NO854753L - PROPOXYLATED SURFACE ACTIVITIES FOR ASSISTED OIL EXTRACTION IN THE AREA BETWEEN HIGH AND LOW SALT CONTENTS. - Google Patents

Description

The present invention relates to a method for extracting oil from an oily, underground formation that contains a salt concentration in the range from approx. 20.0 0 0 to approx. 80,000 parts per million (ppm) total dissolved solids (TDS - Total Dis sol ved Sol i ds). In particular, the invention relates to a method where surfactants are used to improve the oil displacement efficiency for water flows in such formations.

Within the petroleum industry, people have been aware for many years that the natural formation energy in a petroleum reservoir will only produce a part of the crude oil originally found in the reservoir. With water injection, water or another aqueous liquid is introduced into the injection wells to propel the oil through the reservoir to the production wells. However, such injection is ineffective, and leaves in the reservoirs more than half of the reservoir crude oil that is located there.

Much of the oil retained in the reservoir after a typical water injection is in the form of discontinuous globules or discrete droplets that sit in the pore spaces in the reservoir. The normal interfacial tension between the reservoir oil and the water used for injection is so high that these discrete droplets are unable to deform sufficiently to pass through narrow passages in the pore channels of the formation.

It is generally accepted that the interfacial tension between water and the reservoir oil must be reduced to below 0.1 dyne/cm for the injection to provide effective recovery.

Surfactants can be used by injection to reduce or reduce the interfacial tension between the water and the reservoir oil, so that the oil droplets can deform, coalesce and flow together with the flowing water towards a production well. However, the effectiveness of a given surfactant in terms of reducing the interfacial tension varies significantly with the conditions in the reservoir, such as e.g. temperature and the presence and amount of salts and metal ions.

Reservoir brine solutions generally contain over 20 . 0 0 0 ppm TDS sodium chloride and above 5 . 0 00 ppm TDS combined calcium and magnesium chloride . Surfactants which are known to be useful in cleaning in hard water are generally used in sodium chloride concentrations which are no higher than approx. 50 0 ppm TDS, and combined calcium and magnesium chloride concentrations of less than 50 ppm TDS. At salinities above hardness levels that usually occur in reservoirs, such surface-active agents can be precipitated. If the reservoir temperature is high, e.g. above 66°C, such surfactants may be unstable and become ineffective. Consequently, the oil industry has for some time been interested in the development of surfactants that are useful for the conditions in the reservoir.

A number of surfactants have been found to be useful in reservoirs where the salt content and water hardness, i.e. the concentration of divalent ions including calcium and magnesium, are relatively low. Such surfactants include crude oil sulfonates, alkyl sulfates or sulfonates, alkyl aryl sulfates or sulfonates, and quaternary ammonium salts. In general, the reservoir salt content must be less than 5 . 0 0 0 ppm TDS and the concentration of divalent ions in the reservoir must be less than approx. 200 to approx. 500 ppm to allow the use of these surfactants. Although commonly available crude oil sulphonates e.g. become insoluble in water that has a salinity greater than 5.0 00 ppm TDS and/or more than 50 0 ppm divalent ions.

For reservoirs where the salt content and water hardness are higher, various multi-component systems of surfactants have been proposed. Examples of such systems include mixtures of an anionic surfactant and a solubilizing co-surfactant and mixtures of anionic and non-ionic surfactants. However, a number of problems are associated with these systems. As they consist of different species of surfactants, the systems are subject to selective adsorption or chromatographic separation in a reservoir. Such separation or selective adsorption more than outweighs the advantages of the mixture. Furthermore, the concentration ratios between the surfactants, e.g. the concentration of anionic surfactants in terms of the concentration zone of dissolving co-surfactants, usually very critical and varies with the salt content and the concentration of divalent ions. The price of such dissolving co-surfactants can also be several times the price of the primary surfactant, this increases the costs of using the system, possibly to a point where commercial use cannot be defended.

In recent times, surfactants have been proposed which are said to be effective over a wide range of salinity and concentration zones of divalent ions, without the need for co-surfactants or co-solvents.

