Deep Star funded a unique hydrate plug dissociation field test in a 900-psi 4-inch flowline gathering system operated by Kerr-McGee Corporation/Devon Energy in the Powder River Basin in Wyoming. In addition to DeepStar, the field test was supported by seven DeepStar member f m s (Amoco, ARCO, BP, Chevron, Kerr-McGee, Shell, and Texaco) by loan of equipment and analytical and computer services. During the field test, the formation of naturally occurring hydrate plugs was allowed in an existing flowline by stopping methanol injection. After formation, plugs were remediated by a unique one-sided depressurization. This reduction of the pressure on one side of the plug to below the hydrate decomposition curve results in a substantial pressure drop across the plug, as well as partial dissociation of the low pressure end of the plug that is outside the hydrate region. Plug formation and remediation were monitored by five data stations along the line. The pressure drop across a plug was monitored as a function of time to determine plug permeability while the plug was stationary. The pressure drop across a plug and the plug length were used to determine plug yield strength at breakaway. A dual-station gamma-ray system was used to measure slug and plug lengths and velocities at one data station.
Hydrates are ice-like solids that form in hydrocarbon flowlines. Hydrates can deposit on the flowline inner wall and agglomerate in the flow stream, and with time, hydrates can block the flowline. Hydrate operational problems are most severe in high pressure flowlines in cold environments, such as in deep waters around the world where the seawater tem- perature is typically between 34° and 40°F.
Hydrates are members of a chemical class of compounds Hydrocarbon water systems. Hydrates are solids composed of water and a gas (methane, CO,, etc.). The water forms a crystal lattice of cavities with pentagonal and hexagonal surfaces. Each cavity, or cage, may enclose a single mkthane (or other foreign) molecule.
In some ways. hydrates are similar to ice. Hydrates have a density close to that of ice, and have an appearance like frost or ice. However, in contrast to ice, the decomposition temperature of hydrates increases with increasing pressure at pressures below 15,000 psi. A hydrate thermodynamic equilibriumcurve, or decomposition curve, (bold line) for a hypothetical oil/water/gas production system is shown in Fig. 1. The decomposition curve starts at around 100 psi and 34°F and rises to 3,000 psi at around 70°F. Hydrates may form at temperatures below and pressures above the hydrate decomposition curve. Also shown in Fig. 1 is a pressure-temperature profile curve (thin line with arrows) of produced fluids in a flowline from deepwater wells to a shallow water platform 40 to 60 miles away.
