Gas hydrates in the upstream oil and gas industry often cause problems during production, such as plugged pipelines causing down time and loss of revenue. Kinetic hydrate inhibitors (KHIs) have successfully been used in the field for about 2 decades. KHIs work to delay hydrate nucleation and/or crystal growth in the hydrate-stable operating region. KHIs, such as polymers containing N-vinyl amide units, for example, methacrylamide-based KHI polymers with isopropyl groups, have been commercialized and are now used in field operations. However, there are no reports of polymers with n-propyl groups that have been commercialized as a KHI. Using a structure-II-forming natural gas, we have now investigated the KHI performance of homopolymers with n-propyl and isopropyl groups based on the N-vinylformamide (NVF) monomer. A range of copolymers with NVF with higher cloud points were also synthesized and tested because the cloud points of these homopolymers were found to be lower than preferred for most field operations. The polymer series containing nPr-NVF monomer was found to perform better as KHIs than the iPr-NVF series as KHIs at 2500 ppm concentration in deionized water at all copolymer ratios with a similar molecular weight. Two of the best polymers from each of the nPr-NVF and iPr-NVF series were tested at varying concentrations from 1500 to 5000 ppm. A similar trend was found as with the tests of the complete series, in which the nPr-NVF polymer performed better than the iPr-NVF polymer. Poly(N-(n-propyl)-N-vinylformamide) homopolymer gave a similar KHI performance as a commercial sample of polyvinylcaprolactam (PVCap). ■ INTRODUCTIONNatural gas hydrates tend to form at elevated pressures and low temperatures, which are typical conditions in cold-climate upstream oil and gas fields and subsea multiphase pipelines. 1,2 Under these thermodynamic conditions, water molecules form cage-like structures and trap small gas molecules within these hydrate structures. Typical gas molecules are small hydrocarbons, such as methane, ethane, and propane, as well as carbon dioxide and hydrogen sulfide. 2−4 Low-dosage hydrate inhibitors (LDHIs), such as kinetic hydrate inhibitors (KHIs), are used in the oil and gas industry as a method for preventing the formation of gas hydrates; logistically, they can be more efficient and/or less expensive than other hydrate management methods. 2,5−8 KHIs delay the hydrate nucleation, which is the first stage of hydrate formation. 9 When the temperature and pressure conditions reach the hydrate-stable region, hydrate nuclei (clusters of water and gas) come together, grow, and disperse to reach the critical size for hydrate crystal growth. 2,10 Mechanisms for how the inhibition works on a molecular level are not fully understood, but some theories have been proposed. 11,12 It is generally believed that the KHI causes perturbation of the water and/or water/gas (hydrocarbon) interaction; therefore, the hydrate clusters will not reach the critical size, thus delaying the hydrate formation. 13,14 ...

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Formation of gas hydrates is a problem in the petroleum industry, where the gas hydrates can cause blockage of the flowlines. Kinetic hydrate inhibitors (KHIs) are water-soluble polymers that are used to prevent gas hydrate blockages, and they have been used in the field successfully. In this paper we present the first KHI performance results of a series of polymers containing pendant carbamate groups, poly­(hydroxyl-N-alkylcarbamate)­s. Similar polymers have been investigated as KHIs before, some of which have been commercialized. Hydroxyalkylcarbamates with varying alkyl pendant groups from methyl to iso-butyl are reported. It was found that increasing the pendant alkyl chain and branching gave increasing KHI performance; however, the polymer also became significantly less soluble in water or had a very low cloud point temperature (T Cl ). Both solubility and T Cl were slightly improved by copolymerization, and we found that the copolymer with pendant iso-butyl- and methylcarbamate 2:1 and 3:1 gave the best results of average T o = 8.5 and 8.4 °C, respectively. A copolymer of 2.5:1 with pendant iso-butyl- and methylcarbamate was also investigated at concentrations ranging from 1000 to 7000 ppm, where the increased polymer concentration showed increasing KHI performance.

A series of water-soluble polymers has been investigated for comparison of their potential as kinetic hydrate inhibitors (KHIs) for both structure I (SI)- and structure II (SII)-forming gas mixtures. A slow constant cooling test method and steel rocking cells have been used for these experiments. The average hydrate onset temperatures (T o) have been used to rank the polymers according to their KHI ability, with the lower T o values giving a higher rank. The object of the study was a comparison of the KHI performance for SI versus SII inhibition for a range of polymers and not a study to find the optimal polymer for each hydrate structure. The most interesting comparative results were the large deviations of the ethyl derivative of poly­(N-alkylglycine) for the two hydrate systems, where this polymer performed very well with the SI-forming gas but had significantly less effect on the SII-forming gas. Also, poly­(2-ethyl-2-oxazoline) (PEtOx) and poly­(isopropenyloxazoline)-01 (PiPOx-01) ranked better with SI hydrates in comparison to the SII hydrate system. In general, the polymers that work well on the SI-forming gas work well on the SII-forming gas as well. KHI mechanistic insights from this study were discussed. Although other KHIs outside this study may be available with better performance, we found that the best KHIs for SII hydrates were two N-vinylcaprolactam polymers in the synergistic solvent 2-butoxyethanol, and the best KHIs for SI hydrates were a N-isopropylacrylamide homopolymer and the same two N-vinylcaprolactam polymers.

A series of acylamide and amine oxide derivatives of polyethyleneimine, both hyperbranched (HPEI) and linear (LPEI), have been synthesised and their performance as kinetic hydrate inhibitors (KHIs) with a Structure II-forming natural gas mixture investigated in high pressure steel rocking cell experiments. This is the first time the KHI performance of linear and polymers of with the same functional groups have been compared. The importance of quality control in KHI synthesis is also highlighted.In general the amine oxides showed better performance than the acylamides when comparing the structurally-optimised and best polymers from each class. The amine oxide polymers with best KHI performance were the low molecular weight butylated HPEI amine oxides. The performance was similar to those of oligomeric amine oxides reported previously with molecular weights as low as 660 g/mole. This contrasts with other KHI polymer classes where molecular weights below 1000-1500 g/mole generally give poor KHI efficacy. In previous work, the linear LPEI amine oxides were show to be excellent tetrahydrofuran Structure II hydrate crystal growth inhibitors, superior to hyperbranched HPEI derivatives. However, the LPEI amine oxides were poorer gas hydrate KHIs than the HPEI derivatives, suggesting that other mechanisms besides crystal growth inhibition, such as nucleation inhibition, may be the dominant mechanism for this class of KHI.Acyl amide polymers with n-propyl pendant groups performed only a little better than equivalent polymer with iso-propyl groups and far better than polymers with ethyl groups. This trend is in line with a lowering of the cloud points of the polymer as the hydrophobicity increases. Acylamides based on linear LPEI gave better performance than those based on hyperbranched HPEI. The reasons for this are discussed. Linear amine oxides based on LPEI gave lower performance than hyperbranched or oligomeric amine oxides, which may be related to the availability of primary amine groups in HPEI or oligomeric ethyleneamines.