The present invention relates to a process for recovering oil from an oil-bearing formation with the help of surfactant .

In the recovery of oil from a subterranean

formation, only a portion of the oil in the formation generally is recovered using primary recovery methods utilizing the natural formation pressure to produce the oil. A portion of the oil that cannot be produced from the formation using primary recovery methods may be produced by secondary recovery methods such as water flooding. Oil that cannot be produced from the formation using primary recovery methods and optionally secondary methods such as water flooding, may be produced by chemical enhanced oil recovery, also referred to as EOR.

Chemical enhanced oil recovery can utilize

surfactant or a combination of surfactant with other chemicals such as polymer and/or gas to flood an oil- bearing formation to increase the amount of oil recovered from the formation. An aqueous mixture comprising

surfactant and optionally polymer is injected into an oil- bearing formation to increase recovery of oil from the formation, either after primary recovery or after a secondary recovery water flood. Surfactant is thought to enhance recovery of oil by lowering the interfacial tension between oil and water phases in the formation thereby mobilizing the oil for production. Polymer is thought to increase the viscosity of the enhanced oil recovery formulation, preferably to the same order of magnitude as the oil in the formation in order to force the mobilized oil through the formation for production by the polymer containing flood.

The mixture which is produced in an oil recovery process making use of surfactant contains oil, water, surfactant and other compounds which either were present in the fluid which was injected such as polymer or were present in the formation and became incorporated as part of the oil recovery process.

The amount of water in the mixture recovered can vary widely. The oil and water often are present in the recovered fluid as an emulsion of either water in oil or oil in water. Such an emulsion tends to be stabilized by the surfactants which makes that an aqueous phase

containing a relatively large amount of oil would be obtained and an oil product having a relatively high amount of aqueous phase. The latter can cause the oil product to have a relatively high salt content. Refineries tend to have specifications on both the amount of water and the amount of salt which a crude oil is allowed to contain. Crude oil not meeting these specifications will have to be further treated in order to make the crude oils comply with the refinery specifications. Removing

emulsified water from oil can be cumbersome and expensive due to limited differences in specific gravity between the oil and water while their viscosity also can become similar if the aqueous phase contains polymer. Chemical separation can be expensive due to the expense of the chemicals used.

It is general practice to dehydrate crude oil by allowing the oil to stand. However, this can take a long time and/or require large vessels if a substantial amount of surfactant is present in the oil mixture because surfactants tend to stabilize emulsions. Even worse, enhanced oil recovery surfactants are in principle designed to generate micro-emulsions, which are

thermodynamically stable and hence will not demulsify spontaneously given time. Furthermore, any surfactant which remains in the crude oil can also cause problems further downstream. The surfactants and/or thermally cracked derivatives can end up in refinery products thereby causing these to no longer meet the

specifications .

A further complexity is that surfactant which remains in the crude oil cannot again be used in

recovering oil from the formation.

We now have found a process for recovering crude oil from a formation with the help of surfactant making efficient use of various process streams already present in the recovery process to recover a crude oil having reduced surfactant content while the surfactant is present in process streams which again can be used in oil

recovery .

A further advantage of the present invention is that cleaner oil can be obtained by washing oil of relatively high surfactant content.

The present invention relates to a process for recovering oil which process comprises (a) injecting surfactant into the oil-bearing formation and recovering from the formation a mixture containing oil, water and surfactant, (b) treating the mixture recovered in step (a) with salt or an aqueous salt solution and separating the treated mixture into an oil phase containing surfactant and an aqueous phase, and (c) washing the oil phase containing surfactant obtained in step (b) with water and recovering oil having a reduced surfactant content .

The wash water obtained in step (c) generally will contain a substantial amount of surfactant.

Oil containing mixtures which have been recovered from a formation with the help of a surfactant flood, will contain water besides crude oil and surfactant. Water can originate from the aqueous surfactant solution which has been injected and/or from the formation containing water besides oil. The presence of water is undesirable not only because of the water but also because of the salts and other contaminants dissolved in the water.

Without wishing to be bound to any theory, it is thought that the surfactant in the recovered oil can cause relatively stable oil-water micro-emulsions which the current process removes by changing the salinity of the mixture produced by the surfactant flood. The increase in salinity is thought to push the surfactant into the oil phase (also referred to as "over-optimum") thus enhancing the separation of the oil phase from the water phase and coalescence of oil droplets in the water. At the same time, this is thought to separate any other solutes such as polymer which may be present in the water phase.

Furthermore, it is thought that the separation is helped by an increase in the density difference between the oil and water phase due to the high salt content of the aqueous phase besides the viscosity reduction due to the influence of the high salinity on the polymer hydrodynamic radius.

After the oil phase containing surfactant has been separated from the aqueous phase in step (b) , washing with low-saline or fresh water in step (c) is thought to bring the surfactant back to the aqueous phase thereby making it possible to remove surfactant from the oil phase and obtain an aqueous surfactant containing mixture which can be injected into the formation. The water wash in step (c) may have to be repeated one or more times to remove sufficient surfactant from the oil phase to meet the specifications on the water, salt and/or surfactant content of the oil.

