Kinetic hydrate inhibitors (KHIs) have been used for over 25 years to prevent gas hydrate formation in oil and gas production flow lines, but they are some of the most expensive oilfield production chemicals. The main component in KHI formulations is a water-soluble polymer with many amphiphilic groups. Usually, in commercial KHI polymers, the hydrophilic part of these groups is the amide group. In addition, KHI polymers are often incompatible with film-forming corrosion inhibitors. Therefore, we sought to find cheaper but effective KHIs that could also act as a flow line corrosion inhibitor. Continuing earlier work from our group with maleic-based polymers, we have now explored maleic acid/N-vinyl caprolactam (MAcid/VCap) copolymers to introduce VCap, a well-known KHI monomer, together with the cheaper MA monomer. KHI performance screening tests were conducted under high pressure with a structure II-forming natural gas mixture in steel rocking cells using the slow (1 °C/h) constant-cooling test method. Surprisingly, the MAcid/VCap copolymer showed very poor KHI efficacy. GFN2-xTB molecular dynamics simulations revealed that MAcid/VCap exhibits intra-hydrogen bond networks that trap the polymer morphology in the globular form. In this scenario, the caprolactam ring is encapsulated inside the polymer structure due to the intra-hydrogen bonds and the hydrophobic interactions that minimize its ability to interact with the hydrate surfaces, which significantly reduces the MAcid/VCap kinetic inhibition performance. However, the polymer in such globular forms still displays an important amount of its carboxylic groups exposed to water, which explains the water solubility. In contrast to MAcid/VCap copolymers, maleimide derivatives with dibutylamino end groups were effective KHIs and even better with dibutylamine oxide end groups. A terpolymer of MA/VCap reacted with N,N-dibutylaminopropylamine followed by subsequent oxidation of the end groups to dibutylamine oxide and gave the best performance of any maleic-based polymer reported to date. The combination of caprolactam and dibutylamine oxide groups can be thought of as synergism within the same polymer, akin to the excellent synergy of the separate molecules, tributylamine oxide and PVCap.

Kinetic hydrate inhibitors (KHIs) have been used for over 25 years to prevent gas hydrate formation in oil and gas production flow lines. The main component in KHI formulations is a water-soluble polymer with many amphiphilic groups, usually made up of amide groups and adjacent hydrophobic groups with 3–6 carbon atoms. KHI polymers are one of the most expensive oilfield production chemicals. Therefore, methods to make cheaper but effective KHIs could improve the range of applications. Continuing earlier work from our group with maleic-based polymers, here, we explore maleic acid/alkyl acrylate copolymers as potential low-cost KHIs. Performance experiments were conducted under high pressure with a structure II-forming natural gas mixture in steel rocking cells using the slow (1 °C/h) constant cooling test method. Under typical pipeline conditions of pH (4–6), the performance of the maleic acid/alkyl acrylate copolymers (alkyl = iso-propyl, iso-butyl, n-butyl, tetrahydrofurfuryl, and cyclohexyl) was poor. However, good performance was observed at very high pH (13–14) due to the thermodynamic effect from added salts in the aqueous phase and the removal of CO2 from the gas phase. A methyl maleamide/n-butyl acrylate copolymer gave very poor performance, giving evidence that direct bonding of the hydrophilic amide and C4 hydrophobic groups is needed for good KHI performance. Reaction of the maleic anhydride (MA) units in MA/alkyl acrylate 1:1 copolymers with dibutylaminopropylamine or dibutylaminoethanol gave polymers with good KHI performance, with MA/tetrahydrofurfuryl methacrylate being the best. Oxidation of the pendant dibutylamino groups to amine oxide groups improved the performance further, better than poly­(N-vinyl caprolactam).

Several chemical problems can occur during the production of oil and gas through flow lines. This includes corrosion, scale deposition and gas hydrate plugging. Three separate chemicals may be needed to treat these issues. Kinetic hydrate inhibitors (KHIs) are used in cold oil or natural gas production flow lines to prevent the formation and plugging of the line with gas hydrates. They are often injected concomitantly with other production chemicals such as corrosion and scale inhibitors. KHIs are specific low molecular weight water-soluble polymers with amphiphilic groups formulated with synergists and solvents. However, many corrosion inhibitors (CIs) are antagonistic to the KHI polymer, severely reducing the KHI performance. It would be preferable and economic if the KHI also could act as a CI. We have explored the use of maleic-based copolymers as KHIs as well as their use as film-forming CIs. KHIs were tested using a natural gas mixture in high pressure rocking cells using the slow constant cooling test method. A terpolymer from reaction of vinyl acetate:maleic anhydride copolymer with cyclohexy lamine and 3,3-di-n-butylaminopropylamine (VA:MA-60% cHex-40% DBAPA), gave excellent performance as a KHI, better than the commercially available poly(N-vinyl caprolactam) (PVCap). CO2 corrosion inhibition was measured by Linear Polarization Resistance (LPR) in a 1 litre CO2 bubble test equipment using C1018 steel coupons. The new terpolymer gave good CO2 corrosion inhibition in 3.6 wt% brine, significantly better than PVCap, but not as good as a commercial imidazoline-based surfactant corrosion inhibitor. The terpolymer also showed good corrosion inhibition efficiency at high salinity conditions, (density 1.12 g/cm3). VA:MA-60% cHex-40% DBAPA shifted the open-circuit potential to more positive values and significantly decreased the corrosion rate.

