Comparison of three miscible injectants for a high-temperature, volatile oil reservoir - With particular emphasis on nitrogen injection

Gas injection is a proven EOR method in the oil industry with many well-documented successful field applications spanning a period of more than five decades. Miscible nitrogen injection into oil reservoirs is a relatively uncommon EOR technique because nitrogen often requires a prohibitively high pressure to reach miscibility. In this paper, we investigate the potential for applying miscible nitrogen injection for a high-temperature, low-permeability carbonate reservoir, which contains a volatile oil with some H2S and CO2. The field is still in its early development phase but suffers from relatively low throughput rate because of low permeability. At the prevailing reservoir pressures, extensive phase behavior studies confirm that the reservoir fluids develop miscibility with nitrogen, carbon dioxide, and hydrocarbon gas. Nitrogen behaves differently from hydrocarbon gas, despite the fact that the two gases lead to similar minimum miscibility pressures. At the prevailing reservoir pressure, the swelling factor with hydrocarbon gas is four times higher than for nitrogen. Furthermore, the reservoir fluid density increases during swelling with nitrogen, whereas it decreases as a result of hydrocarbon gas swelling. The same trend is observed for viscosity. Injection gas blends with various proportions of nitrogen and carbon dioxide shows that the MMP is constant when more than 35-40% nitrogen is present in the blend. The simulation studies involve a number of sensitivity runs performed on sector models, which are sufficiently fine-gridded to capture the compositional transition zone propagating between injector and producer pairs. Miscible nitrogen injection comes out as a viable option with the potential to increase recovery by 10-20% above the current water flood development scheme. The significantly improved sweep and displacement efficiency are due to N-2 miscibility with reservoir oil under reservoir conditions, possibility of increased PV injected in a N-2 WAG scheme, and the ability to maintain the initial reservoir pressure.