Hydrate phase transition kinetics from Phase Field Theory with implicit hydrodynamics and heat transport

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Most hydrate that forms or dissociates are in situations of constant non-equilibrium. This is due to the boundary conditions and Gibbs Phase rule. At a minimum this leaves a hydrate with two adsorbed phases in addition to hydrate and fluids. One adsorbed phase is governed by the mineral surfaces and the other by the hydrate surface. With pressure and temperature defined by local conditions, hydrate formation will never be able to reach any state of equilibrium. The kinetics of hydrate formation and dissociation are a complex function of competing phase transitions. This requires kinetic theories that include minimization of free energy under constraints of mass and energy transport. Since phase transitions also change density, further constraints are given by fluid dynamics. In this work, we describe a new approach for non-equilibrium theory of hydrates together with a Phase Field Theory for simulation of phase transition kinetics. We choose a three component system of water, methane and carbon dioxide for illustration. Conversion of methane hydrate into carbon dioxide hydrate is a win–win situation of energy production combined with safe long term storage of carbon dioxide. Carbon dioxide is able to induce and proceed with a solid-state exchange, but is slow due to mass transport limitations. A faster process is the formation of new hydrate from injected carbon dioxide and residual pore water. This formation releases substantial heat. This assists in dissociating in situ methane hydrate, making the conversion progress substantially faster, because heat transport is very rapid in these systems. But conversion of liquid water into carbon dioxide hydrate, in the vicinity of the hydrate core will increase temperatures to some portions of the surface. The dissociating regions of the methane hydrate core will show a local decrease in temperature, due to extraction of heat for methane hydrate dissociation from surroundings. Another reason for heat transport implementation is that regions of the system that contains non-polar gas phase will have low heat conductivity and low heat convection. At this stage we apply a simplified heat transport model in which “lumped” efficient heat conductivity is used. We illustrate the theory on the conversion of methane hydrate to mix methane–carbon dioxide hydrate using three initial hydrate sizes: 150Å×150Å, 500Å×500Å and 5000Å×5000Å. The hydrate cores used are spherical because it makes it easier to illustrate the impact of curvature. Symmetrical aspects simplifies the dependency to a two dimensional problem – although there are no such limitations in theory. The mineral surfaces are considered to be water wetting in these examples. It was observed that the smaller sizes convert to a more unstable mix hydrate for some periods of the simulation time, during which there were significant losses of the initial methane hydrate core. These instabilities are caused by local under saturated fluid phases around the hydrate core. Eventually a steady state progress was observed. The largest size system appeared to reach a steady state situation comparable faster than the two smaller systems.