The synergetic effect of a range of different solvents on the kinetic hydrate inhibitor (KHI) performance of poly­(N-vinylcaprolactam) (PVCap) has been investigated. The equipment used was a high-pressure (76 bar) rocking cell apparatus using slow constant cooling (approximately 1 °C/h from 20.5 °C) and a synthetic natural gas mixture forming structure II hydrate. The synergetic effect was investigated by adding 5000 ppm of a range of alcohols, glycol ethers, and ketones to a solution of 2500 ppm of PVCap (M w = 10 000 g/mol). For many of the additives, the ranking of the synergetic effect can be explained with reference to the size, shape, and hydrophobicity of the main alkyl group (“tail”) in the molecule as well as the presence of a glycol ether group. Among all of the solvents investigated, the best synergetic effect was achieved by 4-methyl-1-pentanol. When 5000 ppm of 4-methyl-1-pentanol was added to 2500 ppm of PVCap, no hydrate formation occurred down to the minimum test temperature of 3 °C (subcooling at ca. 16.3 °C) in 15 parallel experiments compared to 10.4 °C for pure PVCap. Predictions for improved glycol ether synergists are given.

Most polymers used in commercial kinetic hydrate inhibitor (KHI) formulations show thermoresponsive behavior in aqueous solution. This means that they exhibit a cloud point (or lower critical solution temperature) which is often low and not far above the equilibrium temperature for gas hydrate formation. For example, poly(N-vinyl caprolactam) has a cloud point of about 30−40 °C and poly(N-isopropylmethacrylamide) has a cloud point of about 35−45 °C depending on the molecular weight and method of polymerization. This report is divided into two parts. First, we review previous KHI studies and show that low cloud point is a useful factor, but not the most critical factor, to be considered within a specific class of polymers in designing a high-performance KHI. This statement is supported in the second part of this report, which is an experimental KHI study. In this study, we present results of KHI tests for a natural gas/deionized water structure II gas hydrate-forming system using low cloud point polymers with a variety of different shapes, molecular weights, functional groups, and sizes of pendant hydrophobic groups. We conclude that the low cloud point, near the hydrate formation temperature, appears to be useful for high KHI efficacy of a polymer but only if certain criteria are met. These include low molecular weight, pendant hydrophobic groups of an optimal size close to the polymer backbone, and the correct hydrophilic functional groups. Possible theories as to why low cloud point for a KHI polymer can be beneficial are discussed, as well as some guidelines for the KHI polymer design.

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The performance of injected kinetic hydrate inhibitor (KHI) polymer solutions can be boosted considerably by judicious choice of the polymer solvent system. We report the excellent KHI synergism of the low-foaming acetylenic diol gemini surfactant 2,4,7,9-tetramethyl-5-decyne-4,7-diol (TMDD) with poly(N-vinyl caprolactam), N-vinyl caprolactam:N-vinyl pyrrolidone copolymer, and poly(N-isopropylmethacrylamide). Highpressure rocking cell tests, using the slow constant cooling method or the isothermal method, were carried out with a natural gas mixture giving structure II hydrates as the preferred thermodynamically stable phase. Poly(oxyethylene) derivatives of TMDD, which are far more water-soluble than TMDD, gave significantly lower synergetic KHI performance with the same polymers. It is conjectured that the low aqueous solubility of TMDD (1700 ppm at 20 °C) and its two isobutyl groups are key features contributing to the synergism. However, when decane was added to the system as a model liquid hydrocarbon phase, the synergetic performance decreases, probably due to partitioning of TMDD to the hydrocarbon phase. This highlights the need to choose synergist systems which are retained in the aqueous phase for optimal performance when condensate or oil is present in the produced fluids. Optimizing the structure and aqueous solubility of the synergist (solvent or otherwise) can be seen as complementary to the known principle of optimizing the structure and solubility of the KHI polymer.

Poly(N-vinylcaprolactam) (PVCap) and related copolymers have been used as kinetic hydrate inhibitors (KHIs) for over 25 years to combat gas hydrate formation in oil and gas field production flow lines. The caprolactam groups in this polymer class have been shown previously to have a particularly strong interaction with hydrate surfaces, inhibiting crystal growth but probably also gas hydrate nucleation. We report here a study on an alternate class of copolymers with pendant caprolactam groups from the 2-methacrylamido-caprolactam (2-MACap) monomer. KHI experiments were carried out in high pressure steel rocking cells using a structure-II-forming natural gas mixture. The KHI performance of some of these copolymers exceeded that of PVCap of similar molecular weight, with further performance enhancement provided by solvent synergists.