2 injection time of 140.83 min. The SBS-CO2 injection method can effectively improve the recovery efficiency of coalbed methane and CO2 injection efficiency, greatly reduce the risk of CO2 consumption and outburst, and significantly reduce the injection and time cost.