No abstract

When clathrate hydrate particles come into contact, they tend to agglomerate together, resulting from capillary liquid bridges that form between the particles. The strength of these capillary bridges is a function of several physical variables, many of which have been directly obtained for clathrate hydrate systems. A less thoroughly explored variable is the contact angle of water on the clathrate hydrate surface. Analyses have shown that the contact angle of a surface can have a strong effect on the cohesion behavior of clathrate hydrates; however, direct measurements of water on a hydrate surface are not prevalent. To better understand this important parameter, a method was developed to directly measure the contact angle of a water droplet deposited onto the surface of a cyclopentane hydrate particle. Using a novel method, the contact angles of water droplets on cyclopentane hydrate surfaces were directly measured. In combination of these new measurements with an updated interfacial tension value for a cyclopentane and water system, the immersion depth of the capillary bridge on a clathrate hydrate particle was estimated. Different anti-agglomerants (AAs) were tested for both contact angle and cohesion force, which showed that the micromechanical force measurement apparatus was capable of ranking the anti-agglomeration tendency of clathrate hydrate particles in the presence of AAs. These tests revealed a correlation between low cohesion force and higher contact angle, corresponding to a hydrophobic surface. Morphological changes were also observed in the hydrate particles upon the addition of AAs, and two main types of morphological changes, water extrusion and hydrate sloughing, were identified.

In order to investigate the mechanism of gas hydrate deposition and agglomeration in gas dominated flowlines, a high-pressure micromechanical force (MMF) apparatus was applied to directly measure CH/CH hydrate adhesion/cohesion forces under low temperature and high pressure conditions. A CH/CH gas mixture was used as the hydrate former. Adhesion forces between hydrate particles and carbon steel (CS) surfaces were measured, and the effects of corrosion on adhesion forces were analyzed. The influences of NaCl concentration on the cohesion force between CH/CH hydrate particles were also studied for gas-dominated systems. It was observed that there was no measurable adhesion force for pristine (no corrosion) and corroded surfaces, when there was no condensed water or water droplet on these surfaces. With water on the surface (the estimated water amount was around 1.7 μg mm), a hydrate film growth process was observed during the measurement. CS samples were soaked in NaCl solution to obtain different extents of corrosion on surfaces, and adhesion measurements were performed on both pristine and corroded samples. The adhesion force was found to increase with increasing soak times in 5 wt% NaCl (resulting in more visual corrosion) by up to 500%. For the effect of salinity on cohesion forces, it was found that the presence of NaCl decreased the cohesion force between hydrate particles, and a possible explanation of this phenomenon was given based on the capillary liquid bridge model.