Development and implementation of a narually fractured reservoir model into a fully implicit, equation-of-state compositional, parallel simulator
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Naturally fractured reservoirs contain a significant amount of the world oil reserves. A number of these fields contain several billion barrels of oil. In naturally fractured reservoirs, fluids exist in two interconnected systems: the rock matrix, which usually provides the bulk of the reservoir volume, and the highly permeable rock fractures. Accurate and efficient reservoir simulation of naturally fractured reservoirs is one of the most important, challenging, and computationally intensive problems in reservoir engineering. Parallel reservoir simulators developed for naturally fractured reservoirs can effectively address the computational problem. A new accurate parallel numerical simulator for large-scale naturally fractured reservoirs, capable of modeling fluid flow in both rock matrix and fractures, has been developed. The simulator is a parallel, 3D, fully implicit, equation-of-state compositional model that uses numerical tools for solving very large, sparse linear systems arising from discretization of the governing partial differential equations. A generalized dual porosity model, the multiple-interacting-continua (MINC), has been implemented in this simulator. The matrix blocks are discretized into subgrids in both horizontal and vertical directions to offer a more accurate transient flow description in matrix blocks. We believe this implementation has led to a unique and powerful reservoir simulator that can be used by small and large oil producers to help them in design and prediction of complex gas and waterflooding processes on their desktops or a cluster of computers. Some features of this simulator, such as modeling both gas and water processes with the ability of two dimensional matrix subgridding for naturally fractured reservoirs, to the best of our knowledge are not available in any commercial simulator. For coupling of the fracture and matrix continua, no analytical approximations are made. Instead, numerical methods are used to treat the transient flow of fluid between matrix and fractures. The development was performed on a cluster of processors, which has proven to be a very efficient and convenient resource for developing parallel programs. The results were successfully verified against analytical solutions and commercial simulators (ECLIPSE, IMEX, and GEM). Excellent agreement was achieved for a variety of reservoir case studies. Applications of this model for several enhanced oil-recovery processes (including gas and water injection) are demonstrated. The effects of matrix subgridding on the accuracy of the results of the simulation runs are investigated. The study showed that in some circumstances the results of simulators without matrix subgridding generated more than 50% error in oil-recovery calculations. Simulation results using the simulator on a cluster of processors are also presented. Excellent speedups were obtained using the simulator in conjunction with solving a variety of problems.