Design and development of N-vinylcaprolactam copolymers as kinetic hydrate inhibitors for sour gas environments

Abstract

Kinetic hydrates inhibitors (KHIs) are mainly water-soluble polymers employed to circumvent gas hydrate formation due to their effectiveness at low dosage, which makes logistics (transport, storage, and pumping) less costly in both onshore and offshore gas operations. Most commercial KHIs contain polymers with amide groups, with poly(N-vinylcaprolactam) P(Vcap) and related copolymers being the most common. In this study, we have synthesized copolymers of N-vinylcaprolactam (Vcap) with N-acryloyl type comonomers and investigated their ability to inhibit the formation of structure-I type gas hydrates under sour environment. Vcap was successfully copolymerized with N-acryloylmorpholine, N-acryloylpyrrolidine, N-Isopropylacrylamide and N-acryloylpiperidine to form a series of low molecular weight copolymers. The compatibility of these copolymers was investigated using cloud point temperature and fouling tests. The performance of all copolymers as KHIs was evaluated with high-pressure rocking cells using a structure-I forming natural gas mixture at 130–140 bar. It was found that KHI performance of copolymers in terms of subcooling temperature exceeded that of the P(Vcap) homopolymer of similar molecular weight. Furthermore, the comonomer with cyclic pendant groups with one heteroatom resulted in a better performance compared to acyclic and cyclic pendant groups with a higher number of heteroatoms. Also, the KHI performance of copolymers increased significantly at higher ratios of acryloyl monomer relative to Vcap. These results describe a facile method to design and develop new copolymers of Vcap as effective KHIs for structure-I type hydrates formed under sour environments.