Microscale Detection of Hydrate Blockage Onset in High-Pressure Gas-Water Systems

A high-pressure stirred autoclave cell equipped with a focused beam reflectance measurement (FBRM) probe and a particle video microscope (PVM) was used to study hydrate formation and plugging in gas-water systems as a function of shear rate. These probes allowed estimates of the mean hydrate particle size and number of hydrate particles to be correlated with the hydrate volume fraction and the hydrate slurry's resistance-to-flow. Before reaching the hydrate volume fraction phi(transition) at which the hydrate slurry first exhibits a measurable increase in resistance-to-flow at approximate to (16 +/- 2) vol %, clear changes in the measured number and size of the hydrate particles were observed. Initially, hydrate particles within the FBRM probe's field of view decreased in size and increased in number until a maximum was reached at concentrations of 2-9 vol % (increasing with shear rate). However, with continued hydrate growth, the number of particles within the FBRM probe's field of view unexpectedly decreased and eventually plateaued at hydrate volume fractions approaching phi(transition). We propose that, following nucleation, particle numbers in the bulk liquid increased until the average particle size and separation became comparable, after which the particles began to interact and, driven by buoyancy forces, formed a moving particle bed located near the gas-water interface. Such findings could facilitate development of a scaling relationship for predicting the onset of hydrate plugging behavior in water-continuous systems.