Experimental study on the optimized anion fracturing fluid for improving coal samples permeability

During coal mining, water is usually injected into the coal seam on site to increase permeability for fracturing. However, the expansion of clay minerals after encountering water can clog the cracks and pores, and reduces the permeability of coal seam. To solve this problem, an anionic fracturing fluid for gas and oil field engineering was optimized with different gel breakers. The influences of the optimized anionic fracturing fluid on the permeabilities of coal were evaluated from the macroscopic perspective with a permeability tester, and from the microscopic perspective with the clay mineral content and pore structure of coal. The optimized anionic fracturing fluid exhibits viscosity of up to 290 mPa·s, good shear stability, and low amounts of gel breaking residues. It reduces the clay mineral content in coal samples by 1.5 times, as compared with deionized water. For the coal samples with different permeabilities achieved by the different weights of 240 g, 250 g, and 260 g, the simulated water injection of the optimized anionic fracturing fluid increases their permeability by 131.3%, 77.4%, and 44%, respectively, as compared with those achieved with the injection of deionized water. These results suggest that the optimized anionic fracturing fluid can greatly improve the permeability of coal seam. The addition of a gel breaker enhances the abilities of the anionic fracturing fluid to increase the permeability of coal seam, which provides a new strategy to improve the efficiency of coal seam water injection.