Wax inhibitors are sometimes used to reduce the rate of wax deposition inpipelines. The efficiency of the inhibitors depends on several factors such asthe right chemistry, injection or introduction at the correct location, targeting the right operating conditions and testing appropriately. It is knownthat bench top tests such as cold finger tests, while useful to qualitativelygauge chemical performance, are not useful to quantitatively predict theperformance of a chemical under field operating conditions. This is because theoperating parameters such as the temperature difference, heat flux, and shearrates experienced in the field cannot be reproduced in such bench top devicessimultaneously.
In this study, we discuss the use of a " cold disk" apparatus for screeningchemicals to reduce wax deposition rates of a relatively waxy offshore crudeoil. Several chemicals were screened by choosing temperature differentialssimilar to those expected in the field where maximum deposition was predictedto occur. The best chemical was then chosen for larger scale flow loop study. Flow loop deposition experiments were performed with the chosen chemical afterappropriately choosing the flow rate and heat flux to drive the deposition. Theresults showed quantitatively good agreement with the cold diskexperiment.
Testing and agreement at such different scales provided a greater degree ofconfidence in the efficiency of the wax inhibitor, although it is understoodthat field performance may still vary depending on various other factors someof which relate to operating conditions, fluid composition, wax content, pipeline size and shear. Modeling deposition in the flow loop also improved theconfidence in these results.
Wax deposition is a key Flow Assurance challenge for many offshore and othercold environment oilfields. Wax deposits as a gel on pipe walls; this gelconsists of solid wax crystals that entrap liquid oil within the crystalnetwork. Initially, a thin layer of wax-oil gel deposits on the cold pipe wall;as time progresses, the deposit grows in thickness and hardens. It has beenshown by several researchers that two conditions must be met for wax depositionto occur in a pipe. These conditions are:
The oil temperature near the wall must be lower than the wax appearancetemperature (Toil < WAT)
The wall temperature must be lower than the oil temperature (Twall <Toil)
Experiments performed with no radial heat flux (Twall = Toil) or an " inward" heat flux (Twall > Toil) have shown no deposition to occur (Hamouda andDavidsen 1995; Singh et al. 2000).
Prevention of wax deposition entails maintaining the fluid in the pipeline at atemperature above the WAT. While this is possible for shorter pipelines such asin-field tiebacks, it is typically not feasible to insulate long oil exportlines (~100 km) to be above the WAT. For oils that exhibit low to moderate waxdeposition rates, the usual mitigation strategy is to pig the line at regularintervals to remove the deposit. For oils that show high deposition rates, waxinhibitor chemicals are sometimes used to manage wax deposition in combinationwith pigging. Note that no wax chemical has been found to eliminate waxdeposition; the primary aim in using these chemicals is to reduce thedeposition rate.