Plugging methods for underground gas extraction boreholes in coal seams: A review of processes, challenges and strategies

Gas extraction is an important method to solve coal mine gas disasters, utilize coalbed methane (CBM) resources and reduce greenhouse gas emissions. Construction of underground extraction boreholes in coal seams is a widely used method for pre-extraction gas in active coal mines in many countries. In most mines, the methane concentration is low and the methane flow decay rate is fast during the extraction process. One of the important reasons is the poor performance of borehole plugging and serious air leakage. Many plugging methods are applied to gas extraction boreholes, but a systematic comparison of the methods has not been conducted. This review analyzes reasons for the poor plugging performance of gas extraction boreholes through reviewing the main plugging methods of gas extraction in active mines. We find that the development range of fractures around a borehole needs to be studied comprehensively and the initial plugging depth and the length of a borehole need to be determined based on theoretical analysis. The grouting pressure cannot meet engineering requirements and plugging materials such as polyurethane and cement cannot effectively plug fractures around a borehole. This review also proposes strategies for improving the plugging performance. Firstly, time-dependent characteristics of the development of fractures around extraction boreholes under multiple damage conditions can be studied. Moreover, new plugging materials can be developed and plugging performances of materials can be tested. Lastly, reasonable plugging length and grouting pressure can be determined. This review can guide the design and optimization of plugging methods to enhance gas extraction efficiency and reduce greenhouse gas emissions.