Impact of Water Film Thickness on Kinetic Rate of Mixed Hydrate Formation During Injection of CO 2 into CH 4 Hydrate Khuram Baig, Bjørn Kvamme, Tatiana Kuznetsova, and Jordan Bauman Dept. of Physics and Technology, University of Bergen, NO-5020 Bergen, Norway DOI 10.1002/aic.14913 Published online July 14, 2015 in Wiley Online Library (wileyonlinelibrary.com) In this work, nonequilibrium thermodynamics and phase field theory (PFT) has been applied to study the kinetics of phase transitions associated with CO 2 injection into systems containing CH 4 hydrate, free CH 4 gas, and varying amounts of liquid water. The CH 4 hydrate was converted into either pure CO 2 or mixed CO 2 ACH 4 hydrate to investi- gate the impact of two primary mechanisms governing the relevant phase transitions: solid-state mass transport through hydrate and heat transfer away from the newly formed CO 2 hydrate. Experimentally proven dependence of kinetic con- version rate on the amount of available free pore water was investigated and successfully reproduced in our model sys- tems. It was found that rate of conversion was directly proportional to the amount of liquid water initially surrounding the hydrate. When all of the liquid has been converted into either CO 2 or mixed CO 2 ACH 4 hydrate, a much slower solid-state mass transport becomes the dominant mechanism. V C 2015 American Institute of Chemical Engineers AIChE J, 61: 3944–3957, 2015 Keywords: natural gas hydrates, nonequilibrium thermodynamics, mixed hydrate, phase field theory Introduction Natural gas hydrates are ice-like crystalline compounds in which water serve as a host for different small nonpolar, or slightly polar guest molecules. Natural gas hydrates occur both onshore in permafrost regions and continental margin sediments consisting mainly of fine-grained clay minerals and organic fragments. The majority of natural gas hydrate depos- its existing around the world are found in fine-grained sedi- ments characterized by low-hydrate saturation, which can be explained by very small pore size and low permeability of clay-rich sediments that hinder mobility of both water and gas, components essential for formation of hydrate. Gas hydrates mostly occur in sand units and are largely absent from mud sequences. 1–3 Permafrost gas hydrate occurrences have been identified in sand-rich deposits in on-shore and near-shore environments, with gas hydrate deposits in Alaska and Canada being the typi- cal examples. Analysis of well log data 4 and pore water geo- chemistry 5 indicates a very level of hydrate saturation at the Mount Elbert site (about 60–75%), which can be attributed to pre-existing free gas and presence of high conductivity faults. 6 Natural gas hydrates are dominated by biogenic sources of methane. Indeed, according to Michael et al., 7 as much as 99% of all gas hydrate deposits may be of biogenic origin. In con- trast to natural gas of thermogenic origin, biogenic methane is very pure and contains only tiny amounts of heavier hydrocar- bons. In this work, we, therefore, focus on hydrates of methane and carbon dioxide and their mixtures, known to form hydrates of structure I. 8,9 Hydrate formation from methane and water can follow a number of different pathways. The most commonly discussed route is hydrate formation on the interface between the hydrate-former phase and water. 8 Numerous experimental data are available in literature (see, for instance, Koh and Sloan 10 for a compilation), though it should be pointed that the bulk of data 10 comes from dissociation point measure- ments, where hydrate is kept at a controlled pressure and slowly increasing temperature. But hydrate can also form from hydrate formers dissolved in water 11,12 and (theoretically) from water dissolved in the hydrate-former phase, 13 although more comprehensive analy- sis is still needed to decide whether the latter route is realistic under mass- and heat-transport limitations. Earlier theoretical studies 14–21 indicate that the critical hydrate nucleus will be about 2.5–3 nm in size, which would require around a hundred of water molecules to come together within a very dilute mix- ture in natural gas or carbon dioxide. Even then, the excess heat of hydrate formation will be rather difficult to dispose of, since both natural gas and carbon dioxide are thermal insulators. Mineral surfaces 13,22,23 will serve as adsorption sites for water and hydrate formers, which can give rise to at least three different formation scenarios even in the simplest case: (1) water and hydrate former, both from adsorbed phase, form hydrate, (2) adsorbed water and hydrate fluid forms hydrate, and (3) adsorbed hydrate former and water from fluid phase forms hydrate. Considering all the possible phases relevant for hydrate formation, hydrate dissociation, and hydrate reforma- tion (CH 4 hydrate over to CO 2 hydrate or mixed CO 2 /CH 4 hydrate), it will be impossible to satisfy the Gibbs phase rule Correspondence concerning this article should be addressed to B. Kvamme at Bjorn.Kvamme@ift.uib.no V C 2015 American Institute of Chemical Engineers 3944 AIChE Journal November 2015 Vol. 61, No. 11