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 N-2 as injected stimulants. Alternatively, injecting mixtures of CO2 and N-2 may potentially optimize recovery of natural gas and beneficially sequester CO2. 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 similar to 20%. Conversely, pure-N-2 injection can increase shale gas recovery by similar to 80%. Injecting mixtures of CO2 and N-2 can increase shale gas recovery between these end-member magnitudes of similar to 20%-similar to 80% depending on the gas composition. We show that a higher proportion of CO2 in the injected CO2-N-2 mixture will result in the decreased recovery of shale gas. However, at the same injection pressure, injecting CO2-N-2 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-N-2 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.