Phase field modeling of CH4hydrate conversion into CO2 hydrate in the presence of liquid CO2

Abstract

We present phase field simulations to estimate the conversion rate of CH₄ hydrate to CO₂ hydrate in the presence of liquid CO₂ under conditions typical for underwater gas hydrate reservoirs. In the computations, all model parameters are evaluated from physical properties taken from experiment or molecular dynamics simulations. It has been found that hydrate conversion is a diffusion controlled process, as after a short transient, the displacement of the conversion front scales with t^1/2. Assuming a diffusion coefficient of D_s = 1.1 × 10⁻¹¹ m² s⁻¹ in the hydrate phase, the predicted time dependent conversion rate is in reasonable agreement with results from magnetic resonance imaging experiments. This value of the diffusion coefficient is higher than expected for the bulk hydrate phase, probably due to liquid inclusions remaining in the porous sample used in the experiment.