BRANCHED ETHER SURFACTANTS AND THEIR USE IN AN ENHANCED OIL RECOVERY PROCESS

Abstract: The present invention relates to a surfactant having general formula (I) in which R3 is C8-12 alkyl, R4 is C2-6 alkyl, A is an alkyleneoxy group derived from an alkylene oxide having 2-3 carbon atoms, p has an average number of from 2 to 6, Y is an alkylene group having 1-3 carbon atoms and optionally containing a hydroxyl group, m1 and m2 is 0 or 1, both m1 and m2 not being 0 at the same time, and r is 0 or 1, or a salt thereof.

Description: BRANCHED ETHER SURFACTANTS AND THEIR USE IN AN ENHANCED OIL RECOVERY PROCESS The present invention relates to a surfactant which forms microemulsions with a very high solubilizing capacity without the use of a cosurfactant. The surfactant and the microemulsion containing said surfactant are suitable for use in enhanced oil recovery. Microemulsions are made from water, an oil component and a surfactant system. The surfactant system traditionally consists of a true surfactant (hereinafter referred to as "surfactant") and a cosurfactant. The surfactant may be anionic, cationic or amphoteric. The cosurfactant,. which is sometimes referred to as a cosolvent or a solubilizing agent, is normally an alkanol having from 3 to 6 carbon atoms, but other types of compounds, such, as glycol ethers and amines, may also be used. The cosurfactant is usually a considerably smaller molecule than the surfactant and one of its roles is to affect the molecular packing at the droplet interface in such a way that formation of microemulsion is energetically favoured. One of the most interesting application areas for microemulsions is surfactant flooding for enhanced oil recovery. Surfactant flooding means injection of a surfactant solution or a microemulsion into a reservoir with a view to decreasing the oil-water interfacial tension and, as a consequence, increasing the amount of oil recovered by flooding. In high surfactant concentration systems a middle phase microemulsion in equilibrium with excess oil and brine is formed if the surfactant system is well balanced. The existence of the middle phase microemulsion is considered a necessary condition to obtain a satisfactory oil recovery. The microemulsion phase gives an extremely low interfacial tension against brine as well as oil. It has the ability to mobilize oil blocked in narrow pores, it causes coalescence of oil droplets and it ultimately brings about formation of a continuous oil bank which is pushed forward towards the production hole by a water slug. The formation and preservation of a microemulsion is, consequently, of utmost importance for a successful result in chemical flooding. However, microemulsions are known to be sensitive to changes in composition. They normally only exist within fairly narrow intervals with regard to the surfactant to cosurfactant ratio. During the flooding process a certain degree of. separation between the surfactant and the cosur¬ factant is likely to occur. This separation may be caused either by one of the components adsorbing more strongly to the surface of the formation than the other, or by- unequiva- lent distribution of the two components between the oil and brine phases. A change in composition due to selective precipitation or degradation of one of the components is also conceivable. Consequently, even with well balanced systems showing only minor differences between the components with regard to adsorption and distribution, a gradual change of the composition will take place in chemical flooding and the microemulsion will normally break. Since a microemulsion in chemical flooding is often transported very long distances, the long-term preservation of the optimum system is regarded a major issue. The problem of component separation can be avoided or reduced by using certain branched surfactants which easily form microemulsions with or without the addition of a cosur¬ factant. For example, U.S. patents Nos. 4 468 335, 4 468 342, and 4 545 912 disclose surfactants with a branched hydro- phobic tail and a polar group consisting of a polyethylene glycol chain with a terminal sulfonate group. These surfac¬ tants have been shown to form microemulsions also without a cosurfactant and they have a high capacity to solubilize oil and have been suggested for use in enhanced oil recovery. U.S. patent No. 4 446 079 discloses a sulfate and sulfonate surfactant of the general formula R 1 0-CH CH-0(CH2_CH20) nR 3-,S03 M + R 2 0-CH 2 where R, and R_ are the same or different and are C..