Kinetic Hydrate Inhibitor Performance Research Articles

Improving the Kinetic Hydrate Inhibition Performance of 3-Methylene-2-pyrrolidone Polymers by N-Alkylation, Ring Expansion, and Copolymerization

Poly(N-vinyl lactam)s have been the dominant class of kinetic hydrate inhibitor (KHI) polymer for many years in oil and gas flow assurance applications. Recently, we reported on the KHI performance of a new, but closely related, polymer, poly(3-methylene-2-pyrrolidone), P(3M2P). It was suggested that, like poly(N-vinyl pyrrolidone), the polymer is not sufficiently hydrophobic for optimum KHI performance. We now report on three improvements to this class: (1) by alkylating the pyrrolidone ring, (2) by expanding the ring size from 5 to 6 atoms, and (3) by copolymerization with more hydrophobic monomers. All new polymers were tested in high pressure rocking cells with a Structure II-forming gas mixture. When methyl, ethyl, and n-propyl groups were introduced onto the pyrrolidone ring, the poly(5-n-propyl-3-methylene-2-pyrrolidone) homopolymer, with the lowest cloud point of the three polymers, showed the best KHI efficacy. Expanding the ring size to the 6-ring piperidone group also lowered the polymer cloud point and improved the KHI performance relative to P(3M2P). Copolymers of 3M2P with N-vinyl caprolactam and N-n-butyl methacrylamide (n-BuMAm) were also synthesized. All copolymers showed good improvements over P(3M2P). The best copolymer was the n-BuMAm:3M2P copolymer with the highest n-BuMAm content and lowest cloud point. The best two copolymers were further investigated at different concentrations (500, 1000, 2500, and 5000 ppm), showing an increase in performance with increasing concentration.

Effects of ethanol on the performance of kinetic hydrate inhibitors

The main types of hydrate inhibitors are thermodynamic (THI), kinetics (KHI) and anti-agglomerants (AA) inhibitors. Kinetic inhibitors are used in low dosages but do not have satisfactory efficiency in large subcoolings. On the other hand, thermodynamic inhibitors are widely used in the industry and require high dosages. In order to study a combined inhibition, the behavior of three different commercial kinetic inhibitors combined with ethanol (THI) was tested under subcooling conditions in cases which unique kinetic inhibition did not yield satisfactory results. With 3% concentrations of kinetic inhibitors and a 10% by mass ratio of ethanol, it was possible to identify a significant increase in induction time for two of the inhibitors tested. Once the growth of the hydrate crystals began to appear, the presence of inhibitors did not result in significant gain in delaying the crystallization process. Some physical characteristics of the formed hydrates were altered in the presence of kinetic inhibitors and ethanol, evidencing a lower capacity of the crystals to adhere. With the same concentration of kinetic inhibitor and a 30% by mass ratio of ethanol the KHI presented low effectiveness.

Study of the Kinetic Hydrate Inhibitor Performance of Poly(N-vinylcaprolactam) and poly(N-isopropylmethacrylamide) with Varying End Caps

Poly(N-vinylcaprolactam) (PVCap) is an effective kinetic hydrate inhibitor (KHI) that has been widely used in both industry applications and laboratory studies. Industrially, organic peroxides are often used as the N-vinylcaprolactam (VCap) polymerization initiator. In our own past studies, we usually make PVCap by polymerizing VCap with azoisobutyronitrile (AIBN), giving a 2-cyanoprop-2-yl polymer end cap. In this study, VCap was polymerized with AIBN and a series of mercaptocarboxylic acid chain transfer agents (CTAs) to modify the end groups of PVCap. These end-group-modified PVCaps were investigated as KHIs in high-pressure rocking cells. Some of them showed better KHI performance than PVCap made with AIBN alone. The mercaptoacetic-acid-group-modified PVCap (PVCapSCH2COOH) had significantly better inhibition performance than normal PVCap on both structure I (sI) and structure II (sII) gas-hydrate-forming systems in aqueous solution as well as a sII hydrate-forming system containing added decane. Howev...

