Experimental research on permeability variation from the process of hydraulic fracturing of high-rank coal

Coal reservoir permeability is a key parameter affecting the productivity of coalbed methane (CBM) wells. Hydraulic fracturing is the main technology used in CBM development. Using GCTS-RTR-1000 rock mechanics experimental system, hydraulic fracturing experiments of high-rank coal rock under triaxial stress were carried out. Through experiments, the deformation and failure of coal samples and the change in permeability during hydraulic fracturing are observed. The relationship between permeability and effective stress of coal samples in the process of hydraulic fracturing is established, and the permeability control mechanism of coal samples is discussed. The experimental results show that the permeability of coal sample increases with the decrease in effective stress in the whole process of hydraulic fracturing. According to the damage degree and permeability change of coal samples, hydraulic fracturing can be divided into three stages: (1) coal rock compaction stage, the permeability changes slowly and even tends to decline; (2) coal rock expansion stage, the permeability increases slowly; (3) coal rock fracture stage, the permeability increases rapidly. The permeability of coal sample after fracturing is 10–250 times higher than that under original reservoir condition. Without proppant, the pore pressure decreases to the initial state, and the fracture closes. The permeability of coal sample after fracture closure is 1.23–78.7 times of the initial permeability. Compared with fractured coal rock, the permeability growth rate of compacted coal rock is higher after fracturing. Pulverized coal and clay particles are produced during fracturing. If the pressure is held for a long time, these micro-particles may infiltrate into the micro-fissures of coal samples, resulting in the decrease of permeability of coal samples. Rapid pressure relief and fracturing fluid back drainage are conducive to the protection of coal reservoirs and the reduction of reservoir damage.