This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper OTC 21716, ’Perdido Startup: Flow Assurance and Subsea Artificial Lift Performance,’ by H.S. Littell, J.W. Jessup, and W.W. Schoppa, Shell International E&P Inc., and M.R. Seay and T.D. Coulon, Shell E&P Company, originally prepared for the 2011 Offshore Technology Conference, Houston, 2-5 May. The paper has not been peer reviewed.
The Perdido regional host is in the ultradeepwater Gulf of Mexico (GOM) and produces from two Lower Tertiary horizons in multiple drill centers—some directly under the host spar, and others offset up to 7 miles and in water depths between 7,800 and 9,600 ft. The commissioning and early-production phases of the development provided opportunities to validate Perdido subsea hydraulic models and operational methods that were considerably different from the typical approach used by major oil companies in deep water.
The Perdido project encompasses the Great White, Tobago, and Silvertip fields and is 250 miles south of Galveston, Texas, in Alaminos Canyon 857, only 6 miles from the Mexico maritime border. The fields all are being developed with subsea wells tied back to the host, which has full processing capabilities and exports gas and oil by pipeline. In Fig. 1, the spar is shown as a hub with three flowlines from offset drill centers back to the manifold system underneath the host. All of the production flowlines lie completely on the seafloor; there are no flowline risers to the spar. The manifolds under the host connect the offset wells and the local producing wells to subsea separation and artificial-lift systems using electrical submersible pumps (ESPs). The host nameplate capacity is 100,000 BOPD, 40,000 BWPD, and 200 MMscf/D, with seawater-injection capability of 80,000 BWPD.
In an effort to understand fluid behavior and risks, and to design for operational flexibility, extensive fluid sampling, analysis, and property evaluations were conducted during the exploration and appraisal phase. The Great White, Silvertip, and Tobago fields initially are being developed in two oil-bearing horizons that are (relatively) low temperature and low pressure, while exhibiting distinct fluid properties. Upper Wilcox fluids are fairly typical of light GOM oils, while Oligocene fluids are a heavily biodegraded, heavy, viscous oil.
Hydrate management presents a significant challenge for the deepwater environment, and hydrates are the primary flow-assurance solids risk for Perdido. Reservoir temperatures are low, gas/oil-ratio (GOR) values are potentially high, and significant expansion cooling in risers as a result of the large pressure drop is a dangerous reality. The hydrate-formation region is especially large for the Upper Wilcox fluid, with a hydrate-dissociation pressure of 160 psig at 40°F, accompanied by particularly aggressive hydrate-formation kinetics. Both fluids demonstrated solid-hydrate-plug formation in laboratory-test cells, even at lower pressures and moderate subcooling. The project leveraged the effect of subsea separation, which presents a beneficial shift of the hydrate curve up to 10 to 20°F for the liquid stream because of degassing, as well as elimination of the extreme gas- expansion cooling effect that would be experienced in a naturally flowing riser.