Numerical simulation of hydraulic fracture propagation in fracture-cavity carbonate formation

There are fracture-cavity systems with different shapes and sizes distributed in the fracture-cavity carbonate formation in Tahe Oil Filed, China, while they are also the main oil and gas reservoirs. Due to the uneven distribution of fracture-cavity systems in the carbonate reservoirs, hydraulic fracturing is used to communicate with the fracture-cavity systems to establish the flow channels between the fracture-cavity systems and the wellbore. So it is worth studying the propagation rule of hydraulic fracture in fracture-cavity carbonate formation, which determines the effect of reservoir reconstruction. In this paper, based on the discontinuous discrete fracture model, we built a hydraulic fracture propagation model in the fracture-cavity carbonate formation. First, we built the fluid-solid coupled stress field model for the fracture-cavity reservoir and then used the discrete fracture model to construct the hydraulic fracture. The model allowed the hydraulic fracture to expand along the initially divided grid, and the minimum strain energy density criterion was used to determine the propagation path. According to the different fracture-cavity distribution rules, we set three fracture-cavity reservoir models with different characteristics in this paper. Based on the simulation result of different fracture-cavity formation characteristics, we found that: In fracture-cavity carbonate reservoirs, the hydraulic fracture would be deflected by the local stress field disturbed around the caves. The larger the cave was, the more pronounced the disturbance was. According to the different relative positions, the disturbance could divide into two cases, frontal repulsion and lateral attraction, which were not conducive to the communication between the hydraulic fracture and the caves. However, when there were nature fractures around the caves, the communication probability was raised as the hydraulic fracture could easily intersect these nature fractures around the cave. By increasing the hydraulic fracture’s net pressure, the leading role of the hydraulic fracture could enhance when it intersected with the fracture-cavity systems so that it was able to break through the repulsion of the cave to communicate with the fracture-cavity systems in the direction of the principal stress. When the hydraulic fracture penetrated the fracture-cavity systems, the injection pressure was mainly controlled by the flow energy loss within the hydraulic fracture and the fracture-cavity systems’ fluid loss rate. Optimizing the fracturing fluid performance could reduce the penetration pressure and improve the propagation range of hydraulic fractures. The results of this paper could provide a reference for the fracturing evaluation of the fracture-cavity carbonate reservoir.