Experimental and theoretical developments in hydrate anti-agglomeration

Gas hydrates---ice-like inclusion compounds consisting of certain natural gas components and water---may form and pose major problems during oil and gas production. Low dosage hydrate inhibitors (LDHI) are desirable because of effectiveness at low concentration. Of the two LDHI types, anti-agglomerants have a clear advantage over kinetic inhibitors from effectiveness at high subcooling often encountered in offshore developments. Massive amounts of commonly-used organic thermodynamic inhibitors may be required in these extreme environments for proper hydrate inhibition.;Results are presented here from tetrahydrofuran (THF) hydrate systems with isooctane as model oil, comparing a quaternary ammonium salt and relatively unproven rhamnolipid biosurfactant in anti-agglomeration effectiveness. A ratio of 1/1 parts (by weight) water/THF has been used throughout the work, but variations in amount of model oil and surfactant are crucial to the study. For mixtures where agglomeration occurs, small amounts of methanol as cosurfactant is employed. The effect of salinity is also examined.;A screening apparatus is employed with temperature bath and thermocouples to determine crystallization and dissociation characteristics. Visual inspection of hydrate agglomeration states is conducted after cooling the mixtures; observations show clear distinction between what is deemed as hydrate plug' or proper anti-agglomeration---indicated by a stable slurry of hydrate. Temperatures employed are as low as -20°C, where subcoolings are roughly 20°C. An assessment of emulsion stability is also made.;Results show rhamnolipid and ammonium salt are effective down to 0.05 and 0.01 wt. %, respectively---an order of magnitude below the current literature limit of antiagglomerants. Agglomeration occurs---even in the presence of surfactant---when model oil amount is reduced to two parts (by weight) or less, but as little as 0.5 wt. % of methanol can be of benefit. To our knowledge, this is the first report of alcohol cosurfactants used in anti-agglomeration. Our results also show---in certain cases---non-monotonic behavior in the agglomeration state patterns of NaCl or MgCl2 brine. The effect of salt valency is also clear. It is also found highly stable emulsions are unnecessary to anti-agglomeration. Some informative theoretical analysis of steric repulsion has also been performed.