Studies of Wax Deposition in the Trans Alaska Pipeline

Abstract

Summary This paper presents the results of a study to investigate mechanisms of wax deposition and to determine the expected nature and thickness of deposits in the Trans Alaska Pipeline System (TAPS) as a function of time and distance. Deposition is believed to occur as a result of lateral transport by diffusion, shear dispersion, and Brownian diffusion. Introduction Operating TAPS at a high flow rate for a period of time will result in the Sadlerochit crude oil remaining at a relatively high temperature along the entire length of the pipeline. When this condition is attained, there will be no significant deposition of waxy crystals from the oil on the pipe wall. Early in the life of the pipeline, however, the oil cools significantly and large amounts of waxy crystals are available for deposition. This condition might be viewed as a startup phenomenon, but with the flow rate at 1.5 million B/D after more than 3 years of operation, deposition is still occurring in the southern portion of the pipeline. The purpose of this study was to investigate mechanisms of wax deposition and determine the expected nature and thickness of deposits in the pipeline as a function of time and distance. The results of the study indicate that deposition during start-up is a consequence of three separate mechanisms which transport both dissolved and precipitated waxy crystals laterally. When oil is cooling, a concentration gradient leads to transport by molecular diffusion with subsequent precipitation and deposition occurring at the wall. In addition, small particles of previously precipitated wax can be transported laterally by Brownian diffusion and shear dispersion. The studies show that a small fraction of the wax crystals that are being carried along in the bulk oil thus can be transported laterally and incorporated into the immobile deposit. The total immobile deposit is believed to consist of approximately 14 to 17% solid phase in a porous structure with the pore spaces being filled with liquid oil. For the Sadlerochit oil, deposition will occur when the oil is flowing at temperatures below a minimum of about 40 degrees C and cooling. In this paper, experimental and mathematical investigations of wax deposition are reported. Measurements of physical properties included a determination of (1) wax solubility, (2) precipitated wax composition, (3) size distribution and settling rates of precipitated solids, and (4) molecular diffusion coefficient of wax dissolved in the crude oil. Studies of the mechanisms responsible for lateral transport and deposition on the pipe wall included theoretical and mathematical analyses as well as the measurement of actual deposition rates in scaled laboratory tubes. The findings of this laboratory study were confirmed in a field test which used larger-scale pipes. The insight and scaling concepts developed were part of the information used by Alyeska Pipeline Service Co. during the development of a pipeline start-up program which was consistent with the anticipated wax deposition. This investigation also has been helpful to Alyeska in understanding subsequent operation of the line. Operating experience appears to have substantiated the concepts developed by the laboratory and field experiments. Characterization of Wax According to Reistle, "The deposits of crude wax or paraffin that accumulate in flow lines . . . consist of very small wax crystals that tend to agglomerate and form granular particles of wax about the size of the grains of ordinary table salt. JPT P. 1075^