LaboratoryTesting and Field Trial of a New Kinetic Hydrate Inhibitor

Abstract

ABSTRACT Gas hydrates, ice-like gas and water cloth rate structures, cause substantial operational safety problems by blocking pipelines, valves, etc. in gas transmission, gas processing, and deep water Milling systems. Traditionally, prevention of gas hydrate formation has been accomplished thermodynamically hrough the use of very large quantities of alcohols, glycols, and occasionally salts. The costs associated with building and maintaining traditional inhibitor delivery/recovery systems canbe extremely high. These costs have been the motivation for establishing a number of research progrm seeking kinetic gas hydrate inhibitors. Kinetic hydrate inhibitors, in concentrations about 100 times lower than thermodynamic inhibitors, do not permanently prevent hydrates, but delay the onset of significant formation long enough for normal water removal to eliminate the threat. This paper describes the prefield trial testing done as part of theresearch program at the Colorado School of Mines, and the successful testing of one kinetic inhibitor (ISP' Gaffix VC-713, a terpolymer of vinyl caprolactam/polyvinyl pyrrolidone / dimethylarninomethacrylate) in a 9.4 km transmission line in the southern North Sea Gas Basin. NATURAL GAS HYDRATES Hydrates are solid crystalline compounds that form when molecules smaller than n-butane contact molecules of water at elevated pressures and reduced temperatures, both above and below the ice point. Produced gases with higher butane and propane contents have a greater tendency for hydrate formation. The formation of hydrates in piping andprocess equipment can cause plugging and reduced production. The subsea conditions of the North Seaand the Gulf of Mexico are extremely conduave to hydrate formation. Conventional prevention is through the use of alcohols and glycols that thermodynamically inhibit hydrate formation. KINETIC HYDRATE INHIBITION - RESEARCH AND DEVELOPMENT Along with 13 other oil companies, ARCO is a member of a consortium coordinated by the Colorado School of Mines (CSM) that has and is actively working to develop chemicals that will kinetically inhibit hydrate formation. This effort has identified various compounds which exhibit promising performance in laboratory studies.' Unlike methanol and mono-ethylene glycol (MEG), these products onIy reduce/deIay the rate of formation so that hydrate crystals cannot gmw beyond small particles and cause plugging on normal oil field time scales. The benefits of this type of inhibition are that signilkantly less chemical is reduced (lOOX reduction) for inhibition and the chemicals examined to date are less toxic than conventional hydrate prevention chemicals. NORTH SEA FIELD PRODUCTION ARCO British Limited operates several gas fields m the North Sea that produce a total of approxirnatel 300 MMSCF/day. Hydrates are problematic for al of these fields. Prevention of hydrate formation is through the use of MEG. AIthough effective, MEG injection is costly due the need for large quantities, transportation, regeneration, recycling to control salt contamination, and maintenance. Although some field trials on kinetic hydrateinhibitors have been conducted, none had been performed in the North Sea. To determine the effectiveness of one of the most pmnising of these new hydrate inhibitors, an 8 inch subsea well flow line was chosen for a field trial. This location waschosen because the field possessed a moderate hydrate problem.