Nitrogen-Driven Chromatographic Separation During Gas Injection Into Hydrate-Bearing Sediments

Abstract

Hydrates are solid phases composed of water cages enclosing gas molecules that may host large quantities of recoverable natural gas and may serve to sequester CO2 on geological time scales. Most hydrate studies focus on hydrates containing a single gas component, such as CH4 or CO2. Yet, there are several settings in which multiple components form hydrate mixtures, or mixed hydrates, including a subsurface injection technique that claims to simultaneously recover CH4 and sequester CO2 called “guest molecule exchange.” Here, we combine multicomponent phase behavior for hydrate-forming systems with a multiphase fluid flow simulator to understand the evolution of hydrate and nonhydrate phases during subsurface injection. We simulate various scenarios for systems composed of H2O, CH4, CO2, and N2. Our study probes the impact of injection composition, initial reservoir composition, and transport of each component through the model domain. We observe chromatographic separation from the combined effect of compositional partitioning in each phase, variable flow speed of each phase, and compositional dependence of phase stabilities. We show that N2 drives chromatographic separation to create a CH4-free zone and a CO2-free zone that are connected by a continuous N2-dominated vapor phase. While our results are theoretical and should be validated experimentally, they imply that guest molecule exchange acts as two sequential processes rather than as a simultaneous process. Furthermore, they show that injections into reservoirs with and without free water have vastly different behaviors, which has implications for the interpretation of the guest molecule exchange field test and various laboratory studies.