Nitrogen injection into naturally fractured reservoirs
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Naturally fractured reservoirs are complex and constitute a large amount of the world’s oil reserves. Their complexity usually is represented by numerical models based on conceptual abstractions. When most recoverable oil is present in low-permeability rocks, the dual porosity model appears to be the most efficient approach for simulating naturally fractured reservoirs. The rhythms of exploitation in a fractured reservoir can cause field pressure to decline drastically and be reflected in the reduction of oil production rates and the formation of a secondary gas cap. Therefore, field pressure maintenance using gas injection can be an attractive method from technical and economical points of view. A similar approach has been implemented in Cantarell field, the largest oil field in Mexico, characterized as a dual porosity system with a black oil fluid. The simulation of the largest nitrogen injection project in the world for a giant naturally fractured reservoir was carried out to perform a detailed study of the nitrogen injection by using a conceptual model. The objectives of this research are 1) Study the impact of nitrogen injection on oil recovery and nitrogen distribution by injecting nitrogen through different scenarios, and using simplified models with homogeneous and heterogeneous properties; 2) Study the injection of different gases at the same reservoir conditions; 3) Determine the variations of temperature that mainly occur in the gas cap and oil zone using a thermal compositional case study; 4) Study the impact of nitrogen distribution by injection at surface and elevated temperatures; 5) Study the nitrogen injection based on uncertain properties that impact the nitrogen distribution and the oil recovery under the gravity drainage mechanism; 6) Investigate the effects of matrix subgridding in vertical and horizontal directions on total oil production by using a stacked conceptual model; and 7) Develop an analytical model that describes the advancement of the nitrogen front through a fracture system for a gas-oil phase equilibrium system under reservoir pressure maintenance. Since the data used for this research is based on black oil fluid characterization and the fractures are responsible for fluid transport, our starting point was the construction of a single porosity case study using a black oil formulation considering only the fracture pore volume. Subsequently, we increased complexity of the modeling to determine the variations of temperature in the reservoir and to investigate the impact of nitrogen distribution by injecting nitrogen at standard and elevated temperatures. Finally, we developed a simple analytical model that describes the movement of nitrogen injected front by using basic equations for fluid-flow in permeable media such as the Buckley Leveret theory. The development of the analytical model will help reservoir engineers to better understand the problem of gravity drainage in fractured carbonate reservoirs and also to grasp the dynamics of fluid-flow in naturally fractured reservoirs.