Starting Behavior of Gathering Lines and Pipelines Filled with Gelled Prudhoe Bay Oil

Abstract

The apparent yield strength of Prudhoe Bay crude oil is influenced by its composition and its thermal and shear history. Laboratory studies were conducted with this gelled crude to determine yield behavior in compressible and essentially incompressible tubes, and it is concluded that with proper design, the proposed Trans Alaska pipeline can be restarted without difficulty. pipeline can be restarted without difficulty. Introduction The discovery of large quantities of oil in arctic regions has led to a number of unconventional difficulties in producing and transporting oil under conditions of low air and soil temperatures. One of the problems aggravated by low temperatures is that of gelling of oils. Although this difficulty has already been encountered during transmission of high-pour-point oils, it is expected that gelling will be much more severe or will be encountered more frequently as ambient temperatures fall far below zero. This paper describes the study of a particular oil, that from the Sadlerochit zone of the Prudhoe Bay field. Although the gelling and rheological behavior will be described in detail for this oil alone, we believe that much of the material presented here will be generally suitable for understanding starting behavior of gathering lines and pipelines filled with other gelled oils. Factors Affecting the Yield Strength of Prudhoe Bay Oil The concept of a "yield strength" of a gel has often proved useful in understanding the behavior of gelled proved useful in understanding the behavior of gelled oils. If it is imagined that a sample of gelled oil in a tube will exhibit a specific yield strength, then experimentally the pressure differential across the length of tube can be increased until the oil just begins to move. The force pushing the gel, D2 p/4, is set equal to tained by monotonically cooling the oil at less than the shear resistance along the wall, DLt, and we can calculate a yield strength of the gel. Although the rheological study described later will show that this concept is quite oversimplified, yield strengths determined this way can reveal the qualitative effects of variables on gel behavior. Using this approach, we have found that the following factors will significantly affect the yield strength of Prudhoe Bay oil:temperature history,shear, Prudhoe Bay oil:temperature history,shear,aging, andcomposition. Many of these same factors have been reported for other oils by previous investigators. Temperature History To investigate the effect of any variable, it is necessary first to return the oil to what is thought to be its reservoir condition. For Prudhoe Bay oil this is accomplished by reheating it to 180 degrees F (approximately the reservoir temperature) and holding for at least 1 hour. After this "beneficiation" treatment, the oil is relatively insensitive to any thermal treatment that it receives at temperatures above about 100 degrees F. However, at less than 100 degrees F three aspects of its subsequent thermal history measurably influence yield strengths. P. 301