OIL AND WATER SEPARATION AT ITS BEST master

Oil and water separation at its best

Looking for faster and more efficient separation of produced water from crude oil as well as increased production? Vessel Internal Electrostatic Coalescers (VIEC) have proved to be the solution for a range of major oil companies worldwide.

Text: KATJA ALAJA Photo: WÄRTSILÄ, SHUTTERSTOCK

The main purpose of any oil and gas production facility is to separate the oil, water and gas produced into their original phases. This is achieved by a stepwise reduction in pressure down to atmospheric pressure, flashing off the gas and then dehydrating the crude oil to meet its export specification of less than 0.5% water in oil.

Conventional gravity-based production separators allow the water to settle at the bottom of the vessel while the oil flows on top of the water phase. A dedicated electrostatic coalescer vessel is normally installed downstream of the production separators to remove the last water fractions.

On the other hand, gravity-based separators only remove the free water and have a limited effect on water emulsified into the oil phase. Emulsion layers in the separators are difficult to monitor and hence difficult to control, which causes carryover of excess water into the oil outlet. Also, traditional electrostatic coalescer vessels are equipped with uninsulated high voltage electrodes which cannot be exposed to gas and the more than 10-15% water remaining in the oil.

A game changer was introduced by the development of an electrostatic device with insulated electrodes, making it possible to handle 100% water as well as any gas present. This allows such devices to be installed into upstream production separators.

“We have successfully delivered and installed Vessel Internal Electrostatic Coalescers (VIEC) into more than 30 test and production separators treating crude oils ranging from API 12–50, with an equal split between retrofits and newbuilds and covering most of the major national and international oil companies,” says Trond Bynes, Director of Separation Technology at Wärtsilä.

CHALLENGES BEYOND EASY OIL

‘Beyond easy oil’ refers to the fact that most of the oil that is easy to extract has already been produced. Existing fields are maturing and experiencing more water coming out of the reservoirs with less oil, exceeding the design basis for the production equipment in use. Reservoir pressures are dropping, which leads to the installation of pumps and once again causes problems with stable emulsions in the separation process. Future oilfield developments will be more challenging as 50% of the world’s remaining oil reserves can be characterized as either heavy or extra-heavy oil.

Traditionally, heavy oils are separated by using huge separator vessels and allowing extra long periods for the water to settle, as well as adding large quantities of chemicals and heating the crude oil to temperatures of up to 150°C. Significant additional operational costs are inevitable.

“Improving the efficiency of the separation process by means of VIEC technology can reduce fluid temperatures below 100°C, allow the optimisation of separator vessel size and reduce the use of chemicals,” says Bynes. “This not only has a positive effect on CAPEX (capital expenditure) and OPEX (operational expenditure), but also on levels of CO2 emissions.”

FASTER AND BETTER SEPARATION

Coalescence of dispersed water in an oil-continuous phase can be greatly enhanced by subjecting the emulsion to high-voltage electric fields. This phenomenon is called electrostatic coalescence. When an emulsion consisting of a polar liquid dispersed in a non-conductive liquid is subjected to electric fields, several physical phenomena cause the droplets to merge. In a VIEC system, two primary effects can be identified. Firstly, an electrical dipole attraction causes droplets to coalesce. Secondly, the electric fields distort and weaken the film, i.e. the surfactant components surrounding the water droplets.

OIL AND WATER SEPARATION AT ITS BEST 1

Separation efficiency can usually be influenced by increasing gravity forces, density difference and the diameter of the water droplets or by reducing viscosity. Droplet growth caused by the electric fields therefore leads to a substantial growth in the settling velocity of the dispersed droplets and enhances separation efficiency. As the surfactant components prevent coalescence, proper chemical treatment such as a demulsifier is also needed.

“A VIEC system typically consists of 150–200 electrodes - depending on the size of the separator - forming a cross-sectional wall within the separator which allows the fluid to pass the electrodes and be exposed to the electrical field,” says Bynes. “Following electrostatic treatment, water quickly settles to the bottom of the separator and is piped to the water treatment system, while the crude oil flows across to the oil section.”

Originally developed by ABB Corporate Research Center in Norway in 1998-2001, VIEC technology is covered by worldwide patents.

INSTALLATION TAKES LESS THAN A WEEK

Production separators are the heart of any oil and gas production facility, offshore or onshore, all the way from the tough conditions in the North Sea to the hot deserts of the Middle East. Any shutdown of these separators will almost immediately hit oil companies’ revenue streams.

Installing VIEC technology has to be planned carefully. The equipment must be a correct fit at the first attempt and the installation process must be performed quickly to reduce downtime.

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In a VIEC system the electrodes usually form a cross-section wall. Water quickly settles to the bottom.

The whole installation also has to be performed through a manhole just 18-24 inches wide. As well as being designed to pass through the manhole, the component parts have to be bolted together inside the separator unit.

“To secure safe and on-time installation, Wärtsilä performs a full-scale test installation prior to equipment delivery,” says Bynes. “While installing a VIEC system inside a separator takes just a few days, additional time is required for depressurizing and cleaning prior to installation, and pressurizing and recommissioning after the installation is complete.”

Making a difference for Qatar Petroleum

Qatar Petroleum were experiencing issues with separation at their Dukhan Field, especially during periods of low winter temperatures when increased fluid viscosity resulted in dense emulsions. Surface tension forces and particles attaching themselves to the water-oil interface made these emulsions appear to be extremely stable.

“We carried out an oil study which involved characterizations of the crude oil and a separation study involving screening of both the chemicals currently being used and alternatives,” says Trond Bynes. “This revealed that the current choice of chemical should be changed and that a significant improvement in separation performance could be achieved by installing a VIEC system.”

Qatar Petroleum decided to install a VIEC System with the key objective of reducing basic sediment and water (BS&W) levels in the production separator from approximately 5% to below 2%.

The VIEC system was installed and commissioned by Wärtsilä in January 2012. The whole project was executed in less than a week.

Performance tests were carried out by a third-party laboratory hired by Qatar Petroleum to verify and confirm the system’s enhanced performance following installation of the VIEC solution. Testing took place over a two-week period.

According to Bynes, the results reported by the third-party laboratory were excellent. The BS&W levels measured were less than 0.2% throughout the test period, well below the target level of 2%. Salt content in the crude oil was also reduced by up to 90%.

OIL AND WATER SEPARATION AT ITS BEST 2

Crude oil goes to the laboratory

To deliver the correct VIEC solution for each customer, Wärtsilä analyzes and characterizes the crude oil that it will be handling. “Crude oils are very complicated liquids and are all very different by nature,” says Trond Bynes.

Customers send a 20–100 litre sample of crude to the Wärtsilä laboratory in Norway. Wärtsilä has already analyzed more than 80 different crude oils from all of the world’s primary crude-oil producing zones. In addition to a library of real crude-oil samples, this has allowed Wärtsilä to build up a unique database of knowledge.

Characterization of the crude oil includes determining its dielectric properties, viscosity, density, surface/interface characterization, asphaltene stability and emulsion stability, as well as inorganic/organic solid quantification and residual water quantification.

Following the characterization process, a range of separation tests is performed to verify the effect of a VIEC in combination with different chemicals. Separation testing focuses on retention time in the electrical field, chemical screening, turbulence, shear, recirculation patterns, flow entrainment of small droplets and other dynamic effects.

“As well as verifying the use of electrostatic technology to treat the crude oil, this testing also provides data for oilfield owners and operators which allows them to optimize and remove bottlenecks from the overall separation process. This can result in smaller separator vessels, reduced processing temperatures and reduced dosing with chemicals.”

VIEC Separation Technology

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