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Molecular mechanisms of methane hydrate dissociation and inhibition

University of British Columbia

Gas hydrates are crystalline compounds with cage-like structures formed by hydrogen-bonded water molecules hosting guest molecules such as light hydrocarbons and CO₂. They are known to: • represent a potential reserve of natural gas embedded in seabed and permafrost sediments • pose a flow assurance challenge to the oil and gas industry Molecular dynamics simulations are employed to study the processes of gas hydrate decomposition and inhibition. To mimic the porous environment of the real gas hydrate reservoirs, hydroxylated silica surfaces are included in the simulations and placed in contact with hydrate and water. Water molecules wet the silica surfaces and form a meniscus, confirming the hydrophilic properties of the hydroxylated silica surface. It is found that the silica surface alters the characteristics of the confined water up to ~6 Å away from the surface. The decomposition of methane hydrate in the presence of silica surfaces, 34 to 40 Å apart, follows a concerted behavior where layers of hydrate cages at the curved dissociation front collapse almost simultaneously. The rate of hydrate dissociation in contact with a silica surface is faster compared to that of a hydrate phase just in contact with bulk water. Additionally, the decomposition leads to the formation of methane-rich regions (nano-bubbles) in the liquid water phase. In more realistic simulations, gas reservoirs are added to the simulations to determine whether the formation of nano-bubbles is a general feature of the hydrate decomposition process. It is found that the nano-bubbles can form under simulation conditions where the dissociation rate is faster than the diffusion rate, thus generating dissolved methane mole fractions of greater than 0.044 that would lead to bubble nucleation. Finally, the binding mechanism of the alpha-helical 37 amino acid residue winter flounder antifreeze protein, which is a candidate as a kinetic hydrate inhibitor to methane hydrate, is determined to be the result of cooperative anchoring of the pendant methyl groups of the threonine and two alanine residues, four and seven places further down in the protein sequence, to the empty half cages at the hydrate surface.

Text

2015-04-07

Attribution-NonCommercial-NoDerivs 2.5 Canada

http://hdl.handle.net/2429/52640

Chemical and Biological Engineering

University of British Columbia

UBCV

http://creativecommons.org/licenses/by-nc-nd/2.5/ca/