Gas hydrate induction times and formation rates in seawater- saturated sand/clay packs are catalyzed in the laboratory by a few ppm of biosurfactants. To determine if biosurfactants are indigenous to sediments near Gulf of Mexico gas-hydrates, such sediment samples were analyzed for catalytic effects on hydrate formation. This paper addresses the relationships of biosurfactants, sand, and clays with gas hydrate formation rates, induction times, and form (dispersed, nodular, stratified, massive), relating these factors to gas hydrate occurrence in GOM sediments.
While searching for a surfactant to retard gas hydrate formation, Bishnoi first reported a catalytic effect of some surfactants on gas hydrate formation (1). Zhong and Rogers (2) first reported not only the catalytic effect of some anionic surfactants on hydrate formation but the additional facilitation of self-packing hydrates on metal surfaces as they form and complete reaction of interstitial water. These properties have been incorporated into a conceptual design of a large gas- storage facility (3), and DOE is currently sponsoring a scale- up of the design for industrial gas storage. Vysniauskas and Bishnoi developed Equation 1 with data taken from a chilled, hydrocarbon-gas and water, stirred system. The vigorous stirring continually renewed the gas- water interfacial boundary to prevent hydrate barrier films from developing (4).
Significantly, Equation (1) was found to also apply to a non-stirred, anionic-surfactant water solution in which the hydrates migrate and adsorb on the surface of a metal at the water-gas interface (2). The migration of forming hydrate crystals clears the water surface of any hydrate film and gives about the same rate of formation as when vigorously stirred. Whether by mechanical stirring or by crystal migration, the reacting surface is kept clear of a barrier film; mass transfer is not a rate-limiting factor in either case.
The results of the synthetic surfactants on the gas storage process suggested a study of the effect of biosurfactants on hydrate formation in sea floors. There, water-borne microbes, in order to feed on the organic matter in ocean-floor sediments, would be expected to produce biosurfactants to make that insoluble matter accessible. Consequently, those biosurfactants could catalyze gas hydrate formation. Results of our investigation address the possibility.
In the gas storage process, the surfactant's catalytic effect on gas hydrate formation is caused by the formation of surfactant micelles. The sodium dodecyl sulfate (SDS) anionic surfactant used in the gas storage process forms a micelle. The 12-carbon alkyl groups form a spherical micelle that solubilizes hydrocarbon gases (5). Water associates around the periphery of the micelle in close proximity to the solubilized gas. Thus, micelles act as nucleation sites for hydrate crystals. The threshold concentration of surfactant to form micelles occurs at the critical micellar concentration (CMC), and this CMC is traditionally measured by surface tension at ambient conditions.