Phase behavior and the interaction of multiple gas molecules in hydrate-dominated geological flow processes
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Hydrate is a non-stoichiometric, ice-like solid compound of water and gas molecules that forms at low temperatures and high pressures. The stability of a particular hydrate is affected by the molecular composition of the environment in which it forms. For example, salt causes freezing point depression of hydrate much like it does for ice. In addition, a gas molecule, such as methane, that ordinarily forms hydrate at one pressure-temperature condition, may not form hydrate if the gas is mixed with another molecule, such as nitrogen, that requires increased pressure or decreased temperature to form hydrate. Here, I develop a modeling framework that incorporates the phase stability of gas mixtures to understand the coupling of equilibrium thermodynamics and fluid flow that governs hydrate-dominated geological flow processes. I first present a benchmark study that utilizes standard hydrate models to demonstrate the complex phase stability that occurs when salt and only methane are considered. The results show the impact that three-phase equilibrium, or the co-existence of a gas phase, a liquid water phase, and a hydrate phase, has on the evolution of hydrate systems. I then develop compositional phase diagrams for systems composed of water, methane, carbon dioxide, and nitrogen that elucidate how multiple hydrate-forming components interact to alter the composition of hydrate, completely de-stabilize hydrate, or create three-phase equilibrium conditions. I finally incorporate these compositional phase diagrams into a mathematical framework that describes multi-phase fluid flow that I use to simulate a subsurface injection strategy designed to simultaneously sequester carbon dioxide as hydrate and produce methane gas. The modeling framework illuminates the processes that govern the dynamic behavior of multiple hydrate-forming components. Simulations of subsurface injection demonstrate behaviors that support field and laboratory observations and clarify how composition impacts internal reservoir dynamics. The modeling framework developed here is general and flexible, so it can be modified to model additional components or to include additional physics. In particular, the modeling framework presented here is well-suited to simulate the buoyant ascent of thermogenic gas mixtures through marine sediments or the out-gassing of hydrate layers within the interior of icy planetary bodies like Enceladus.