U.S. patent no. 4, 293, 428 describes a class of surface-active agents containing many discrete component compounds which exhibit similar surface-active properties in a given reservoir salt water solution and thereby substantially reduce the problems usually associated with chromatographic separation during injection operations. Each compound contains propoxy groups (which are lipophilic) positioned between a lipophilic group consisting of a linear or branched alkyl radical, an alkaryl radical, or etalkyl-or alkylaryl-substituted benzene radical and an ethoxy chain/hydrophilic group. If a chromatography-type separation of the component compounds takes place, an effective reduction of the oil-water interfacial tension is still achieved. As a result of this, variations in the salt concentrations inside the reservoir do not affect the effect of the surface-active agent to the extent that with previously used surface-active agents. The surfactant described provides a high degree of surface activity in reservoirs that have a high concentration of inorganic salts, especially concentrations greater than approx. 1 0 0 . 0 0 0 ppm TDS.

U.S. patent no. 4,340,492 describes a process for the recovery of oil by the use of a surface-active agent whereby the problems of chromatographic separation and selective adsorption are avoided in that the agent includes a nonionic polyalkylene oxide hydrophilic group and an anionic sulfonate hydrophilic group which are linked on the molecular plane to the same lipophilic base by means of a dinuclear aryl group with a mononuclear or fused ring. The polyalkylene oxide group contains at least three alkylene oxide units which are derived from ethylene oxide or propylene oxide or mixtures thereof. The 1 ipof i le base is an aliphatic group, an aliphatic substituted succinimido group, or the corresponding succinic acid derivative of said aliphatic-substituted succinimido group. The surface-active agent is water-soluble and is used in an essentially oil-free, aqueous liquid which differs from a microemulsion. It is claimed to be particularly suitable for reservoirs where the original water shows a relatively high salt content, including divalent metal ions. The surfactant is also claimed to be useful in water streams where the available injection water shows a relatively high salt content. The preferred use of the surfactant is in reservoirs or injection water having a concentration of divalent ion in the range from 500 ppm to 24,000 ppm. Experimental results showed that the surfactant showed surface activity in salt water solutions with from 4.8 to 20.4% by weight, or 48,000 ppm to 204,000 ppm TDS.

The present invention relates to an injection process for assisted extraction of oil from an oil-bearing underground formation that contains a salt concentration in the range of approx. 20 . 000 to approx. 80,000 ppm TDS. In this process, a liquid is injected that has a salt content in the range from approx.

20 . 0 0 0 to approx. 80 . 00 0 ppm TDS and which contains a surfactant in the formation. The surfactant has the general formula:

where

R is a linear or branched alkyl or alkylaryl radical containing from approx. 1 0 to approx. 20 carbon atoms;

x has an average value from approx. 2.0 to 15;

R' is a linear or branched alkyl radical containing from 0 to approx. 5 carbon atoms; and

M is a cation.

Figure 1 is a graphical presentation of the optimal salt contents in i-tridecyl ethoxylated ether sulfates and i-tridecyl propoxylated ether sulfates as a function of the number of ethoxy or propoxy groups. Figure 2 is a graphical representation of fraction 1 oil recovery or oil j fraction as a function of pore volumes of liquid formed by a core injection.

It has surprisingly been found that propoxylated surfactants without the presence of ethoxy groups can be used very effectively to assist the recovery of oil from oily reservoirs containing saline water with a moderately high level of salts by using the surfactants in a microemulsion having a salt content corresponding to the salt content in the reservoir. That is in an environment containing approx. 2% to approx. 8% TDS, or approx. 20 . 0 0 0 to approx. 80,000 ppm TDS, are surfactants that have the general formula:

where

R is a linear or branched alkyl or alkylaryl radical containing from approx. 10 to approx. 20 carbon atoms;

x has an average value of from approx. 2.0 to approx. 15;

R' is a linear or branched alkyl radical containing from 0 to approx. 5 carbon atoms; and

M is a cation, such as e.g. K<+>, Li<+>, MEA+ , Ca<++>or Mg++ ,

or preferably, Na<+> or NH 3

very effective surfactants for the extraction of oil. These surfactants are referred to in the present invention as propoxylated surfactants.

Propoxy groups do not contribute to the amphiphilic nature of the surfactant in the same way that ethoxy groups do. Ethoxy groups interact very strongly with water. Propoxy groups interact less strongly with water. As a result, ethoxy groups and propoxy groups have opposite effects on the optimum salt content for a surfactant. That is , as shown in Figure 1, addition of ethylene oxide to a lipophilic portion of a surfactant will cause the optimal salt content of the surfactant to increase, while addition of propylene oxide to a lipophilic portion of a surfactant will cause the optimal salt content of the the surfactant is reduced. Surprisingly, however, it has been found that the optimal salt content for bare-propoxylated surfactants is moderately high, and the surfactants are very effective at moderately high salt contents varying from approx. 20 . 0 0 0 ppm to approx. 80,000 ppm TDS, although no ethoxy groups are present to improve the solubility of the molecule in water.

