Flow Increase in the Trans Alaska Pipeline Through Use of a Polymeric Drag-Reducing Additive

Abstract

Summary The first commercial use of a polymeric drag-reducing additive to increase the flow rate in a crude oil pipeline began during 1979 in the Trans Alaska Pipeline System (TAPS). This paper discusses the details of the two drag-reduction experiments conducted in TAPS that ultimately led to the decision for full-scale additive use. The logistics pertaining to and the benefits resulting from the current application also are discussed. Introduction Drag reduction is a well-known phenomenon first observed in the laboratory almost 35 years ago.1 Although extensive experimental studies of drag reduction are continuing, the mechanism of drag reduction is not well understood. In spite of this limitation, there is a wide potential application for drag reduction in the oil industry in both production and transportation operations. Drag reduction, as defined by Savins,2 is the increase in pumpability of a fluid caused by the addition of small amounts of an additive to the fluid. The effectiveness of a drag reducer is normally expressed in terms of percent drag reduction. At a given flow rate, percent drag reduction (DR) is defined asEquation 1 where ?p is the base pressure drop of the untreated fluid and ?pp is the pressure drop of the fluid containing drag-reducing polymer. Percent drag reduction is a measure of drag-reducing additive performance, but it does not reflect the primary end use of drag reducers. Normally, the increased pumpability is used to increase flow rate without exceeding the safe pressure limits within the flow system. The relationship between percent drag reduction and percent throughput increase (TI) can be estimated using the following equation.3Equation 2 where %DR is the percent drag reduction as defined in Eq. 1. Eq. 2 assumes that pressure drop for both the treated and untreated fluid is proportional to flow rate raised to the 1.8 power. The volumetric capacity and/or horsepower of the existing pumps may limit the amount of throughput increase. The installations of additional pump capacity may be necessary to obtain the full benefits of drag reducer use. The use of drag reducers in the oil industry has been limited primarily to hydraulic fracturing operations.4 Water-soluble polymers have been effective in reducing hydraulic horsepower requirements and/or increasing pump rates during fracturing treatments. Many water gels used to improve the sand-carrying capacity of the fracture fluid have excellent drag-reduction properties. Oil-soluble polymers have been used to a lesser extent in hydraulic fracturing with oil-base fluids. Drag reducers have not been used routinely in standard crude oil pipeline operations.