Reduction of Hydrate Film Growth Rate and Cohesive Force By Surface Active Components in Crude Oil

During the production of offshore oil and gas, the cooling of hydrocarbons toward seafloor temperatures enables the formation of gas hydrates, which may restrict fluid flow and ultimately block the flowline. As hydrates aggregate and begin to form a blockage, a hydrate film may grow between individual particles or between hydrate particles and the pipeline wall, respectively resulting in a higher slurry viscosity or a reduced hydraulic diameter. The hydrate cohesive force and hydrate film growth rate are fundamentally controlled by the properties of the interface. While it has been shown previously that hydrate cohesive force may be affected by naturally-occurring surfactants in the crude oil, there is limited data available to inform whether hydrate film growth rate may also be affected by these components. In this study, we have used a micromechanical force (MMF) apparatus to quantify the film growth rate of cyclopentane hydrate at a moderate subcooling. Naturally-occurring surface active species were obtained from two Australian crude oils by solvent extraction, resulting in the separation of three oil fractions: asphaltenes, binding resins, and free resins. Each oil fraction was then added to a chemically inert hydrocarbon phase, and was injected in the MMF experimental cell to quantify its impact on hydrate film growth rate. At mass fractions below 500 ppm, the hydrate film growth rate was reduced by at least one order of magnitude by asphaltenes; both resin classes were less effective at reducing hydrate film growth rate. Despite variations in the film growth measurements, cohesive force trials with the MMF revealed that each of the three fractions was able to reduce the hydrate cohesive force by two orders of magnitude. The presence of each fraction also caused an increase in the wetting angle of the water droplet on the hydrate particle surface, which suggested that these naturally-occurring components may adsorb to the hydrate particle surface.