Study on molecular motion behavior during the wall sticking process of waxy crude oil emulsion in gathering and transportation pipeline

Abstract

China possesses abundant reserves of waxy crude oil. However, as the operational temperature decreases in gathering pipelines, wax molecules within the crude oil progressively precipitate. This phenomenon triggers an increase in the viscosity of the crude oil. These changes exacerbate the wall-sticking propensity, leading to accelerated pipeline blockage.In this study, the wall-sticking behavior of waxy crude oil under varying water cut conditions is investigated through molecular dynamics simulations and experimental approaches. The simulation results revealed that as the water content increased, the density distribution value of water molecules on the pipe wall rose from 0.17 g/cm³ to 5.44 g/cm³, indicating enhanced distribution behavior of water on the wall. The wall sticking behavior of system is weakened. Under conditions of lower oil temperatures and increased wall surface roughness, the wall sticking behavior of waxy crude oil also became more pronounced. Furthermore, as wall surface roughness increases, the peak concentration of C₁₂H₂₆ molecules within microscopic wall grooves rises from 6.49 to 9.47, whereas the peak concentration of C₁₈H₃₈ molecules decreases from 4.63 to 0.01. This comparative analysis reveals that with prolonged simulation time, light components exhibit a stronger tendency to infiltrate irregular surface regions. Notably, as the surface roughness escalates, the structural reinforcement trend of the gelled crude oil becomes progressively more pronounced. Experimental analysis of wall sticking under different wall materials and water content levels demonstrated that as the water content increased from 70 % to 95 %, the accumulation area of gelled crude oil decreased from 468.0 mm² to 177.0 mm², indicating a gradual weakening of wall sticking behavior. The accumulation area of gelled crude oil for glass and stainless steel tubes were measured at 321.5 mm² and 358.2 mm², respectively. Consistent with molecular dynamics simulation results, it was observed that rougher tube wall surfaces exhibited more pronounced sticking behavior. The experimental and simulation results not only contribute to understanding the wall-sticking characteristics of gelled crude oil from the perspective of molecular motion at micro scales but also provides insights for the removal of wall sticking and reduction of accumulation of gelled crude oil in engineering applications.