Enhanced Recovery by Injection of Nitrogen and Carbon Dioxide Mixtures in Tight Reservoirs: A Combination of Partial Pressure Reduction and Competitive Adsorption

ABSTRACT

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 CO₂advances 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.

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