Development of tight oil reservoirs mainly relies on massive hydraulic fracturing, which can generate complex fracture networks in the reservoirs. These highly discrete fracture networks bring great challenges for reservoir simulation. An efficient model applying an unstructured discretization method and automatic differentiation is proposed. The flexibility in unstructured control volume shapes enables the gridding of complex fracture systems. By introducing the concept of half-transmissibility for each grid, transmissibility list including connections of matrix-matrix, matrix-fracture, fracture-fracture is established. Nonlinear flow and transport equation system is solved by a modified Newton’s method, in which the Jacobian matrix is computed by automatic differentiation (AD). Accuracy of the model was validated by performing simulations using a commercial simulator. We implemented our model in several cases with a uniform physical domain (radial model with diameter=1 km) but different fracture properties. Results show that fracture configuration and property have significant impacts on the production. Moreover, our model shows a high efficiency in the densely fractured system with fracture density up to 1018/km2. The novelty of the model is in the ability to represent complex fracture systems individually and explicitly, and in the application of automatic differentiation, which greatly facilitates the model establishment and improves computational efficiency.


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