Produced fluids from the oilwell are diverse, complex fluid mixtures that contain various amounts of water, oil and gasses. Crude oil is a mix of over 200 different compounds. The different combinations of these compounds affect the properties of the fluid. Produced water may contain high amounts of brine, may have high amounts of hardness or contain dissolved corrosive gasses.
Some (but not all) of the properties of the produced fluids are:
- Emulsion tightness
- API Gravity of the oil
- Specific Gravity of the produced water
- Gas Oil Ratio (GOR), Oil / Water Cut
- Scaling Tendencies of the Water
- Paraffin Forming Tendancies of the Oil
- Inlet Pressure & Temperature
- Desired Outlet Pressure, Temperature & percent oil outlet
- Corrosiveness of the production fluids
- Presence of Hydrogen Sulfide
- Operation and Maintenance Requirements
The fluid properties vary with temperature, temperature and treatment process. Designing of the overall system requires consideration of several factors.
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Oil Well
The oilwell is the point at which the oil leaves the formation and enters the gathering system. There are many different designs depending on the method of extraction of fluid. Types of well depend if the well is on:
Manifold & Measurement
From the wellhead, the production fluids either enter the gathering system directly or go through some type of measurement apparatus. This might be in the form of an Individual Well Tester (IWT) or Group Well Tester (AWT).
Some oilfields that have a high density of wells will group the collection facilities together by a manifold before putting the production into the gathering system. Other fields will put some sort of testing device on each well before entering into the gathering system. In some cases, the well might produce directly into a tank or a portable separator.
Gathering System
The gathering system will consist of smaller diameter lines combining to form larger diameter lines. In areas where lease agreements require or government leases, multiple gathering systems may exist. Some oilwells are very sensitive to backpressure on the annulus and additional pumping may be required. Some systems may produce at high enough pressures to flow directly into the oil cleaning plant.
Treatment Plant
The oil treatment (dehydration) plant starts with removing the water from the production stream. This is done by a freewater knockout which removes the water from the production. Crude oil emulsion leaving the FWKO may be as much as 30 to 40 percent water. Water leaving the FWKO is sent to a water treatment facility.
If gas is not removed at the FWKO, it will be removed in the wash tank. Emulsion breaking chemicals may be injected into the stream upstream of the FWKO to aide in removal of the water.
Following the FWKO comes a wash tank or gun barrel tank which is a large holding vessel that allows for gravity settling for the water to continue to drop out of the oil. Typically, these are one third water and two thirds oil. The emulsion is dispursed into the tank by a spreader that is below the liquid level. This provides a gentle agitation where the water in the emulsion comes in contact with the small entrained water drops and coalesces assisting the removal of the water from the tank.
Following the wash tank, the dehydrated oil will be sent to a settling tank, a degassing tank or another FWKO. This additional step is treat the oil a final time before it enters a LACT tank or a sales tank before entering the LACT unit for sale.
LACT Unit
Any time oil changes custody it must pass throught a LACT unit. The LACT unit ensures the oil meets the requirements of the pipeline and measures the oil. If the oil coming from the LACT tank does not meet temperature or cut requirements, it will either be sent back into a reject tank or the sales tank.
Pipeline
The final step before reaching the refinery is shipment. Typically this is done via pipeline but it may also be be trucked out of the oil cleaning plant depending on volumes.
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