Recent advances in injection pressure operated gas lift spring valve technology have proven to be beneficial in the Western Operated Area (WOA) of the Prudhoe Bay Oil Field. The temperature independent nature of the spring valves combined with their ability to operate at injection pressures as high as 2400 psig at valve depth make them the proven choice over conventional bellows charged gas lift valves. This cost effective technology has simplified both gas lift design and operation in the WOA. Two case wells which exemplify the benefits derived from the utilization of spring valve technology are described in this paper.
The Prudhoe Bay field is located some 250 miles north of the Arctic Circle. As a consequence of geographical location, there is a permafrost zone across the field extending from several inches below the surface to approximately 2000 feet true vertical depth. This frozen soil is a heat sink which creates a large differential temperature in the well bore between shut in and flowing-conditions.
WOA oil production began in June, 1977, with approximately 75 naturally flowing wells on line. The typical well at start up produced 10,000+ BFPD at a solution GOB of 650 SCF/STB and no water cut. Production now comes from 500 wells of which nearly 200 are gas lifted using some 400 MMSCFD of lift gas. The typical matured gas lifted well now produces 3200 BFPD at 51% water cut and a producing GOR ranging from 700 to 2000 SCF/STB. Where artificial lift has been needed, the only viable means has been gas lift.
Gas lift operations in the WOA are unique because there is effectively an unlimited supply of lift gas available. Maximum surface injection pressure (kickoff pressure) ranges from 1700 to 2000 psig. Operating injection pressure is regulated from individual surface control valves and ranges between 1000 and 1800 psig depending upon the producing characteristics of a given well. Tubing sizes range from 3.5 to 5.5 inches in diameter. Most wells are equipped with 5 or 6 gas lift mandrels which normally accommodate 1.5 inch gas lift valves. In general, the gas lift operations may be classified as continuous high volume lift.
Utilizing gas lift at Prudhoe Bay is complicated by the fact that a well will typically have a shut in temperature of some 60 degrees F at 3000 true vertical depth, TVD, which is usually the location of the uppermost gas lift mandrel. However, the flowing temperature at the same depth may be as high as 200 degrees F.