Performance Improvement of Various Kinetic Hydrate Inhibitors Using 2-Butoxyethanol for Simple Gas Hydrate Formation in a Flow Mini-Loop Apparatus

The effect of 2-butoxyethanol as an additive on simple gas hydrate formation in the presence of kinetic hydrate inhibitors such as modified starch, polyvinylcaprolactam, and Gaffix VC-713 under various conditions in a flow mini-loop apparatus has been studied. A laboratory flow mini-loop apparatus has been designed and manufactured to measure the induction time of simple gas hydrate formation. Hydrate formers (such as C1, C2, C3, i-C4, and CO2) are contacted with water containing dissolved inhibitor in the presence of 2-butoxyethanol as an additive at desired temperature and pressure. The effect of 2-butoxyethanol on the induction time during gas hydrate formation was investigated in the presence and absence of modified starch, polyvinylcaprolactam, and Gaffix VC-713 as kinetic inhibitors. Results show that the induction time is prolonged in the presence of Gaffix VC-713 compared to polyvinylcaprolactam and modified starch as inhibitors. Moreover, the induction time in the presence of 2-butoxyethanol is greater than in the absence of this additive for simple gas hydrate formation. As a result, the performance of kinetic hydrate inhibitors was enhanced in the presence of 2-butoxyethanol for natural gas components during gas hydrate formation.

As Green As It Gets: An Abundant Kinetic Hydrate Inhibitor from Nature

We report the development of a kinetic hydrate inhibitor (KHI) based on a cheap, abundantly available natural product that is readily biodegradable and low in toxicity. The optimal product, called KHI530, was determined using KHI screening tests by the slow constant cooling method in multiple rocking cells. The performance was better than a low molecular weight poly(N-vinylpyrrolidone) for a synthetic natural gas mixture (SNG) forming primarily structure II hydrate and about the same performance for structure I methane hydrate. Isothermal tests using the SNG mixture indicate that an active dosage of 0.5 wt % can protect against hydrate formation for 48 h at a subcooling of 6–7 °C. KHI530 exhibits no cloud point up to 95 °C in deionized water or NaCl brines up to 7 wt %. KHI530 can be biostabilized in monoethylene glycol (MEG) or using a biocide such as tetrahydroxymethylphosphonium sulfate (THPS), which does not affect the KHI performance. KHI530 was found to be fully compatible at all ratios with thermod...

Synergetic Performance of the Mixture of Poly(N-vinylcaprolactam) and a Pyrrolidinium-Based Ionic Liquid for Kinetic Hydrate Inhibition in the Presence of the Mineral Oil Phase

In our previous studies, it was proven that ionic liquids can be a good synergist with poly(N-vinylcaprolactam) [PVCap, a polymeric kinetic hydrate inhibitor (KHI)] against methane or natural gas hydrate formation. In particular, 1-hexyl-1-methylpyrrolidinium tetrafluoroborate (HMP-BF4) showed the best performance for natural gas hydrate inhibition among the ionic liquids tested. In this study, we report the result of performance tests on kinetic inhibition of natural gas hydrate formation with a mixture of PVCap and HMP-BF4 in the presence of an additional oil phase. At various water cuts (20, 50, and 80%) with 3.5 wt % aqueous NaCl solution and mineral oil, the KHI performance of PVCap alone and a mixture of 0.5 wt % PVCap and 0.5 wt % HMP-BF4 was evaluated. This was performed by measuring the induction time for the synthetic natural gas hydrate formation, in addition to the pressure, temperature, and torque of the system investigated. Calculations were carried out to determine the amount of water conve...

Synergism of tert-Heptylated Quaternary Ammonium Salts with Poly(N-vinyl caprolactam) Kinetic Hydrate Inhibitor in High-Pressure and Oil-Based Systems

Quaternary ammonium ionic liquid salts (QAILs) are well known as synergists for kinetic hydrate inhibitor (KHI) polymers such as poly(N-vinyl caprolactam) (PVCap). Earlier work showed that branching of the pentyl tails of tetra(n-pentyl)ammonium bromide to make isohexyl groups and tert-heptyl groups gave a significant improvement in the synergistic KHI performance. We have now evaluated in more detail the KHI performance of tris(tert-heptyl)-N-pentyl-1-ammonium bromide (tris(tert-heptyl)PeAB) or tris(tert-heptyl)-N-propyl-1-ammonium bromide (tris(i-heptyl)PrAB) blended with poly(N-vinyl caprolactam) (PVCap) in steel rocking cells at 130 bar. Due to tris(tert-heptyl)PeAB being sparingly soluble in water we used a water–gas–decane system in order to get this compound dissolved in the fluids. These QAILs were also compared to the known QAILs, tetra(n-butyl)ammonium bromide (TBAB), tetra(n-pentyl)ammonium bromide (TPAB), tetra(n-hexyl)ammonium bromide (TnHexAB), tetra(isohexyl)ammonium bromide (TiHexAB), tetr...

