A simulation study of the effect of injecting carbon dioxide with nitrogen or lean gas on the minimum miscibility pressure



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Gas injection projects are used for enhanced oil recovery purposes. Many gases are used as injected gas such as carbon dioxide, nitrogen, and lean gas. The minimum miscibility pressure of the carbon dioxide is low compared to the other injection gases. Moreover, carbon dioxide injection reduces its concentration in the atmosphere, which makes it environmental solution. However, capturing and storing carbon dioxide is not economical. Thus, lean gas can be used as an injection gas. Instead of storing lean gas, which raises the cost of the gas injection project, lean gas can be re-injected instead of carbon dioxide. This process of re-injection of lean gas lowers the cost but raises the cost of the injection project due to its high minimum miscibility pressure compared to the carbon dioxide. Moreover, nitrogen is also used as an injection gas. Nitrogen is available everywhere, which makes the use of nitrogen more favorable. Nonetheless, nitrogen has an extremely high minimum miscibility pressure, which is much higher than carbon dioxide and lean gas. Therefore, nitrogen’s high minimum miscibility pressure is one of its biggest disadvantages. In reservoirs with high pressure, nitrogen is a good gas to be injected. When carbon dioxide is used in injection gas projects, the availability of carbon dioxide is not enough to complete the project; therefore, some additional gases can be mixed with the available amount of carbon dioxide. In the first part of this project, different mixture of carbon dioxide and lean gas with different percentages are simulated using Schlumberger Eclipse 300 to measure the minimum miscibility pressure values. Based on the result, mixing carbon dioxide to the lean gas lowers the minimum miscibility pressure of the 100% lean gas. A small amount of total gas is required to be injected to produce a good amount of oil compared to the other mixtures. The second part of the project is using nitrogen instead of lean gas to be mixed with carbon dioxide. The minimum miscibility pressure was simulated and compared to the result of the minimum miscibility pressure of the 100% carbon dioxide and 100% nitrogen. Unlike the 50%-50% carbon dioxide and lean gas mixture, the 50%-50% carbon dioxide and nitrogen mixture has a high minimum miscibility pressure, but not as high as the 100% nitrogen’s minimum miscibility pressure. Even though the amount needed of the total gas injected (nitrogen mixture) is not high compared to the other mixtures, the amount of oil produced is too low. This project also studies the effect of temperature on injection gas mixtures’ minimum miscibility pressure consisting of two or more gases. It is found that the higher the temperature is, the higher the minimum miscibility pressure is, in general. Temperature has more effect on the 100% lean gas’s and 100% nitrogen minimum miscibility pressure than the minimum miscibility pressure of the 100% carbon dioxide. Therefore, in carbon dioxide - lean gas and carbon dioxide - nitrogen mixtures, the higher the amount of lean gas or nitrogen in the injected gas mixture results in a higher effect of the reservoir temperature on the minimum miscibility pressure.



Minimum miscibility pressure, Gas injection, EOR