What is Enhanced Oil Recovery ?

Enhanced oil recovery (abbreviated EOR) is the implementation of various techniques for increasing the amount of crude oil that can be extracted from an oil field. Enhanced oil recovery is also called improved oil recovery or tertiary recovery (as opposed to primary and secondary recovery). According to the US Department of Energy, there are three primary techniques for EOR: thermal recovery, gas injection, and chemical injection.Sometimes the term quaternary recovery is used to refer to more advanced, speculative, EOR techniques. Using EOR, 30 to 60 percent, or more, of the reservoir's original oil can be extracted, compared with 20 to 40 percent using primary and secondary recovery.

Primary recovery techniques: This implies the initial production stage, resulted from the displacement energy naturally existing in a reservoir. 

Secondary recovery techniques: Normally utilized when the primary production declines. Traditionally these techniques are water flooding, pressure maintenance, and gas injection. The recovery factor can rise up to 50%. 

Tertiary recovery techniques: These techniques are referred to the ones used after the implementation of the secondary recovery method. Usually these processes use miscible gases, chemicals, and/or thermal energy to displace additional oil after the secondary recovery process has become uneconomical. The recovery factor may arise up to 12% additionally to the RF obtained with the secondary recovery method. 

Primary Recovery In this recovery process oil is forced out of the petroleum reservoir by existing natural pressure of the trapped fluids in the reservoir. The efficiency of oil displacement is primary oil recovery process depends mainly on existing natural pressure in the reservoir. This pressure originated from various forces: Expanding force of natural gas Gravitational force Buoyancy force of encroaching water An expulsion force due to the compaction of poorly consolidated reservoir rocks Among these forces, expanding force of high-pressure natural gas contributes mainly to oil production. These forces in the reservoir either can act simultaneously or sequentially, depending on the composition and properties of the reservoir. The gravitational force is more effective in steeply inclined reservoirs, where it facilities the drainage of oil. This force alone may not be effective in moving large amounts of oil into a production well. Another, more effective, force for displacement oil is encroachment of water from the side or bottom of a reservoir. In some fields, edge water encroachment from a side appears to be stationary. The ability of the edge water to encroach depends upon the pressure distribution in the reservoir and the permeability. Compaction of the reservoir as fluids are withdrawn also is a mechanism for movement of oil to production wells. Part of the oil will be expelled due to the decrease in the reservoir volume.

Secondary Recovery When the reservoir pressure is reduced to a point where it is no longer effective as a stress causing movement of hydrocarbons to the producing wells, water or gas is injected to augment or increase the existing pressure in the reservoir. Conversion of some of the wells into injection wells and subsequent injection of gas or water for pressure maintenance in the reservoir has been designated as secondary oil recovery. 

When oil production declines because of hydrocarbon production from the formation, the secondary oil recovery process is employed to increase the pressure required to drive the oil to production wells. The purposes of a secondary recovery technique are: Pressure restoration Pressure maintenance The mechanism of secondary oil recovery is similar to that of primary oil recovery except that more than one well bore is involved, and the pressure of the reservoir is augmented or maintained artificially to force oil to the production wells. The process includes the application of a vacuum to a well, the injection of gas or water .

Water Injection In water injection operation, the injected water is discharged in the aquifer through several injection wells surrounding the production well. The injected water creates a bottom water drive on the oil zone pushing the oil upwards. In earlier practices, water injection was done in the later phase of the reservoir life but now it is carried out in the earlier phase so that voidage and gas cap in the reservoir are avoided. Using water injection in earlier phase helps in improving the production as once secondary gas cap is formed the injected water initially tends to compress free gas cap and later on pushes the oil thus the amount of injection water required is much more. The water injection is generally carried out when solution gas drive is present or water drive is weak. Therefore for better economy the water injection is carried out when the reservoir pressure is higher than the saturation pressure. 

Water is injected for two reasons: 

1) For pressure support of the reservoir (also known as voidage replacement).

 2) To sweep or displace the oil from the reservoir, and push it towards an oil production well. 

The selection of injection water method depends upon the mobility rate between the displacing fluid (water) and the displaced fluid (oil). 

The water injection however, has some disadvantages, some of these disadvantages are: 

• Reaction of injected water with the formation water can cause formation damage. 

• Corrosion of surface and sub-surface equipment. As part of water injection it is also common to find the water flooding technique. 

Water flooding consists of water Water is injected into the reservoir through injection wells. The water drives oil through the reservoir rocks towards the producing wells. For water flooding the most common pattern of injection and production wells is a five-spot configuration as shown in figures 3 and 4 show from different angles, where the water is injected in the central well displacing oil to the four surrounding production wells. 

Gas Injection It is the oldest of the fluid injection processes. This idea of using a gas for the purpose of maintaining reservoir pressure and restoring oil well productivity was suggested as early as 1864 just a few years after the Drake well was drilled. 

The first gas injection projects were designed to increase the immediate productivity and were more related to pressure maintenance rather to enhanced recovery. Recent gas injection applications, however, have been intended to increase the ultimate recovery and can be considered as enhanced recovery projects. In addition, gas because of its adverse viscosity ratio (higher mobility ratio) is inferior to water in recovering oil. Gas may offer economical advantages. Gas injection may be either a miscible or an immiscible displacement process. The characteristics of the oil and gas plus the temperature and pressure conditions of the injection will determine the type of process involved. The primary problems with gas injection in carbonate reservoirs are the high mobility of the displacing fluid and the wide variations of permeability. It is required a much greater control over the injection process than the one necessary with water-flooding. In order to evaluate the weep efficiency of the planned gas injection, a short-term pilot gas injection test should be driven. At the same time, this test would provide the necessary data to calculate the required volumes of gas; this in turn, will aid in the design of compressor equipment and estimating the number of injection well which will be required. In some cases gas injection can increase the ultimate recovery of oil such cases like having carbonate reservoirs. The benefits obtained by the gas injection are dependent upon horizontal and vertical sweep efficiency of the injected gas. The sweep efficiency depends on the type of porosity system present.