Micellar Slug for Oil Recovery

1. Field of the Invention

This invention relates to a micellar slug suitable for use in a micellar drive for recovering oil from subterranean reservoirs. More specifically, it relates to a micellar slug capable of recovering oil from subterranean reservoirs containing subterranean water having a very wide salt concentration at a high oil recovery efficiency.

2. Description of the Prior Art

It is well known in the art that the so-called "primary recovery" methods, including pumping methods, can recover only a portion of the petroleum or crude oil (referred to as "oil" hereinafter) from subterranean reservoirs and leave substantial amounts of oil in the subterranean reservoirs.

In order to recover the remaining large amounts of oil from the subterranean reservoirs, so-called "secondary recovery" methods have been proposed, wherein, for example, water or gas is injected into the subterranean reservoirs from an injection well at a pressure sufficient to increase the flowability of oil, steam is injected into subterranean reservoirs so as to effect the displacement of oil toward a production well, or oil in subterranean reservoirs is partially burned to heat the subterranean reservoirs so as to decrease the viscosity of the oil and increase the flowability of the oil.

Furthermore, the so-called various "tertiary recovery" methods, including a combination of secondary recovery methods and improved secondary recovery methods, have been proposed, wherein surfactants or water-soluble polymers are utilized. These methods are generally called "enhanced oil recovery" (EOR) methods.

Of the surfactant EOR methods, the recent "micellar drive" methods are to be noted. According to these methods, a micellar slug, that is, a clear micro-emulsion derived from water and oil such as petroleum, petroleum distillates, or fuel oil, is injected under pressure into the subterranean reservoirs for the recovery of oil in the subterranean reservoirs. These EOR methods are disclosed in, for example, U.S. Pat. Nos. 3,506,070, 3,613,786, 3,740,343, 3,983,940, 3,990,515, 4,017,405, and 4,059,154. These prior arts disclose that various kinds of surfactants including anionic-, nonionic-, and cationic-type surfactants can be used alone or in any mixture thereof in the formation of micellar slugs. Examples of such surfactants are petroleum sulfonates, alkylaryl sulfonates, dialkyl sulfosuccinates, alkane sulfonates, polyoxyethylene alkylether sulfates, alpha-olefin sulfonate, polyoxyethylene alkylethers, polyoxyethylene alkylphenylethers, polyol fatty acid esters, alkyltrimethyl ammonium salts, and dialkyldimethyl ammonium salts.

The micellar slugs should have the following characteristics to recover oil from subterranean reservoirs at a high efficiency:

Sufficiently low interfacial tensions between oil and the micro-emulsions and between formation water and the micro-emulsions; stability of the micro-emulsions and the small change in the interfacial tensions during sweeping even when the salt concentration of the formation water is changed; salinity tolerance of the micellar slugs at a wide salt concentration range, since the salt concentration range of the formation water extends widely from a low to high concentration; and low cost availability of the micellar slugs, since a large amount of a micellar slug is necessary to recover oil from subterranean reservoirs.

The object of the present invention is to provide, for the recovery of oil, a micellar slug having an excellent oil-micro-emulsion and formation water-micro-emulsion interfacial tension decreasing capabilities and an excellent salinity tolerance at a wide salt concentration range and being capable of maintaining the stability of the micro-emulsions during sweeping in the subterranean reservoirs even when the salt concentration of the formation water is changed.

A further object of the present invention is to provide an oil recovery process using a micellar slug.

Other objects and advantages of the present invention will be apparent from the following description.

In accordance with the present invention, there is provided a micellar slug, for the recovery of oil, consisting essentially of a hydrocarbon, an aqueous medium, a surfactant, and a cosurfactant, the surfactant containing, as an essential component, an internal olefin sulfonate having 10 to 26 carbon atoms and a disulfonate content of about 20% by weight or less.

The micellar slugs desirably used for the recovery of oil are transparent or translucent micro-emulsions containing about 2% to about 90% by weight of a hydrocarbon, about 4% to about 95% by weight of an aqueous medium, about 1% to about 30% by weight of a surfactant containing, as an essential constituent, an internal olefin sulfonate having 10 to 26 carbon atoms, and about 0.1% to about 20% by weight of a cosurfactant.

