Due to its superior properties in releasing hydrocarbons while reducing carbon foot-print, CO2 injection has been considered a promising technique for Enhanced Recovery in tight/shale reservoirs. However, although it has the potential to increase reserves, CO2 does not necessarily result in faster production. In this work, numerical modeling is used to demonstrate that a combination of N2 and CO2 as the injection gas mixture provides superior results in terms of both production performance and CO2 sequestration. The competing effects of Knudsen and molecular diffusion are incorporated by using the Maxwell-Stefan equations and the Dusty-Gas approach. The total flux of gas considers contribution of viscous flow, molecular and Knudsen diffusion, adsorption/desorption and surface diffusion. The 1D model is solved using the Method of Lines, and validated against counter-diffusion experiments available in the literature. Once validated, the model is used to perform numerical experiments demonstrating the effects of transport mechanisms during injection of N2 and CO2 mixtures. We evaluate average pressure, breakthrough times and chromatographic separation effects in the presence of a wider range of components (C1, C2, C3 and C4+) in flow through adsorbing porous media.
During validation, it is demonstrated that the Maxwell-Stefan equations are able to properly model friction between molecules, which is critical in evaluating displacement front advancement during gas injection. This demonstrates the superior properties of the Maxwell-Stefan equations, when compared to classical Fick’s law, which is commonly used in upstream modeling.
Due to adsorption and diffusion effects, the mobility of N2 and CO2 molecules in the medium is significantly different. It is observed that N2 moves quickly, since it does not interact strongly with the pore walls. As a result, breakthrough times are short. Behind the displacement front, N2 composition is high, and hydrocarbons are released due to partial pressure reduction. CO2, on the other hand, is readily adsorbed by the organic sites. This results in a self-sharpening behavior of the displacement front. Whenever a molecule of CO2 advances ahead of the concentration shock, it is quickly adsorbed, keeping a distinct front, in a piston-like fashion. Therefore, breakthrough times for CO2 molecules are considerably longer. Competitive adsorption of CO2 results in release of heavier hydrocarbon fractions, which forms concentration banks that are transported through the medium by the fast flowing N2.
The use of Maxwell-Stefan equations for evaluating chromatographic separation provides a well-founded approach to track frontal velocities and properly predict heavier hydrocarbon fractions production. It is demonstrated that a combination of N2 and CO2 injection in tight reservoirs helps in enhancing production while retaining injected CO2.