Petroleum is found in subterranean formations and reservoirs in which it has accumulated, and recovery is initially accomplished by pumping or permitting the petroleum to flow to the surface of the earth through wells drilled into and in fluid communication with subterranean formations. Petroleum can be recovered from subterranean formations to any degree only if certain conditions exists. There must be an adequately high concentration of petroleum in the pore space of the formation, and there must be adequate permeability or interconnected flow channels throughout the formation to permit the flow of fluids therethrough if sufficient pressure is applied to the fluid. When the subterranean, petroleum-containing formation has natural energy present in the form of an underlying active edge or bottom water drive, solution gas or a high pressure gas cap above the petroleum, this natural energy source is utilized to recover petroleum. In this primary phase of petroleum recovery, petroleum flows to the surface to wells drilled into the formation. When the natural energy source is depleted, or in the instance of these formations which do not originally contain sufficient natural energy to permit primary recovery operations, some form of supplemental recovery process must be applied to the reservoir.
Waterflooding is a commonly employed method for recovering additional amounts of petroleum beyond the amount recoverable by primary means, and comprises injecting field water into the formation to displace petroleum through the formation to the production well. Water does not displace petroleum efficiently, however, since oil and water are immiscible and the interfacial tension between water and oil is quite high. After completion of primary and secondary recovery, it is common to find that from 50 to 70 percent of the oil originally present in the formation still remains unrecovered in the formation.
It is recognized in the prior art that waterflooding can only recover a fraction of the oil present in the formation, and many prior art references disclose the use of water containing additives which decrease the interfacial tension between the injected water and formation petroleum. Petroleum sulfonates have been disclosed in many references for use in oil recovery operations, but petroleum sulfonates are limited with respect to formation water salinity, hardness, and other factors which greatly reduce their applicability.
U.S. Pat. Nos. 3,508,612; 3,792,731; 3,811,504; 3,811,505; 3,811,507; 3,827,497; 3,858,656 and 4,016,932 describe oil recovery methods employing fluids containing a combination of surfactants which permit application of surfactant waterflooding processes to formations containing higher salinity and/or higher concentrations of divalent ions such as calcium and magnesium.
While the foregoing multi-component surfactant systems effectively recover oil from some formations, other formations present problems in the use of at least certain of these systems. With respect to those systems requiring the use of a nonionic surfactant as a solubilizing co-surfactant with a primary surfactant such as petroleum sulfonate or other organic sulfonate, commonly available nonionic surfactants exhibit cloud point phenomena which regularly restrict their applicability. The cloud point of a nonionic surfactant is the temperature above which the nonionic surfactant is relatively insoluble, and nonionic surfactants must be soluble in order to solubilize the primary anionic surfactant for use in high salinity environments. It is believed that problems which have been encountered employing fluids containing nonionic surfactants in high temperature formations, which are manifest in phase separation phenomena, are associated with the cloud point of a nonionic surfactant. It is known in the art that the cloud point of polyethoxylated alkyl phenols, for example, increase with the degree of ethoxylation, i.e. with the average number of ethoxy groups contained in the molecule. It is also known that the cloud point decreases as the salinity of the surfactant solution increases. For example, an ethoxylated nonyl phenol having 10 ethoxy groups per molecule will ordinarily have a cloud point of about 130.degree. F. in essentially pure water, but the cloud point is reduced to about 85.degree. F. in a 10 percent sodium chloride brine solution. It is further known that the maximum detergency using an aqueous solution of polyethoxylated alkyl phenol occurs at the cloud point of a particular ethoxylate. See for example, "Nonionic Surfactants" by Ed Martin, J. Schick, published by Marcell Dekker Inc., New York 1967. It is not taught in the literature, however, that how the cloud point of an ethoxylated nonionic surfactant which is being used in combination with one or more dissimilar surfactants in an oil recovery process being applied to a formation containing high concentrations of divalent ions such as calcium and magnesium as well as high salinities, affects the efficiency of a surfactant fluid for low surface tension displacement of petroleum.
In view of the foregoing discussion, it can be readily appreciated that there is a significant commercial need for an oil recovery method which can be applied to formations containing high salinity and hard water, which formations are at temperatures greater than 80.degree. F. (27.degree. C.), e.g. from about 80.degree. to about 180.degree. F. (27.degree. C. to about 82.degree. C.).