GB2139270A - Micellar slug for oil recovery - Google Patents

Abstract

A micellar slug consists of a hydrocarbon, an aqueous medium, a surfactant, and a cosurfactant. The surfactant contains, as essential constituents, (a) a petroleum sulfonate having an average molecular weight of 350 to 550 and (b) an internal olefin sulfonate having 10 to 26 carbon atoms. The weight ratio (a) to (b) is within the range of from 99/1 to 30/70.

Description

SPECIFICATION Micellar slug for oil recovery BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a micellar slug suitable for use in a micellar drive for recovering oil (i.e., petroleum) from subterranean reservoirs. More specifically, it relates to a micellar slug having improved salinity tolerance and low interfacial tension and capable of recovering oil from subterranean reservoirs at a high oil recovery efficiency and at a low cost.

Description of the Prior Art Micellar drive is known in the art as one of the so-called "enhanced oil recovery (EOR)" methods for recovering oil from oil-bearing subterranean reservoirs. According to the micellar drive method, a micellar slug, that is, a micro-emulsion obtained by emulsifying water and oil with a surfactant, is injected under pressure into the subterranean reservoirs for the recovery of oil remaining in the subterranean reservoirs. Various processes and chemicals have been heretofore studied in the art. The micellar drive method is disclosed in, for example, U.S. Patent Nos. 3506070, 3613786, 3740343, 3983940, 3990515, 4017405, and 4059154. These prior arts disclose various kinds of surfactants suitable for use in the formation of micellar slugs.

Examples of such surfactants are petroleum sulfonates, alkylaryl sulfonates, dialkyl sulfosuccinates, alkane sulfonates, polyoxyethylene alkylether sulfates, alpha-olefin sulfonates, polyoxyethylene alkylethers, polyoxyethylene alkylphenylethers, polyol fatty acid esters, alkyltrimethyl ammonium salts, and dialkyldimethyl ammonium salts.

Numerous oil production wells or oilfields are present in the world and, therefore, subterranean reservoirs have a wide variety of properties. Accordingly, oilfields to which EOR methods are applied have various different properties and also contain a variety of available oil stratum water, for example, from soft water containing a very small amount of inorganic salts to brine containing large amount of inorganic salts and polyvalent metallic ions. Furthermore, the desired soft water is often not available as a formation water (i.e., water used in the formation of the micro-emulsions. For these reasons, surfactants used in the formation of micellar slugs must have good salinity tolerance and hard-water resistance. The surfactants must also have good thermal stability so as not to be affected by the temperature of the subterranean reservoirs.

Furthermore, the micellar slugs must have, in addition to good salinity tolerance and hard-water resistance, sufficiently low interfacial tension between oil and the micro-emulsions and between formation water and the micro-emulsions and must form micro-emulsions which can be stably maintained during sweeping until an oil bank is formed. Of the known surfactants usable in the micellar drive method, petroluem sulfonates have been studied in detail and actually used in field tests. However, petroleum sulfonates have poor salinity tolerance and hard-water resistance. Furthermore, the interfacial tension and the stability, during sweeping, of micro-emulsions prepared from petroleum sulfonates are unsatisfactory.

SUMMARY OF THE INVENTION The object of the present invention is to provide, for the recovery of oil, a micellar slug having excellent salinity tolerance and hard-water resistance and capable of forming micro-emulsions, which have sufficiently low interfacial tension and which can be stably maintained during sweeping in the subterranean reservoirs even with different salt concentration of the formation water.

Another object of the present invention is to provide a micellar slug capable of recovering oil from subterranean reservoirs at a high recovery efficiency and at a low cost.

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 use in the recovery of oil, consisting essentially of a hydrocarbon, an aqueous medium, a surfactant, and a cosurfactant. The surfactant contains, as essential constituents, (a) a petroluem sulfonate having an average molecular weight of 350 to 550 and (b) an internal olefin sulfonate having 10 to 26 carbon atoms. The weight ratio of the component (a) to the component (b) is within the range of from 99/1 to 30/70.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The micellar slugs desirably used for the recovery of oil are transparent 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 surfactants containing, as essential constituents, (a) a petroleum sulfonate and (b) an internal olefin sulfonate, and about 0.1 % to about 20% by weight of a cosurfactant.

