Study on fracture morphology of hydraulic fracturing in heterogeneous elastoplastic coal

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circular 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 show 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 crack initiation criterion and the BK mixed fracture energy criterion as the crack extension criterion,which can accurately simulate the extension morphology of hydraulic fractures under complex stress states. ② In the case of a single cluster of injection holes,the hydraulic fracturing simulated fluid injection in coal seams with nonhomogeneity coefficients m of 2-5 .③ In coal seams with nonhomogeneity factor m of 1.5-5.0,the hydraulic fracture extension will show bifurcation and deflection,and coal seams with nonhomogeneity factor too low (m<1.5) will only form a single long straight fracture,while coal seams with too high (m>5) will form a symmetrical biplane curved fractures,with coal seams with m=2 forming the most complex fracture pattern after hydraulic fracturing.