Controls of CO₂–N₂ gas flood ratios on enhanced shale gas recovery and ultimate CO₂ sequestration

Primary production of less than 25% of original gas in place (OGIP) may be elevated by enhanced shale gas recovery (ESGR) using either pure CO₂ or N₂ as injected stimulants. Alternatively, injecting mixtures of CO₂ and N₂ may potentially optimize recovery of natural gas and beneficially sequester CO_2. We develop a dual-porosity, dual-permeability finite element (FEM) model coupled with multi-component gas flow and sorption behavior to 1) explore the evolution of sorption-induced strain resulting from competitive adsorption and its influence on the matrix and fracture permeability; 2) define cumulative production of CH₄ and 3) evaluate the amount of CO₂ sequestered in the reservoir; Results show that pure-CO₂ injection can increase shale gas recovery by ∼20%. Conversely, pure-N₂ injection can increase shale gas recovery by ∼80%. Injecting mixtures of CO₂ and N₂ can increase shale gas recovery between these end-member magnitudes of ∼20%–∼80% depending on the gas composition. We show that a higher proportion of CO₂ in the injected CO₂–N₂ mixture will result in the decreased recovery of shale gas. However, at the same injection pressure, injecting CO₂–N₂ mixtures with a higher proportion of CO₂ does not always result in more CO₂ sequestered in the reservoir. Indeed, when the CO₂ injection ratio is >70%, as explored in this study, increasing the CO₂ injection ratio will result in less CO₂ sequestered. This is because, as the CO₂–N₂ gas ratio increases, shale gas recovery decreases and results in more CH₄ left in the reservoir to compete with CO₂ for sorption sites and finally resulting in less CO₂ sequestered.