Nonequilibrium hydrate growth on an expanding gas-liquid interface: insights from Xenon-water experiments and phase-field modeling

Growth of hydrates at the gas-liquid interface is a ubiquitous process within hydrate-bearing sediments. An improved understanding of interfacial hydrate growth in porous media can help elucidate many outstanding questions such as the origin of different hydrate growth habits within the pore space and how free gas migrates through the hydrate stability zone. Interfacial hydrate growth is also intrinsically a thermodynamically nonequilibrium phenomenon, where the coexistence of hydrate, gas and liquid phases can persist under conditions not prescribed by the triple-point. This makes modeling efforts in an equilibrium framework challenging.

In this work, we develop a continuum-scale phase-field model to study gas-liquid-hydrate systems far from thermodynamic equilibrium [4]. We design Gibbs free energy functional for methane (CH₄)-water (H₂O) and xenon (Xe)- H₂O mixtures that recover the isobaric temperature-composition phase diagram predicted from CSMGem [2]. The proposed free energy is incorporated into a phase-field model to study the dynamics of hydrate formation on a gas-liquid interface. We elucidate the similarities and differences in how hydrate grows on an interface in both systems, and argue for the soundness of using Xe-H₂O as an analog to the CH₄-H₂O system.

We then apply our model to study the volumetric expansion of a hydrate-crusted gas capsule induced by depressurization from the ambient liquid. Understanding of this problem has implications in the formation mechanisms of hydrate-controlled craters recently reported in arctic seafloors [1] and the evolution dynamics of hydrate-crusted gas bubbles/fingers in sediments [3, 5]. We first present a high-pressure microfluidic experiment to study the depressurization-controlled expansion of a pocket of Xe gas in a water-filled Hele-Shaw cell. The evolution of the pocket is controlled by three processes: (1) volumetric expansion of the gas; (2) rupturing of existing hydrate films on the gas-liquid interface; and (3) spontaneous new hydrate formation along interfaces. These processes result in gas fingering leading to a complex labyrinth pattern. We present high-resolution numerical simulations of our model, which illustrate the emergence of complex crustal fingering patterns as a result of gas expansion dynamics modulated by hydrate growth at the interface, as observed in the experiments.

_{[1] K. Andreassen, A. Hubbard, M. Winsborrow, H. Patton, S. Vadakkepuliyambatta, A. Plaza-Faverola, E. Gudlaugsson, P. Serov, A. Deryabin, R. Mattingsdal, J. Mienert, S. Bünz, Science, 356, 6341 (2017);}

_{[2] A. Ballard, E. Sloan, J. Supramol. Chem. 2, 385-392 (2003);}

_{[3] L. Chen, N. Li, C-Y Sun, G-J Chen, C. A. Koh, B-J Sun, Fuel, 197, 298 (2018);}

_{[4] X. Fu, L. Cueto-Felgueroso, and R. Juanes, Phy. Rev. Lett., 120, 144501 (2018) .}

_{[5] D. Myer, P. Flemings, D. DiCarlo, K. You, S. Philips, T. Kneafsey, J. Geophys. Res., in press (2018)}