-C--. hydrocarbyl, R^ is C Q _-. alkyl, M is a cation and n is a rational number from 2 to 6. Normally the polyethylene glycol chain consists of 3-5 ethyleneoxy groups to exhibit optimum performance. Insertion of a polyethylene glycol chain between the sulfate or the sulfonate end group and the 1.3-dialkoxypropyl hydrophobic tail gives a surfactant which has an excellent ability to solubilize oil in the middle phase of .a micro¬ emulsion and also a good tolerance to high concentrations of divalent cations, such as in sea-water. The solubilizing effect is generally considered an essential property of the microemulsion to obtain a high " oil recovery. We have now surprisingly found that when performing surfactant flooding it is particulary advantageous to use surfactants having two alkyl groups, one . being a C„ , --alkyl and the other a C-,_,-alkyl. Surfactants having this structure have superior properties in comparison with the surfactants disclosed in the working examples of US patent No 4 446 079, where the alkyl groups have the same number of carbon atoms. The surfactants of the present invention may be illustrated by the following general formula 3 . - CH. CH-O-(A) - (Y) r - S0 3 H 4 ir - CH. in which R_ is a C . - alkyl, R, is a C__ g alkyl, A is an alkyleneoxy group derived from an alkylene oxide having 2-3 carbon atoms, p has an average number of from 2 to 6, Y is an alkylene group having 1-3 carbon atoms and optionally containing a hydroxyl group, m. and m„ is 0 or 1 , both m. and m_ not being 0 at the same time, and r is 0 or 1, or a salt thereof. Particular preferred surfactants are those having C R _- n alkyl and C._, alkyl. Examples of suitable alkyl groups are octyl, nonyl, decyl and their isomers and 2-ethylhexyl and tripropyl. Other suitable alkyl groups are 2-methyl- propyl, 1-butyl, 2-methylbutyl, 1-pentyl and 1-hexyl. Both m, and τn_ are preferably 1, while Y is preferably an alkylene group having 2-3 carbon atoms. When used at a high temper¬ ature, e.g. above 70 C, and for a long fluid distance, compounds are preferred where r is 1. The alkyleneoxy groups are preferably ethyleneoxy groups or a mixture of ethyleneoxy groups and propyleneoxy groups, the number of ethyleneoxy groups being preferably from 2-4. The salt is normally an alkali metal salt but the ammonium salt or an organic ammonium salt is also suitable. The present surfactants are more useful for surfactant flooding than the previously described surfactants since they are not as sensitive to variations in salinity, temperature and equivalent alkane carbon number of the oil. In practice, such an insensitivity is necessary, at least in large scale operations, such as in off-shore flooding, since the condi¬ tions in the reservoir vary considerably. When injecting an aqueous solution of the novel surfac¬ tants into an oil reservoir they will form a large middle phase microemulsion in equilibrium with excess oil and brine. At equilibrium the surfactants are mainly localized in the middle phase microemulsion and their concentration in both the brine phase and the oil phase is very low. This is a necessary condition to keep the need of surfactants on a low level. Preferably, the surfactants are used without any cosurfactants, but the use of additional surfactants and cosurfactants are also within the scope of this invention. In order to increase the solubility of the surfactants in the aqueous injection media, the surfactants could be added in the form of an inorganic or organic ammonium salt or combined with a solubilizing agent, such as a lower alcohol or a lower ethylene glycol monoalkyl ether. SUBSTITUTE SHEET The surfactants of the present invention can be prepared by known synthesis methods. Typical procedures are outlined below: 0 OH /\ acid I R 3 0H+C1CH 2 CHCH 2 - R- j O - CH 2 CHCH 2 C1 > 0 R,OCH., base /\ R.OH > Rj-O - CH2„CHCH„ . > _ CH - OH acid / / R 3 OCH 2 ethylene oxide C 1CH -,CH = CH . - CH - 0-(CH 2 CH 2 0) p H → ' base R .OCH„ 4 2 R-OCH, I - + Na HSO3 CH - O - (CH 2 CH2θ) p CH 2 CH 2 CH 2 S0 3 Na R .OCH„ 4 2 In the formula above, R,, R. and p have the meaning earlier mentioned. As an alternative, the ethoxylate intermediate can be reacted with epichlorohydrin or thionyl chloride followed by a reaction with a sulphite salt. In order to prepare compounds of the general formula above in which m, or ir is 0, the corresponding c -olefin oxide may be used as starting material. The first reaction step is then: R .OCH- 4 2 or O R 3 OCH 