Study Of Hydraulic Fracture Crack Extension In Coal Reservoirs Based On Non-homogeneity Of Mixed Fracture Energy

Conventional reservoir CBM mining,which mainly uses hydraulic fracturing to modify the coal body for seepage enhancement,accurately simulates the seam network morphology of the elastoplastic coal body after hydraulic fracturing,which can provide a reference for construction process improvement and reservoir recovery value assessment.The finite discrete element method(FDEM)is an effective method to simulate the internal crack extension morphology of elastoplastic materials.To obtain the mixed fracture energy based on the B-K criterion in the intrinsic model used for the simulation,a semi-disc three-point bending(SCB)test with variable prefabricated fracture inclination was performed on the coal;a test method for hydraulic fracture studies using a cubic specimen of epoxy-coated coal samples is proposed to restore the in-situ The test method is proposed to restore the expansion process of hydraulic fracture in the in-situ condition to verify the accuracy of the numerical simulation method.Considering the non-homogeneity of coal reservoirs in the natural state,the Weibull probability distribution in statistics is introduced,the shape parameter m in the density function is defined as the non-homogeneity coefficient,and the strength and fracture energy of the cohesive type units in the numerical model are randomly assigned according to the Weibull distribution to simulate the fracture expansion patterns of coal reservoirs with different non-homogeneity degrees in hydraulic fracturing operations,and fully consider the gradual transformation of fracture flow from Darcy flow to Poiseuille flow.The results indicate that.(1)the finite discrete element method is suitable for modelling the extension of hydraulic fractures in elasto-plastic coals,using the quadratic nominal stress criterion as the fracture initiation criterion and the B-K mixed fracture energy criterion as the fracture extension criterion,which can accurately simulate the extensional morphology of hydraulic fractures in complex stress states.(2)The injection pressure-time curve obtained from hydraulic fracturing simulations in coal seams with non-homogeneity coefficients m of 2～5 under single cluster of injection holes shows obvious pressure peaks,which is consistent with the injection pressure curve in actual engineering.(3)In coal seams with non-homogeneity factor m ranging from 1.5 to 5,the expansion of hydraulic fractures will show bifurcation and deflection,and coal seams with non-homogeneity factor too low(m<1.5)will only form single long straight fractures,while those with too high(m>5)will form symmetrical biplane curve fractures,among which the fracture pattern formed in coal seams with m=2 is the most complicated after hydraulic fracturing.