Concentrations of Oil in the Lower 48 States

        EOR is fairly advanced in some of these regions.  For instance, steam flooding is used to enhance production from many California fields because the oil can be very viscous.  Carbon dioxide flooding is common in the fields in New Mexico, West Texas, western Oklahoma and Wyoming because pipelines deliver the gas to this region from natural sources or from natural gas purification plants.  CO₂ is also available for some fields in Mississippi and Louisiana.   However, EOR is not common in most of the rest of the nation because steam is not needed or carbon dioxide is not available.  The purpose of this note is to describe techniques that companies could apply without risking large amounts of capital. 

     Gas Versus Water Injection:  The first decision that must be reached is whether to inject a gas or to either start or continue a waterflood.  The facts are that waterfloods (without surfactants) leave more oil in the reservoir than gas floods, but waterfloods can also recover oil faster than gas if the permeability of the reservoir is high.  Usually, if the permeability of the reservoir is above 50 md, a waterflood will work well, whereas if the permeability is below 25 md, gas will recover oil faster than water because more gas can be injected.  In addition, if a waterflood has not been successful because of poor sweep, a gas flood should be profitable.

 Gas Injection Options

      Reinjection of Reservoir Gas:  The first option to consider in a low permeability reservoir is reinjection of reservoir gas.  This can only occur in a relatively unproduced, newer reservoir that originally was nearly saturated with gas.  The gas will have already been produced from older reservoirs.  If this option is possible, the reservoir pressure is controlled so as to optimize gas production, gas recycle, and oil production.

      Nitrogen or Air Injection:  If reservoir gas is not available, injecting either nitrogen or air to increase oil recovery is common.  Several dozen nitrogen injection projects are being operated at any one time, and approximately one dozen air injection projects are currently active.  Nitrogen, extracted from air using membranes or pressure swing adsorption, is relatively inert because it contains less than 4 percent oxygen.  It is best used in shallower reservoirs to enhance production by increasing reservoir pressure.  Produced nitrogen can be recompressed and recycled so that very little new gas is needed and the production can continue for several decades.

      Air is used in deeper, hotter (> 150º) reservoirs where the air spontaneously reacts with the oil to form flue gas.  The CO₂ in the flue gas dissolves in the oil.  In addition, water and light oil evaporate from the combustion zone. This means that three mechanisms (pressure maintenance, swelling and waterflooding by condensed steam) combine to increase oil recovery.  Some air-injection projects have operated for nearly 30 years.

      CO₂ Sequestration: When a CO₂ pipeline is not nearby but CO₂ is available from plants in a nearly pure form and is not being sold for another purpose, it can be injected into light-oil reservoirs where the CO₂ and oil could become miscible.  In the few instances where these conditions exist, carbon dioxide is the best choice for recovering oil for decades from a low-permeability, deeper reservoir where gas will not override the reservoir fluids.

 Improving Waterfloods

      Selective Gel Blockage:  If a waterflood is practical, one method to control high water production is selective gel blockage.  When the zone needing treatment is identified, a mixture of a polymer and crosslinker is injected to retain water in a Jello^®-like material.  The treatment can work for years and is usually very cost effective.

      Polymer Flooding:  Waterfloods can also be improved by polymer flooding.   Addition of polymer makes the water more viscous so that oil is produced faster.  Obviously, this is not an good idea in a low permeability reservoir or one with a high clay content that can adsorb the polymer.  However, polymer-augmented waterfloods can be profitable.

      Surfactant-Enhanced Waterfloods:  Three types of chemical floods exist.  The first is an alkaline-augmented polymer flood.  Another is an alkaline-surfactant polymer flood.  The third is a micellar or low interfacial tension flood.  In a survey conducted in 2001, the average chemical cost per incremental barrel was $1.96 for secondary floods and $6.14 for tertiary.  Payout occurs in two to three years since production costs are reduced much more than the chemicals cost.  These projects are only practical in sandstones where adsorption of chemicals is not high or the water is not so hard that the alkali precipitates.  The projects are very profitable if injectivity of the chemicals is high and oil recovery is rapid.   This means that projects are likely to be more profitable if the Permeability·Depth/Pattern Area is greater than 0.3, i.e., 0.5 PV is injected in less than two years.  This means that projects with small well spacing can be shallow, but that larger patterns should be deeper and have higher permeability.

 Thermal EOR Methods

      This primarily means steam drives, soaks or SAGD and is practiced on shallower, viscous-oil reservoirs in the US in California and Texas.  The technique is the oldest EOR technique and is responsible for most EOR production but is seldom used for light oil.