Low dosage hydrate inhibitors (LDHIs) are a recent and alternative technology to thermodynamic inhibitors for preventing gas hydrates from plugging oil and gas production wells and pipelines. LDHIs are divided into two main categories, kinetic inhibitors (KHIs) and anti-agglomerants (AAs), both of which are successfully being used in field applications. This paper reviews the research and development of LDHIs with emphasis on the chemical structures that have been designed and tested. The mechanisms of both KHIs and AAs are also discussed.

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Tetraalkylammonium salts, particularly with several n-butyl or n-pentyl or iso-pentyl groups, have previously been shown to be excellent structure II (SII) gas and tetrahydrofuran (THF) hydrate crystal growth inhibitors. We have investigated the ability of quaternary ammonium salts with isoalkyl groups with 5 to 7 carbons atoms to inhibit the growth of tetrahydrofuran (THF) hydrate crystal. Two new quaternary salts were synthesized for the first time: tetra(iso-hexyl)ammonium bromide (TiHexAB) and tetra(iso-heptyl)ammonium bromide (TiHepAB). It was found that tetra(iso-pentyl)ammonium bromide (TiPeNB) gave poorer crystal growth inhibition than isomeric tetra(n-pentyl)ammonium bromide (TPAB) but similar performance to tetra(n-butyl)ammonium bromide (TBAB). However, TiHexAB gave better inhibition than any quaternary ammonium previously reported, including TPAB. TiHepAB was a poorer THF hydrate crystal growth inhibitor, similar in performance to isomeric tetra(n-hexyl)ammonium bromide. We believe the reason for these results is related to the optimal length of the n-alkyl/isoalkyl groups and the improved van der Waals interaction with open SII hydrate cages with the isoalkyl branching at the end of the chains, compared to a straight alkyl chain. The superior inhibition performance of TiHexAB was illustrated by testing its ability as a synergist for the well-known kinetic hydrate inhibitor (KHI) polyvinylcaprolactam (PVCap). In high pressure rocking cell tests using a SII-forming natural gas mixture, TiHexAB clearly outperformed TBAB, TPAB, and all the other quaternary ammonium salts tested. In addition, tetra(n-hexyl)ammonium bromide (THexAB) gave KHI synergism with PVCap superior to that of TBAB, even though TBAB was a better THF hydrate crystal growth inhibitor. We speculate that adsorption onto hydrate crystal surfaces may not be the only synergistic mechanism operating and that the more hydrophobic THexAB is perturbing the nucleation of hydrate more than the less hydrophobic TBAB. In addition, it was investigated whether replacing a carbon atom with an oxygen atom (ether linkage) in the alkyl chains of TPAB would affect the THF hydrate crystal growth inhibition. Thus, tetra(alkoxyethyl)ammonium bromides were prepared for the first time where the alkoxy group is ethoxy or methoxy. Both of these quaternary ammonium salts gave negligible THF hydrate crystal growth inhibition. ■ INTRODUCTIONIt has been known for about two decades that certain onium salts (quaternary ammonium and phosphonium salts) are capable of perturbing and inhibiting the growth of structure II (SII) clathrate hydrates. 1−4 Shell, the oil and energy company, was the first to discover this fact from tests with a study on SII tetrahydrofuran (THF) hydrates. The most active inhibitors of THF hydrate were found to have either n-butyl, n-pentyl, or iso-pentyl groups on the quaternary N or P atom (Figure 1). Shell's patents on this subject also cover sulphonium salts, which can have a maximum of three alkyl groups attached to the sulfur atom, althoug...

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Kinetic hydrate inhibitors (KHIs) have been studied, developed, and used in the oil and gas industries for more than two decades. The main active ingredients in commercial KHI formulations are water-soluble polymers. When dosed at low concentrations (0.1−2.0 wt % active chemical), they are able to retard the gas hydrate formation process and facilitate reliable oil and gas transportation. A considerable amount of research effort on KHI technologies has contributed to an abundance of KHI knowledge, applications, and tailor-made solutions. Whereas previous reviews have concentrated on the chemistry of KHIs, this review article has a particular emphasis on the experimental equipment, hydrate detection tools, and test methods commonly applied in KHI investigations. The underlying mechanisms of KHIs are still not fully understood. The major hypotheses proposed in the literature and supporting experimental and computational evidence are also reviewed.

