Fracture permeability damage and recovery behaviors with fracturing fluid treatment of coal: An experimental study

The water-based fracturing fluid is commonly used in coalbed methane (CBM) industry. The fracturing fluid invades into both stimulated and natural fractures. The fracture permeability are pivot parameter for fracturing stimulation and production planning. In this study, laboratory studies have been conducted to measure the fracture permeability evolutions for both artistically propped and natural fractures of anthracite. Three commonly used water-based fracturing fluids, including the slickwater, the guar gel and the viscoelastic surfactant (VES) fracturing fluids, were used for the experiments. The anthracite core specimens were prepared with either artistically propped (simulating hydraulic fracture) or natural fractures. The damage of the water-based fracturing fluid to the permeability of anthracite was comprehensively analyzed. Our experimental results show that both the slickwater and the guar gel can induce apparent damage to the permeability of the artificially propped and natural fractures. The VES fracturing fluid has less damage than the slickwater and the guar gel. The degree of permeability damage to the artificially propped fracture generally increases with an increase of concentration or the viscosity of the fracturing fluid. The fracturing fluid gives higher permeability damage to the artificially propped fracture supported by smaller proppant than that for large proppants. For each type of fracturing fluid, the size and density of the residue mass increased with the increase of the concentration of the fracturing fluid. For the permeability damage induced by the invasion of the fracturing fluid, the intrinsic reason is the blockage and occupation of the residue to the internal space of the fracture.