Investigation of Lactam- Based Kinetic Hydrate Inhibitors and the Synergetic Effect of Solvents

Gas hydrate formation in flow lines is of notorious concern for the oil and gas industry, and it only gets bigger because of the never-ending pursuit of oil and gas compels the industry into deeper and colder waters. Gas hydrate can form and agglomerate into plugs, jeopardizing hydrocarbon production. Therefore, a variety of methods have been developed to inhibit gas hydrate formation. One of these is to utilize chemicals in the form of low dosage hydrate inhibitors (LDHI), which consists of two categories, kinetic hydrate inhibitors (KHI) and anti-agglomerants (AA). KHIs, the focus of this thesis, are mixtures of one or more water-soluble polymers in solvents and synergists. In order to find KHI chemicals with better inhibiting capabilities and/or environmental properties, the mechanism as to how KHIs operate must be better understood. The understanding of gas hydrate formation and the effect of different KHIs has been enriched by a large number of experimental studies using a plethora of research techniques, gaining insight into the gas hydrate nucleation and growth processes, coupled with the possible modes of action of KHIs. Investigations were carried out on both the macroscopic and microscopic scale. Despite all these endeavours, no clear consensus on the inhibition mechanism exists in the hydrate community, which limits the ability to design improved KHIs. Therefore, investigating new KHI polymers and synergists can help understand the structure-activity relationship and factors involved in the KHI mechanism. This PhD study consists of two main agendas, 1) investigating different compounds for their potential capabilities as synergist with probably the most well-known industrial KHI polymer, poly(N-vinyl caprolactam) (PVCap) and 2) investigating novel alternatives for PVCap as KHIs which also contain pendant caprolactam rings. Several excellent new synergists were discovered and a new class of acrylamido-based caprolactam polymers were synthesized and developed to give good KHI performance. In addition, the stability of PVCap and related KHI polymers was studied at a wide range of conditions including temperature and pH. This is relevant for field applications at extreme conditions. In another study, literature and experimental studies were used to determine if there is a correlation between polymer cloud point and KHI inhibition performance. The results showed that a low cloud point, near the hydrate formation temperature, was useful for high KHI performance 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 neighbouring hydrophilic functional groups. Finally, a new class of non-amide-based polymers, polyvinylaminals, were investigated as KHIs. For testing the inhibition capabilities, for both the synergist mixtures and the novel polymers, high-pressure rocking cells with synthetic natural gas mixture with either slow constant cooling or isothermal test regime were mainly used. These studies have resulted in nine journal publications

Further Investigation of Solvent Synergists for Improved Performance of Poly(N-vinylcaprolactam)-Based Kinetic Hydrate Inhibitors

Poly(N-vinylcaprolactam) (PVCap) and related copolymers have been used as kinetic hydrate inhibitors (KHIs) to combat gas hydrate formation in oil and gas field production flow lines. It is known that the addition of certain solvents to the KHI polymer can enhance its ability to hinder gas hydrate formation. In an earlier study, a wide range of alcohols, glycol ethers, and ketones were investigated as synergetic solvents with PVCap. In that study, an outstanding synergetic effect was achieved by 4-methyl-1-pentanol (iHexOl). This report builds on that study by investigating iHexOl in more detail as well as some newly synthesized solvents predicted by the first study to have good synergism. Both slow constant cooling (SCC) and isothermal KHI experiments were conducted in high-pressure steel rocking cells using a structure II-forming natural gas mixture. The KHI polymer concentration, solvent concentration, and mixed solvent systems were investigated. The solvent synergist water solubility, also in brines, and partitioning to the liquid hydrocarbon phase are shown to be important factors to consider for optimizing KHI performance. Further, it was observed that the optimal molecular weight distribution for the KHI polymer when used with a solvent synergist is not the same as the optimum distribution when using the polymer alone.publishedVersio

Powerful Synergy of Acetylenic Diol Surfactants with Kinetic Hydrate Inhibitor Polymers - Choosing the Correct Synergist Aqueous Solubility

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). High-pressure 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.publishedVersio

