Development of a Crystal Growth Inhibition Based Method for the Evaluation of Kinetic Hydrate Inhibitors

Over the past decade, low dosage hydrate inhibitors (LDHIs) - which include anti-agglomerants (AAs) and kinetic hydrate inhibitors (KHIs) - have seen increasing use as a cost effective technology for gas hydrate control in the oil and gas industry, offering significant CAPEX/OPEX advantages when compared with traditional thermodynamic inhibitors (e.g. methanol, glycols). While AAs prevent agglomeration/plugging, KHIs are primarily understood to be nucleation inhibitors, inducing an extended ‘induction time’ at a specific subcooled condition before hydrate nucleation can proceed to growth. The best known KHIs are water soluble poly-n-vinylamides such as poly-n-vinylcaprolactam (PVCap), poly-n-vinylpyrrolidone (PVP) and related polymers. As KHIs are seen primarily as ‘nucleation delayers’, evaluation is typically undertaken through measurement of induction times as a function of various parameters at the conditions of interest. However, as nucleation is stochastic by nature, obtaining repeatable/transferrable data is often highly problematic and time-consuming, making robust evaluation difficult. Here, we demonstrate that less well investigated aspect of KHI polymers - their ability to inhibit crystal growth - is considerably simpler to quantify than nucleation inhibition. Beginning at low aqueous concentrations (e.g. > 0.1 mass% aqueous), PVCap and other KHI polymers induce a number of highly repeatable, well-defined hydrate crystal growth inhibition (CGI) regions as a function of subcooling. Discernible by step changes in relative growth rates − commonly by an order of magnitude − CGI regions range from complete inhibition (even hydrate dissociation), through severely to moderately reduced growth rates, ultimately to final rapid/catastrophic growth as subcooling increases. Closely related to induction time data, CGI regions are readily measurable using conventional hydrate laboratory equipment, with subcooling extents − which it is speculated are polymer crystal surface absorption related phenomena − providing a means to assess KHIs more rapidly and reliably, while giving an increased confidence in performance under worst case scenario (hydrate present) field conditions.