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Various Crude oil Recovery methods

Crude oil recovery methods are techniques used to extract oil from reservoirs. These methods are generally categorized into three main stages: primary, secondary, and tertiary (or enhanced) recovery. Each stage involves different technologies and approaches depending on the reservoir characteristics and the amount of oil that can be economically recovered. 

Stages of Oil Recovery

Stages of Oil Recovery  

Primary(15-20%)

Oil comes  to the well bore because of the gas pressure available  within the reservoir. 

Secondary (20-50%)

Water/Gas is injected  into the reservoir to maintain the energy so as to facilitate oil /gas to flow

Tertiary (50-70%)

Enhanced oil recovery   is a method to recover   the oil of nature which is not possible  to recover  as above,

I. Primary Recovery (15-20%)

Primary Recovery Method

In the early stages, the well naturally flows because of the pressure within the reservoir. Over time, as the reservoir pressure drops, the production rate of these wells usually decreases. To manage this, production is carefully controlled to ensure the well can keep producing for a specific period. Regular activities on a producing well involve monitoring, safety and security measures, maintenance tasks, and occasional downhole servicing. This servicing is done using tools like a wireline unit or a workover rig to help maintain steady production.However, primary recovery typically only extracts a small percentage (around 15-20%) of the oil in place.   

Artificial Lift

A. Sucker Rod Pump (SRP)

This is similar to the ground water pump and is attached to a rigid rod which is lowered down the tubing. These SRPs are very commonly used onshore in ONGC.

Sucker Rod Pump (SRP)

B. Gas Lift System

 Gas Lift System

Gas lift is a form of artificial lift, possibly the cheapest, easiest and most common method, and is used at all stages of the well cycle. It is used in most producing wells in ONGC. The process involves injecting compressed natural gas down through the well casing.

C. ESP (Electrical Submersible Pump)

ESP (Electrical Submersible Pump)

In this method, an electrical submersible pump (ESP) is inserted into the well. The entire assembly, consisting of a long narrow motor and a multiphase pump, is suspended by an electrical cable with tension members below the tubing.

II. Secondary Recovery Methods (EOR) (20-50%)

Only 15-20% of oil can be extracted from primary production. When the natural reservoir pressure reduces, it leaves a considerable amount of oil in the reservoir. Secondary methods are adopted to extract this amount of oil, which is also called enhanced oil recovery (EOR). Water injection and gas injection are the most important enhanced oil recovery methods, this method can extract 20% to 50% of oil.

Waterflooding:

A. Water Injection

CO2 / Water Injection

Water injection is the most widely used method for secondary oil recovery, both on land and offshore. This technique involves pumping water into the reservoir to boost oil production. The water is injected directly into the oil-bearing zone, and because water is denser than oil, it pushes the oil upward and drives it toward the production well. To make this happen, high-pressure pumps are used, typically operating at pressures between 150 and 250 bar. The success of water injection depends on factors like the mobility ratio (how easily oil and water flow relative to each other) and the specific geology of the reservoir. These factors determine how effective the water flooding process will be in recovering more oil.This method significantly increases oil recovery compared to primary recovery.

Gas Injection

B. Gas Injection

Gas Injection

Gas injection is another popular method for secondary oil recovery, especially used in areas like the Ankleshwar asset. Similar to water injection, this technique involves injecting natural gas, nitrogen, or carbon dioxide into the reservoir to maintain pressure and help push out more oil. The gas is pumped into the reservoir through dedicated injection wells. Instead of being injected directly into the oil-producing zone (like water), the gas is usually added to the gas cap—the upper part of the reservoir. As the gas expands, it helps drive the oil toward the production wells. The injected gas eventually flows out along with the oil and is collected for reuse or processing.

Similar to waterflooding, gas (such as natural gas or nitrogen) is injected to maintain reservoir pressure and push oil towards production wells.   

III.Tertiary Recovery-Enhanced Oil Recovery - EOR ( 50-70%)

Tertiary Recovery Methods

Tertiary recovery comes into play when secondary methods, like water or gas injection, aren’t enough to keep oil production going—but only if extracting the remaining oil is still profitable. These methods are more advanced and costly, but they can recover up to 70% of the original oil in the reservoir. The main goal of tertiary recovery is to reduce the oil’s thickness (viscosity), making it easier to flow and boosting production. This stage is often seen as the final push to get as much oil out of the reservoir as possible.

Commonly Used Tertiary Recovery Methods

Chemical Methods

Polymer Flooding: In this method, a substance called polyacrylamide (a high-molecular-weight chemical) is added to the water being injected into the reservoir. This helps thin out the oil, making it flow more easily. It also improves how effectively the water sweeps through the reservoir, leading to higher oil recovery.

Surfactant Flooding: This technique involves adding special chemicals (surfactants) to the injected water. These chemicals reduce the surface tension that traps oil in the tiny pores of the rock, helping to release more oil.

Water Alternate Gas (WAG)

This method, first used in the Gandhar offshore field, involves alternating injections of water and natural gas into the reservoir. The combination helps maintain pressure and improves the overall efficiency of oil recovery.

Tertiary recovery methods are used to recover the remaining oil that is not accessible through primary or secondary methods. EOR can recover an additional 10-30% of the oil but is more complex and expensive.

Thermal EOR

Steam Injection 

Steam Injection

This method involves pumping steam into the reservoir, similar to how water is injected in water flooding. The steam heats up the oil, causing some of it to evaporate and mix into the surrounding hot water as the steam condenses. This process thins out the oil (reducing its viscosity) and improves its ability to flow through the rock (increasing permeability). To make this method work effectively, the steam injection process needs to be repeated in cycles.Steam Flooding Steam is injected to heat heavy oil, reducing its viscosity and making it easier to flow.

In Situ Combustion (Fire Flooding)

In Situ Combustion (Fire Flooding)

This method, also known as fire flooding, involves creating a controlled fire inside the reservoir. A special heater in the well ignites the oil, and a continuous flow of air keeps the fire burning. As the fire moves through the reservoir toward the production wells, it heats up the oil, making it thinner and easier to flow. The heat also turns the reservoir water into steam. Together, the steam, hot water, and gases produced by the combustion push the oil toward the production wells. ONGC has been successfully using this technique to enhance oil recovery.In-Situ Combustion Air is injected to ignite part of the oil, creating heat and pressure to push oil toward production wells.

Life Cycle of a producing Field

Primary Stage

During the primary stage, the well flows naturally to the surface using its own energy from the reservoir. At this stage, only about 20% of the oil is typically recovered.

Secondary Stage

As the reservoir loses its natural energy, it can no longer push the oil to the surface on its own. To keep production going, artificial methods like gas lift are used. Gas lift works by injecting gas into the well to lighten the fluid column, making it easier for the oil to flow to the surface. Additionally, water is injected into the reservoir to help maintain pressure. With these methods, the recovery factor can increase to around 50%.

Tertiary Stage (In-Situ Combustion)

In this stage, a technique called in-situ combustion (or fire flooding) is used near the wellbore. This involves heating the oil to reduce its thickness (viscosity) and improve its ability to flow. By the end of the field's life, this method can recover up to 70% or more of the oil. However, once this stage is complete, further production is no longer commercially viable.