The hydrocarbons usable in the present invention include, for example, petroleum, iiquefied petroleum gas, crude gasoline (naphtha), kerosine, diesel oil and fuel oil. Recovered petroleum is preferably used due to its low cost and availability as well as its composition, which is similar to that of the oil contained in the subterranean reservoirs.

As mentioned above, the micellar slugs of the present invention can contain about 2% to about 90% by weight of a hydrocarbon. The desirable concentration of the hydrocarbon 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 a hydrocarbon is not economical.

The aqueous medium usable in the formation of the micellar slug of the present invention includes soft water and brine containing a medium amount of inorganic salts. 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. It is believed in the art that the maximum inorganic salt concentration of the brine is about 2% by weight when only petroleum sulfonate is used as a surfactant. However, according to the present invention, brine having an inorganic salt concentration of up to about 5% by weight can be used as an aqueous medium. Thus, the inorganic salt concentration of an aqueous medium preferably used in the present invention is O to about 5% by weight, more preferably about 0.5% to about 4% by weight.Typical examples of alkali metal salts contained in the brine are NaCI, KCI, Na2SO4, and K2SO4.

The petroleum sulfonates usable as component (a) in the present invention are those generally used in the production of conventional micellar slugs. Typical examples of the petroleum sulfonates are the alkali metal or ammonium salts of petroleum sulfonate having an average molecular weight of about 350 to 550, more preferably 380 to 500, which can be prepared by sulfonating crude oil or gas oil fractions, followed by neutralization. Although these petroleum sulfonates generally contain unreacted hydrocarbons and inorganic salts, either the purified or unpurified petroleum sulfonates may be used for the purpose of the present invention.

The internal olefin sulfonates usable as component (b) 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: R-CH = CH-R' wherein R and R' are independently straight- or branched-chain saturated hydrocarbon radicals having 1 or more carbon atoms provided that the total number of carbon atoms 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 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 internal olefin sulfonates suitable for use in the present invention are those containing about 40% by weight or more, more preferably about 45% to about 90% by weight, of the hydroxyalkane sulfonates and about 20% by weight or less, more preferably about 0.1 % to about 15% by weight of the disulfonate. When these internal olefin sulfonates are used, microemulsions having a sufficiently low interfacial tension can be formed and, therefore, the desired oil recovery efficiency is increased.

The internal olefin sulfonates usable in the present invention can be alkali metal salts, ammonium salts, and organic amine salts thereof. The preferable counter cations are Na, K, Ca, NH4, 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, 13, 14, 15, 16, 18, 20, 22, 24, 12-16, 13-14, 14-16, 14-18, 15-17, 16-18, 17-20, and 20-24 carbon atoms. These sulfonates may be used alone or in any mixture thereof.

The micellar slugs for the recovery of oil according to the present invention contain as the essential surfactant constituents the components (a) and (b). The component (b) (i.e., the internal olefin sulfonates) functions to provide the desired salinity tolerance, hard-water resistance, and sufficiently low interfacial tension to the micro-emulsions. On the other hand, since internal olefin sulfonates are inferior to petroleum sulfonates in, for example, the starting material cost, the reactivity, and the product quality the use thereof for the production of micellar slugs, which are injected into subterranean reservoirs in a large amount, is not economical.Accordingly, in order to produce a micellar slug having a good salinity tolerance, hard-water resistance, and sufficiently low interfacial tension and capable of being economically recovered oil from the subterranean reservoirs having a relatively high salt concentration at a high oil recovery efficiency, the weight ratio of the component (a) to the component (b) should be within the range of 99/1 to 30/70, preferably 95/5to 40/60, and more preferably 90/10 to 50/50.

As mentioned above, the micellar slugs of the present invention contain about 1 % to about 30% by weight of surfactant. However, the micellar slugs desirably contain about 3% to about 25% by weight of surfactant, taking into consideration both low interfacial tension and reasonable cost.

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 alcohols selected from alcohols having 4 to 8 carbon atoms and the ethyleneglycol monoethers and the diethyleneglycol monoethers thereof. Typical examples of such alcohols are butanols, pentanols, hexanols, 2-ethylhexanol and the other octanols, butoxyethanols, octyloxyethanols, and diethyleneglycol monobutylethers.

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 preferable concentration of the cosurfactants is within the range of about 1 % to about 10% 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, as essential surfactants, the petroleum sulfonate and the internal olefin sulfonates. However, other auxiliary surfactants can also be included, together with the petroleum sulfonates and the internal olefin sulfonates.