In this range of moderately high salt contents, the dissolution parameters of bare-propoxylated surfactants are high compared to the solubility parameters of commercially available surfactants.

The solubility parameters provide a measure of surface activity. The higher the dissolution parameter, the lower the interfacial tension will be between the microemulsion and the reservoir fluids - oil and salt water solution. It is therefore desirable to have high dissolution parameter values for the surfactants to be used in assisted oil extraction.

Comparison of surfactants on the basis of the dissolution parameters and the optimal salt contents and the determination of such parameters and salt contents are methods known to the person skilled in the art. Basically, these methods take into account that when water, an oil and a surfactant are brought to equilibrium at concentrations of surfactant above the critical micelle concentration, one or more microemulsions are formed. All surfactant injection processes can include microemulsions in situ. A microemulsion can be defined as a stable, translucent micelle solution of oil, water which may contain electrolytes, and one or more amphiphilic compounds such as e.g. surfactants. When the salt content increases, the amount of oil dissolved in the surfactant increases, the amount of water dissolved in the surfactant decreases, and vice versa. At the point where the amount of water dissolved by the surfactant equals the amount of oil dissolved by the surfactant, the maximum oil recovery from a stream of surfactant can be achieved. This point occurs at, and is known as, the optimum salt content for the surfactant. m.a.o. is the optimal salt content for a given surface-active agent the concentration of inorganic substances in the oil-water microemulsion system whereby the low interfacial tensions are equal for the microemulsion-oil interface and the microemulsion-water interface.

The optimum salt content for the surfactant varies with temperature and properties of the oil and water in the microemulsion. In general, a surfactant whose optimum salt content at the reservoir conditions is approximately the same as, or is close to, the salt content of the reservoir water, will be most effective in assisted oil recovery in a stream of surfactant.

For a detailed discussion of the estimation of optimal salinity and dissolution parameters, see R. N. Healy, R. L. Reed, and D . G. Stenmark, "Mul tiphase Microemulsion Systems", Siciety of Pet roleum Engineers Journal, pages 147-160, June 1976. For further discussion of microemulsions, see the article by R.N. Healy and R.L. Reed entitled "Physiochemical Aspects of Microemulsion Flooding", published in the October 1974 issue of the Society of Petroleum Engineers Journal on page 491; see also the chapter entitled "Some Physiochemical Aspects of Microemulsion Flooding: A Review" by R. L. Reed and R. N. Healy in "Improved Oil Recovery by Surf actant and Polymer Flooding", published by Academic Press, Inc. in 1977.

The present invention relates to the sulfate and sulfonate versions of bare-propoxylated surfactants. When R' is 0, the bare-propoxylated surfactant is a sulfate; when R' is greater than 0, the just-propoxylated surfactant is a sulfonate. Sulfates and sulfonates are prepared somewhat differently, but they behave the same in the present invention, except at temperatures above approx. 71°C. Above this temperature the sulfate begins to become hydrolytically unstable. Consequently, at such high temperatures, propoxylated only surfactants which are sulfonates are preferred.

Barely propoxylated surfactants which are sulfates can be prepared in a variety of ways well known to those skilled in the art. The techniques of alkylation and propoxylation (or alkoxylation) and sulfation which can be used to prepare such propoxylated surfactants are also discussed in the aforementioned patent No. 4,293,428.

Although this discussion in the aforementioned patent took as its starting point the preparation of propoxylated, ethoxylated surfactants, it is also relevant with respect to the preparation of bare-propoxylated surfactants. Barely propoxylated sulfate surfactants can be prepared by the same method as propoxylated, ethoxylated surfactants, except that the ethoxylation step is omitted for bare propoxylated surfactants.

Barely propoxylated surfactants which are sulfonates can also be prepared in a number of ways. For example they can be prepared in the same manner as bare-propoxylated sulfates where sulfonation is substituted for the sulfation step, as described in the aforementioned U.S. Patent No. 4,293,428. Below follows an example that describes the preparation of bare-propoxylated sulfonate surfactants for investigation of the present invention.