Boronic and Organic Acids as Synergists for a Poly(N-vinylcaprolactam) Kinetic Hydrate Inhibitor

A range of boronic acids have been investigated as synergists for the kinetic hydrate inhibitor (KHI) polymer, poly(N-vinylcaprolactam) (PVCap, Mw ≈ 10,000 g/mol) using high pressure rocking cells, a natural gas mixture, and a slow constant cooling (1 °C/h) test method from 76 bar. Surprisingly, unlike other classes of synergists such as alcohols and quaternary ammonium salts, the boronic acids that gave the best synergy had an alkyl or cycloalkyl tail with a maximum of a 3 carbon atom distance from the boron atom. The tail-branched iso-butylboronic acid was the best of these, yet it showed a negligible KHI effect when tested alone. However, consistent with the other classes of synergists, tail branching or use of a cyclic alkyl group was beneficial. Interestingly, boronic acids with chains of 5 to 6 carbon atoms, i.e., n-pentyl- and n-hexylboronic acids, were antagonistic to the PVCap KHI performance. For comparison, several organic acids were also investigated as synergists with PVCap. The same trend as for the boronic acids regarding the size and branching of the acid was seen. 3-Methylbutanoic acid gave the best synergy although worse than that of iso-butylboronic acid. The synergistic performance of sodium salts of some organic acids differed markedly to that of the free organic acids. Sodium 3,3-dimethylbutanoate gave the best synergy with PVCap.publishedVersio

Solvent Synergists for Improved Kinetic Hydrate Inhibitor Performance of Poly(N-vinylcaprolactam)

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 (Mw = 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.publishedVersio

Oxyvinylenelactam Polymers-A New Class of Lactam-Based Kinetic Hydrate Inhibitor Polymers

The deployment of kinetic hydrate inhibitors (KHIs) is a chemical method for the prevention of gas hydrate plugging in gas, condensate, and oil production flow lines. Polymers made using the monomer N-vinylcaprolactam (VCap) are one of the most common KHI classes. Alternative classes of polymers containing caprolactam groups are rare. Here, we present a study on oxyvinylenelactam polymers and copolymers with pendant piperidone or caprolactam groups. Low-molecular-weight homo- and copolymers were obtained. The nonrotating vinylene groups impart rigidity to the polymer backbone. Poly(oxyvinylenecaprolactam) (POVCap) was insoluble in water, but poly(oxyvinylenepiperidone) (POVPip) and OVPip:OVCap copolymers with 60+ mol % OVPip were soluble with low cloud points. KHI screening tests were carried out using the slow constant cooling method in steel rocking cells. POVPip was water soluble with no cloud point up to 95 °C but showed a poor KHI performance. In contrast, OVPip:OVCap copolymers with about 60–70 mol % OVPip were also water soluble and showed a reasonable KHI performance, better than that of poly(N-vinylpyrrolidone) but not as good as that of poly(N-vinylcaprolactam). Surprisingly, several additives known to be good synergists for VCap-based polymers showed negligible synergy or were antagonistic with the 62:38 OVPip:OVCap copolymer with regard to lowering the onset temperature of hydrate formation. However, a blend with hexabutylguanidinium chloride showed a strong effect to delay the onset of rapid hydrate formation.publishedVersio

Reliability and Performance of Vinyl Lactam-Based Kinetic Hydrate Inhibitor Polymers after Treatment under a Range of Conditions

Well-known kinetic hydrate inhibitors (KHIs) such as poly(N-vinylcaprolactam) (PVCap), poly(N-vinylpyrrolidone) (PVP), and 1:1 N-vinylcaprolactam:N-vinylpyrrolidone (VCap:VP) copolymer have been subjected to a range of treatments to determine their reliability and whether the treatment conditions could affect the KHI performance, both positively or negatively. This included thermal aging (at varying temperatures, at varying pH, and in monoethylene glycol (MEG) solvent), treatment with microwaves or ultrasound, ball-milling, and oxidizing agents (household bleach or hydrogen peroxide, also with heat). In addition, samples of commercial polymer solutions kept for up to 15 years were also tested for KHI performance to determine their long-term reliability. Testing was carried out using a synthetic natural gas mixture in steel rocking cells using slow constant cooling starting at ca. 76 bar. All samples of PVCap and 1:1 VP:VCap showed good KHI performance to the first sign of hydrate formation, but older samples showed a better ability to inhibit crystal growth. KHI polymer testing after treatment with microwaves or ultrasound, or thermal aging (at varying temperatures, varying field pH, and in MEG solvent up to 160 °C) showed little loss of performance. Oxidizing agents, particularly sodium hypochlorite solution, worsened the KHI performance.publishedVersio