Low dosage hydrate inhibitors (LDHI) offer a recently developed hydrate control technology that can be more cost‐effective than traditional practices, such as the use of thermodynamic inhibitors (e.g., methanol and glycols). One class of LDHI, called kinetic inhibitors, is already being successfully used in the field. This paper describes efforts to develop a new class of kinetic inhibitor that shows various improvements over existing commercial technology. The polymer chemistry of the inhibitors and experiments carried out in high pressure cells and wheel/loops is described.

Poly(N-vinyl azacyclooctanone) (PVACO) has been synthesized for the first time. Dependent upon the method of polymerization and polymer molecular weight, the cloud point of a 1.0 wt % solution in water can be varied between approximately 14 and 22°C. Using identical polymerization conditions for the four N-vinyl lactams with 5−8-membered rings, the polymer molecular weight decreases as the ring size increases. This is probably due to the relative steric effect of the monomers in the polymerization process. In high-pressure rocking cell experiments with a structure-II-forming hydrocarbon gas mixture, PVACO was shown to be a more powerful kinetic hydrate inhibitor (KHI) than the other 5−7-ring poly(N-vinyl lactam)s of similar molecular weight made using an otherwise identical method to PVACO. The synergistic effect of mono-nbutyl glycol ether with PVACO and the effect of the polymer molecular weight on KHI performance are also discussed. ■ INTRODUCTIONKinetic hydrate inhibitors (KHIs) are now a well-known technology for preventing gas hydrate plugs in upstream oilfield operations. 1−4 KHIs are water-soluble polymers, often with added synergists that improve their performance. KHIs delay the nucleation and usually also the crystal growth of gas hydrates. The nucleation delay time (induction time), which is the most critical factor for field operations, is dependent upon the subcooling (ΔT) in the system: the higher the subcooling, the lower the induction time. The absolute pressure is also an important factor. 5−8Probably the commonest class of polymers used in commercial KHI formulations are homo-polymers and copolymers of the N-vinyl lactams N-vinyl pyrrolidone (VP) and N-vinyl caprolactam (VCap). 9−16 Recently, we showed that the homo-polymer of the 6-ring N-vinyl lactam monomer N-vinyl piperidone (VPip) had an intermediate KHI gas hydrate performance between that of poly(N-vinyl pyrrolidone) and poly(N-vinyl caprolactam). 17,18 Thus, the KHI performance increases with an increasing lactam ring size. The structures of the homo-polymers of VP, VPip, and VCap are given in Figure 1. It was therefore of great interest to investigate whether the homo-polymer of an even larger ring N-vinyl lactam would perform better than homo-polymers with the smaller lactam rings. The 8-ring N-vinyl lactam monomer N-vinyl azacyclooctanone (VACO) has not been reported previously nor have any polymers from this monomer. We were also uncertain if the homo-polymer poly(N-vinyl azacyclooctanone) (PVACO) was even water-soluble and, therefore, could be tested as a KHI, because the cloud points of the poly(N-vinyl lactam)s decrease with lactam ring size. For example, PVCap as a 1.0 wt % solution in water has a cloud point of about 30−40°C depending upon the polymerization method and molecular weight, whereas cloud points for PVPip are generally in the range of 60−80°C. 17,18 In this paper, we report the synthesis and structure II (SII) gas hydrate KHI performance of PVACO for the first time and compare the results to other poly(N-vinyl la...

Kinetic hydrate inhibitors have been used successfully in the field for about the last 13 years to prevent gas hydrate formation mostly in gas-and oilfield production lines. They work by delaying the nucleation and often also the growth of gas hydrate crystals for periods of time dependent upon the subcooling in the system. Current commercial kinetic hydrate inhibitors are used for field applications where the subcooling is as high as about 10°C. In the Norwegian sector of the North Sea, very few of the commercial kinetic hydrate inhibitors are available for use offshore because of poor environmental properties usually related to biodegradability. We have designed and synthesized a class of kinetic hydrate inhibitor, which appears to show good biodegradability (OECD306, >20% in 28 days). Inhibitor performance tests have been carried out in stirred autoclaves (titanium and sapphire) using a natural gas blend and saline water giving structure II hydrates. In the presence of solvents, we have obtained a fairly good performance of the new inhibitors but a little lower than that of a current commercial inhibitor Luvicap 55W.