Water-based flow assurance issues include corrosion, scale, and gas hydrate formation. Chemical solutions to mitigate these issues usually require separate inhibitors, which sometimes can lead to compatibility difficulties. Herein, we report studies on maleic-based copolymers to combat hydrate and corrosion with a view to optimizing for scale inhibition also. The product of maleic anhydride:N-vinyl caprolactam copolymer reacted with 3-dibutylaminopropylamine (MA:VCap-DBAPA) and its amine oxide derivative (MA:VCap-DBAPA-AO) were the kinetic hydrate inhibitor (KHI) polymers investigated. Due to limited CO2 corrosion inhibition (CI) by the polymers alone, various oxygen-, sulfur-, and nitrogen-based additives were screened for improved CO2 CI and possible KHI synergy. KHI performance screening tests were conducted under high pressure with a structure II-forming natural gas mixture in steel rocking cells using the slow (1 °C/h) constant cooling method. CO2 corrosion inhibition was measured by linear polarization resistance in a stirred 1 L CO2 bubble test apparatus using C1018 steel coupons and 3.6 wt % brine at 20.5 °C. Several sulfur-based additives improved the CI efficiency of the maleic polymers, especially butyl thioglycolate and 2-aminoethanethiol, without a negative effect on the KHI performance. For example, 2500 ppm MA:VCap-DBAPA plus 1000 ppm butyl thioglycolate gave an average hydrate onset temperature (T o) of 4.4 °C (12.7 °C below the T o for no additive) and 99.7% CI efficiency. In contrast to a classic fatty acid imidazoline surfactant corrosion inhibitor, butyl thioglycolate was also found to greatly enhance the CI efficiency of industrial KHIs, poly­(N-vinyl caprolactam) (PVCap) and N-vinyl pyrrolidone:N-vinyl caprolactam copolymer, with no antagonism to the KHI performance of the polymer. In addition, butyl thioglycolate boosted the KHI performance of PVCap. The use of small synergists such as butyl thioglycolate avoids the use of classical surfactant corrosion inhibitors which can lead to tighter emulsions and poor overboard oil-in-water quality.

Kinetic hydrate inhibitors (KHIs) are applied in oil and gas fields to prevent gas hydrate formation, most often in cold subsea flow lines. The main component in industrial KHI formulations is a water-soluble polymer with many amphiphilic groups of which the hydrophilic part is most commonly the amide functional group. In the last decade, we have investigated polyamine oxides as alternatives to polyamides due to the strong hydrogen bonding of the amine oxide group. Here, we report the KHI performance of maleic and methacrylic homopolymers with dialkylamine and dialkylamine oxide pendant groups. Performance screening experiments were conducted under high pressure with a Structure II-forming natural gas mixture in steel rocking cells using the slow (1 °C/h) constant cooling test method. Polymers with dibutylamine groups gave much better KHI performance than polymers with dimethylamine or diethylamine groups. Polyamines formed from polymaleic anhydride reacted with 3-(dibutylamino)-1-propylamine (DBAPA) or 2-(dibutylamino)-ethanol (DBAE) gave good water solubility and good KHI performance, probably due to self-ionization between the dibutylamino and carboxylic acid groups. The lack of self-ionization for the methacryl homopolymers of DBAPA and DBAE explains why these polymers are not water-soluble. Oxidation of the maleic or methacryl polyamines to polyamine oxides gave water-soluble polymers with good compatibility with brines (0.5−7.0 wt % NaCl), but only the DBAPA-based polyamine oxides gave improved KHI performance compared to the polyamines. Poly(3-(dibutylamino oxide)-1-propyl methacrylamide) gave a similar performance to commercial N-vinyl pyrrolidone:N-vinyl caprolactam 1:1 copolymer and without a cloud point in deionized water up to +95 °C.