The formation and stability of emulsions in the oilfield are known to be affected by a number of factors: temperature, pH, heavy fraction in crude oils, solids and droplet size.
Temperature affects the viscosity of the oil, as well as the properties of the produced water, interfacial films and emulsifier solubility in the crude oil and water.
Likewise, temperature causes the energy of a fluid to rise, thereby increasing the number of droplet collisions and coagulation.
Changing pH levels influences the ionisation in the interfacial films of organic acids and bases and asphaltenes and solids. Heavy fraction in crude oil also affects emulsion formation and stability, as naturally-occurring emulsifying agents are found in higher boiling point, polar fractions. Fine solid particles can stabilise a crude oil emulsion by diffusing to the oil-water interface, where a film forms and hinders the coalescence of droplets. And Stoke's Law, which describes the settling of particles in an ideal situation, helps explain that an emulsion that has a smaller average droplet size is generally more stable.
When treating emulsions, the addition of chemicals is most common. Demulsifiers are surface active compounds that migrate to the oil-water interface and rupture or weaken the rigid film, thereby enhancing coalescence of water droplets. A demulsifier molecule can operate by one or more of several mechanisms:
- Preferential adsorption onto the oil-water interface and displacement of pre-existing stabilising emulsifying agents, thus removing the steric barrier.
- Spreading of the demulsifier can then occur via hetero-droplet interaction and can help to explain why some demulsifiers act immediately to cause rapid coalescence of water droplets.
- Good demulsifiers adsorb to the interface as the interfacial film is stretched. Strong partitioning and mobility to the interface are important characteristics, and the demulsifier causes the film to rupture and the water droplets to coagulate.
- Solids wetting characteristics are preferentially changed by effective demulsifiers to either totally-oil-wet or totally-water-wet, thus encouraging solids off the interface and into one discreet phase. It is generally more desirable to move solids to the water phase.
It is hardly surprising that any given demulsifier formulation is highly specific to an individual field or process system because they are sensitive to crude oil type, pH, salt content and temperature.
Clariant Oil Services has a recognised, proven track record in the development of total treatment solutions to complex separation challenges. Demulsifier-based chemistries and the development of formulated products are key to competency within the organisation. Over the past five years alone, our research and development into demulsifier chemistries has continued with the introduction of many new and environmentally acceptable bases offering a range of different advantages. These new chemistries are developed and synthesised in-house at our research and development location in an integrated part of our manufacturing plant.
Looking for new and innovative chemistries and variation developments is a continual improvement process. Once demulsifier bases have been established and approved for field trial, they are screened in a variety of locations in order to identify the best performing chemicals within a series or family, which leads to the final scale up and commercialisation of the top performing blended formulations as final products.
Research testing is frequently performed with a mix of produced fluids in order to identify new chemistries for the more challenging environments, such as the North Sea, California, Venezuela, Canada and Africa.
As a result of our targeted research and development, we have developed new demulsifier intermediates to be classified as 'gold' with no substitution warnings - the very best environmental category in the North Sea.
This case history tracks the continual improvement cycle for the fluid separation process from a heavy oil field over a period of three years. One of the major challenges posed by the operator of this 13 to 16° API crude oil was to discontinue the injection of two separate demulsifier formulations and begin injecting a single demulsifier product. This was not an easy task due to the very different conditions that existed at the two injection locations. The first location was at a series of injection points upstream of the gathering stations out in the field, prior to separation, and the second was at the process plant. The operator required a very strict 0.5 per cent BS&W on the crude exiting the plant for sales.
A schematic of the battery plant has been given in Fig. 1. Injection of two different demulsifier products in the field and at the plant occurred for several years (both were mixtures of EO/PO block polymers and alkoxylated amine based resin molecules) with relatively trouble-free operation - the export BS&W was in specification. However, in mid-2007 it was a requirement to not only move to single demulsifier products for application in the field and the battery, but also to decrease the BS&W to 0.2 per cent. The reasons for these changes were to ease logistical and operational issues by moving to one product and the BS&W specification was to have more of a buffer for the sales specification.
Fig. 2 shows the evolution of the export BS&W with total oil and water flow rates. There is a direct relationship between the BS&W and the volume of water the field produced and handled at the plant. This would suggest that the water handling limitations of the plant were being exceeded.
Residence times would also decrease as the produced water rate increased from 8,000 to 16,000 m3/day.
A slow increasing trend in the BS&W occurred from early 2005 to the end of 2006 (A), and this could be explained by increasing water production rates and generally decreasing demulsifier injection rates. At the beginning of 2007, an increasing trend was experienced in the BS&W (B), and this was commensurate with a marked increase in the produced water rate. By June 2007, the new single demulsifier product was introduced and, even though further increases in produced water rates occurred, the increasing trend in BS&W was halted, where it has maintained an average of 0.4 per cent (C). While the change in demulsifier did not decrease the BS&W to the target of 0.2 per cent, the change was positive because it ensured faster water drop and more efficient separation at the plant, even with massive increases in gross fluid throughput, and resulted in maintaining a constant BS&W. Optimisation continues with the new product combination to reduce the injection rate to the most economical concentration.
From field testing, sampling and the demulsifier optimisation exercise, the following conclusions can be made:
- By injecting two separate demulsifier products prior to mid-2007, operations were relatively trouble-free. A change to the BS&W requirement and a desire to move to a single demulsifier product applied in the field and at the process plant gave significant treatment challenges.
- Redesigning the bottle test procedure was required in order to more accurately mimic the field application of a single demulsifier product.
- The general increase in export BS&W was most likely attributable to the large increase in water production.
- The change in demulsifier chemistry was positive because it halted the increasing trend in BS&W commensurate with water rate increases.
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Jonathan J Wylde is UK Business Manager, Clariant Oil Services, Dyce, Aberdeen, UK. www.oil.clariant.com