Oil and gas processing normally occurs offshore on platform, unlike refining that takes place onshore. The aim of processing is to take raw produce or well fluid and turn it into a marketable product, i.e. crude oil, gas, and condensate. These products are often known by their benchmark name for example Brent Crude, West Texas Intermediate etc.
The reason for processing to take place offshore as opposed to onshore, is the need to reduce the products volatility and remove all contaminants i.e. water and sand, all of which can prove problematic during transport, whether by ship or subsea pipeline.
The process of raw produce is everything that takes place between the well Christmas trees and export lines. Dependent on the reservoir the well is coming from this can be in the form of oil, gas or a mixture.
As a very basic example of offshore processing, a diagram below has been used to show processing from mixed raw produce.
Diagram of production process
Christmas Tree To Production Separator
Between the Christmas tree and the production separator, the line pressure often has to be reduced to a lower pressure by means of a choke valve. This is normally when the well is relatively new, when there is enough downhole pressure to lift the produce. As the reserves start to deplete, the pressure will drop to become low pressure (LP). A modern day well will reach a point in its life when it is deemed necessary to inject either fluids or gas back in to the reservoir to increase pressure. This is known as artificial lift or gas lift.
The first true part of the production process is when the raw produce hits the production separators. At this point the fluid pressure can be up to 50 times atmospheric and have a temperature that is likely to be above 100°C. In a typical gravity separator, the fluid remain inside for around 5 minutes, allowing the gas to escape and rise above the oil content whilst the water content rests to the bottom. The three main components, crude oil, gas and produced water then get released from the separator to continue their process.
This process will be repeated, typically another two times before the raw well fluid is deemed to have completely separated in to its three different products, each time the oil continuing to the next separator.
The 2nd stage separator takes the oil from the 1st stage separator to further break down the different products. It can typically receive the fluid at 10 times that of atmospheric pressure and at a temperature below 100°C. It will also take fluid back off of scrubbers from gas processing.
The 3rd stage separator takes oil from the 2dn stage separator. At this stage the fluid will be at atmospheric pressure. The 3rd stage separator is also known as the flash drum. In purely gas processing, this can often be replaced by a knockout drum.
It’s common for crude oil to be put through a coalescer as its final stage of processing. Typically an Electrostatic coalescer will get the water content in crude oil down to below 0.5% volume.
Gas separated from oil and water within the separators must be further cleaned and compressed to ready it for export.
Gas leaving the 1st stage separator will head to a scrubber, allowing further fluid to be removed before it can enter the high pressure (HP) compressor. This must be done to remove any chance of damage to the compressors. Fluid removed from this gas is returned to the 2nd stage low pressure (LP) separator.
Gas separated via the 2nd stage LP separator will head to LP compression due to its low pressure. It, like the gas from the 1st stage separator must go through a scrubber to remove further fluids.
Once this gas has been scrubbed and put through the LP compressor, it will rejoin the HP gas as it enters the HP compressor.
All water removed from the well fluid is classed as produced water. This must be treated and cleaned before it can be returned to sea.
It is common for offshore oil platforms to store crude oil produced, in tanks, within its legs. For example the Hibernia, a concrete gravity base structure (GBS) platform offshore East Canada, has a storage facility within its structure capable of holding 1.2 million barrels of crude oil.
Metering And Export
Metering, specifically known as fiscal metering is the last and arguably the most important stage of the production process as effectively this is the point of sale. Each barrel or joule that leaves this point would now have already been sold through exchanges across the world and it is at this point ownership changes hand to the purchaser. It is also at this point that governments will collect tax on production.
Metering at this point will also be able to provide full product data to clients to ensure the specification is as benchmark i.e. Brent Crude or as ordered, including viscosity and water content.
Last updated on 10:34AM - 05/01/15