Introduction to Enhanced Oil Recovery (EOR)
The extraction rate of crude oil from an offshore well depends on the pressure in the reservoir, which forces the oil up the well to the production platform. Once this pressure starts to drop, the output from the reservoir also reduces. To counteract this, a system known as enhanced oil recovery, which involves the pumping of high pressure fluids or gases into the reservoir to recover the remaining oil, is used.
This is an article on the method of recovering crude oil from a mature reservoir, and we begin with an overview of offshore crude oil production.
Crude Oil Production
Initially oil is extracted due to the pressure inside the oil reservoir which forces the crude up the well onto the production platform where sand, condensates, gas, and water are removed before being pumped ashore via the subsea pipelines.
This is the primary production phase may remove only about 15% of the recoverable oil from the reservoir so when extraction rates fall below a certain level, the secondary production phase takes over.
One method of secondary production is the use of Enhanced Oil Recovery where different fluids and gases are pumped through injection wells drilled into the reservoir rock. This enables the fluids or gases to gather the oil from pockets in the rock and shepherd it towards the production wells.
Fluids and Liquids Used in Enhanced Oil Recovery
- Water – usually the first fluid used as there is plenty of it about
- Inert Gas – this has to be produced from ambient air in an on-board specialized plant
- Steam – used where the oil is particularly viscous
- CO2 – used to increase the reservoir pressure
- Micellar Polymer Flooding – a relatively new method that will be addressed in a future article
Water Injection Enhanced Oil Recovery
Also known as water flooding, this is usually the first type of EOR used as there is plenty of water about offshore.
However seawater is not the first choice. Produced water is used in most cases, and as this contains hydrocarbons, it needs to be treated before being disposed of overboard. This is a cheap means of getting rid of it, but it still requires cleaning up and filtration before use.
Produced water, as the name suggests, is water removed from the oil, gas, and condensates during production processing.
It is supplied to the injection wells by high pressure and by flow-dedicated water injection pumps that can be to sixteen stage pumps running at 5000 RPM. If seawater is to be used, it is normally taken from the platforms deoxygenating unit as an anti-corrosion precaution.
Water injection is used for two purposes;
1. Pumped directly into the oil reservoir it sinks to the bottom where it gradually builds up pressure, displacing the oil floating on top and forcing it up the production well.
2. Pumped into injection wells drilled in selected locations throughout the reservoir rock. Being under high pressure it pushes any oil trapped in the porous rock towards the production wells.
Both methods require a substantial amount of time to begin to show improvements, however once established, represent a long-time investment, extracting up to 35% of oil which would otherwise remain stored in the reservoir.
CO2 Enhanced Oil Recovery
CO2 is separated from natural gas as part of the production process on an offshore gas platform, it can then be pumped over to a crude oil production platform and used in EOR.
There are several ways that CO2 can be used, the two popular ones being shown below:
1. Gas Lift
This method involves the fitting of specialized valve (may have been fitted at initial well drilling and casing assembly) which monitors and controls the gas pressure difference in the well casing and the reservoir. This enables the gas to be injected directly into the reservoir where it bubbles through the oil aerating it and lowering its density thus making it flow easier.
2. Liquid CO2 (this will be covered in future article)
Thermal Enhanced Oil Recovery
Three methods employed in this type of EOR
1. Cyclic Steam Stimulation (CSS)
Here the steam is injected into the reservoir on a continuous basis. After some time the crude becomes heated and turns less viscous. As the steam pressure builds up, the oil begins to flow up the production well.
2. Steam Assisted Gravity Drainage (SAGD)
Two horizontal wells are drilled into the reservoir, and the top well being supplied with steam sprays it onto the heavy oil making it less viscous and able to flow downwards towards the lower well.
The lower horizontal well collects the heated oil which flows towards the production well and up to the platform.
3. Steam Flooding
Numerous vertical wells are sunk into reservoir rock through which steam is injected. The steam gathers the oil in front of it pushing the oil towards the production well.
Due to the large amount of heavy oil reservoirs abandoned because of the cost involved in extraction, thermal enhancement oil recovery is becoming popular in American oil fields.