2 Λ R 3 oH 1 R 4 CH 2 CHCH 2 ~ » CH - OH acid I R 4 CH 2 Sulfates may be produced by reaction of the ethoxylate intermediate and SO 3 . The invention is further illustrated by the examples below. Example 1 Two surfactants of the present invention were prepared and compared in flooding experiments with a surfactant of the previously known type with a higher number of ethyleneoxy groups. The novel surfactants were the following: C 4 H 9 - CHCH 2 - 0 CH I CH - O - (CH 2 CH 2 0) 2 - CH 2 CH 2 CH 2 S0 3 Na an (2) CH - O (CH 2 CH 2 0) 3 - CH 2 CH 2 CH 2 S0 3 a I CH„ CH 3 CH The previously known surfactant used for comparison was CH 3 CH 2 (3) C 4 H 9 - CHCH 2 - 0 CH I CH - O - (CH CH-O) - CH-CH.CH SO Na CH 2 C 4 H g - CHCH 2 - 0 CH 3 CH 2 The surfactants were all prepared according to the general procedure given above. A detailed description of the synthesis is given below for surfactant No. 1. The other two surfactants were prepared analogously. (Step 1) Epichlorohydrin is added dropwise to 2-ethylhexanol catalysed with 1% (by weight) of SnCl 4 at 70 " C. (Step 2) 1.5 Equivalents of NaOH is added dropwise as a 30% aqueous solution to the chloroglyceryl ether formed in Step 1 under vigorous stirring at 80 C. After completed addition, the reaction mixture is kept at 80 C and the formation of product is monitored by repeated titrations of epoxide content in the organic phase. After completed reaction, agitation is stopped and the organic and aqueous layers are carefully separated. (Step 3) The glycidol ether formed in Step 2 is reacted with 1-hexanol in an analogous procedure to that described in Step 1. 1-(2-ethylhexyl) -3-hexylglycerol is formed. (Step 4) The secondary alcohol of Step 3 is reacted with 2 eς. of ethylene oxide per mole of alcohol. The reaction temperature is kept at 70 C and SnCl 4 (1% by weight) is used as catalyst. (Step 5) The ethoxylate of Step 4 is mixed with allyl chloride (1 eq.) , cetyltriethylam oniu bromide (0.05 eq.) , solid NaOH (2 eq.) and water (0.30 eq.) . The vigorously stirred reaction mixture is heated at 40 C for 5 h and' then at 70 C for 3 h, giving the allyl ether which is freed from salts bv filtration. (Step 6) The allyl ether of Step 5 is charged portionwise to a 200 ml solution of tetrabutylammoniu hydrogen sulphate (0.1 eq.) in a 65:35 mixture (by volume) of 2-propanol and water buffered at pH 8. An aqueous saturated solution of sodium hydrogen sulphite (1.1 eq.) is added dropwise at 50 C under a continuous oxygen purge. When the addition is completed, the mixture is left under stirring at 50 C for 8 h. The disappearance of allyl ether is monitored by H - NMR analysis. After completed reaction, water and 2-propanol are added, the aqueous layer is saturated with Na_CO, at 60 C, the phases are separated and the organic phase concentrated by rotary evaporation to yield the desired sulphonate. The active content of the product is typically around 80%. The degree of ethoxylation of surfactant No. 3 was chosen so that it was optimally balanced for the test system used. In the flooding experiments a 1:1 mixture of nonane and decane was used as model oil, the aqueous component was sea-water and the core temperature was 70 C. All three surfactants gave three phase systems under these conditions with a middle phase microemulsion in equilibrium with excess oil and water. The symmetrical surfactant 3, however, gave a very high viscous middle phase. On addition of an alcohol, such as 2-butanol, the viscosity of the system was considerably reduced. Table 1 shows the solubilization parameters and the salinity windows of the three surfactants. The solubilization parameter is widely used as a measure of solubilizing power of the surfactant. It is . defined as the amount of oil (or water) solubilized in the middle phase divided by the amount of surfactant used at the point of equal solubilization of oil and water. The salinity window is given in sea-water equivalents, i.e. multiples of synthetic sea-water. Table 1 Surfactant Solubilization Salinity parameter window 1 18 0.8-1.2 2 19 0.8-1.3 3 20 (gel-like) 1.0 (gel-like) 3 + 2-butanol 18 0.9-1.0 As can be seen from Table 1, surfactants 1 and 2 do not give larger solubilization parameter than surfactant No. 3. However, their salinity window is much larger. Surfac¬ tant No. 3 gives a viscosity of the middle phase too high to be of practical interest unless alcohol is added. Weight ratio of alcohol to surfactant is 2:1. Flooding experiments were performed in a Berea sandstone core of 