Establishment of new equipment for testing low dosage hydrate inhibitors

Master's thesis in Environmental technologyGas hydrate formation posses a notorious concern for the oil and gas industry, and it only gets bigger because of that the never-ending pursuit of oil and gas compels the industry into deeper and colder waters. Here gas hydrate can from and agglomerate into plugs, jeopardizing pipelines and process equipments. Therefore a variety of methods have been developed to inhibit gas hydrate formation, one of them being to utilize low dosage hydrate inhibitors, which consists of kinetic hydrate inhibitors and anti-agglomerants. Low dosage hydrate inhibitors are relatively expensive, and it is therefore important to determine effective concentrations in laboratory apparatuses. Test apparatuses and methods are numerous, and the majority are THF rigs, rocker rigs, autoclaves, pipe wheels and flow loops. Prior methods for assessing hydrate inhibitors performance concentration tend to suffer form not being repeatable in addition to be inconsistent. Thus there are always possible for new hydrate inhibition test methods and apparatuses. A prototype table top wheel was developed for testing low dosage hydrate inhibitors. It consisted of a wheel submerged in a water bath. Unique features of the table top wheel was its small size, the acrylic top disk and the mode of moving the liquid in the apparatus. A swirling circular motion made the liquid move based on the "Euler disk" mechanical movement, hence no pumps or internal equipment for propelling the liquid was utilized. Preliminary tests were conducted to approve the apparatus for future experimental work, by providing a gas hydrate formation method of a consistent manner. The result for the same anti-agglomerant range in both 1.5wt% NaCl solution and distilled water, obtained in the table top wheel was compared to the rocker rig RCS20. This was done to confirm if this apparatus indeed could be used to rank inhibitors, thus a validation of the apparatus. It was confirmed that the table top wheel result for the same anti-agglomerant range in both 1.5wt% NaCl solution and distilled water had the same trend as the results from the rocker rig. The table top wheel was thereby validated and approved at least for this anti-agglomerant range. However, the concentration required for an adequate inhibition was higher in the table top wheel compared to the rocker rig, hence the table top wheel was a more conservative test apparatus than the rocker rig. Unfortunately the table top wheel could not be operated safely after 59 pressurized experiments, due to cracks in the acrylic top disk. This was a severely design flaws which must be sorted out for the apparatus to progress. However, as far as being a prototype the table top wheel approved to be adequate. It yielded reliable and predictable test results and provided a consistent method for both hydrate formation and inhibitor testing

Synthesis and Investigation of Polymers of 2‑Methacrylamido-caprolactam as Kinetic Hydrate Inhibitors

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.publishedVersio

High Cloud Point Polyvinylaminals as Non-Amide-Based Kinetic Gas Hydrate Inhibitors

In recent years, we have explored non-amide-based classes of kinetic hydrate inhibitor (KHI) polymers to determine if the same level of performance can be achieved as commercial KHI polymers, which all contain amide groups. Here, we have investigated a series of polyvinylaminals as KHIs for the first time. These polymers with pendant alkyl or cycloalkyl groups of varying sizes and shapes were synthesized in a simple one-step procedure from polyvinylamine. Their performance as tetrahydrofuran (THF) hydrate crystal growth inhibitors and as high-pressure KHIs was studied with a structure II-forming natural gas mixture in both sapphire and steel rocking cells. A structure–performance analysis was carried out. The best KHI was obtained with a polyvinylaminal with pendant cyclohexyl groups and gave a similar performance to the well-known KHI poly(N-vinyl caprolactam) and without the disadvantage of a low cloud point.publishedVersio