Ranking of kinetic hydrate inhibitors using a high pressure differential scanning calorimeter

A High Pressure Differential Scanning Calorimeter (HP-µDSC) was used to characterize the kinetic inhibition performance of several kinetic hydrate inhibitors (KHIs) up to 390 bar (39 MPa) of model natural gas. A novel protocol was devised to increase the spread between effective KHIs to obtain their performance ranking. The natural gas used in the study was a mixture of 90 mol% methane and 10 mol% propane. Linear cooling ramps at a constant cooling rate of 0.5 K/min were used under isobaric conditions. Both the formation temperature of natural gas hydrate during a linear cooling ramp and the amount of hydrate formed were used for comprehensive analysis. The second method was based on the growth inhibition of natural gas hydrates and opened a new possibility that only several repeat measurements may be sufficient for fast screening of good KHIs. The results showed that HP-µDSC is a promising tool for the evaluation of kinetic inhibition performance of KHIs that can only be obtained in minute quantities.

Comparison of Kinetic Hydrate Inhibitor Performance on Structure I and Structure II Hydrate-Forming Gases for a Range of Polymer Classes

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 (To) have been used to rank the polymers according to their KHI ability, with the lower To 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 hy...

Poly(alkyl ethylene phosphonate)s: A New Class of Non-amide Kinetic Hydrate Inhibitor Polymers

All commercial kinetic hydrate inhibitor (KHI) formulations are based on polymers with amide (or imide) functional groups. In our continuing work to explore non-amide-based KHI polymers, a series of poly(alkyl ethylene phosphonate)s (PPns) has been synthesized. These polymers were investigated for their performance as KHIs in high-pressure rocking cells using a structure-II-forming gas mixture and the slow constant cooling test method over 24 h. All of the PPns gave better KHI activity than tests with no additive. However, despite several of the polymers being designed to have low cloud points, a factor that is often useful for good KHI performance, none of the PPns gave lower onset temperatures than poly(N-vinylcaprolactam), a well-known commercial KHI with a low cloud point. A random PPn copolymer with eight ethyl and eight n-hexyl groups gave a biodegradation of about 31% using the marine OECD 306 test protocol.

Evaluation of copolymers from 1-vinyl-3-alkylimidazolium bromide and N-vinylcaprolactam as inhibitors of clathrate hydrate formation

This work presents experimental results on the inhibition performance of kinetic hydrate inhibitors (KHIs) of structure-II hydrates. The studied KHIs, namely derivatives of copolymers of N-vinylcaprolactam (VCap) and alkylimidazolium salts, were synthesized at the Mexican Petroleum Institute with the objective of producing copolymers with higher cloud points than that exhibited by the poly(N-vinylcaprolactam) homopolymer. Three experimental methods were used to evaluate the hydrate inhibition performance of the synthesized copolymers: the capacity to inhibit the formation of tetrahydrofuran (THF) hydrates (under atmospheric pressure and at a temperature of −0.5 °C; i.e., a subcooling for THF hydrates of about 3.4 °C); the capacity to inhibit the formation of natural gas hydrates under high pressure (about 6 MPa) and low temperature conditions (about 1 °C; i.e. a subcooling of 16 °C for the studied natural gas sample); and the determination of the cloud point temperature of the studied copolymers in aqueous systems of varying salinity (under atmospheric pressure and temperatures ranging from 2 to 81 °C). The inhibition performance and the high solubility (in highly saline aqueous systems) of the synthesized copolymers seems to indicate that these compounds can be considered as promising low dosage inhibitors to prevent or delay the formation of hydrates during the production and transport of hydrocarbons.

High throughput synthesis and characterization of PNIPAM-based kinetic hydrate inhibitors

Libraries of kinetic hydrate inhibitors (KHIs) based on poly(N-isopropylacrylamide) (PNIPAM) were synthesized by post-synthetic modification of an existing PNIPAM copolymer. Unlike other synthetic routes this generates accurately controlled libraries of KHIs with similar molecular weight (M‾w), molecular weight distribution, end groups and composition. This allows for accurate interpretation of the effect of each of these components and highlights key functional groups that can improve performance. This was assessed using a high throughput KHI ranking method based on its inhibition performance of Structure II (sII) forming cyclopentane (c-C5) hydrate under atmospheric pressure. Hydrate inhibition tests showed that performance of PNIPAM-based KHIs can be improved by careful inclusion of select groups. In addition, cloud point data demonstrates that polymers with higher deposition points could be generated using this method. The presence of side products has a pronounced effect on cloud point but minimal effect on hydrate inhibition performance. These data provide valuable insights into polymers that can improve the performance of KHIs. The method can also be applied to a number of structural motifs to develop chemicals to overcome issues associated with natural gas transport.

