Oil recovery in oil and gas refers to the comprehensive process of extracting crude oil from a reservoir, particularly focusing on methods that enhance the amount of oil produced beyond what natural reservoir pressure can achieve. As defined, oil recovery is "the process of extracting crude oil from an oil field that would not be achievable otherwise." It encompasses various techniques aimed at maximizing the extraction of hydrocarbons from underground formations throughout the life of an oil field.
Why is Oil Recovery Essential?
Even with the most advanced drilling and production technologies, only a fraction of the oil originally present in a reservoir can be brought to the surface using natural reservoir energy alone. This is because oil is trapped within microscopic pores of rocks, often adhering to the rock surface or becoming bypassed by initial fluid movement. Oil recovery methods are essential to:
- Maximize Resource Utilization: Extract more of the valuable hydrocarbons from a given field.
- Extend Field Life: Keep oil fields productive for longer periods.
- Improve Economic Viability: Make extraction from less accessible or more challenging reservoirs profitable.
Stages of Oil Recovery
Oil recovery is typically categorized into three main stages, each employing different techniques to enhance production:
1. Primary Oil Recovery
This initial stage relies on the natural energy present within the reservoir to push oil towards the wellbore. This energy can come from:
- Natural Pressure Depletion: The pressure difference between the reservoir and the wellbore drives oil flow.
- Solution Gas Drive: As pressure drops, dissolved natural gas comes out of solution, expanding and pushing oil.
- Water Drive: Natural aquifers connected to the reservoir push oil as pressure declines.
- Gas Cap Drive: An overlying natural gas cap expands, pushing oil downwards.
Primary recovery typically yields only a small percentage of the original oil in place, often ranging from 5% to 20%.
2. Secondary Oil Recovery
Once the natural reservoir energy is depleted, secondary recovery methods are employed to maintain or restore reservoir pressure. The most common techniques involve injecting external fluids into the reservoir:
- Waterflooding: Water is injected into injection wells to push oil towards production wells. This is the most widely used secondary recovery method.
- Gas Injection: Natural gas, flue gas, or nitrogen can be injected to maintain pressure and displace oil.
Secondary recovery can significantly increase the total oil recovered, often bringing the cumulative recovery to between 20% and 50% of the original oil in place, including primary recovery.
3. Tertiary Oil Recovery (Enhanced Oil Recovery - EOR)
Tertiary recovery, also known as Enhanced Oil Recovery (EOR), involves advanced techniques that alter the properties of the oil or the reservoir rock to facilitate further oil extraction. These methods are crucial for recovering oil that would otherwise be left behind after primary and secondary methods. The reference highlights that oil recovery involves "altering the wettability properties of oil to enhance the recovery process, often by making the rock surface more water-wet."
EOR methods can typically recover an additional 5% to 20% of the original oil in place, significantly boosting overall field recovery.
Key EOR Techniques:
- Thermal EOR: Involves injecting heat into the reservoir to reduce oil viscosity, making it flow more easily.
- Steam Injection: The most common thermal method, including cyclic steam stimulation and steamflooding.
- In-Situ Combustion: Burning a portion of the oil underground to generate heat and gases.
- Chemical EOR: Involves injecting chemical solutions to improve fluid displacement efficiency. This is where altering wettability is crucial.
- Surfactant Flooding: Injects surfactants (soap-like chemicals) to reduce the interfacial tension between oil and water, making the oil less sticky and more easily displaced. This method directly contributes to "making the rock surface more water-wet," allowing water to push oil out more effectively.
- Polymer Flooding: Injects polymers to increase the viscosity of the injected water, improving its sweep efficiency and preventing it from bypassing oil.
- Alkaline Flooding: Injects alkaline solutions to react with acidic components in the oil, forming natural surfactants.
- Miscible EOR: Involves injecting a fluid that is miscible (mixes completely) with the crude oil, reducing the interfacial tension to zero and allowing the fluids to move as one.
- CO2 Flooding: Carbon dioxide is injected, which can dissolve in oil, swelling it and reducing its viscosity.
- Hydrocarbon Gas Injection: Injecting lighter hydrocarbons (like methane or ethane) that can mix with the reservoir oil.
Understanding Wettability Alteration
Wettability refers to the preference of a solid surface (the reservoir rock) to be in contact with one fluid (oil or water) rather than another. Most reservoir rocks are naturally oil-wet or mixed-wet, meaning oil tends to stick to the rock surface.
By altering the wettability properties (as mentioned in the definition of oil recovery), EOR methods, particularly chemical EOR using surfactants or alkaline solutions, aim to make the rock surface more water-wet. When the rock becomes more water-wet, the injected water can more easily spread across the rock surface and displace the oil, releasing trapped oil and allowing it to flow towards the production wells. This is a critical mechanism for recovering oil that is tightly adsorbed to the rock matrix.
Summary of Recovery Methods and Factors
The table below summarizes the different stages of oil recovery and their approximate recovery factors:
Recovery Method | Primary Mechanism | Typical Recovery Factor (Approximate) | Additional Notes |
---|---|---|---|
Primary | Natural reservoir pressure, gas expansion, water drive | 5-20% of Original Oil in Place (OOIP) | Relies on inherent reservoir energy. |
Secondary | Fluid injection (water, gas) to maintain pressure | 20-50% (cumulative with primary) | Focuses on pressure support and sweep efficiency. |
Tertiary | Altering fluid/rock properties (EOR) | Additional 5-20% (on top of P+S) | Addresses residual oil by modifying oil or rock behavior. |
Oil recovery is a complex and continually evolving field in the oil and gas industry, crucial for meeting global energy demands by maximizing the potential of existing reservoirs.