How Does Enhanced Oil Recovery Work?

Enhanced Oil Recovery (EOR) works by injecting substances into oil reservoirs to extract additional crude oil that remains after primary and secondary recovery methods have been exhausted.

This advanced technique can increase oil recovery from 20-40% to an impressive 30-60% of the reservoir’s original oil in place. EOR encompasses three method types:

1. Thermal
2. Gas
3. Chemical methods

Each method alters the physical properties of the oil, making it flow more easily through the reservoir to production wells. In this guide, we’ll explain each method after covering the basics of oil recovery.

The Three Stages of Oil Recovery

Oil recovery typically happens in three phases:

1. Primary recovery:  relies on the natural pressure within the reservoir to push oil to the surface. This natural pressure comes from dissolved gas, water, or gravity forces acting on the oil.
2. Secondary recovery: employed when the natural pressure depletes. They typically involve water or gas injection to maintain pressure in the reservoir and push additional oil toward production wells.
3. Tertiary recovery: this is where EOR comes into play. It becomes necessary when primary and secondary methods can no longer effectively extract oil from the reservoir.

Without EOR techniques, about 60-80% of a reservoir’s original oil would remain trapped underground. This represents a significant untapped resource that can be accessed with the right application of EOR methods.

Thermal Enhanced Oil Recovery Methods

Thermal EOR methods work by introducing heat into the reservoir to reduce oil viscosity. Heavy crude oil can be as thick as molasses at reservoir conditions, making it difficult to extract.

When heat is applied through steam injection, the oil becomes less viscous – more like water than molasses. This allows it to flow more easily through the reservoir rock.

Steam injection is the most common thermal EOR technique. In this process, steam generated at the surface is injected into the reservoir through specially designed wells.

As the steam moves through the reservoir, it heats the oil and causes it to expand and become less viscous. The steam may also condense into liquid water, which helps to push the oil toward production wells.

Thermal methods account for over 40% of U.S. EOR production, with California oil fields being the primary beneficiaries due to their heavy oil characteristics. The steam not only reduces oil viscosity but also creates steam distillation effects that further enhance recovery.

In-situ combustion is another thermal technique where a portion of the oil in the reservoir is ignited. The combustion generates heat that reduces the viscosity of the surrounding oil.

This burning process creates a combustion front that moves through the reservoir, pushing heated oil toward production wells. While effective, this method requires careful control to maintain the combustion at desired levels.

Gas Injection EOR Techniques

Gas injection EOR involves introducing gases into the reservoir. Popular gases include:

• Carbon dioxide (CO2)
• Nitrogen
• Natural gas

These gases interact with the oil to enhance its flow characteristics.

CO2 injection is particularly effective because it becomes a supercritical fluid that mixes with oil under reservoir conditions. This creates a less viscous mixture that flows more readily through the reservoir rock.

When CO2 dissolves in oil, it causes the oil to swell and reduces its viscosity. This swelling effect helps to push additional oil out of tiny pores in the rock that would otherwise remain trapped.

Gas injection accounts for approximately 60% of EOR production in the United States, with CO2 injection being the dominant method. In fact, CO2 EOR now represents over 5% of total U.S. oil production.

The process typically involves injecting the gas into the upper portion of the reservoir. As the gas moves through the reservoir, it forms a miscible zone with the oil that washes away previously immobile oil from the rock.

Nitrogen and natural gas are also used as injection gases, though less commonly. These gases work through similar mechanisms but are chosen based on specific reservoir conditions and availability.

An added benefit of CO2 injection is its potential for carbon sequestration. The CO2 used for EOR can be sourced from industrial emissions, thereby reducing greenhouse gas releases to the atmosphere.

Chemical Injection EOR Methods

Chemical EOR methods involve injecting specialized chemical formulations that alter the interaction between oil, water, and reservoir rock. These chemicals help mobilize oil that would otherwise remain trapped due to capillary forces.

Key methods include:

• Polymer flooding: increases the viscosity of injected water, which helps push oil more uniformly through the reservoir, preventing the water from finding easy paths around the oil. When water alone is injected into a reservoir, it tends to finger through the oil (leaving much of the oil behind), so polymers help to create a more uniform displacement front.
• Surfactant flooding: involves chemicals similar to detergents. These surfactants reduce the surface tension between oil and water, allowing the oil to detach from rock surfaces more easily. By reducing the interfacial tension, surfactants help oil droplets coalesce and flow more readily through the tiny pores in reservoir rock.
• Alkaline flooding: introduces alkaline materials like sodium hydroxide into the reservoir. These materials react with acidic components in the oil to naturally generate surfactants within the reservoir. The in-situ generation of surfactants provides similar benefits to surfactant flooding but can be more cost-effective in reservoirs with oils that have the right chemical composition.

Factors Affecting EOR Success

The success of any EOR project depends on multiple factors related to both the reservoir characteristics and operational parameters. The most important factors are:

• Reservoir temperature
• Pressure
• Rock properties
• Oil composition

The depth of the reservoir also matters. Thermal methods become less efficient at greater depths due to heat loss, while gas injection methods may work better in deeper formations where pressure is higher.

Oil price is another critical factor affecting EOR implementation. Since EOR methods are more expensive than primary and secondary recovery, they require higher oil prices to be economically viable.

The availability of injection materials can also limit the application of specific EOR techniques in certain regions. Infrastructure for transporting these materials to the oilfield must be considered.

Environmental regulations and concerns are increasingly important considerations for EOR projects. Water usage, potential for induced seismicity, and management of produced fluids factor into project planning and execution.

The Future of Enhanced Oil Recovery

The future of EOR lies in technological innovations that can make these methods more efficient and environmentally sustainable. New approaches are continuously being developed to address the limitations of current techniques.

For example, smart water flooding is showing promise for improving oil recovery without the need for expensive chemicals. This method works by altering the wettability of the reservoir rock to release more oil.

Nanotechnology is emerging as another potential game-changer for EOR. Nanoparticles can be designed to target specific reservoir conditions and oil properties, potentially offering more precise control over the recovery process.

Combinations of multiple EOR methods in hybrid approaches are also gaining traction. For example, using thermal and chemical methods together can address different aspects of oil recovery simultaneously, potentially leading to higher overall recovery rates.

As environmental concerns grow, carbon capture utilization and storage (CCUS) is becoming increasingly integrated with CO2 EOR. This synergy provides both enhanced oil recovery and a pathway for reducing industrial carbon emissions.

Enhanced oil recovery will keep maximizing production from existing oilfields, reducing the need for new exploration while meeting global energy demands in the coming decades.