Kinetic Hydrate Inhibition with N-Alkyl-N-vinylformamide Polymers: Comparison of Polymers to n-Propyl and Isopropyl Groups

Abstract

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 ...

Chemically Influenced Self-Preservation Kinetics of CH4 Hydrates below the Sub-Zero Temperature

“…During constant ramping, an initial pressure reduction is observed due to a decrease in temperature and CH 4 solubility. The onset nucleation temperature T o is identified as the temperature at the start of hydrate nucleation when the first macroscopic detection of hydrate formation occurs before rapid hydrate formation [68]. It is also possible that nucleation may have occurred earlier but could not be detected due to a lack of visual observation.…”

Insights into Kinetics of Methane Hydrate Formation in the Presence of Surfactants

“…Only two repetition in each experiments are performed. According to Malcolm et al [26], T 0 refers to the first macroscopic detection of hydrate formation, while T a indicates the slow hydrate growth region before rapid hydrate formation, where the influence of SDS on hydrate behavior is minimal.…”

“…During the constant cooling experiment, T a was also studied and indicates the slow hydrate growth region before rapid hydrate formation. The difference between T 0 and T a reflects the ability of the chemicals to hinder the rapid crystal growth of hydrates after nucleation begins [26]. Table 4 summarizes the difference between T 0 -T a for the fresh SDS solution.…”

Inhibition Effect of Kinetic Hydrate Inhibitors on the Growth of Methane Hydrate in Gas–Liquid Phase Separation State

“…As one of the typical LDHIs, kinetic hydrate inhibitors (KHIs), with the advantages of low dosage and high efficiency [5,6], have attracted the attention of many researchers. KHIs can inhibit hydrate formation by effectively delaying the hydrate nucleation or inhibiting hydrate growth [7][8][9][10][11][12][13]. Polyvinylpyrrolidone (PVP) is one representative KHI found by Long et al [14].…”