Experimental study on stress-permeability of high rank coal samples before and after hydraulic fracturing

Hydraulic fracturing technology has become a key technical means to improve coalbed methane (CBM) yield. The evolution of CBM reservoir permeability before and after hydraulic fracturing reflects the induced fracturing level. By means of fracturing and stress-permeability experiments on four high rank coal samples of No.3 coal seam of the Permian Shanxi Formation in Sihe Coal Mine, Jincheng Mining Area, Shanxi Province, the permeability changes before and after fracturing are tested, and the evolution of pillar coal sample permeability with stress before and after fracturing is revealed. The results show that under the condition of constant confining pressure and axial pressure, the pore pressure of coal samples increases gradually with the increase of injection pressure. When the injection pressure is greater than the fracture pressure, the coal sample breaks, and the fracture pressure of the coal sample increases linearly with the increase of the confining pressure. Under the same injection pressure, the effective stress increases with the increase of confining pressure and axial pressure. The permeability of coal samples before and after hydraulic fracturing decreases exponentially with the increase of effective stress, and the permeability after fracturing is significantly greater than that before fracturing. The permeability improvement coefficient (PIC) of CBM reservoir is used to evaluate the permeability improvement effect of coal samples. The PIC of four test coal samples increase exponentially with the increase of effective stress, but decrease with the increase of confining pressure. The permeability of CBM reservoir mainly depends on the degree of fracture development and the size of fracture width in coal. The permeability of fractured reservoir is in direct proportion to the 3rd power of fracture width. After hydraulic fracturing, the average fracture length, fracture porosity and fracture width increase by 70.81%−253.25%, 171.88%−383.02% and 20.31%−32.43%, respectively. The fracture width decreases with the increase of effective stress in a negative exponential function. The test results are compared with the measured coal reservoir permeability before and after hydraulic fracturing. The two have good consistency, which explores an effective way for the evaluation of the effect of hydraulic fracturing of coal reservoirs.