This invention relates to the recovery of oil from subterranean oil reservoirs and more particularly to improved waterflooding operations involving the injection of multifunctional anionic-nonionic surfactant systems which are compatible with high salinities and high concentrations of divalent metal ions.
In the recovery of oil from oil-bearing reservoirs, it usually is possible to recover only minor portions of the original oil in place by the so-called primary recovery methods which utilize only the natural forces present in the reservoir. Thus a variety of supplemental recovery techniques have been employed in order to increase the recovery of oil from subterranean reservoirs. The most widely used supplemental recovery technique is waterflooding which involves the injection of water into the reservoir. As the water moves through the reservoir, it acts to displace oil therein to a production system composed of one or more wells through which the oil is recovered.
It has long been recognized that factors such as the interfacial tension between the injected water and the reservoir oil, the relative mobilities of the reservoir oil and injected water, and the wettability characteristics of the rock surfaces within the reservoir are factors which influence the amount of oil recovered by waterflooding. Thus it has been proposed to add surfactants to the flood water in order to lower the oil-water interfacial tension and/or to alter the wettability characteristics of the reservoir rock. Also, it has been proposed to add viscosifiers such as polymeric thickening agents to all or part of the injected water in order to increase the viscosity thereof, thus decreasing the mobility ratio between the injected water and oil and improving the sweep efficiency of the waterflood.
Processes which involve the injection of aqueous surfactant solutions in order to reduce the oil-water interfacial tension are commonly referred to as low tension waterflooding techniques. Thus far, most low tension waterflooding applications have employed anionic surfactants. For example, a paper by W. R. Foster entitled "A Low-Tension Waterflooding Process", JOURNAL OF PETROLEUM TECHNOLOGY, Vol. 25, Feb. 1973, pp. 205-210, describes a promising technique involving the injection of an aqueous solution of petroleum sulfonates within designated equivalent weight ranges and under controlled conditions of salinity. The petroleum sulfonate slug is followed by a thickened water slug which contains a viscosifier such as a water-soluble biopolymer in a graded concentration in order to provide a maximum viscosity greater than the viscosity of the reservoir oil and a terminal viscosity near that of water. This thickened water slug is then followed by a driving fluid such as a field brine which is injected as necessary to carry the process to conclusion.
One limitation encountered in waterflooding with anionic surfactants such as petroleum sulfonates is the tendency of the surfactants to precipitate from solution in the presence of even moderate concentrations of divalent metal ions such as calcium and magnesium ions. Thus, as taught for example in the Foster paper, the surfactant slug may be preceded by a protective slug which functions to displace the reservoir waters ahead of the subsequently injected surfactant slug. Another limitation imposed upon the use of anionic surface-active agents resides in the fact that desired low interfacial tensions can seldom be achieved, even in the absence of divalent metal ions, at salinities significantly in excess of 2 to 3 weight percent. Thus, the protective slug as well as the surfactant slug normally will be of a relatively low salinity.
Surfactant adsorption poses a serious problem to the application of low tension waterflooding. To counter this, it has been proposed, as taught in the aforementioned paper by Foster and also in U.S. Pat. No. 3,474,864 to Hurd, to employ a surfactant slug having a salinity consistent with the desired low interfacial tension, then to follow the surfactant slug with a slug of a somewhat lower salinity in order to move the surfactant through the reservoir. In addition, as taught by these references, sacrificial agents such as sodium tripolyphosphate and/or sodium carbonate may be employed in order to decrease adsorption of the surfactant.
In addition to the use of anionic surfactants, it also has been proposed to carry out waterflooding employing nonionic surfactants, either alone or in combination with anionic surfactants. For example, U.S. Pat. No. 3,332,486 to McGhee discloses waterflooding employing a combination of nonionic and anionic surfactants which are said to be blended such that the hydrophilic-lipophilic balance of the surfactant slug is approximately the same as that associated with the crude oil-injection water system. Various weight ratios of anionic to nonionic surfactants in the system are disclosed within the range of about 6.5/3.5 to about 8.5/1.5.
Another low tension waterflooding process, employing anionic-nonionic surfactant systems in the presence of salt concentrations within the range of 0.5 to 15.0 percent by weight, is disclosed in U.S. Pat. No. 3,792,731 to Feuerbacher et al. Anionic surfactants proposed for use in the Feuerbacher et al. patent include petroleum sulfonates, alkyl sulfonates, alkyl sulfates, and sulfosuccinates. Nonionic surfactants suggested by the patentees include ethoxylated alkyl phenols, ethoxylated alcohols, polymers or copolymers of ethylene oxide and/or propylene oxide, ethoxylated thioethers, and ethoxylated amines. The desired surfactant concentration is said to be within the range of 0.5 to 2.5 percent by weight with the surfactant system containing about 20 to 50 percent by weight of the nonionic surfactant with the balance being the anionic surfactant.
A number of recent patents are directed to the use of anionic-nonionic surfactant systems in low tension waterfloods carried out in the presence of high divalent-metal ion concentrations. For example, U.S. Pat. No. 3,811,505 to Flournoy et al. discloses such a system for use in formations containing water having concentrations of divalent ions such as calcium and magnesium within a range of about 500 to about 9000 parts per million (0.05 to 0.9 weight percent). The nonionic surfactants employed in the Flournoy et al. process include polyethoxylated alkyl phenols in which the alkyl group has 5 to 20 carbon atoms and polyethoxylated aliphatic alcohols having from 5 to 20 carbon atoms. The surfactants are said to contain 6 to 20 polyethoxy groups. The anionic surfactants employed include alkyl sulfonates and phosphates having from 5 to 25 carbon atoms and alkylaryl sulfonates and phosphates having from 5 to 25 carbons in the alkyl groups. Both the anionic and nonionic surfactants may be employed in concentrations within the range of 0.05 to 5.0 percent with the ratio of anionic surfactant to nonionic surfactant being about 1/10 to about 10.
U.S. Pat. No. 3,811,504, also to Flournoy et al., is directed to a low tension waterflood process for use in environments exhibiting a polyvalent ion concentration of about 1500 to about 12,000 parts per million and which employs a three-component surfactant system containing two anionic surfactants and one nonionic surfactant. One of the anionic surfactants is an alkyl or alkylaryl sulfonate and the other anionic surfactant is an alkyl polyethoxy sulfate. The nonionic surfactant may be a polyethoxylated alkyl phenol or a polyethoxylated aliphatic alcohol as disclosed in the previously mentioned Flournoy et al. patent or it may take the form of a fatty acid dialkanolamide or a fatty acid monoalkanolamide in which the fatty acid contains from 5 to 20 carbon atoms. In this process as in the previously described Flournoy et al. patent, a thickening agent such as a polyacrylamide or polysaccharide may be added to the surfactant slug or to a subsequently injected slug. In addition the surfactant slug may be preceded by a sacrificial agent such as sodium polyphosphate or sodium carbonate.