Effect of oil composition on minimum miscibility pressures using mixtures of CO2, N2 and lean gas

In gas flooding projects where miscibility is the desired displacement mechanism, carbon dioxide (CO2) is the best performing gas due to its ability to be miscible with oil at low pressures compared to other gases. This minimum miscibility pressure (MMP) is of high importance because it represents the first limiting factor in the feasibility of a miscible flooding project. However, when it comes to other limiting factors such as availability and cost, lean gas and nitrogen (N2) sometimes offer a larger degree of freedom compared to CO2. Mixing CO2 with lean gas or with nitrogen can allow the design of an injection gas that will satisfy the availability and cost constraints while also having a reasonable and reachable miscibility pressure. The objectives of this paper are to investigate the effect of adding nitrogen or lean gas to CO2 on MMP, and to study the effect of oil composition in the MMP. The MMP estimation method used in this study is the numerical simulation method, so the first step is to validate it. Comparing the results from actual experiments and correlations with the results from the numerical simulation, it is shown that the numerical simulation method successfully captures the miscibility mechanism during oil displacement and provides trustable MMP results. The effect of oil composition is observed by varying the molar fraction of each of the grouped components which are the light weight components, the intermediate weight components, and the heavy weight components, as well as the heptane plus molecular weight. It is observed that when injecting pure CO2, the main factor is the molecular weight, followed by the intermediate weight component molar fraction. When mixing CO2 with lean gas or nitrogen, the ratio of light to heavy component fractions becomes the third parameter affecting the MMP. In this paper, the effectiveness of nitrogen or lean gas is expressed as the additional MMP per percent of solvent added to the pure CO2 injection gas. It is also observed that lean gas is at least twice as effective as nitrogen in terms of effectiveness. This effectiveness is also used to determine the maximum solvent fraction in the injection gas in order to satisfy a maximum allowable pressure. It is shown that this number can vary greatly depending on the maximum pressure constrain.