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

Abstract

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 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 ∼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.