Microwave processing heats up oil drill cuttings
UK researchers are looking to scale-up a microwave system to treat oil-contaminated waste from fossil fuel exploration on and offshore.
BP Exploration and British Gas are supporting the research, which is led by a team at the University of Nottingham. The technology is said to be more energy efficient and cost effective than existing thermal desorption methods, as well as providing a small, portable and modular system suitable for in situ activity on offshore rigs.
Drilling activities in mature fields, such as the North Sea, generally require oil-based muds (OBMs) for deeper well sections, high angle wells and poorly consolidated rock formations. The discharge of cuttings contaminated with OBMs can have a detrimental impact on the marine environment and are subject to stringent regulations governing their disposal.
The UK’s implementation of European legislation (OSPAR 2000/3), for example, dictates that drill cuttings discharged into the North Sea must have residual oil levels of less than one per cent in weight. Cuttings from North Sea-based rigs, however, typically contain OBMs of 15% or higher.
Researchers at Nottingham have used microwave processing to treat 750kg/hr of drill cuttings in a pilot plant, reducing oil levels to under 0.1%, which is well below the threshold for marine disposal and also applicable for onshore landfill.
Only substances with a high dielectric loss convert microwave energy to heat. The team has exploited this to selectively heat the water in the matrix. The rock fragments and oil in the mix are low dielectric loss materials, meaning they allow microwaves to pass through them with little absorption.
Professor Sam Kingman at Nottingham explains, ‘The water is in the structure of the material and the oil tends to be located on the surface. If you superheat the water, it carries the oil droplets with it as it leaves the rock’. Steam distillation then facilitates oil removal at lower temperatures, allowing the oil to be reused while the water and treated cuttings are ready for disposal.
Selective heating of the water, which is typically 10% of the mass, is said to make this an energy efficient and safer process. Current indirect thermal desorption (ITD) methods heat the bulk material. Onshore ITD systems typically employ a hollow rotary heat exchanger through which oil is passed to heat the cuttings, while offshore the mechanical energy of a friction-based system heats the waste.
The latter ‘is an expensive process to run and is not always cost effective versus shipping cuttings for onshore treatment’, explains Paul Page, Drilling Environmental and Waste Management Specialist at BP. ‘It is also difficult to install on many existing mobile drilling rigs, where space is a premium. Microwave processing opens up a much wider onshore and offshore market for the use of OBMs.’
Furthermore, compared to conventional thermal processing, where energy transfer is limited by conductivity, microwave energy is delivered through molecular interaction with the electric field, enabling heating of all individual elements of the material instantaneously. Processing can therefore take less than one per cent of the time traditionally required. The energy cost, says Kingman, is about 75-80kWhr/t, depending on the water content.
He adds, ‘We want to keep the electric field as high as possible to give us the highest possible power density in the water phase. The higher the density, the more watts per metre cubed in the water and the more efficient the process due to less heat transfer’.
The fact that the base oil can be reused at the site is an added benefit. Kingman says, ‘In the conventional process, you heat the oil to high temperatures (about 300ºC), so you change the properties. But because we collect it in liquid form at low temperatures, there is little change to its structure’.
Engineering a change
Scaling up the microwave process, however, will be a challenge, requiring the collaboration of specialists in microwave and electromagnetic engineering, bulk solids handling and process engineering. Engineers at Stellenbosch University in South Africa and Greenwich University, UK, are offering their expertise.
‘One of the major challenges is to design an applicator and cavity for high value power density heating that is even across the width,’ says Kingman.
‘Another point is that the oil and water content of the cuttings will vary greatly, which means designing an applicator and cavity that allows us to process material of variable properties. The other method is to develop a way to control the feed, if necessary, by blending in dry materials (residual cuttings produced by the system) to bring down the water content and improve throughput.’
The team is negotiating with oilfield service companies to license the technology for
‘The operational system must be light, small, cheap to operate and maintain, and have a high process rate and built-in redundancy,’ notes Dominic Thomas, Team Leader in Drilling at British Gas. ‘Cost effectiveness will depend on the final design.’