28.8 cm length and a diameter of 3.75 cm. The permea¬ bility was 520 D, the core was oriented vertically, the overburden pressure applied was 40 bar and the back-pressure 20 bar. The core having a residual oil saturation of 40% was flooded with sea-water (1.7 pore volumes) , a 2% solution of surfactant in sea-water (0.5 pore volumes) , a polymer solution (0.5 pore volumes) and, finally, sea-water (1.7 pore volumes) . Table 2 shows the effect of the three surfactants tested in terms of total oil recovery. Table 2 Surfactant Oil recovery (%) 1 90 2 86 3 64 3 + 2-butanol 77 From the results it is evident that the surfactants of the invention are considerably more efficient than the known surfactant in mobilization of model oil from the sandstone core. Example 2 One sulphate surfactant of the present invention was prepared and compared in flooding experiments with a sur¬ factant of the previously known type. The novel surfactant was the following: CH 3 CH 2 (1) C .H Q — CHCH— — \ C :HH. \ CH - 0 - (CH-CH O) 3 - S0 3 a / C ClH 2 / CH 3 CH 2 CHCH 2 - 0 CH 3 CH 2 The previously known surfactant used for comparison was: CH 3 CH (2) C 4 H 9 - CHCH 2 - 0 \ CH - 0 - (CH 2 CH 2 0) S0 3 a CH. / C 4 H g - CHCH 2 - 0 CH 3 CH 2 The surfactants were prepared according to the general procedure given above. The syntheses were run as described by Steps 1-4 of Example 1. The ethoxylate obtained from step 4 was reacted with gaseous SO-, for 1 h at 60 C followed by neutralization with aqueous NaOH. The active content of surfactant 1 was 78% and of surfactant 2, 80%. The degree of ethoxylation of the surfactants, 3 and 5 respectively, is chosen so that both surfactants are optimally balanced for the test system used. Flooding experiments were performed in a Berea sandstone core at 70 C using nonane as model oil and sea water as aqueous component. The testing procedure was the same as already given in Example 1. Both surfactants gave three phase systems under the conditions used with a middle phase microemulsion in equilibrium with excess oil and water. The symmetrical surfactant 2, however, gave a very high viscous middle phase unless an alcohol, such as 2-butanol, was added. Table 3 shows solubilization parameters and salinity windows of the two surfactants. Table 3 Surfactant Solubilization Salinity parameter window 1 22 0.6-1.3 2 26 (gel-like) 1.0 (gel-like) 2 + 2-butanol 22 0.9-1.0 As can be seen from Table 3, the novel surfactant No. 1 does not give as large a solubilization parameter as sur¬ factant No. 2. However, its salinity window is much larger. Surfactant No. 2 gives too high a viscosity of the middle phase to be of practical interest unless alcohol is added. The weight ratio of alcohol to surfactant is 2:1. Table 4 shows the effect of the surfactants tested in core flooding in terms of total, oil recovery. Table 4 Surfactant Oil recovery (%) 1 79 2 50 2 + 2-butanol 61 As can be seen, the surfactant of the invention is considerably more efficient than the known surfactant in mobilization of model oil from the sandstone core.

Claims: CLAIMS 1. A surfactant having the general formula R. (O)m. - CH. 'CH-O-(A) - (Y) so 3 κ 4 m. - CH. in which R-. is '8-12 alkyl, R. is ' 2-6 alkyl, A is an alkyleneoxy group derived from an alkylene oxide having 2-3 carbon atoms , p has an average number of from 2 to 6, Y is an alkylene group having 1-3 carbon atoms and optionally containing a hydroxyl group, m, and m_ is 0 or 1 , both m- and m-, not being 0 at the same time, and r is 0 or 1, or a salt thereof. 2. The surfactant according to claim 1, where R- is C R _ 10 alkyl and R 4 is C 4_g alkyl. 3. The surfactant according to claim 2 or 3, where 1 and „ both are 1. 4. The surfactant according to any one of claims 1-3, where r is 1. 5. The surfactant according to claim 4, where Y is an alkylene group containing 2-3 carbon atoms. 6. The surfactant according to any one of claims 1-5, where A is an ethyleneoxy group or the group (A) is a mixture of ethyleneoxy groups and propyleneoxv groups, the number of ethyleneoxy groups being 2-4. 7. Use of a surfactant according to any one of claims 1-6 in chemical flooding of an oil well with sea-water.

Inventors: GILJE EIMUND, HELLBERG PER-ERIK, HOLMBERG KRISTER, SONESSON CLAS, SVENNBERG STIG, GILJE, EIMUND, HELLBERG, PER-ERIK, HOLMBERG, KRISTER, SONESSON, CLAS, SVENNBERG, STIG
Applicants: BEROL NOBEL AB, NORSKE STATS OLJESELSKAP, DEN NORSKE STATS OLJESELSKAP A.S
Application year: 1991
Main classification: E21B43
IPC codes: B01F17, C09K8