Example 1

Sodium hydride (60% dispersion in mineral oil, 4 g) was added to a four-necked round bottom flask equipped with an overhead stirrer, reflux condenser, additional funnel and thermometer. It was washed three times with dry hexane. After inoculation, 80 ml of dry hexane was added. During a period of 5 minutes, n-C-| 5H33( C3H5O )3H (32.8 g) added dropwise. The temperature rose to 35°C. The mixture was brought to 70°C and heated under reflux for 40 minutes. The mixture was then cooled to room temperature, after which propanesulfone (97%, 12.59g) was added dropwise. The mixture was then heated to 70°C under reflux for 4 hours. After desalting and removal of oil, 16.5 g of surfactant, n-C-| 6H33-(CjHgO)3CH2CH2CH2SO3Na. Oso-octane can be substituted for hexane in this reaction. Iso-octane has a higher boiling point than hexane, this can lead to a greater yield of surfactant. Furthermore, n-C-j 6H33( C3H6° )3H is more soluble in iso-octane than in hexane.

The aforementioned U.S. Patent No. 4,293,428 also discusses other alkylation, propoxylation and sulfonation techniques that can be used.

The surfactants prepared as described above and in said U.S. Pat. patent no. 4,293,428 will not be pure, i.e. they will not consist of just one structure such as e.g. RO (C3H6O )x SO5M<+>where x is 6 for all molecules in the surfactant. In contrast, the surface-active agent will contain different molecules of the same species, i.e. RO (C3H5O )x SO^M4" where x will vary. Overall, however, the surface-active agent will have a certain average value for x.

For each average value of x, there is a distribution of the number of moles of propylene oxide. The addition or omission of one mole of propylene oxide changes the optimum salt content for the surfactant relatively little compared to the addition or omission of one mole of ethylene oxide. Consequently, the propoxylated surfactants are less sensitive to moderate changes in the well's salt content, temperature and oil composition than more conventional surfactants such as ethoxylated alcohol sulfates or mixtures of alkylarylsulfonates with ethoxylated alcohols.

An optimal average value of x for the bare-propoxylated surfactant can be determined for a given reservoir. In general, this value would be one where the surfactant has an optimum salt content that is approximately equal to, or close to, the salt content of the reservoir.

The exact compositions of different portions of surfactants with the same average x values may vary. The propoxylation reactions cannot be controlled to the extent that the compositions are exactly reproducible. Furthermore, the raw materials and the operation of the plant can change, this affects the composition which gives a certain average value for x. Consequently, the phase behavior seen for different portions of the surfactant may vary.

Mixtures of the bare-propoxylated surfactants with average x-values that provide optimal salinities, above and below the desired reservoir salinity, show reproducible phase behavior. Consequently, mixtures allow greater flexibility regarding raw materials or plant operation, without affecting the phase behavior of the mixed product. It also widens the distribution of moles of propylene oxide for the surfactant thereby increasing the flexibility of the surfactants for moderate changes in reservoir temperature, brine present, salinity and oil composition. Accordingly, mixing the bare propoxylated surfactants is preferred.

Mixtures of bare-propoxylated surfactants do not appear to be susceptible to chromatographic separation in the reservoir at all, and clearly not to the same extent as observed with mixtures of more conventional surfactants.

The wider propylene oxide distribution and the resulting flexibility that can be achieved by mixing bare-propoxylated surfactants may make it advantageous to use mixtures of bare-propoxylated surfactants instead of propoxylated, ethoxylated surfactants as described in the aforementioned U.S. Pat. patent no. 4,293,4-28 in reservoirs that have only moderately high salt contents, i.e. saltwater solutions where the salt content is in the range from approx. 20,000 ppm to approx. 80,000 ppm TDS. Although propoxylated ethoxylated surfactants are effective in this moderately high salt range, the average number of ethoxy groups in a propoxylated ethoxylated surfactant that is optimal at such reservoir salinity must be very low, probably in the range of only approx. 1 to 2. There is therefore little leeway for the mixing of the surface-active agents to achieve the improved flexibility and the other above-mentioned advantages which are possible by mixing.

Best results when using the present invention can be achieved by determining and using the mixture of bare-propoxylated surfactants that have an optimal salt content that corresponds or is close to the salt content of the reservoir's salt water solution at the reservoir temperature.

The suitable average number of propoxy groups in the mixture of propoxylated surfactant only, i.e. the optimum average value for x is preferably determined by examination of surfactants with fixed salt content in the water, molecular weight of the oil and temperature. Preferably, the salt content of the water, the molecular weight of the oil and the temperature are determined at values that are close to the values in the reservoir where the surfactant is to be used. In such experiments, surfactants with different average numbers of propoxy groups, or in the case of mixtures of surfactants, the different ratios of two or more surfactants with different average numbers of propoxy groups, are mixed with water and oil to form or attempt to to form a microemulsion. Various bare-propoxylated surfactants or mixtures of bare-propoxylated surfactants are investigated until an optimal agent or mixture is found. At the optimum point, the formed microemulsion will contain equal parts of oil and water and the interfacial tension for the microemulsion-oil will be low and equal to the interfacial tension for the microemulsion-salt water solution.