The aqueous medium usable in the formation of the micellar slug of the present invention includes soft water, water containing inorganic salts, and brine. For example, rain water, river water, lake water, formation water, oil stratum water, and seawater can be freely used in the formation of the micellar slug of the present invention.

The micellar slugs of the present invention contain the internal olefin sulfonates having good salinity tolerance and hard-water resistance as an essential constituent of the surfactant. Therefore, brine having a salt concentration of up to about 10% by weight can be used in the formation of the micellar slugs of the present invention. Further, brine having a salt concentration of up to about 15% by weight can be used when other surfactants or cosurfactants are used together with the internal olefin sulfonate. Furthermore, the micellar slugs of the present invention have an excellent hard-water resistance. For example, brine usable in the formation of the micellar slugs of the present invention can contain a relatively large amount of multivalent metal ions, for example, about 5,000 ppm of a Mg ion (i.e., about 2.6% by weight of MgSO.sub.4). Thus, water (or brine) usable in the formation of the micellar slugs of the present invention can contain 0% to about 15% by weight, desirably about 0.5% to about 12% by weight and more desirably about 1% to about 10% by weight, of inorganic salts. Typical examples of the inorganic salts contained in the water (or brine) are NaCl, KCl, Na.sub.2 SO.sub.4, and K.sub.2 SO.sub.4. For instance, seawater contains about 3.5% by weight of inorganic salts including about 1,600 ppm, in terms of a Mg ion, of divalent metal ions. This salt concentration is within the desirable salt concentration range of the present invention.

The internal olefin sulfonates usable as an essential surfactant in the present invention are those obtained by sulfonating internal olefins containing as a main constituent vinylene-type monoolefin having 10 to 26 carbon atoms, desirably 12 to 24 carbon atoms and having a general formula:

wherein R and R' are independently straight-or branched-chain saturated hydrocarbon radicals having 1 or more carbon atoms provided that the total carbon atom numbers of R and R' is 8 to 24, desirably 10 to 22, and optionally containing about 33% by weight (about one third of the olefins) or less of tri-substituted type monoolefins, followed by neutralizing the sulfonated products with appropriate bases and, then, optionally, hydrolyzing the neutralized products. The internal olefin sulfonates thus prepared generally contain about 10% to about 60% by weight of alkenyl sulfonates having a double bond and about 90% to about 40% by weight of hydroxyalkane sulfonates and also contain about 80% by weight or more of monosulfonates and about 20% by weight or less of disulfonates. It should be noted, however, that internal olefin sulfonates having the compositions different from the above-mentioned composition ratios can be prepared by appropriately selecting the sulfonation conditions and hydrolysis conditions. Generally speaking, the increase in the carbon atom number of the internal olefin tends to result in an increase in the composition ratio of the alkenylsulfonate. On the other hand, the increase in the mol ratio of the sulfonating agent to the internal olefin during the sulfonation tends to result in an increase in the composition ratio of the disulfonate.

The content of the disulfonate in the internal olefin sulfonates usable in the present invention should be about 20% by weight or less. A disulfonate content of more than 20% by weight undesirably decreases the interfacial tension decreasing capability of the internal olefin sulfonates and, therefore, decreases the oil recovery efficiency of the micellar slugs. Thus, a small content of the disulfonate in the internal olefin sulfonates increases the salinity tolerance and the resistance against the change in the salt concentration of the brine, without impairing the interfacial tension decreasing capability of the internal olefin sulfonates. Accordingly, the content of the disulfonates in the internal olefin sulfonates is desirably about 0.5% to about 15% by weight, more desirably about 1% to about 12% by weight.

The internal olefin sulfonates usable in the present invention can be alkali metal salts, alkaline earth metal salts, ammonium salts, and organic amine salts thereof. The desirable counter cations are Na, K, Mg, Ca, NH.sub.4, and alkanolammonium.

Examples of internal olefin sulfonates usable in the formation of the micellar slugs of the present invention are: internal olefin sulfonate having 12, 14, 16, 18, 20, 22, 24, 12-16, 13-14, 13-16, 14-16, 14-18, 15-18, 16-18, 16-20, 17-20, 18-20, and 20-24 carbon atoms. These sulfonates may be used alone or in any mixture thereof.

