A Summary of Successful Field Applications of A Kinetic Hydrate Inhibitor

Abstract

Abstract A kinetic hydrate inhibitor based on a copolymer of vinylcaprolactam andvinylmethylacetamide has been successfully deployed in a number of fields, including both in-land and offshore applications. The inhibitor has a lowtoxicity and manages a high degree of subcooling, up to 20°F. The effectivedosage ranges from 550 ppm to 3000 ppm, depending upon the severity of theoperating condition in each field. The performance of this inhibitor, the fieldconditions in which it was applied, and the benefits are discussed in thispaper. These field cases helped establish the application technology for properdeployment of kinetic hydrate inhibitors. The success of these programs hasconfirmed the viability of using kinetic inhibitors as an effective hydratecontrol method. This kinetic inhibition technology not only provides anattractive cost-saving alternative to thermodynamic inhibitors; it alsoimproves the safety of operation while lowering the environmental impact. Introduction Formation of natural gas hydrates can present a serious problem in oil andgas production. Hydrates are crystalline, ice-like solids that form when gasmolecules are trapped in hydrogen-bonded water cages under high pressure andlow temperature conditions.1,2 These conditions are oftenencountered in deepwater operations, such as subsea flowlines carrying wetgases, and in cold-weather operations in northern climates. Formation of gas hydrates can be inhibited by several methods.1,2The principle of these methods is to control or eliminate one of the fouressential elements necessary for hydrate formation. The four essential elementsare: the presence of hydrate-forming components in natural gas (e.g., methane), the presence of water, conditions of low temperature and high pressure. Theabsence of any of these four elements would make hydrate formation impossible. For example, the element of low temperature can be removed from the equation byproper heat management techniques using external heating or thermal insulation. Similarly, lowering the pressure by choking-back the production can reduce thetendency for hydrates to form in a production system. Water, another necessaryelement in hydrate formation, can be removed by dehydration of the naturalgas. Although all of these methods can theoretically prevent hydrate formation, some may not be feasible or desirable in the field, especially in offshoreenvironments. For instance, dehydration may not be an option for offshoreoperation due to space limitations for the processing equipment. Therefore, inorder to transport the unprocessed, wet gas production streams, operators oftenrely on chemical inhibitors such as methanol and ethylene glycol. Theseinhibitors shift the hydrate equilibrium condition so that the operatingcondition falls outside of the hydrate formation region. These chemicals areoften classified as "thermodynamic" inhibitors because of their ability toshift the hydrate equilibrium curve toward higher pressures and lowertemperatures by changing the activity of water molecules. Methanol and ethyleneglycol are the most popular choices because of their low cost and widespreadavailability.