Examples of such auxiliary surfactants are anionic surfactants and nonionic surfactants such as 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.

When the viscosity of the micellar slugs of the present invention is desired to be increased, 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, naphtalenesulfonic 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 miceliar 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.

As mentioned hereinabove, the micellar slugs of the present invention contain petroleum sulfonates and internal olefin sulfonates. Therefore, the following advantageous features can be obtained.

(1) The present micellar slugs can be applied to oilfields containing oil stratum water having a high salt concentration, (2) Either soft water or seawater can be freely used for the production of the micellar slugs; (3) The micro-emulsions are stably maintained during sweeping in subterranean reservoirs, since the present micellar slugs have resistance against changes in the salt concentrations; (4) The micellar slugs according to the present invention have sufficiently low interfacial tension; (5) A high oil recovery efficiency can be obtained; and (6) The present micellar slugs are economically available at a low cost.

EXAMPLES 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 I Micro-emulsions were prepared by weighing 12% or 10.5% of mixtures of (a) sodium petroleum sulfonate having an average molecular weight of 410 and (b) sodium C14-Ca6 internal olefin sulfonates (Ct4-C,610S-Na) or sodium C16-C,8 internal olefin sulfonate (C16-C18 lOS-Na) having various mixing ratios as a surfactant, 6% or 4.5% of n-amyl alcohol as a cosurfactant, 20% or 30% of fuel oil (ASTM No. 2 oil) as a hydrocarbon, and 62% or 55% of an aqueous solution of 2% of sodium chloride dissolved in demineralized water as a brine or soft water in a beaker. The resultant mixture was stirred at 100 rpm for 30 minutes at a temperature of 71"C.

The micro-emulsion forming capabilities were determined from the visual appearance of the micro-emulsions according to the following: o . . A transparent and homogeneous micro-emulsion was formed x . . . . An opaque suspension was formed or the mixture was separated into two phases.

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

The viscosities were measured at 25"C by a Brookfield viscometer.

The results, together with the compositions of the ingredients, are shown in Table 1. Table 1 Sample No. 1 2 3 4 5 6 7 8 9 10 11 12 Arionic surfactant Petroleum sulfonate Na 12 12 11.5 10.6 8.7 6.4 4.4 10.1 8.3 6.6 5.8 4.0 (Ave. M.W. = 410) C16- C18 IOS-Na - - 0.5 1.4 3.3 5.6 7.6 0.4 - 3.9 - 6.5 C14-C16 IOS-Na - - - - - - - - 2.2 - 4.7 Cosurfactant n-Amyl alcohol 6 6 6 6 6 6 6 4.5 4.5 4.5 4.5 4.5 Hydrocarbon Fuel oil (No. 2 oil) 20 20 20 20 20 20 20 30 30 30 30 30 Aqueous medium Soft water 62 Brine (NaCl 2%) 62 62 62 62 62 62 55 55 55 55 55 Property Visual appearance o x o o o o o o o o o o Interfacial tension 19.3 - 12.2 7.96 5.43 4.46 4.38 12.6 6.05 4.74 4.68 4.45 (x 10-3 dyne/cm) Viscosity (CP) 8 - 10 12 15 17 20 10 15 18 21 24 Example 2 Micro-emulsions were prepared by weighing 8.7% of sodium petroleum sulfonate having an average molecular weight of 410 and 3.3% of C16-C18 lOS-Na as a surfactant, 6% of n-amyl alcohol as a cosurfactant, 20% of fuel oil (ASTM No. 2 oil) as a hydrocarbon, and 62% of an aqueous solution of a given amount of sodium chloride dissolved in demineralized water as brine in a beaker. The resultant mixture was stirred at 100 rpm for 30 minutes at a temperature of 71"C.

The evaluation of the micro-emulsion forming capabilities and the measurement of the interfacial tensions were carried out in the same manner as in Example 1.

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"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 recovered amount of the fuel oil.

The results are shown in Table 2.