As mentioned above, the micellar slugs of the present invention contain about 1% to about 30% by weight of the surfactant. However, the micellar slugs desirably contain about 3% to about 25% by weight of the surfactant, taking into consideration both low interfacial tensions and reasonable cost. The amount of the higher internal olefin sulfonates having 10 to 26 carbon atoms should be at least 50% by weight, desirably 60% by weight or more, based on the total amount of the surfactants contained in the micellar slugs.

The hydrocarbons usable as an oil component in the present invention include, for example, petroleum, liquefied petroleum gas, crude gasoline (naphtha), kerosine, diesel oil, and fuel oil. The recovered petroleum is desirably used due to its low cost and availability as well as its composition, which is similar to that of the oil contained in subterranean reservoirs. As mentioned above, the micellar slugs of the present invention can contain about 2% to about 90% by weight of hydrocarbons. The desirable concentration of hydrocarbons is within the range of about 3% to about 40% by weight whereby an oil-in-water (O/W) type emulsion is formed, since the use of a large amount of hydrocarbons is not economical.

The cosurfactants used in the formation of the micellar slugs of the present invention are an essential constituent for forming micro-emulsions associated with the surfactants. The cosurfactants usable in the present invention are those having an alcoholic hydroxyl group. The desirable cosurfactants are alcohols having the general formula:

wherein n is a number of from 0 to about 4 and R" is an alkyl or alkenyl group having 4 to 8 carbon atoms when n is zero and an alkyl or alkenyl group having 6 to 15 carbon atoms, a phenyl group, or an alkylphenyl group having 7 to 16 carbon atoms when n is not zero. The aliphatic groups of R" may be straight-chain or branched-chain groups.

Examples of such alcohols are butanols, pentanols, hexanols, 2-ethylhexanol or other octanols, polyoxyethylene hexylethers (n=1), polyoxyethylene decylethers (n=4), polyoxyethylene tridecylethers (n=4), polyoxyethylene butylphenylethers (n=2), polyoxyethylene nonylphenylethers (n=3), and polyoxyethylene dodecylphenylethers (n=4).

As mentioned above, the micellar slugs of the present invention can contain about 0.1% to about 20% by weight of the cosurfactants. However, the desirable concentration of the cosurfactants is within the range of about 1% to about 15% by weight from the viewpoints of the stability of the micro-emulsions and the decreasing capacity for the interfacial tensions.

As mentioned above, the micellar slugs of the present invention contain internal olefin sulfonates having 10 to 26 carbon atoms as an essential or major constituent of the surfactants. However, other auxiliary surfactants can also be included, together with the internal olefin sulfonates.

Examples of such auxiliary surfactants are anionic surfactants and nonionic surfactants such as petroleum sulfonates, alkylbenzene sulfonates, polyoxyethylene alkylether sulfates, dialkyl sulfosuccinates, alpha-olefin sulfonates, paraffin sulfonates, soaps, higher alcohol ethoxylates, alkylphenol ethoxylates, polyol fatty acid esters, fatty acid alkylol amides, and polyoxyethylene fatty acid amides.

The viscosity of the micellar slugs of the present invention can be suitably adjusted by selecting the kinds and amounts of the components of the micro-emulsions. However, when a micellar slug having a high viscosity is desired, an appropriate known thickening agent such as a water-soluble polymer can be added to the micellar slugs. Examples of thickening agents usable in the formation of the micellar slugs are heteropolysaccharides produced by microbes, naphthalenesulfonic acid-formaldehyde condensates, polyacrylamides, polyacrylates, hydroxyethylcelluloses, and carboxymethylcelluloses.

The micellar slugs of the present invention can be readily obtained by any known method of production. For example, the hydrocarbons, the surfactants, the aqueous medium, and the cosurfactants can be mixed in any mixing order by using conventional mixing devices, mixing temperatures, and mixing pressures.

