Gas hydrate anti-agglomerants (AAs) are a class of low-dosage
hydrate
inhibitor that are used to prevent plugging of gas hydrates in oil
and condensate upstream flow lines. Industrial AAs are mostly cationic
surfactants which are “hydrate-philic”, i.e., they are
designed to interact with and modify gas hydrate crystal growth. Tetrahydrofuran
(THF) hydrate crystal growth studies have been used for many years
to determine useful functional groups to incorporate into AA surfactants.
In particular, quaternary ammonium and phosphonium salts with optimized
alkyl groups show good THF crystal growth inhibition, which is a key
property for AAs. AAs are often screened and tested in model brines
containing sodium chloride despite the produced water containing various
divalent cations. Recent studies have shown that AAs performed better
when tested in brines containing both sodium and calcium ions rather
than just sodium ions. Here, we present THF hydrate crystal growth
studies on quaternary ammonium and phosphonium salts and other related
molecules including guanidinium salts and amine oxides. Tests were
carried out with a variety of cations including sodium, calcium, magnesium,
and lithium at identical pre-determined subcooling, in order to investigate
the effect of the ion size and charge density on the crystal growth
inhibition. We also investigate the effect of using the more polarizable
iodide ions compared to chloride ions. Our results show that crystal
growth inhibition in solutions with calcium ions is somewhat greater
than that with sodium ions, in agreement with past studies on the
effect of AA performance with mono- and divalent cations. However,
the variation does not seem to be primarily related to the charge
density and polarizing ability of the cations. This study therefore
provides evidence that AAs should be tested in brines containing all
the ions present in the produced water and not just sodium chloride
brine.