Technology Update: Oil-Soluble LDHI Represents New Breed of Hydrate Inhibitor

Abstract

Natural-gas hydrates may stand alone among oil and gas production problems in both the rapidity with which a severe problem can develop and the potentially catastrophic safety, environmental, and financial risks raised by worst-case scenarios. Unlike corrosion or scale or paraffin buildup, which typically take months or years to evolve into production problems, gas-hydrate plugs have been known to form in subsea flowlines and pipelines in an hour or less. Fig. 1 depicts a gas-hydrate plug that has been removed from a production line by means of pigging. Once a hydrate plug has formed, it can take weeks or even months to dissociate it safely. Meanwhile, downtime and production losses mount day by day, and this impact can be huge, compared with other production problems. Gas hydrates are ice-like crystals composed of water and natural gas, in which methane or carbon dioxide has become trapped in hydrogen-bonded cages. Unlike ice formed from H2O, though, gas hydrates are quite flammable and explosive. A cubic foot of gas hydrate at standard conditions contains 0.8 ft3 of water and 182 ft3 of methane. The hydrate plug can explode with a huge destructive force that may rupture subsea flowlines, damage production equipment, place workers at high risk, and endanger the ocean environment. Given these risks and the difficulty of remediating gas-hydrate plugging, most operators view prevention as their only real strategy for dealing with the problem. Hydrate Conditions in Deep Water Hydrates are formed thermodynamically whenever natural gas and water are present in the right combinations of low temperature and high pressure. Unlike pure ice, hydrates can form at temperatures much greater than 32 degrees Fahrenheit (°F). Hydrates have been shown to form in gas/water mixtures at temperatures of 40°F with pressure as low as 166 psi. It has been demonstrated, furthermore, that if pressures are great enough, hydrates can form in some gas/water compositions at temperatures as warm as 80°F. The extreme temperature and pressure regimes encountered on the world's ultradeepwater exploration and development frontiers combine to create almost perfect conditions for hydrate creation. In thousands of feet of water with long subsea tiebacks, production cools rapidly as it travels through flowlines from the well to the producing facility. The farther the flowstream travels, the more it is cooled by the surrounding seawater and the greater its risk of gas-hydrate formation somewhere within the production system.