Experimental study on waxy oil-water horizontal flow at temperatures above the wax appearance temperature

Abstract

Temperature sensitivity of waxy crude oils makes it difficult to study their flow behaviour in the presence of water especially near their wax appearance temperature (WAT). In this study, a method was proposed and implemented to mitigate such difficulties by predicting mixture temperatures prior to experimental flow of a typical Malaysian waxy crude oil and water in a designed horizontal multiphase flow loop. To observe this method in action, mixture temperatures, pressure drops and liquid holdups were experimentally measured for mixture velocity ranging from 0.2 to 1.7 m/s in a carbon steel horizontal pipe at three different temperatures slightly above the WAT. Several correlations were also applied to predict the pressure gradients and their results were compared with the experimental values. Accordingly, flow patterns were determined by considering a combination of visual observations, pressure drop interpretations and free water measurements. Moreover, the effect of emulsified water droplets on accelerating the wax crystallization process above the WAT under dynamic and static conditions was examined in connection with the results of the two-phase flow experiments. The results showed the success of the proposed method in predicting the mixture temperature with an accuracy of ±0.5 °C. The results of pressure drop revealed a dependency on mixture velocity, input water fraction, flow pattern and the parameters that flow pattern is a function of (such as pipe wettability, superficial velocities, and oil composition). In dual continuous flows, the performance of twofluid model was comparatively better than homogenous model with average deviation of 17.9 and 26.7%, respectively. Despite operating the experiments above the WAT, the deposition of wax crystals on the pipe wall was evidenced for some of the flow patterns which, by implication, authenticates the influence of emulsified water on elevating the WAT in dynamic flow conditions. Classification of the flow patterns based on the wax deposition yielded an original flow pattern map composed of nine patterns among which new configurations were evidenced for annular flows. In addition, all the flow patterns were affected by the entrance effect and a layer of water-in-oil emulsion was observed for all the flow conditions. From the experiments under the static conditions, a sharp increase in the WAT was found with the presence of water in the system, regardless of the volume of water. Greater deviations became apparent at higher water volume fractions and rotational speeds, which resulted in the formation of a larger number of droplets. The results of this study provide a progressive introduction to help flow assurance engineers to understand the process of wax crystallization and deposition under two-phase flow conditions in horizontal pipelines, and to ultimately develop more effective wax management strategies.