Relationship between the interfacial tension and inhibition performance of hydrate inhibitors

In this study, we tried to establish the relationship between the kinetic inhibition performance of KHIs and their effects on interfacial properties, such as the interfacial tension between hydrate-forming gases and aqueous solutions. The hydrate onset time and gas/liquid interfacial tension for systems composed of the same natural gas and aqueous phases containing different kinetic hydrate inhibitors (KHIs) and KHI synergists were measured in parallel. Five amide class KHIs, PVP-E, PVP-A, PVP-BP developed by our group, as well as PVP and Inhibex 501 were tested. Four familiar alcohols, methanol, ethanol, glycol and triglycol, were used as synergists. All of the experimental results with different types or dosages of KHIs and synergists show that a lower interfacial tension corresponds to a longer onset time, i.e., a better kinetic inhibition performance. We therefore suggested a lowest gas/liquid interfacial tension rule for developing amide class KHIs or KHI synergists as well as for determining their suitable applied dosages. Additionally, triglycol was found to be a good synergist for PVP-BP, and its best applied dosage is 2.3wt% or so. Finally, we proposed new insights into the mechanisms of both the kinetic inhibition of KHIs and the synergistic effect of alcohols.

N,N-Dimethylhydrazidoacrylamides. Part 3: Improving Kinetic Hydrate Inhibitor Performance Using Polymers of N,N-Dimethylhydrazidomethacrylamide

A series of homopolymers of N,N-dimethylhydrazidomethacrylamide (DMHMAM) and copolymers with (N-isopropylmethacrylamide) (IPMAM) have been synthesized and investigated as kinetic hydrate inhibitors (KHIs) with a structure-II-forming synthetic hydrocarbon gas mixture in high-pressure steel rocking cells. The same polymerization method to give similar low molecular weights was used to aid KHI performance comparison. The homopolymer polyDMHMAM was found to give good KHI performance, significantly better than the related homopolymer, poly(N,N-dimethylhydrazidoacrylamide) (polyDMHAM), which does not have methyl groups in the backbone. This is in agreement with previous studies on N-alkylmethacrylamides. PolyDMHMAM also has no cloud point in deionized water up to 95+ °C, making it suitable for injection in high-temperature wells. A 1:1 N,N-dimethylhydrazidomethacrylamide/N-isopropylmethacrylamide copolymer (1:1 DMHMAM/IPMAM copolymer; Mn = 2100) also exhibited no cloud point in deionized water and showed an improved KHI performance over polyDMHMAM homopolymer (Mn = 2300) and poly(N-isopropylmethacrylamide) (Mn = 1300) of similar low molecular weights. Other ratios of monomers in DMHMAM/IPMAM copolymers did not show an improvement on the performance compared to the other copolymers. Protonation of polyDMHMAM to give only quaternary dimethylhydrazinium groups strongly lowers the KHI performance. High pH (8–9) also gave a worse performance than at pH 6.5, suggesting that partial protonation of polyDMHMAM is optimal for the best KHI performance.

Investigation of the performance of low-dosage hydrate inhibitors combined with wax inhibitors for a mixed CH4-CO2 gas hydrate formation

Within the oil and gas industry, low-dosage hydrate inhibitors (LDHIs) are a proven technology to control hydrates. Besides hydrate inhibitors, wax inhibitors (WIs) are frequently injected to prevent wax buildup in the crude oil pipeline. However, little attention has been focused on the effect of wax inhibitors on the performance of LDHIs. In this study, performance tests of 3 LDHIs in the presence of wax inhibitors were carried out for a 67% CH4/33% CO2 gas hydrate formation. Using the isothermal cooling method at pressures of 9 MPa and temperatures of 4 °C (subcooling is 9 °C), the results showed that the induction time of CH4-CO2 gas hydrate formation with LDHI/WI was shorter than the system with only LDHI. During the growth period, when the concentration of the WIs was 1 mass%, the growth time of the system with LDHI/WI was prolonged. Taking the induction time and the growth time into consideration, it was found that WIs had a more negative impact on the kinetic hydrate inhibitor performance at low dosage. The effect of WIs at high concentration could be negligible.