The techniques for carrying out such investigations of surfactants are known to the person skilled in the art. For further discussion of such techniques, seeM. C. Puerto and R. L. Reed, "A Three Parameter Representation of Surfactant--Oil-Brine Ineraction", Proceedings of the Third Joint SPE/DOE Enhanced Oil Recovery Symposium, Tulsa, Oklahoma, pages 51-75 (April 4-7 , 1982).

In the present invention, the bare-propoxylated surfactant can generally be used in any injection process where a surfactant is introduced into a formation for the purpose of extracting crude oil. The method according to the present invention finds particular application in aqueous solutions of surfactants and in microemulsions. The surfactants can also be used in liquid hydrocarbon solutions in the recovery techniques where an oil solvent is used to provide a miscible displacement of the crude oil inside the formation.

In all applications of the method according to the present invention, the environment for the surfactant must have a salt concentration between approx. 20 . 0 00 ppm and approx. 80 . 0 0 0 ppm TDS. The salt water solution present in the reservoir may already have such a salt concentration, or the salt concentration may be changed by pre-flushing the reservoir with a salt water solution containing such a salt concentration. The surfactant can also be included in a microemulsion with such a salt concentration for injection into the reservoir.

In a preferred application of the present invention, however, the bare-propoxylated surfactants are used in a microemulsion which has a salt concentration that is approximately equal to the salt concentration for the reservoir and the salt concentration range for the reservoir is between approx. 20 . 0 0 0 ppm and approx. 80 . 00 0 ppm TDS.

Microemulsions can be formed for injection into a reservoir as well as formed in situ with streams of surfactant. However, the concept is similar, and the meaning of the term "microemulsion" is the same.

Injection of a surfactant used in micro-emulsions instead of in aqueous solution is generally preferred in assisted oil recovery processes because the surfactant is usually insoluble in optimal salt water solution and consequently stops formation again.

Production of microemulsions for use in assisted oil extraction is known to the person skilled in the art. E.g. microemulsions suitable for the present invention can be water-external, oil-external or fall into the class of micelle structures where there is apparently no identifiable external phase. The microemulsions can be single-phase solutions that can absorb additional amounts of oil or water without phase separation. The microemulsions can be immiscible with oil, water or both.

In general, microemulsions for injection into a reservoir are formulated on the basis of information regarding the reservoir. Such information will preferably include analysis of the physical and chemical properties of the reservoir crude oil, determination of the amount and type of ionic substances present in the reservoir water or brine and determination of the reservoir temperature.

For the purposes of the present invention, the oil component in the composite microemulsions must not have the same physical and chemical properties as the reservoir crude oil. The aqueous medium used in the composition of the microemulsions, however, preferably has a salt concentration corresponding to the salt water solution in the reservoir and such a salt concentration will vary between approx. 20 . 0 0 0 ppm and approx. 80.0 0 0 ppm TDS. The amphiphilic component of the microemulsion will be propoxylated surfactants alone or as a mixture. Co-surfactants or co-solvents are not required by the present invention. However, one or more co-surfactant agents or co-solvents can also be used in the microemulsion, e.g. to regulate the viscosity of the microemulsion.

When producing the microemulsions according to the present invention, the proportions of oil, water and surfactant are not particularly critical as long as the quantity e is large enough to provide a microemulsion. Accordingly, the amount of water or the amount of oil can vary within wide limits. It is sufficient in the microemulsions according to the invention that the surface-active agent is used in an amount which effectively produces the desired microemulsion. In most cases, the just-propoxylated surfactant is used in an amount of from approx. 0.5 to approx. 12% based on the volume of the microemulsion, and preferably between approx. 1.5% and approx. 5%. Usually the lowest possible concentration is used, so that the correspondingly large storage size can be used to compensate for any losses in the reservoir; the upper limits are based on economic considerations.

Mobility control agents can be used with the propoxylated bare surfactants or microemulsions containing propoxylated bare surfactants. Mobility control agents can also sometimes be added to, or incorporated into, such a microemulsion. Typical mobility control agents or thickeners for injection operations include: polysaccharides, such as marketed under the trademark "Kelzan XC" by Kelco Corporation, and under the trademark "Biopolymer 1035" by Pfitzer, Inc.; and especially hydrolyzed polyacrylamides, such as e.g. marketed under the trademark "Pusher" by the Dow Chemical Company, and many others too numerous to list.