The recovery of oil from subterranean reservoirs can be carried out by means of any conventional micellar drive method by using the micellar slugs of the present invention. For instance, the micellar slugs are injected under pressure into at least one injection well of the subterranean reservoirs. Then, at least one driving fluid such as flood water and/or aqueous solution of the above-mentioned thickening agent is injected into the injection well so as to transfer or drive the remaining oil toward an oil production well and to recover the oil from the production well. The suitable amount of the micellar slugs injected into the injection well is about 5% to about 25% by volume of the porosity of the subterranean reservoirs.

The suitable salt concentration of the subterranean water in the subterranean reservoirs where the micellar slugs of the present invention can be applied is 0% to about 15% by weight, desirably about 0.1% to about 12% by weight and, more desirably, about 0.5% to about 10% by weight. Although the salt concentration of water used in the formation of the micellar slugs is not necessarily the same as that of the subterranean water, they are desirably the same from the viewpoints that the salt concentration is changed during the sweeping.

As mentioned hereinabove, the micellar slugs of the present invention contain the internal olefin sulfonates having a disulfonate content of about 20% by weight or less as an essential component of the surfactant. The resultant micellar slugs thus have an excellent salinity tolerance and hard-water resistance and can form micro-emulsions by using water having a very wide salt concentration from soft water to brine having a high salt concentration. Furthermore, the micellar slugs of the present invention have very small interfacial tensions between water and the micro-emulsions and between oil and the micro-emulsions and have resistance against the changes in the salt concentrations. As a result, the following remarkable advantageous features can be obtained:

(1) Either soft water, seawater, or subterranean water having a high salt concentration can be freely used;

(2) The micellar slugs injected into the subterranean reservoirs are subjected to no substantial adverse effects by inorganic salts present in the subterranean reservoirs;

(3) The micellar drive method can be readily applied in subterranean reservoirs containing oil having a low viscosity to a high viscosity; and

(4) A high oil recovery efficiency can be attained since stable micro-emulsions are maintained in the subterranean reservoirs until oil banks are formed.

The present invention now will be further illustrated by, but is by no means limited to, the following examples, in which the component ratios or amounts of samples used are based on "% by weight" unless otherwise specified.

EXAMPLE 1

Micro-emulsions were prepared by weighing 10.5% of sodium C.sub.14 -C.sub.18 internal olefin sulfonates (C.sub.14 -C.sub.18 IOS-Na) having various disulfonate contents or petroleum sulfonate TRS-10 (manufactured by Witco Chemical Corp.) as a surfactant, 4.5% of amyl alcohol as a cosurfactant, 17% of fuel oil (ASTM No. 2 oil) as a hydrocarbon, and 68% of an aqueous solution of 8% of sodium chloride dissolved in demineralized water as a brine in a beaker. The resultant mixture was stirred at 100 rpm for 30 minutes at a temperature of 71.degree. C.

The IOS-Na samples having various disulfonate (DS) contents used as a surfactant were prepared by changing the molar ratio of the starting internal olefin and SO.sub.3. The samples having a small DS content were prepared by extracting the disulfonates from the reaction products by isobutyl alcohol to adjust the desired DS contents.

The micro-emulsion forming capabilities, the interfacial tension decreasing capabilities, and the oil recovery efficiencies of the micro-emulsions thus prepared were evaluated as follows. The results are shown in Table 1.

The micro-emulsion forming capabilities were determined from the visual appearance of the micro-emulsions according to the following:

o . . . A transparent or translucent and homogeneous micro-emulsion was formed.

x . . . An opaque suspension, rather than a micro-emulsion was formed.

The interfacial tensions were measured by a spinning drop type tensiometer at 71.degree. C. in an appropriately diluted system.

