(29a) Salt Effect On the Rheology of Hydrate-Forming Emulsions

Clathrate hydrate formation within pipelines is a major problem in the oil and gas industry. It causes flow interruption and significant equipment damage. Hydrate, a crystalline compound, forms when the hydrogen-bonded water cage is stabilized by a light hydrocarbon molecule at low temperatures and high pressures, conditions predominantly found in subsea pipelines. An efficient and cost effective mitigation of plug formation requires a mechanical understanding of the process involved in hydrate formation. Since offshore drilling occurs under the seafloor, the salinity of the aqueous phase in the aquifers is an important factor to be considered for the formation of hydrate. Salt is a thermodynamic inhibitor of hydrate. We study the effect of salinity on a 40% (v/v) aqueous phase density-matched cyclopentane hydrate-forming emulsion through a combination of experimental techniques using a micro-differential scanning calorimeter (µ-DSC) and rheometer. Cyclopentane hydrate-brine system liquidus line is plotted using calorimetric data. Equilibrium temperature and thermodynamical conversion of water to hydrate particles in the emulsion are obtained from the hydrate-brine phase diagram. Rheological properties - viscosity critical time, viscosity evolution time and final viscosity - of the hydrate-forming emulsion are extracted from the rheological experiments. A correlation of the rheological properties of the hydrate-forming emulsion to the thermodynamic driving forces is obtained. Viscosity critical time and evolution time decrease with higher subcooling. Final viscosity dependence on the thermodynamical water to hydrate conversion shows that both aqueous phase and hydrate content are intrinsically important to the strength of formed network like structures. Maximum viscosity is attained when the thermodynamic water to hydrate conversion is in the 70-80% range. The hydrate-forming emulsion final structure exhibits shear thinning effects, which are fitted to the Ostwald-de Waele model.