Although the general procedure for injection of microemulsions and extraction of crude oil will be known to the person skilled in the art, a brief overview can be illustrative. In the preferred embodiment of the present invention, a microemulsion containing bare-propoxylated surfactants is first injected into the reservoir in the form of a shock, followed by the injection of thickened water and then non-thickened water. The shock of microemulsion is injected into the underground formation in an amount that is chosen so that it is large enough to effectively displace the crude oil in the formation to one or more production wells. The person skilled in the art can easily determine the volume to be injected. The thickened water injected after the impact of the microemulsion can be any conventional thickened water used as a propellant in microemulsion injection processes. After the injection of thickened water, unthickened water is injected as an injection medium. The thickened and non-thickened water act as propellants that propel the microemulsion portion through the reservoir and the portion of the microemulsion displaces the crude oil that is in it. The displaced oil is transported to the production facility and then to the surface of the earth.

The use of bare-propoxylated surfactants for assisted oil recovery in salt concentration areas from approx. 20,000 to approx. 80.0 00 ppm TDS appears from the following laboratory tests.

Phase behavior experiments

Phase behavior experiments, especially investigations of surfactant as described above, were carried out with the following surfactants:

a) Alkyl aryl sulfonates (commercially available) of the general structure b) alkyl aryl sulfonates (commercially available) of the general formula c) propoxylated, ethoxylated sulfates of the general structure i-C13<H>270 (C3H60)4(C2H402) 2SO^Na+j d) propoxylated, ethoxylated sulfates of the general structure C-| 3H270 (C5H60)5(C2H/).02)2 SOjNa"1";

e) bare-propoxylated sulfates of the general structure

C13H27° (C5H60)5SOjNa"<1>-;

f) bare-propoxylated fulfats of the general structure

C 13 H 27 O (C 3 H 6 O) 6 SO 5 Na<+>;

g) bare-propoxylated sulfates of the general structure

C15H270 (C3H60) SOjNa<+>j and

h) mixtures of bare-propoxylated sulfates of the general structures indicated under e) and g)

above, where the mixture has an average structure as follows

The test conditions were as follows: temperature -49°C; molar volume for crude oil - 310 cm^/g-mol; salt 53 . 0 0 0 ppm TDS in water, including approx. 49,500 ppm sodium chloride and approx. 3,500 ppm calcium and magnesium ions. This salt concentration is roughly equivalent to a salt content in the salt-water solution in the reservoir of 60 . 0 0 0 ppm TDS .

The test results are reproduced in the following table.

The phase behavior was poor for the bilinear alkylaryl sulfonates (a and b in Table I above). Even when a significant amount of alcohol co-solvent was added, a microemulsion was not formed. Consequently, the optimum salt contents for these sulphonates under the prevailing experimental conditions were not determined. Mixtures of the two surfactants. C-i^Ho^SO-jNa<*>and C17H35

with the crude oil and the brine solution gave

significant emulsification and some visible precipitation.

The phase behavior for the propoxylated, ethoxylated surfactants and for the bare-propoxylated surfactants was very good with low emulsification and moderately high dissolution of oil and salt water at all optimal salt contents.

However, the propoxylated, ethoxylated surfactants had optimal salt contents that were higher than the salt content of the reservoir solution. Optimal recovery with these surfactants would be expected in a reservoir at 49°C with a salt content in the salt water solution of approx. 80,000 ppm or 90,000 ppm TDS instead of approx. 60,000 ppm TDS. Optimum extraction from a reservoir at 40 °C with a salt content in the salt water solution of approx. 60.0 0 0 ppm TDS will more likely be achieved with a surfactant that has an optimal salt content of approx. 60 . 0 0 0 ppm than with these propoxylated, ethoxylated surfactants. Such optimal extraction will consequently take place with greater probability in such a reservoir at 40 °C which has a salt content in the salt water solution of approx. 6 0 . 0 0 0 ppm TDs , with the investigated bare-propoxylated surfactants. Of the bare-propoxylated surfactants that have been examined, i-tridecyl (CjHgO^ ether sulfate may be the optimal choice for such a reservoir, but the mixture that has an average structure of C3H27O ( C3H5O )3 fQ SOjNa"4" would be preferred due to greater flexibility as discussed above.

