Optimization and application of sealing material and sealing length for hydraulic fracturing borehole in underground coal mines

Considering the problems happening to sealing materials for hydraulic fracturing borehole in underground coalmines, such as shrinkage, poor sealing effect, high cost, and improper sealing length, a new hydraulic fracturing sealing material consisting of cement, early strength water-reduced agent, polypropylene fiber, and mixing water is under development. Through indoor laboratory experiment, the sealing material ratio at the optimum compressive strength and shrinking percentage is optimized. A mechanical model for hydraulic fracturing borehole sealing has also been established, which leads us to the relationship between the maximum water pressure that can be carried by sealing materials and such sealing parameters as properties of materials and sealing length. A proper sealing length is determined and verified through a site experiment. The result demonstrates that the optimal ratio of the new material is 1: 0.6: 0.03: 0.005 for concrete, water, early strength water-reduced agent, and polypropylene fiber, respectively. The maximum water pressure that can be carried by the sealing material amplifies with the increase of the strength, elasticity modulus of the sealing material, and sealing length until a constant value is nearly attained when the sealing length reaches a certain point. In addition, taking Chongqing Songzao mining area in China for example, the reasonable sealing lengths of the fracture borehole for through beds fracturing and along beds fracturing are proved to be 10 and 13 m, respectively, which is consistent with theoretical analysis.