In the beginning of field life, unless the pressures are very low, the well fluid comes out of wells on its own pressure. This kind of production of oil on its own pressure is called Primary Production. A primary production facility comprises of:
· Manifold on top of the well, called Christmas Tree.
· Equipment and systems for separation of oil, gas and water.
· Equipment and systems to make the oil and gas as free of water as specified by the customer (dehydration of oil & gas).
· Equipment and systems for measurement and transportation of oil & gas to the customer.
· Equipment and systems to treat water for disposal.
During Primary Production, 25-30 percent of the oil in the reservoir can be recovered by the natural reservoir drive. Other techniques are used to recover some of the remaining oil. Secondary and Tertiary methods of recovery of hydrocarbons are used for maximum extraction of hydrocarbons from the reservoir. These are also known as Enhanced Oil Recovery (EOR) methods.
Secondary And Tertiary Methods of Production
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Method of Recovery
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Process
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Use
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WATER-FLOODING
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Water
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Water is pumped into the reservoir through injection wells to force oil toward Production Wells.
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Method most widely used in secondary recovery.
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IMMISCIBLE GAS INJECTION
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Natural gas, flue gas, nitrogen
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Gas is injected to maintain formation pressure, to slow the rate of decline of natural reservoir drives.
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Secondary Recovery
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MISCIBLE GAS INJECTION
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Carbon-dioxide, Propane, Ethane, Nitrogen
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Under pressure, carbon dioxide becomes miscible with oil, vaporizes hydrocarbons, and enables oil to flow more freely. Often followed by injection of water.
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Secondary recovery or tertiary recovery following water flooding. Considered especially applicable if carbon-dioxide supplies located with a feasible distance.
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CHEMICAL FLOODING
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Polymer
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Water thickened with polymers is used to aid water flooding by improving fluid-flow patterns.
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Used during secondary recovery to aid other processes during tertiary recovery.
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Micelllar Polymer
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A solution of detergent – like chemicals miscible with oil is injected into the reservoir.
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Almost always used during tertiary recovery after secondary recovery by water flooding.
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THERMAL RECOVERY
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Steam drive
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Steam is injected continuously into heavy-oil reservoirs to drive the oil toward production wells.
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Used when oil is too viscous for water flooding. Tertiary recovery by water flooding.
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In-Situ Combustion
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A part of the oil in the reservoir is set on fire, and compressed air is injected to keep it burning. Gases and heat advance through the formation, moving the oil toward the production wells.
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Used with heavy-oil reservoirs during Primary recovery when oil is too viscous to flow under normal reservoir conditions.
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Sometimes the pressures of the reservoir are low at the early stages of production. In such cases artificial methods are used even during primary production.
A very popular method used for low-pressure shallow wells is Sucker Rod Pumps to pump out the oil. These pumps having huge size of their drive systems, which moves up & down, make a magnificent sight in the oilfield, where often an array of such pumps can be seen. The plunger goes down the well moving up and down pumping out the oil.
WATER INJECTION
Water is first treated to meet reservoir specification for particulate content, dissolved solids content, oxygen content etc. Then it is injected around the periphery of the producing well.
Use of water injection can boost the recovery by another 15-20% of the original oil in reservoir, raising the recovery level to 40-45%. Water injection is sometimes considered primary production method and falls under the category of artificial lift, meaning lifting the oil out of the well by artificial means rather than its own pressure.
The essential equipments in water injection systems are filters, de-oxygenating tower and chemical injection system.
Enhanced Oil Recovery (EOR) methods are tried after the water injection. It can further increase by another 15-20% leading to recovery of around 60% of the oil in the reservoir.
STEAM INJECTION
Steam is injected down injection wells to heat the heavy oil to reduce its viscosity and make it more fluid. The steam also produces drive to push the oil toward producing wells.
Main equipment are water treatment plant (for boiler quality water). Rugged type of boilers are used to produce high-pressure steam (above reservoir pressure).
IN-SITU COMBUSTION
This method of EOR is used for very viscous crude oils. It is also used as primary production method where crude oil is too viscous to flow up through the well on its own.
In-Situ combustion has been effectively used to North Gujrat Oilfield in India to produce very viscous crude oil, which is almost like semi-solid in the ambient temperature.
In the process, air and water are injected into the oil reservoir in alternate cycles. At first air is injected around the outer layer of the reservoir and the oil is ignited as a result of presense of oxygen (air). The heat generated raises the temperature of oil thus reducing the viscosity.
But due to combustion, there is loss of some amount of crude oil. Once the desired temperature level in the reservoir is reached, air injection is stopped.
The flame in the reservoir is quenched with injection of water. Water injection is stopped once the flame is quenched. As the temperature falls, injection of air and ignition of the oil is done again. This cycle goes on repeating according to the time cycle decided by reservoir engineers:
Gas Injection and Gas Lift
Gas lift is injection of gas in the well tubing to make the density of oil column in the well lighter. As a result the hydraulic head of the fluid in the well becomes less and oil flows out more easily.
But gas injection involves injection of the gas directly to the reservoir to provide drive to push out oil.
Both processes involve compression of the gas coming out in the field to high enough pressure to be put back either to the well or to the reservoir.
These were some of the EOR processes.
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