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| Steps of Oil Recovery Methods |
Oil recovery is the process of extracting crude oil from underground reservoirs. Depending on the reservoir's natural pressure and the complexity of extraction, oil recovery is divided into three main stages: Primary, Secondary, and Tertiary. Each stage uses different techniques to maximize oil production.
Oil recovery is a multi-stage process that evolves as reservoir conditions change. Primary recovery relies on natural pressure, secondary recovery uses water or gas injection, and tertiary recovery employs advanced chemical and thermal techniques. Each method has its advantages and challenges, but together, they help maximize oil extraction from reservoirs, ensuring efficient energy production. ONGC utilizes a combination of these techniques across its onshore and offshore fields to optimize oil recovery.
1. Primary Oil Recovery (15-20% Recovery)
In the initial stage, oil flows naturally to the surface due to the high pressure inside the reservoir. However, as oil is extracted, the pressure decreases, reducing the flow rate over time. To maintain production, engineers adjust extraction rates and perform regular maintenance on wells.
Methods Used in Primary Recovery:
(a) Sucker Rod Pump (SRP) Method
This method works similarly to a hand pump used for drawing groundwater. A long rod (called a sucker rod) is connected to a pump and lowered into the well. As the rod moves up and down, it lifts oil to the surface. This is one of the most common methods used by ONGC (Oil and Natural Gas Corporation) in onshore fields.
(b) Gas Lift Method
Gas lift is a cost-effective and widely used technique where compressed natural gas is injected into the well casing. The injected gas mixes with the oil, reducing its density and helping it rise to the surface. This method is commonly used in ONGC’s production wells.
(c) Electrical Submersible Pump (ESP) Method
In this method, an electric pump is submerged deep inside the well. The pump consists of a motor and a multi-stage pumping system, all powered by an electric cable. ESPs are highly efficient but require careful maintenance.
2. Secondary Oil Recovery (20-50% Recovery)
After primary recovery, a significant amount of oil remains trapped in the reservoir due to declining pressure. To recover more oil, secondary recovery methods (also called Enhanced Oil Recovery or EOR) are used. The two most common techniques are water injection and gas injection.
(A) Water Injection (Water Flooding)
This is the most widely used secondary recovery method in both onshore and offshore fields. High-pressure water is pumped into the reservoir, pushing the oil toward production wells. Since water is denser than oil, it displaces the oil, forcing it upward.
How It Works:
- Water is injected at pressures between 150 to 250 bars.
- The efficiency depends on the rock structure of the reservoir and the oil-water mobility ratio.
- ONGC uses this method extensively to maintain reservoir pressure and improve oil recovery.
(B) Gas Injection
Instead of water, gases like natural gas, nitrogen, or carbon dioxide (CO₂) are injected into the reservoir to maintain pressure.
How It Works:
- Gas is usually injected into the gas cap (the upper part of the reservoir) rather than directly into the oil zone.
- The gas expands, pushing the oil toward production wells.
- This method is particularly effective in ONGC’s Ankleshwar oilfield.
3. Tertiary Oil Recovery (50-70% Recovery)
When secondary methods are no longer effective, tertiary recovery (also called Enhanced Oil Recovery – EOR) is used. These methods are expensive but can recover up to 70% of the original oil reserves. Tertiary recovery mainly focuses on reducing oil viscosity to improve flow.
A. Chemical Methods
(a) Polymer Flooding
- A thick polymer (like polyacrylamide) is mixed with injected water to increase its viscosity.
- This helps push oil more effectively through rock pores, improving recovery rates.
(b) Surfactant Flooding
- Surfactants (soap-like chemicals) are added to water to reduce surface tension.
- This helps free trapped oil droplets from rock surfaces, making them easier to extract.
(c) Water-Alternate-Gas (WAG) Injection
- A combination of water and gas is injected alternately into the reservoir.
- This method was first tested in ONGC’s Gandhar offshore field and helps improve sweep efficiency.
B. Thermal Methods
(a) Steam Flooding
- Steam is injected into the reservoir to heat the oil, reducing its viscosity.
- The heat also causes some oil to evaporate, which later condenses into liquid form, making it easier to extract.
- This method is cyclical, requiring repeated steam injections.
(b) In-Situ Combustion (Fire Flooding)
- A controlled fire is ignited inside the reservoir using a special heater.
- Air is continuously pumped in to keep the fire burning, which heats the surrounding oil, reducing its viscosity.
- The heat and gases produced push the oil toward production wells.
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