Table 2 Sample No. 13 14 15 16 17 18 19 Petroleum sulfonate 8.7 8.7 8.7 8.7 8.7 8.7 8.7 (Ave. M.W.=410) C16-C18 lOS-Na 3.3 3.3 3.3 3.3 3.3 3.3 3.3 Brine NaCI (%) 0 1 2 3 4 5 7 Visual appearance o o o o o o x Interfacial tension 5.72 5.58 5.43 5.40 5.51 5.64 (x 10~3dyne/cm) Oil recovery (%) 87 89 90 91 90 88 Example 3 Various micro-emulsions were prepared by weighing 10% of mixtures of (a) ammonium petroleum sulfonate having an average molecular weight of 410 or sodium petroleum sulfonate having an average molecular weight of 500 and (b) C14-C16 lOS-K or C16-C20 lOS-NH4 at various mixing ratios as a surfactant, 4% of isopropyl alcohol or n-amyl alcohol as a cosurfactant, 25% of fuel oil or-35% of kerosine as a hydrocarbon, and 57% or 67% of an aqueous solution of a given amount of sodium chloride dissolved in demineralized water as brine in a beaker. The resultant mixture was stirred at 100 rpm for 30 minutes at a temperature of 71"C.

The evaluation of the micro-emulsion forming capabilities and the measurements of the interfacial tensions and viscosities were carried out in the same manner as in Example 1.

The results, together with the compositions of the ingredients are shown in Table 3.

Table 3 Sample No. 20 21 22 23 24 25 26 27 28 29 30 31 Anionic surfactant Petroleum surfactant Petroleum sulfonate.NH4 10 9.6 8.4 7.3 6.5 5.2 - - - - - (Ave. M.W. = 410) Petroleum sulfonate.Na - - - - - - 10 9.6 8.4 7.3 6.5 5.2 (Ave. M.W. = 500) C14-C16 IOS-K - 0.4 1.6 2.7 3.5 4.8 - - 0.7 - 1.1 C16-C20 IOS-NH4 - - - - - - - 0.4 0.9 2.7 2.4 4.8 Cosurfactant Isopropyl alcohol - - - - - - 4 4 4 4 4 4 n-Amyl alcohol 4 4 4 4 4 4 - - - - - Hydrocarbon Kerosine - - - - - - 35 35 35 35 35 35 Fuel oil 25 25 25 25 25 25 - - - - - Aqueous (NaCl 1%) 67 - - - - - 67 - - - - medium (NaCl 2.5%) - 67 67 67 - - - 57 57 57 - Brine (NaCl 4%) - - - - 67 67 - - - - 57 57 Property Visual appearance x o o o o o x o o o o o Interfacial tension - 12.8 7.67 6.05 5.42 5.35 - 11.6 6.26 4.26 4.27 4.22 (x 10-dyne/cm) Viscosity (CP) - 10 14 17 21 24 - 11 17 20 25 28

Claims (5)

1. A micellar slug for use in the recovery of oil, said slug consisting essentially of a hydrocarbon, an aqueous medium, a surfactant, and a cosurfactant, said surfactant containing, as essential constituents, (a) a petroleum sulfonate having an average molecular weight of 350 to 550 and (b) an internal olefin sulfonate having 10 to 26 carbon atoms, the weight ratio of the component (a) to the component (b) being within the range of from 99/1 to 30/70.

2. A micellar slug as claimed in claim 1, wherein said surfactant is an internal olefin sulfonate having 1 2 to 24 carbon atoms.

3. A micellar slug as claimed in claim 1, wherein said micellar slug consists essentially of 2% to 90% by weight of the hydrocarbon, 4% to 95% by weight of the aqueous medium, 1 % to 30% by weight of the surfactant, and 0.1 % to 20% by weight of the cosurfactant.

4. A micellar slug as claimed in claim 1, wherein said cosurfactant is at least one component selected from the group consisting of alcohols having 4 to 8 carbon atoms and the ethyleneglycol monoethers and the diethyleneglycol monoethers thereof.

5. A process for producing oil from an oilbearing subterranean reservoir penetrated by wells, which comprises the steps of: (1) injecting into said reservoir through an injection well a micellar slug consisting essentially of a hydrocarbon, an aqueous medium, a surfactant; and a cosurfactant, said surfactant containing, as essential components, (a) a petroleum sulfonate having an average molecular weight of 350 to 550 and (b) an internal olefin sulfonate having 10 to 26 carbon atoms, the weight ratio of the component (a) to the component (b) being within the range of from 99/1 to 30/70; (2) injecting into said reservoir at least one driving fluid; and (3) recovering oil from said reservoir through a production well.