Tertiary Recovery Definition / Meaning
Tertiary Recovery, also known as Enhanced Oil Recovery (EOR), is the third stage of oil production applied to a reservoir after primary and secondary recovery methods have been exhausted. It involves injecting fluids or energy into the reservoir to mobilize trapped oil that cannot be produced by natural pressure or waterflooding alone. Tertiary recovery targets the residual oil saturation left behind after conventional methods, which can account for 50% to 70% of the original oil in place (OOIP).
How Tertiary Recovery Works
Primary recovery relies on natural reservoir energy (e.g., gas cap drive, solution gas drive, or aquifer influx) to push oil to the wellbore. Secondary recovery, typically waterflooding or gas injection, maintains reservoir pressure and sweeps oil toward producing wells. Tertiary recovery goes further by altering the physical or chemical properties of the oil, the rock, or the injected fluid to improve displacement efficiency. The main mechanisms include:
- Reducing oil viscosity (e.g., thermal methods like steam injection)
- Lowering interfacial tension between oil and water (e.g., surfactant flooding)
- Miscible displacement (e.g., CO2 or hydrocarbon gas injection)
- Improving sweep efficiency (e.g., polymer flooding to thicken water)
Common Tertiary Recovery Methods
| Method | Description | Typical Recovery Factor Increase |
|---|---|---|
| Thermal (Steam Injection) | Injects steam to heat heavy oil, reducing viscosity and allowing flow. | 10%–30% of OOIP |
| Gas Injection (Miscible) | Injects CO2, N2, or hydrocarbon gas that mixes with oil, swelling it and lowering viscosity. | 5%–20% of OOIP |
| Chemical Flooding | Injects surfactants, polymers, or alkaline solutions to reduce interfacial tension or improve sweep. | 5%–15% of OOIP |
| Microbial EOR | Uses microorganisms to produce biosurfactants or gases that help mobilize oil. | 2%–10% of OOIP |
Practical Industry Context
Tertiary recovery is capital-intensive and technically complex, often requiring pilot tests before full-field implementation. It is typically applied in mature fields where primary and secondary methods have declined. For example, the Permian Basin in West Texas has seen extensive CO2 flooding since the 1970s, recovering billions of barrels of additional oil. In heavy oil reservoirs, steam-assisted gravity drainage (SAGD) is a common thermal method in Canada’s oil sands. The choice of method depends on reservoir characteristics such as depth, temperature, permeability, oil viscosity, and remaining oil saturation.
Key Factors for Success
- Reservoir heterogeneity: Fractures or high-permeability streaks can cause early breakthrough of injected fluids.
- Fluid compatibility: Injected chemicals must not react adversely with formation water or minerals.
- Economic viability: High oil prices and favorable tax regimes often justify the investment.
- Environmental regulations: CO2 injection may qualify for carbon credits or tax incentives.
Usage Example
After 30 years of waterflooding, the field operator initiated a tertiary recovery program using CO2 injection, which increased the recovery factor from 35% to 55% of the original oil in place.
Challenges and Limitations
Tertiary recovery is not always successful. Common issues include injectivity problems, scaling, corrosion, and high operational costs. In some cases, the incremental oil recovered may not justify the expense. Advances in reservoir simulation, real-time monitoring, and smart well technology have improved the predictability and efficiency of these projects.
Relation to Reservoir Management
Effective reservoir management integrates tertiary recovery planning from the early stages of field development. This includes selecting the optimal EOR method, designing injection patterns, and monitoring performance through tracers, production logs, and time-lapse seismic. A well-managed tertiary recovery project can extend the economic life of a field by decades.