This invention relates generally to the field of Enhanced Oil Recovery (EOR) and more specifically to an Oil Recovery Process employing mixtures of amphoteric surfactants. The amphoteric surfactants are betaines containing both saturated and unsaturated hydrophobic hydrocarbyl groups and are derived from naturally occurring oils and fatty acids rendering them green and biodegradable.
This invention also relates to the recovery of oil from subterranean oil reservoirs and more particularly to improved chemical flooding operations involving the use of certain mixtures of amphoteric surfactants that are suitable for use in brines containing relatively high concentrations of divalent metal ions and at high temperature ranges.
Crude oil is recovered from oil-bearing reservoirs generally by three processes designated primary, secondary and tertiary recovery. In primary recovery the oil is produced through a producing well by taking advantage of the pressure exerted on underground pools of oil by gas or water present with the oil. Approximately 20% of the original oil in place (OOIP) is recovered by this process. Once this pressure has been exhausted other means of recovering the remaining oil must be employed. In secondary recovery the well may be re-pressurized with gas or water injected through one or more injection wells to recover approximately an additional 20% of the OOIP. Other secondary recovery methods include acidizing and/or fracturing to create multiple channels through which the oil may flow. After secondary recovery means have been exhausted and fail to produce any additional oil, tertiary recovery can be employed to recover additional oil up to approximately 60% OOIP. Tertiary oil recovery processes include, but are not limited to, steam flooding, polymer flooding, microbiological flooding, and chemical flooding.
Chemical flooding includes the use of surfactants for lowering the interfacial tension (IFT) between the injection brine and the residual oil usually to an ultra-low value of below 1×10−2 mN/m. Mobility control agents such as polymers are usually employed along with surfactants to adjust the mobility ratio between the oil and the injection brine. It has also been found that alkali, when included in the injection brine, can react with the acidic material present in the trapped oil to form surface-active salts that enhance the effectiveness of the injected surfactant. Alkali also is preferentially adsorbed onto the reservoir and therefore reduces the loss of surfactant and polymer through adsorption.
Alkaline-Surfactant-Polymer Flooding (ASP) has been the subject of numerous studies, papers and patents, for example U.S. Pat. No. 4,004,638 issued to Burdyn et al. in 1977 and U.S. Pat. No. 6,043,391 issued to Berger et al. in 2000. Several other tertiary chemical processes for enhanced oil recovery include Alkaline Surfactant (AS), Alkaline Polymer (AP), and Alkaline flooding. The alkali commonly used in these applications are inorganic alkali including, but are not limited to, sodium hydroxide, sodium carbonate, the combination of sodium hydroxide and sodium carbonate, and sodium silicates.
There are many examples of the prior art that discuss the use of different types of surfactants and/or surfactant formulations for EOR including amphoteric surfactants. As is known by those who are familiar with the art, amphoteric surfactants have the advantages of providing low IFT, tolerance to salt and di-valent cations and fair to low adsorption loss to the formation. U.S. Pat. No. 4,216,097 to Stournas, discloses a process for the recovery of oil from subterranean reservoirs employing an aqueous solution of an amphoteric surfactant. The amphoteric surfactant is used at a relatively low concentration within the range of 0.001 to 0.1 weight percent and is injected in a relatively large pore volume amount of at least 0.5 pore volume. U.S. Pat. No 4,554,974 to Kalpakei, et al. discloses a method for recovering petroleum using a surfactant slug comprising an aqueous solution containing about 0.001 to about 5% by weight of an amphoteric surfactant and an effective amount of high molecular weight homopolysaccharide gum thickener derived from the fungus strains of genus Schlerotium. 
Although the prior art employ amphoteric surfactants as part of various formulations for the recovery of oil, we have unexpectedly found that the degree of unsaturation and the distribution of carbon chain lengths in the lipophilic base is of extreme importance to lower IFT for a wide range of different gravity oils and brines. Furthermore, relatively low pore volumes of the injection fluid including the mixture of the amphoteric surfactants is required for effective oil recovery. This present invention provides improved performance and economics over the prior art.