Quantitative kinetic inhibitor comparisons and memory effect measurements from hydrate formation probability distributions

In contrast with conventional experiments for ranking the performance of kinetic hydrate inhibitors (KHIs), the high pressure automated lag time apparatus (HP-ALTA) is not limited by the stochasticity of hydrate formation because it can make large numbers of formation measurements rapidly. An improved procedure for analysing HP-ALTA data is presented here in which formation probability density histograms with uniform temperature bin widths are constructed. Operations such as numerical integration and subtraction can then be applied to entire formation distributions without the need to regress the data to model analytic functions. This approach enables apparatus effects on the measured formation distributions to be quantified and mitigated. Using analogue natural gas mixtures, a demonstration of the improved method is reported here for six KHIs that were also tested using a conventional rocking cell. The cumulative formation probability distribution functions determined by numerical integration of the histograms were used to quantitatively (1) assess the performance of the KHIs, (2) study the memory effect in pure water, and (3) assess its impact on the performance of each KHI. The rankings of the KHIs obtained from the various performance metrics accessible through the HP-ALTA data were generally consistent with the rocking cell rankings, with the average absolute deviation in net subcooling for the two apparatus being 2 to 3K. In the quiescent HP-ALTA samples, the three worst performing KHIs still had expected subcoolings of about 14K unless the memory effect was present, in which case the expected net subcoolings of the three worst performing KHIs ranged from 0–6K. In contrast the memory effect reduced the expected subcoolings of the three best performing KHIs by only 1–2K. Using the new method of analysis, hydrate formation distributions measured with the HP-ALTA can also be used to identify regions of good and poor performance for a given KHI, which might aid in the determination of improved inhibition strategies or assist the development of improved hydrate inhibitors.

First Investigation of the Kinetic Hydrate Inhibitor Performance of Polymers of AlkylatedN-Vinyl Pyrrolidones

A series of water-soluble homopolymers and copolymers of 1-alkyl-vinyl pyrrolidones (1-alkyl-VPs) with alkyl groups, including methyl, ethyl, isopropyl, and n-propyl, has been synthesized. Homopoly...

Investigation of the Kinetic Hydrate Inhibitor Performance of a Series of Copolymers ofN-Vinyl Azacyclooctanone on Structure II Gas Hydrate

A series of copolymers of N-vinylazacyclooctanone (VACO) with more hydrophilic monomers, including the 5–7 ring N-vinyl lactams, N-vinyl-N-methyl acetamide (VIMA), and N-vinyl acetamide (NVA), have been synthesized. Their performance as kinetic hydrate inhibitors (KHIs) was compared to poly(N-vinylazacyclooctanone) (PVACO) homopolymer in high pressure rocking cells using a Structure II hydrate-forming natural gas mixture. The best 1:1 copolymer between any two of the four N-vinyl lactams was found to be for VP/VACO copolymer. 1:1 VACO/VIMA copolymers were also shown to be a superior KHI to PVACO, whereas copolymers of other ratios of these two monomers were less effective KHIs. 2-Butoxyethanol (BGE) was also found to be a good solvent and synergist for 1:1 VACO/VIMA copolymers. Finally, 1:1 N-vinyl lactam/NVA copolymers were shown to have worse KHI performance than the equivalent 1:1 N-vinyl lactam/VIMA copolymers at similar molecular weights.

Can cyclopentane hydrate formation be used to rank the performance of kinetic hydrate inhibitors?

The equilibrium temperature for cyclopentane hydrate formation is approximately 7.7°C. Since these hydrates are Structure II and are formed at atmospheric pressure it was of interest to determine if cyclopentane hydrate-forming fluids could be used in the performance ranking of kinetic hydrate inhibitors (KHIs), avoiding the use of high pressure equipment and flammable gases. We have carried out a series of tests with well-known classes of KHIs in four parallel cells using cyclopentane hydrate-forming fluids. The superheated hydrate method was used since it was found that cyclopentane hydrate formation was inconveniently slow using fluids with no hydrate history. The amount of KHI needed to obtain significant kinetic inhibition was found to be in the region of 100–200ppm, a fraction of that needed for high pressure Structure II gas hydrate inhibition with the same KHI. This may be due to the fairly low subcooling used (7.8°C) as well as the lower solubility of cyclopentane in water compared to small hydrocarbon hydrate-formers at elevated pressures. The results of testing a range of KHIs using this superheated CP hydrate test method correlated fairly well with tests carried out on Structure II-forming gas hydrates in high pressure stirred autoclaves, but there were some notable exceptions, which meant that this method is unlikely to be reliable for ranking all KHIs under atmospheric conditions. Furthermore, raising the KHI concentration from 100ppm to 400ppm did not lead to the expected increase in KHI performance for all the KHIs tested.

Poly(N-vinyl azacyclooctanone): A More Powerful Structure II Kinetic Hydrate Inhibitor than Poly(N-vinyl caprolactam)

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-n-butyl glycol ether with PVACO and the effect of the polymer molecular weight on KHI performance are also discussed.