Fully hydromechanical coupled hydraulic fracture simulation considering state transition of natural fracture

The natural fracture in the reservoir presents different properties in the inactivated and the activated state. To model the hydromechanical behaviors of the natural fractures in the two states, the element partition method (EPM) is employed. The EPM is a fracture simulation method, which allows a crack run across an element without introducing any extra degrees of freedom. For the element containing an inactivated fracture segment, it is treated as a potential partitioned element, but its permeability is related to the normal stress to the embedded fracture. A Mohr-Coulomb type of activation criterion is developed for the inactivated fracture. Once the fracture is activated, this potential element is transformed into a partitioned element. The constitutive relation with consideration of shear dilation and the fully hydromechanical coupled equation of a partitioned element are derived. Based on this method, the activation process of a fracture and its propagation are simulated. The simulation results suggest that the present method can capture the full hydromechanical properties of a natural fracture from its inactivated to activated state. Besides the natural fracture, the randomized cavities are another type of discontinuities in some reservoir. The effect of the cavity on hydraulic fracture(HF) is also considered. It is suggested that the cavity strongly repels the HF due to the surrounding stress concentration, but whether the HF can connect with the cavity depends. If there are some cracks adjunct to the cavity, the HF may connect with the cavity through these adjunct cracks. The simulation results suggest that the present method is a simple and efficient simulation method for the HF in the complex reservoir. It can comprehensively consider the behaviors of the natural fracture and cavity.