Investigation of Wax Depositional Behaviour in Straight and Curved Pipes – Experiments and Simulation

Abstract

Deposition wax is one of the chronic flow assurance problems that continue to attract attention in the oil and gas field, particularly as the hydrocarbon explorations and productions in recent years move into a colder environment. The accumulation of the wax deposit in the pipeline reduced or interrupted the production, and in the worst-case completely clogged the pipeline, resulting in equipment failure or field abandonment. Although significant progress has been made in mitigating the problem, many questions about the full understanding of the factors and the physical process mechanisms of deposition remain unanswered. Predominant researches are being conducted in straight pipes. The impact of pipe curvatures such as pipe bends, elbows, and inclinations (found in production and transportation systems) on the increased severity of the deposition is not well studied, only very limited studies being reported. In this thesis, the deposition of wax was studied in a single-phase flow using a novel laboratory-scale flow loop accommodating three different pipes configurations: a straight pipe and curved pipes with 45°and 90°bends. Series of experiments were carried out using a crude blend with and without chemical inhibitor at different flow rates–under laminar(Q!#; 3, 5, 7 l/min) and turbulent flow (Q!#; 9, 11 l/min) conditions and at different ambient cooling temperatures of 10, 15, 25 °C(T$!!#≤pour point temperature of the oil) and 30, 35, 40 °C(T$!!#≥WAT of oil). The experimental period was varied from 2, 4, 6, 8 hours. Prior to the flow loop studies, three different waxy crude oil samples and four commercial wax inhibitors are screened and characterised through different standard techniques. The wax content of crude oil samples A, B,and C was found to be 12.05, 20.05,and 14wt%, n-paraffin distributions ranges from C15-C74, WAT and pour point respectively. Similarly, SARA (saturated/aromatic/resin/asphalte)fractions, wax appearance temperature, viscosity, pour point, and the colloidal instability index of the samples was obtained. Anew wax inhibitor was developed by blending different fractions of commercial inhibitors improved the performance and transformed the needle-like or rod-like crystals structure of the oil samples into an agglomerate with small particles dispersed in the oil matrix. The flow loop studies showed that the rate of wax deposition without inhibitor is significantly higher in curved pipes than in the straight pipe. In horizontal flow study –the test section with 90°bends pipes produces the highest wax deposition rate compared to the straight pipe test section and a test section with 45obends pipes.For instance, under a low case scenario (i.e. at T$!!#,15°C andQ!#,5 l/min), around 22%of the wax deposition rate was seen in straight pipe test section after 2 hours compared to36% and 43.5% of the wax deposition rate in test sections with 45Obend pipes and 90O bend pipes sections.This implies at the same conditions where the system experienced 0% wax deposition in the straight pipe it underwent 14% and 19% deposition, respectively, for the pipe with 45Oand 90Obends. However, the severity of wax deposition in curved pipes simultaneous decreases with increasing coolant temperature and flow turbulence and vice versa.On the other hand, increase in elevation of the pipes, however, led to a more wax deposition;higher percentage of wax deposit was observed in 45-degree compared to 90-degree curved pipe.Therefore,contrary to some theory established in the literature, this research concluded that gravity settling mechanism plays a role in the deposition process alongside with other mechanisms.The study with the new blended inhibitor produced a significant reduction of wax deposition.At 1500-ppmandata cooling temperatureof15°C, around 80% of the wax deposition problem could be reduced under a turbulent flow regime. At the same flow rate but at a relatively higher temperature of 25°C(i.e. pour point temperature), 100% wax prevention was reached even at a lower concentration (500-ppm). Whereas, in the laminar flow regime (e.g.7 l/min), 100% wax deposition prevention could only be achieved at the higher concentrations of1000 and 1500-ppm at 30°C.On the other hand, the experimental results under different conditions were simulated with OLGA software.The predicted results are in good agreement with the experimental data, particularly in the straight pipe.Base on the flow loop design, anew correlation using pressure drop a relationship was developed for quantification of wax deposit thickness in curved pipes. Generally, the established model relates wax deposit thicknesses, with a bend angle of the pipes and pressure drop due to the wax build-up with reasonable agreements to the published data, especially at steady-state conditions.