Abstract
The rheological properties of Prudhoe Bay oil, as with any other mixture of hydrocarbons, are markedly affected by the lowering of temperature. The Trans Alaska pipeline traversing the state in a north-south direction is pipeline traversing the state in a north-south direction is subjected to severe ambient temperatures during the winter months. A prolonged flow interruption would result in inevitable heat losses from the trapped crude oil. The temperature decline would cause a significant alteration of the flow behavior. A fundamental heat-transfer study and laboratory measurements were combined to forecast the rheological response and subsequent start-up requirements of Prudhoe Bay oil in gathering lines and in the Trans Alaska pipeline.
Introduction
The Prudhoe Bay reservoir is the largest reservoir discovered in North America. The recoverable reserves are estimated at 9. 7 × 109 bbl while the recoverable gas reserves are estimated at 26 × 10–12 scf The Trans Alaska pipeline, roughly 800 miles long, joins the North Slope fields with the port of Valdez. The flow rate is 1. 68 × 106 B/D as of Nov. 1982. During winter. the unburied portion of the pipeline (roughly one-half the total length) is subjected to ambient temperatures that may reach -70deg. F. The flowing temperature of 140deg. F is sustained through a 4-in. thick insulation and the addition of kinetic energy through the various pumping stations. About 80% of Alaska is underlain by permafrost. The entire North Slope and much of the state's western half are in a continuous zone of permafrost, where the phenomenon is found nearly everywhere. The seasonally phenomenon is found nearly everywhere. The seasonally frozen active layer is commonly only 1 ft thick in these regions, but the underlying permafrost is more than 1,000 ft thick in many places. Most of the rest of the state is in a discontinuous zone, where permafrost is progressively more sporadic from north to south, progressively more sporadic from north to south, diminishing to isolated, small masses of permanently frozen ground. Major soil-engineering problems arise where permafrost occurs in poorly drained, fine-grained soils. permafrost occurs in poorly drained, fine-grained soils. especially silts and clays that are "thaw-unstable. "Such soil generally contains much ice. The volume of ice can be much greater than the void volume of the thawed soil. In fact, while the soil may be pure, thawing would produce excess moisture. The result can be loss of strength, produce excess moisture. The result can be loss of strength, settlement, and soil containing so much moisture that it becomes mobile. These possibilities were accounted for in the design of the pipeline. In much of the discontinuous zone, the permafrost is just below 32deg. F, and the addition of as little as 0. 5deg. F in some places will induce thawing. This is termed "warm permafrost" as compared to the "cold permafrost" of permafrost" as compared to the "cold permafrost" of the North Slope. To keep the permafrost stable around the pipeline, about half its length is erected above ground on a unique system of vertical support members (VSM's) that permit the oil to be moved through the pipeline without disturbing the stability of the permafrost below the line. An interruption of the flow would result in lowering of the fluid temperature, with subsequent elevation in viscosity. Further decline in temperature would result in gelling or even waxing of the oil. Perkins and Turner calculated the starting behavior of Prudhoe Bay crude on the basis of laboratory measurements. They found that several factors affected the yield strength of Prudhoe Bay oil: temperature history, shear history, aging, and composition. Ackroyd et al. and Ford et al. reported similar factors affecting the behavior of other oils. While Perkins and Turner dealt only with gelled oils, our paper reports the viscosity levels as well as the thixotropic and pseudoplastic phenomena observed for a wide range of temperatures.
SPEJ
p. 408
Knowledge Co-production in Contested Spaces: An Evaluation of the North Slope Borough – Shell Baseline Studies Program
