Case Study: Heating Offshore Production Tubing Electrically for Crudes that Have Wax Issues

Abstract

Wax buildup that would restrict the flow of the oil from the reservoir to the offshore platform was expected in several wells offshore New Zealand. Alternatives such as hot oiling, scraping and chemical injection were rejected, since electric downhole heating was the most efficient, economical, reliable, and environmentally friendly solution. The use of a unique, skin-effect electrical heat tracing system inside the production tubing was selected as the most efficient and cost-effective method. This heat tracing system involved a sealed coiled tubing, with an internal cable and filled with di-electric oil, that was installed in the production tubing. This created a skin-effect heater with maximum heat transfer and also ease of installation. The use of heaters inside the production tubing provide a more effective and efficient system for flow assurance than external heaters on the tubing would. The heaters, called skin-effect heat tracers, covered the length from the reservoir to the topsides, some 1,700–2,000 meters (5,577–6,562 ft.). The reservoir temperature was only 50 0C (122 0F) and the crude had a Wax Appearance Temperature of 48 oC (118.4 0F), so even minor cooling could potentially cause wax buildup problems. Typically well flows were heated effectively during operations to ensure that the tubing head temperatures were 60-65 0C (140- 149 0 F) so that wax was never deposited on the tubing. The heaters can easily be adjusted at the surface to provide the required heat under varying operating conditions. Skin-effect heating is an efficient, reliable, and easily adjustable method to prevent flow assurance issues that can be retrofitted to existing offshore or onshore wells The total system is very space efficient both downhole and on the platform. Some of the conclusions include that the flow assurance issues of wax buildup and the resulting restricted oil flow were resolved in an economical and efficient manner. Also, this unique, first application of skin-effect heat tracing downhole for a deep offshore well provided reliability and economic returns on investment. Finally, this method can also be used to prevent other flow assurance issues such as viscous oils and to prevent hydrate formation.