The oil recovery tests were carried out by using Berea sandstone core having a size of 3.8 cm diameter and 28 cm length and having a permeability of about 500 mD and a porosity of about 20%. A core sufficiently saturated with brine was set in a core holder and, then, fuel oil was injected under pressure into the core at a feed rate of 6 cc/min until no brine was discharged. Then, brine was injected under pressure at the same feed rate in a water drive method until the content of the fuel oil in the effluent became less than 0.1%. Thus, the fuel oil was recovered. After the water drive method, the core holder and the micro-emulsions were placed in a constant temperature bath at a temperature of 71.degree. C. for a micellar drive method. The micro-emulsions were first injected under pressure into the core in an amount of 10% by volume of the pore volumes, a polymer solution (i.e., 1000 ppm of Xanthan gum solution in a brine solution) was then injected under pressure in an amount of 100% by volume of the pore volume, and, finally, brine was injected under pressure in an amount of 100% by volume of the pore volume. Thus, the fuel oil was recovered. The injection rate under pressure was 2 feet/day. The oil recovery efficiency was determined by measuring the amount of water in the core after the test in a toluene azeotropic method to convert the recovery amount of the fuel oil.

Micro-emulsions were prepared by weighing 10.5% of C.sub.14 -C.sub.18 IOS-Na containing 7% (based on the effective component) of the disulfonate as a surfactant, 4.5% of amyl alcohol as a cosurfactant, 17% of fuel oil (ASTM No. 2 oil) as a hydrocarbon, and 68% of an aqueous solution of a given amount of sodium chloride dissolved in demineralized water, or an aqueous solution of a given amount of CaCl.sub.2 or MgCl.sub.2 dissolved in the above-prepared aqueous sodium chloride solution in a beaker. The resultant mixture was stirred at 100 rpm for 30 minutes at a temperature of 71.degree. C.

The micro-emulsion forming capabilities, the interfacial tensions, and the oil recovery efficiencies of the micro-emulsions were evaluated in the same manner as in Example 1. The results are shown in Table 2 below.

Micro-emulsions were prepared by weighing 10.5% of C.sub.13 -C.sub.14 IOS-Na, C.sub.18 -C.sub.20 IOS-Mg, or C.sub.20 -C.sub.22 IOS-K as a surfactant, 4.5% of amyl alcohol as a cosurfactant, 17% of fuel oil (ASTM No. 2 oil), and 68% of an aqueous solution of 8% of sodium chloride dissolved in demineralized water in a beaker. The resultant mixture was stirred at 100 rpm for 30 minutes at a temperature of 71.degree. C.

The micro-emulsion forming capabilities, the interfacial tensions, and the oil recovery efficiencies of the micro-emulsions were evaluated rn the same manner as in Example 1. The results are shown in Table 3 below.

Micellar slug compositions containing anionic surfactants, cosurfactants, hydrocarbons, and aqueous media were prepared.

The anionic surfactant used was sodium C.sub.14 -C.sub.18 IOS-Na (DS=7%), C.sub.20 -C.sub.24 IOS-Na (DS=9%), petroleum sulfonate TRS-10 (manufactured by Witco Chemical Corp.). The cosurfactants used were amyl alcohol or isopropyl alcohol. The hydrocarbons used were kerosine or fuel oil (ASTM No. 2 fuel oil). The aqueous media used were seawater having a saline content of about 3.5% and an alkaline earth metal ion concentration of about 1,600 ppm, or soft water.

The micellar slugs were prepared by first mixing the surfactant and the aqueous medium and, then, adding the cosurfactant and the hydrocarbon to the mixture, while stirring it.

The compositions and properties of the micellar slugs thus obtained are shown in Table 4. Visual appearance was determined according to the following:

o . . . A micro-emulsion was formed.

x . . . A suspension, rather than a micro-emulsion was formed.

Micellar slug samples were prepared in the same manner as described in Example 4 except that various brines obtained by dissolving sodium chloride in demineralized water to predetermined saline concentrations were used.

The compositions and the properties of the samples are shown in Table 5.

Micellar slug samples were prepared in the same manner as described in Example 4 except that various brines were obtained by adding 1% by weight of sodium chloride and magnesium chloride and/or calcium chloride so as to provide the predetermined bivalent metal ion concentration to demineralized water.

The compositions and the properties of the samples are shown in Table 6.

Micellar slug samples were prepared in the same manner as described in Example 4 except that the counter ions of the internal olefin sulfonates were changed. The compositions and the properties of the samples are shown in Table 7A (monovalent cations) and Table 7B (divalent cations).

Micellar slug samples were prepared in the same manner as described in Example 4 except that two or more internal olefin sulfonates were used together.

The compositions and the properties of the samples are shown in Table 8.