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

Erik G Dirdal,Malcolm A Kelland

doi:10.1021/acs.energyfuels.1c03567

Erik G Dirdal, Malcolm A Kelland

Open Access

https://doi.org/10.1021/acs.energyfuels.1c03567

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Abstract

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.

Highlights

Gas hydrates are nonstoichiometric crystalline solids with many similarities with ice
Given the good synergetic effect that 4-methyl-1-pentanol achieved with PVCap polymer from the first study, this compound was further investigated at varying concentrations in addition to PVCap being polymerized in it (Table 2)
We have expanded our initial study on solvent synergists for PVCap with focus on 4-methyl-1-pentanol using both slow constant cooling (SCC) as well as isothermal tests.[24]

Summary

Introduction

Gas hydrates are nonstoichiometric crystalline solids with many similarities with ice. In gas hydrates, gases of certain molecular weight stabilize hydrogen-bonded molecular water cages. If suitable low-molecular weight hydrocarbon gases combine with water under specific conditions of temperature and pressure, typically favoring conditions at temperature and pressure in the ranges of 30 bar, respectively, gas hydrates will form.[1−4] These requirements are not uncommon to encounter when producing or transporting oil and gas, and if it is left untreated, formation of gas hydrate plugs can occur, potentially jeopardizing operations and posing health hazards.[4−12] it is important to treat the system in such a manner that the risk for forming gas hydrate plugs is eliminated. Polymers and copolymers based on the monomers N-vinylcaprolactam (VCap), N-vinylpyrrolidone (VP), and N-isopropylmethacrylamide as well as hyperbranched poly(ester amide)s based on diisopropanolamine reacted with various cyclic anhydrides make up the bulk of commercially available KHIs.[5]

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