Engine injection test

A core injection was performed with i-tridecyl ether (031150)5sulfate to determine the efficiency of an oil recovery process using this bare-propoxylated surfactant in a reservoir having a moderately high concentration of salts. The results of this injection are reproduced in Figure 2. Approx. 65% of the oil was recovered, which shows that the surfactant is very effective in assisted recovery of oil under the investigated test conditions.

The test conditions, simulating a reservoir where the surfactant was assessed to be used, were as follows: Temperature - 26°C; molar volume for reservoir crude j e - 296 cm^/g mol; salt concentration in reservoir solution - 53.0 0 0 ppm TDS, including approx. 49.50 0 ppm TDS sodium chloride and approx.

1,400 ppm divalent ions.

Investigations of the salt content had shown that this surfactant had an optimal salt content of approx. 53,000 ppm TDS under these test conditions. The examination of salt content is carried out in the same way as examinations of surface-active agent described previously, except that in examination of salt content the salt content is varied and the surfactant used is the same, while in examinations of surface-active agent the surfactant and salt content are varied used is the same. In the same way as the conduct of investigations of surfactant, the conduct of investigations of salt content is known to the person skilled in the art.

A microemulsion solution was prepared with the surfactant and water containing approx. 60 . 000 ppm TDS and oil osm had physical and chemical properties approximately the same as the previously mentioned reservoir oil.

The core used in the experiment was a piece of Berea sandstone that had a cross section of 2.54 cm x 2.54 cm and a length of 61 cm. Each experimental core had a permeability to salt water of approx. 4. 00 0 millidarcies and was mounted in epoxy with threaded fittings at each end for injection and production of fluids.

Before the experiment was carried out, the core was flushed with oil and brine solution to approximate the saturation with oil and water that would exist in an oil reservoir that has been flowed to the point that no further oil can be produced. In this run-through operation, the core was first saturated with salt water, which had the salt water concentration described above. The core was then perfused with the reservoir oil previously described until no further brine could be produced. The core was then again flushed with brine to remove all the oil that could be recovered in a conventional water injection process. At this point, the amounts of oil and water remaining in the core were approximately equal to the amounts in a reservoir that was water-flooded at 1 residual oil saturation. The residual oil in this core amounted to approx. 32% of the pore volume in the core; the remaining 68% was saturated with saline solution.

After the core was flushed with water to residual oil saturation, the following tests were carried out in the core. The composition of the microemulsion solution described above was injected into the core. The injection of this microemulsion was continued until approx. 20% pore volume of liquid was injected. The microemulsion solution was followed by a saline solution containing approx. 750 ppm of a polymeric thickener and approx. 6% TDS until 1 oil production ceased. The viscosity of this salt water solution was approx. 15 centipoise. These liquids were injected with an average front velocity of approx. 0.30 mpr. day. It. final oil saturation was obtained by measuring the growing oil recovery.

The experiment was repeated with mixtures of bare-propoxylated surfactants <C>13<H>27O (C^H^O^ SO-jNa"1", hereafter referred to as "(P0)5" and C13H27O (C-jHgO ^ SO^Na<*>,

hereafter referred to as "(P0)7<n>. The experimental conditions were

the same as in the phase behavior experiments discussed above. The concentration of polymer and microemulsions and the amount of microemulsion used in each experiment, as well as the experimental results, are listed in Table II below.

Comparison with Other Alkoxylated Surfactants An important feature of the surfactants in the previously mentioned U.S. patent no. 4,293,4-28 is the presence of propoxy groups located between the lipophilic group and the ethoxy chain/hydrophilic group in the surfactant molecule. The propoxy groups reduced the total interaction between the ethoxy chain/hydrophilic group and water, and allowed a balancing of the lipophilic and hydrophilic groups, so that a high surface activity and resulting good oil recovery could be achieved. The combination of the propoxy groups m. h. t. the ethoxy groups and the position of the two groups relative to each other were critical to the invention.

Data in the aforementioned patent demonstrated that surfactants with such combination and placement of propoxy and ethoxy groups were superior to surfactants referred to in the patent as "only-ethoxylated surfactants", i.e. surfactants containing only ethoxy groups, such as C- 13H27O (C2H4O)2,5 SO-jNa*.

