Oil Drag Reducer Performance in Horizontal Two-Phase Annular Regime

Abstract Most of the work done on drag reduction, in literature, has been with water-soluble polymers/surfactants. Drag reduction for hydrocarbon system is more challenging, especially for a two-phase system; hence very little work is done in this area. In this work, the experiments are done at 40o C and 10 bars with crude oil (about 29oAPI gravity) and nitrogen gas (90% purity). The used polymer is a high molecular weight oil-soluble poly-alpha-olefin. The limited published work on drag reduction in two-phase systems leaves the effect of liquid and gas superficial velocities on drag reduction (DR) unresolved. The controversial conclusions in literature on the effect of superficial liquid and gas velocities may partly be due to the shear degradation of the polymer in the system where at below a certain velocity the shear degradation dominates, hence DR appears to decrease with increasing liquid velocity. The other part is related the entrainment of liquid in gas phase which is high at low gas velocity. Polymer stretching and velocity profiles demonstrate that at highest tested superficial liquid velocity (~2 m/s), large increase in the degree of stretching and in the elasticity, respectively. While at the intermediate velocities, inconsiderable differences in polymer stretching and elasticity are observed. Introduction The application of drag reducer polymers at low concentration has shown to reduce the pressure drop in single-phase and multiphase fluid flow and thereby increase the production rate (1–6). The first large-scale commercial use of drag reducing polymers took place in the Trans-Alaska Pipeline in 1979. Sylvester and Brill (1976)14 found that the highest drag reduction occurred at the highest superficial gas velocities, which were approximately 35% of air-water two-phase flow using polyethylene oxide at 100 ppm. Al-Sarkhi and Hanratty (2001a)8 studied drag reduction in an annular air-water two-phase flow for 9.53 cm diameter pipe. They used co-polymer of polyacrylamide and sodium-acrylate as drag reducer. They obtained drag reduction in the range of 10–63% for Ugs and Uls in the range of 28–52 m/s and 0.03–0.20 m/s, respectively. These findings are in agreement with Hamouda and Islam (2009)30 where drag reduction increased with increasing Uls and decreased with increasing Ugs. Kang and Jepson (199911, 200012) and Daas et al. (2000)9 studied drag reduction in horizontal or slightly inclined slug flow and annular flow using oil, gas (CO2), and, in some cases, a water phase for 10 cm diameter horizontal pipe at pressure 1.3 bar. They found that the drag reduction decreased from 50% at Uls=0.5 m/s to 30% at Uls=1.0 m/s, which means that drag reduction decreased with increasing superficial liquid velocity. Fernandes et al. (2004)7 used a two-phase, gas-condensate (methane and condensate close to that of decane) annular flow for 19 mm diameter pipe. They used poly-alpha-olefin as drag reducer. The experiments were performed at pressure of 11 bar. Superficial gas velocities 10.4–21.3 m/s and superficial liquid velocities 0.01–0.7 m/s were used. They concluded that drag reduction increases with increasing superficial liquid velocity. They also found that drag reduction decreases from 62% to 44% with increasing Ugs from 10.4 m/s to 21.3 m/s. Al-Sarkhi and Hanratty (2001)8 drew similar conclusions for a different system (water/air). The objective of this paper is to investigate the effect of liquid superficial velocity on drag reduction in annular flow regime. It is also intended to verify the model proposed by Fernandes et al. (2004), which was applied to gas condensate / methane systems.