Can kinetic hydrate inhibitors inhibit the growth of pre-formed gas hydrates?

Abstract

Low dosage kinetic hydrate inhibitors (KHIs) are one of the most promising techniques for inhibition at the earliest stages of hydrate formation. Although KHI polymers are generally considered to inhibit nucleation, they also show powerful properties in terms of crystal growth inhibition. Some KHIs may even induce anomalous gas hydrate dissociation inside the hydrate stability zone depending on the conditions. While KHIs have been proven for both inhibition of hydrate nucleation and initial crystal growth, their performance when it comes to pre-formed hydrates – i.e. where significant hydrate has already formed prior to KHI injection/contact with the KHI – has yet to be investigated. In this work, the ability of PVCap polymer to inhibit pre-formed hydrates has been evaluated for a methane-water system. Results are consistent with KHIs being able to inhibit further hydrate growth where a considerable volume of hydrate is already present in the system prior to chemical injection. Not only that, but that in the case of the PVCap system studied here, injection can, under certain conditions, directly result in the partial to complete anomalous dissociation of all pre-formed hydrates, even though conditions are inside the hydrate region. It is speculated this behaviour is related to the transition from metastable to stable hydrate structures/phases; PVCap not preventing the dissociation (of unstable phases) step of this process but inhibiting subsequent regrowth of stable equilibrium structures.