The number of ethoxy groups in such bare-ethoxylated surfactants is an average number, since the surfactants are in reality a mixture of several pure component surfactants having the formula<C>13<H>27° (C2<H>4°)nSO ^Na<4>-, where n=0, 1, 2, 3, 4, etc. The bare-ethoxylated surfactants showed effectiveness in reducing the interfacial tension in environments containing high concentrations of salts ( 1 90.0 0 0 ppm TDS ) , but were subjected to chromatographic separation into the individual components of surfactants in long-lasting core injection experiments or field applications. The separated surfactant components did not show acceptable surface activity in the reservoir brine solution. Addition or exclusion of an ethoxy group on the surface-active molecule significantly changes the optimal salt content for the molecule. Consequently, the optimal salinity for many of the surfactant components deviated significantly from the salinity of the reservoir solution.

The above-mentioned patent also describes results comparing i-tridecyl ether (C^HgOV sulfates with the propoxy, ethoxy surfactants according to the patent.

Laboratory trials were conducted with three similar sulfates, referred to in the patent as "only-propoxylated surfactants", to assess their applicability in oil reservoirs with high salinity brines and/salt water with high concentrations of metal ions. The reservoir system with which the experiments were carried out. thought of, and which the experiments simulated, had a salt water solution with 1 04 . 0 00 ppm TDS, of which sodium and chloride ions accounted for approx. 1 0 0 . 0 0 0 ppm and the remaining 4 . 0 0 0 ppm TDS was made up of calcium and magnesium ions and other trace elements. The temperature in the reservoir was 26°C.

The three bare-propoxylated surfactants investigated were i-tridecyl ether (031150)2 sulfate, i-tridecyl ether (C3H5O) j, sulfate and i-tridecyl ether (C3H5O )4 sulfate. The first of these formed a gel at the reservoir conditions, so that no core injection simulating water injection into a reservoir with the surfactant could be performed. Core injections were carried out with the latter two sulphates at conditions close to the optimum values, but the second sulphate did not give good oil recovery, and the third sulphate gave only moderately good recovery. In the mentioned patent, the conclusion was drawn that only propoxylated surfactants often give unsatisfactory oil recovery even when injections are made under near-optimal conditions.

In contrast to this, it has since been found that propoxylated surfactants work well under optimal conditions when the environment for the surfactants has a salt concentration in the range from approx. 20 . 00 0 ppm to approx. 80 . 0 0 0 ppm TDS . The principle of the invention and the application of this principle which is considered best is described. It should be noted that the foregoing is an illustration and that other devices and techniques may be used without departing from the scope of the present invention as defined in the following claims.

Claims (2)

1. Method for extracting oil from an oil-bearing formation, with a salt content in the salt solution in the range from approx. 20 . 0 00 ppm to approx. 80 . 00 0 ppm TDS, characterized in that it includes injecting a liquid into the formation that has a salt content in the range from approx. 20,000 ppm to approx. 80 . 0 00 TDS and containing an effective amount of surfactant having the general formula: where R is a linear or branched alkyl or alkylaryl radical containing from approx. 10 to approx. 20 carbon atoms; x has an average value from approx. 2.0 to approx. 15; R' is a linear or branched alkyl radical containing from 0 to approx. 5 carbon atoms; and M is a cation; and the liquid is driven through the formation, so that oil is displaced from the formation and the displaced oil is extracted. 2. Method according to claim 1, characterized in that the liquid is a microemulsion. 3. Method according to claim 2, characterized in that the salt content in the microemulsion is approximately the same as the salt content in the formation. 4. Method according to claim 2, characterized in that the microemulsion is driven through the mold with the help of a bursty portion of thickened water. 5. Method according to claim 2, characterized in that the microemulsion includes a thickening agent. 6. Method according to claim 2, characterized in that the microemulsion does not include co-solvent or co-surfactant. 7. Method according to claim 1, characterized in that the liquid forms a microemulsion after injection into the formation. 8. Method according to claim 1, characterized in that the surface-active agent is an i-tridecyl ether (C3H5O )x sulfate, where the value of x varies from 2.0 to approx. 15 and the value of R' is 0 . 9 . Method according to claim 1, characterized in that the surface-active agent is an i-tridecyl ether (C-jHgCOxsulfonate, where the value of x varies from approx. 2.0 to approx. 15 and the value of R' varies from approx. 1 to approx. 5 . 1 0 . Method according to claim 1, characterized in that the formation contains a concentration of divalent ions of more than 20. 0 0 0 ppm TDS. 11 . Method according to claim 1, characterized in that the surface-active agent is a mixture of two or more surface-active agents which have different values for x. 1 2 . Method according to claim 1, characterized in that the liquid is an aqueous solution.