Enhanced Recovery Techniques (EOR)

=Overview=

The life of an oil well goes through at least three distinct phases, with various techniques employed to keep oil production at maximum levels. Enhanced oil recovery (EOR) is the third and most advanced stage in this process, whereby oil is forced into the well-head where it can be pumped to the surface. EOR can substantially improve the efficiency of extraction.

EOR can refer to any method of increasing oil production from a reservoir, using sophisticated technological techniques to add energy to a reservoir to stimulate oil production and increase recovery factor, or the amount of the reservoir's total oil that is extracted. EOR also has some considerable drawbacks, including the relatively high cost of its implementation and, in some cases, the unpredictability of its effectiveness.

According to the Petroleum Technology Transfer Council, about 10 percent of all oil produced in the United States in 2009 used enhanced recovery techniques.

=Three stages of oil field development=

In the first stage of an oil field's development, oil is forced out by pressure generated from gas present in the oil (also known as associated gas). The natural pressure of the reservoir, or gravity, drives oil into the wellbore, combined with artificial lift techniques such as pumps which bring the oil to the surface. Only about 10 percent of a reservoir's original oil in place is typically produced during primary recovery.

In the secondary stage, the reservoir is flooded with water or injected with gas to maintain sufficient pressure levels to displace oil and drive it to the production wellbore. This stage extends a field's productive life and results in the recovery of 20 to 40 percent of the original oil in place.

EOR refers to the tertiary stage of development, which involves the introduction of fluids that the reduce the viscosity, or thickness, of the oil and improve its flow. A variety of fluids are used for this purpose. These fluids typically consist of gases that are miscible (form a homogeneous mixture) with oil, steam, air or oxygen, polymer (long-chained molecule) solutions, gels, or microorganism formulations. Tertiary recovery enables producers to extract up to or over half of a reservoir's original oil content, depending on the reservoir and the technique used.

=Categories of EOR techniques=

The primary categories of EOR are thermal recovery, gas injection and chemical injection.


 * Thermal EOR has historically been the most widely applied. This method involves the introduction of heat, most commonly in the form of steam, into a reservoir to reduce the viscosity of the oil to be extracted.


 * Gas injection uses gases such as nitrogen or carbon dioxide that expand in a reservoir to push additional oil to the production wellbore. Other gases can also be used to dissolve in the oil to lower its viscosity and increase its flow rate. Other gases, such as hydrocarbon gases and flue gases (the combustion exhaust gas of a power plant, for example), can also be used in this method of EOR.


 * Chemical EOR generally involves the flooding of a reservoir with water-soluble polymers, or long-chained molecules, to help reduce the surface tension that often prevents oil from moving through a reservoir. This effects a more efficient displacement, and therefore better recovery, of moderately viscous oils.

A number of other EOR processes have also evolved, including the injection of carbonated water, microorganisms, foams, alkaline, and other substances. These have shown varying degrees of promise but require additional development to enter into more common use.

According to the US Department of Energy in December 2011, thermal techniques accounted for over 40 percent of EOR production in the United States, gas injection accounted for nearly 60 percent, and chemical techniques accounted for about one percent.

=References=