Numerical simulation of fracture propagation in deflagration-hydraulic composite fracturing of unconventional reservoirs

Based on continuum-discontinuum element method, the numerical simulation of fracture propagation during deflagration-hydraulic composite fracturing was constructed by considering deflagration stress impact induced fracture creation, deflagrating gas driven fracture propagation, and hydraulic fracture propagation, exploring the effects of in-situ stress difference, deflagration peak pressure, deflagration pressurization rate, hydraulic fracturing displacement and hydraulic fracturing fluid viscosity on fracture propagation in deflagration-hydraulic composite fracturing. The deflagration-hydraulic composite fracturing combines the advantages of deflagration fracturing in creating complex fractures near wells and the deep penetration of hydraulic fracturing at the far-field region, which can form multiple deep penetrating long fractures with better stimulation effects. With the increase of in-situ stress difference, the stimulated area of deflagration-hydraulic composite fracturing is reduced, and the deflagration-hydraulic composite fracturing is more suitable for reservoirs with small in-situ stress difference. Higher peak pressure and pressurization rate are conducive to increasing the maximum fracture length and burst degree of the deflagration fractures, which in turn increases the stimulated area of deflagration-hydraulic composite fracturing and improves the stimulation effect. Increasing the displacement and viscosity of hydraulic fracturing fluid can enhance the net pressure within the fractures, activate the deflagration fractures, increase the turning radius of the fractures, generate more long fractures, and effectively increase the stimulated reservoir area. The stimulated reservoir area is not completely positively correlated with the hydraulic fracturing displacement and fracturing fluid viscosity, and there is a critical value. When the critical value is exceeded, the stimulated area decreases.