A model that combines the Dusty-Gas approach and Darcy’s Law is used to investigate the dynamics of production enhancement by gas injection in unconventional reservoirs. A comparison between CH4, N2 and CO2 injection in both Huff-n-Puff and Flooding operation modes is performed.
The mechanism of production enhancement for each gas is different. CO2 can be injected to preferentially adsorb into the shale matrix, releasing hydrocarbons. In this case, the dominant mechanism is competitive adsorption. Due to stronger affinity with adsorption sites, CO2 injection would suggest high cumulative production. In spite of that, frontal displacement is very slow in this case, resulting in poorer short-term production when compared to N2 and CH4.
N2 injection induces the release of hydrocarbons solely by partial pressure reduction. Frontal velocities are fast, resulting in high short-term production. Yet, since N2 is deemed inert, it does not replace components retained in the adsorbed phase.
CH4 injection also prompts desorption of heavier hydrocarbons by partial pressure reduction. However, as heavier fractions are desorbed, CH4 molecules occupy the vacant sites. In this case, combined mechanisms of partial pressure reduction and uptake by the adsorption sites results in efficient release of heavier hydrocarbons.
In this work, we demonstrate the impact of the presence of heavier hydrocarbon fractions in modeling gas transport during enhanced gas recovery processes. Multicomponent gas flow affects average reservoir pressure, produced gas composition and natural gas liquids (NGLs) yields, which is relevant for development of wet-gas and dry-gas unconventional reservoirs. Moreover, we demonstrate that injection gas composition significantly influences transport behavior of chemical species through the porous medium, and we highlight the relevant transport mechanisms during enhanced gas recovery in tight reservoirs.