Types of Offshore Drilling Rigs: A Comprehensive Guide

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.

What is Primary Recovery?

Primary oil recovery is the initial stage of extracting crude oil from a reservoir. It relies on the natural energy of the reservoir, such as pressure from gas caps, water drive, or the expansion of oil itself, to push hydrocarbons to the surface. When natural pressure is insufficient, artificial lift methods like pumps are used to maintain production.

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.

Factors Affecting Oil Recovery Efficiency

Efficient oil recovery depends on multiple factors that influence how much oil can be extracted from a reservoir. Understanding these factors is critical for designing effective extraction strategies.

1. Reservoir Characteristics

Porosity and Permeability: Higher porosity allows more oil storage, while higher permeability enables easier fluid flow.
Reservoir Pressure: Declining pressure reduces natural flow, requiring artificial lift or injection methods.
Rock Type and Structure: Heterogeneous or fractured formations may complicate fluid movement.

2. Oil Properties

Viscosity: Thick, heavy oils flow less easily, reducing recovery efficiency.
Composition: Oil with high sulfur or wax content may require specialized recovery methods.
API Gravity: Lower gravity oils are heavier and harder to extract, affecting method selection.

3. Technological Factors

Choice of Recovery Method: Primary, secondary, or tertiary recovery techniques must match reservoir conditions.
Equipment Efficiency: Pumps, injection systems, and monitoring tools must function optimally.
Enhanced Oil Recovery (EOR) Techniques: Properly applied thermal, chemical, or gas injection methods can significantly improve recovery.

4. Operational Factors

Well Placement and Spacing: Properly located wells ensure maximum sweep efficiency.
Injection Rates and Patterns: Optimized water or gas injection improves pressure maintenance.
Monitoring and Maintenance: Real-time monitoring and timely interventions prevent production loss.

Summary:

Oil recovery efficiency is determined by a combination of geological, physical, technological, and operational factors. By carefully evaluating and managing these factors, engineers can maximize production, reduce costs, and extend the productive life of oil fields.

Typical Recovery Rates and Limitations

Recovery Rate: Primary recovery usually extracts 15–20% of the total oil in place.
Limitations: Efficiency depends on reservoir pressure, oil viscosity, and well design. Once natural energy is depleted, production rates decline, making secondary or tertiary recovery necessary.

Examples of Primary Recovery Methods

Rod Pumping (Beam Pumping): Uses a surface-driven pump to lift oil.
Electric Submersible Pumps (ESP): High-volume submersible pumps for deep wells.
Gas Lift: Injected gas reduces fluid density and helps lift oil to the surface.
Natural Flow: Wells that flow freely without artificial assistance, typically in high-pressure reservoirs.

Summary:

Primary recovery is the foundation of oil extraction, providing initial production while preserving reservoir energy for secondary and tertiary recovery methods. Understanding this stage is crucial for effective well planning and maximizing overall hydrocarbon recovery.

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.

What is Secondary Recovery?

Secondary oil recovery is the process of extracting additional oil after primary recovery becomes inefficient. It involves maintaining reservoir pressure and pushing remaining oil toward production wells using external energy sources, typically water or gas injection. This stage significantly increases the total amount of recoverable oil.

(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.

Typical Recovery Rates and Benefits

Recovery Rate: Secondary recovery can typically extract 20–50% of the original oil in place, depending on reservoir characteristics.

Benefits: Extends the productive life of oil fields, improves overall recovery, and delays the need for tertiary methods.

Limitations

Requires careful reservoir monitoring and well placement.

Water or gas breakthrough can reduce efficiency if not properly managed.

Not effective for extremely viscous oils without additional measures.

Summary:

Secondary oil recovery is a critical step in maximizing hydrocarbon extraction. By maintaining reservoir pressure and displacing residual oil, it bridges the gap between primary recovery and advanced tertiary or enhanced oil recovery techniques.

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.

What is Tertiary or Enhanced Oil Recovery?

Tertiary oil recovery, also known as Enhanced Oil Recovery (EOR), is the stage used to extract oil that remains after primary and secondary recovery. EOR involves advanced techniques that alter the properties of oil or reservoir conditions to mobilize residual hydrocarbons and maximize recovery.

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.

Benefits of Tertiary Recovery

Can increase total oil recovery to 50–70% or more of the original oil in place.

Extends the productive life of mature oil fields.

Improves economic efficiency by extracting oil that would otherwise remain trapped.

