Dingwei Weng

Summary: Quantitative prediction of hydraulic fracture (HF) propagation in horizontal wells has become an important research topic in recent decades. The thermal-hydraulic-mechanical (THM) behavior of HFs in deep ductile rock is complicated, especially when the fracturing fluid is compressible and its viscosity is temperature/pressure sensitive. Traditional HF models ignore the ductile behavior of rock breakage and the changes in thermal/physical properties of fracturing fluid. For this study, we developed a coupled THM 3D fracture model (MFrac3D) for multistage hydraulic fracturing in deep ductile rocks. Fracture opening, rock deformation, and heat transfer are coupled with fluid flow inside the HF. Compressibility and temperature/pressure-sensitive viscosity of fracturing fluid are first considered in this model. The cohesive zone model (CZM) is used to determine the ductile failure at the fracture tip of deep rock. MFrac3D is accurately verified by the analytical solutions and physical fracturing experiments. The temperature field inside the HF is dominated by the convective heat carried by the fracturing fluid. The ductile failure of deep rock affects not only the fracture tip geometry but also the injecting pressure and HF width distribution. For simultaneously propagating HFs, the ductile failure of deep rock also affects the stress interference strength among them.

Summary: The uneven propagation of multifractures is a key factor restricting production growth due to stress shadow and heterogeneity. To date, limited-entry fracturing techniques, nonuniform perforation, and in-stage diversion have been commonly used to promote even multifracture growth. In this study, a fully coupled multiple pseudo-3D (P3D) fracture simulator has been developed to examine the competitive propagation of multifractures during multicluster fracturing in a horizontal well. The present model considers stress interaction among multiple fractures, perforation erosion, fluid distribution among clusters, and in-stage diversion. The results of the model are validated against the reference data. Using the model, a series of numerical simulations are performed to investigate multifracture propagation with nonuniform perforation and in-stage diversion fracturing. We estimate the value of stress interaction for different fractures and time based on the approximate solution of Perkins-Kern-Nordgren (PKN) fracture in the viscosity-dominated regime and improve the dimensionless parameter that characterizes the competition between stress interaction and perforation friction. The fluid distributes evenly when the dimensionless parameter is less than unity (perforation friction is larger than stress interference). Based on this dimensionless parameter, a method to design nonuniform perforation and in-stage diversion is proposed. Results show that in the case of homogeneous in-stage stress, the perforation parameters should be selected under the condition that the dimensionless parameter is less than unity. In the case of heterogeneous in-stage stress and based on the perforation parameters selected under homogeneous stress conditions, the perforation holes in the high-stress cluster should be increased, making the reduction of perforation friction equal to the value of the in-stage stress heterogeneity. The stress heterogeneity can be balanced by decreasing the perforation friction of the high-stress clusters. In this way, nonuniform perforation under heterogeneous in-stage stress conditions can be designed quantitatively without numerical simulation. For in-stage diversion treatment, a method to design the number of ball sealers is proposed based on the results of nonuniform perforation, and only several or even zero groups of simulation are necessary to find the optimal number of ball sealers. A series of numerical simulations shows that the proposed design method is reliable and achieves a satisfactory result in an actual field case. The results can be helpful for nonuniform perforation and in-stage diversion design for multicluster fracturing in a horizontal well.