A STUDY ON HYDRAULIC FRACTURE PROPAGATION BEHAVIOR IN NATURALLY FRACTURED CAVERNOUS CARBONATE RESERVOIRS

摘要

In naturally fractured cavernous carbonates, hydraulic fracture propagation behavior is influenced by natural fractures and cavities; for example, fracture turning or generation of multiple fractures. In this paper, we studied the hydraulic fracturing propagation behavior of naturally fractured cavernous carbonates. Based on the governing equations of fluid/solid coupling in porous medium and the theory of damage mechanics, a finite-element computing model was developed to simulate hydraulic fracture propagation with the effect of natural fractures and cavities. Based on the model, extensive numerical simulations were conducted to investigate fracture propagation behavior. The results show that natural fractures, cavities, and hydraulic fractures change the orientation and magnitude of the horizontal principal stress. The change in the horizontal principal stress and the low residual tensile strength of natural fractures are the main reasons causing turning of the hydraulic fracture. Turning of the hydraulic fracture is determined not only by the intersection angle between the natural fracture and the maximum horizontal principal stress, residual tensile strength, and friction coefficient of natural fractures, but also by the length of the natural fractures, diameter of the cavities, and pump rate. Multiple hydraulic fractures may be generated when a hydraulic fracture enters into a cavity with multiple natural fractures connected to the cavity face. The hydraulic fracture width reduces after turning. The reduction extent depends on the intersection angle between the natural fractures and the maximum horizontal principal stress as well as the difference between the two horizontal principal stresses. The width of hydraulic fracture decreases sharply when the hydraulic fracture enters into a cavity. The downhole pressure increases when the hydraulic fracture meets a natural fracture or a fractured cavity and turns into the natural fracture.