Limitations

Higher operational costs compared to primary and secondary recovery.

Requires careful reservoir evaluation and monitoring.

Some methods may have environmental considerations (e.g., CO₂ injection).

Summary:

Tertiary or enhanced oil recovery is the most advanced stage of oil extraction. By employing thermal, chemical, and gas techniques, EOR significantly boosts overall recovery, ensuring maximum utilization of hydrocarbon reservoirs.

Comparison of Recovery Methods

Understanding the differences between primary, secondary, and tertiary oil recovery methods helps engineers choose the most effective approach for each reservoir.

1. Primary Recovery

Mechanism: Relies on natural reservoir pressure or artificial lift.

Recovery Rate: Typically 15–20% of original oil in place.

Advantages: Simple, low-cost, minimal equipment required.

Limitations: Limited recovery, production declines as pressure drops.

2. Secondary Recovery

Mechanism: Uses water flooding or gas injection to maintain pressure.

Recovery Rate: Typically 20–50% of the original oil in place.

Advantages: Extends field life, increases overall recovery efficiency.

Limitations: Requires careful reservoir monitoring and injection management.

3. Tertiary / Enhanced Oil Recovery (EOR)

Mechanism: Uses thermal, chemical, or gas methods to mobilize remaining oil.

Recovery Rate: Can achieve 50–70% or more of the original oil in place.

Advantages: Maximizes hydrocarbon extraction, improves economic viability.

Limitations: Higher operational costs, requires advanced technology and monitoring.

Key Considerations for Selecting a Method

Reservoir type and characteristics

Oil viscosity and composition

Economic feasibility and operational complexity

Environmental and regulatory constraints

Future Trends in Oil Recovery

The oil and gas industry continues to evolve, and innovations in recovery technology are helping operators maximize production and efficiency. Key future trends include:

1. Advanced Enhanced Oil Recovery (EOR) Techniques

Thermal, chemical, and gas methods are becoming more efficient and cost-effective.

The development of hybrid EOR techniques allows customized approaches for complex reservoirs.

2. Digital Oilfield Technologies

Real-time monitoring systems track reservoir performance, fluid properties, and equipment conditions.

Data analytics and AI enable predictive maintenance and optimized injection strategies.

3. Automation and Smart Reservoir Management

Automated control systems adjust pumping rates and injection pressures for maximum efficiency.

Smart reservoirs integrate sensors and predictive models to optimize production.

4. Environmentally Sustainable Practices

CO₂ utilization and recycling in EOR helps reduce carbon emissions.

Water management and chemical optimization improve environmental compliance.

5. Integration with Renewable Energy

Some operators are exploring hybrid power sources for pumps and injection systems to reduce reliance on fossil fuel electricity.

Innovations in energy-efficient equipment reduce operational costs and carbon footprint.

FAQs with Answers Oil Recovery Stages Explained: Techniques, Methods & Processes

1. What are the main stages of oil recovery?

Oil recovery typically occurs in three stages: Primary recovery, using natural reservoir pressure; Secondary recovery, using water or gas injection; and Tertiary or Enhanced Oil Recovery (EOR), using advanced methods like thermal, chemical, or gas techniques.

2. What is primary oil recovery?

Primary recovery relies on the natural pressure of the reservoir and artificial lift systems (like pumps) to bring oil to the surface. It usually recovers 5–15% of the total oil in place.

3. What is secondary oil recovery?

Secondary recovery involves injecting water or gas into the reservoir to maintain pressure and push additional oil toward production wells, often increasing recovery to 30–50% of the total oil.

4. What is tertiary or enhanced oil recovery (EOR)?

Tertiary recovery uses advanced techniques like thermal methods, gas injection, or chemical flooding to extract remaining oil that primary and secondary methods cannot recover, boosting total recovery to 60–70% or more.

5. What factors affect oil recovery efficiency?

Factors include reservoir type, rock permeability, fluid viscosity, pressure, temperature, and the choice of recovery technique. Proper reservoir management is crucial for maximizing output.

6. How does enhanced oil recovery differ from secondary recovery?

While secondary recovery maintains reservoir pressure using water or gas injection, EOR involves altering reservoir conditions or oil properties (like reducing viscosity) to extract additional oil that standard methods cannot recover.

7. Why is understanding oil recovery stages important for engineers?

Knowledge of oil recovery stages helps engineers plan effective extraction strategies, optimize production, reduce operational costs, and extend the life of oil fields.