The process of the present invention employs a surfactant which generally is effective in reducing the interfacial tension between oil and water in the oil- bearing formation. This is thought to mobilize the oil for production from the formation.

The surfactant may be any enhanced oil recovery surfactant. Preferably, the surfactant is chosen from the group consisting of anionic and cationic surfactants .

Suitable cationic surfactants may be ammonium compounds, more preferably quaternary ammonium compounds, most preferably benzylic ammonium salts. Cationic

surfactants are especially suitable for use with carbonate formations .

The anionic surfactant may be a sulfonate-containing compound, a sulfate-containing compound, a carboxylate- containing compound, a phosphate-containing compound, a phenolate-containing compound or a blend thereof. The anionic surfactant may be chosen from the group consisting of an alpha olefin sulfonate compound, an internal olefin sulfonate compound, a branched alkyl benzene sulfonate compound, a propylene oxide sulfate compound, an ethylene oxide sulfate compound, a propylene oxide-ethylene oxide sulfate compound, or a blend thereof. Anionic surfactants are especially suitable for use with sandstone formations.

The surfactant preferably contains from 2 to 28 carbons, more specifically of from 2 to 20 carbons, more specifically of from 12 to 20 carbons. The surfactant of the oil recovery formulation may comprise an internal olefin sulfonate compound containing from 15 to 18 carbons or a propylene oxide sulfate compound containing from 12 to 15 carbons, or a blend thereof, where the blend contains a volume ratio of the propylene oxide sulfate to the internal olefin sulfonate compound of from 1:1 to 10:1.

The surfactant generally is present as part of an oil recovery formulation which can contain further compounds. The oil recovery formulation may contain an amount of the surfactant effective to reduce the interfacial tension between oil and water in the formation and thereby mobilize the oil for production from the formation. The oil recovery formulation may contain from 0.05 % by weight (wt%) to 5 wt% of the surfactant or combination of surfactants, or may contain from 0.1 wt% to 3 wt% of the surfactant or combination of surfactants based on total amount of formulation. This amount can be gradually decreased during operation when the amount of surfactant recovered from the crude oil gradually

increases and wash water and/or regeneration water injected into the formation start to contain a substantial amount surfactant .

The oil recovery formulation can also contain further compounds such as polymers and/or alkali.

Polymer can be present to provide the oil recovery formulation with a viscosity of the same order of

magnitude as the viscosity of oil in the formation under formation temperature conditions so the oil recovery formulation may drive mobilized oil across the formation for production from the formation with a minimum of fingering of the oil through the oil recovery formulation and/or fingering of the oil recovery formulation through the oil. The polymer can be a single compound or can be a mixture of compounds. Preferably, the polymer is selected from the group consisting of polyacrylamide ; partially hydrolyzed polyacrylamide; polyacrylate ; ethylenic co ^¬ polymer; carboxymethylcelloluse ; polyvinyl alcohol;

polystyrene sulfonate; polyvinylpyrrolidone; biopolymers; 2-acrylamide-methyl propane sulfonate (AMPS) ; styrene- acrylate copolymer; co-polymers of acrylamide, acrylic acid and/or acrylate, AMPS and n-vinylpyrrolidone in any ratio; and combinations thereof.

Examples of ethylenic co-polymers include co ^¬ polymers of acrylic acid and acrylamide, acrylic acid and lauryl acrylate, and lauryl acrylate and acrylamide .

Examples of biopolymers include xanthan gum, guar gum, schizophyllan and scleroglucan .

Most preferably, the polymer is (hydrolyzed) polyacrylamide . The latter includes but is not limited to copolymers of acrylamide and acrylic acid or sodium acrylate such as polymers which are being sold by SNF Floerger under the trade name Flopaam 3630S and Flopaam EM533.

The concentration of the polymer in the oil recovery formulation to be injected into the formation preferably is sufficient to provide the oil recovery formulation with a dynamic viscosity of at least 0.3 mPa.s (0.3 cP), more specifically at least 1 mPa.s (1 cP), or at least 10 mPa.s (10 cP), or at least 100 mPa.s (100 cP), or at least 1000 mPa.s (1000 cP) at 25°C or at a temperature within a formation temperature range. The concentration of polymer in the oil recovery formulation preferably is from 250 ppm to 10,000 ppm, or from 500 ppm to 5,000 ppm, or from 1,000 to 2, 000 ppm.

The molecular weight number average of the polymer in the oil recovery formulation preferably is at least 10,000 dalton, or at least 50,000 dalton, or at least 100,000 dalton. The polymer preferably has a molecular weight number average of from 10,000 to 3,000,0000 dalton, or from 100,000 to 15,000,000 dalton.

Furthermore, the oil recovery formulation can be prepared with alkali so that the formulation which is injected has a pH of at least 8, more specifically at least 8.5, most specifically at least 9. Alkali compounds which are especially suitable are sodium hydroxide, soda ash and ammonia. In order to prevent damage to the formation, the pH preferably is at most 12, more

specifically at most 11, most specifically at most 10.5. The oil recovery formulation can further contain a low molecular weight alcohol as co-solvent, preferably in an amount of from 100 ppm to 50,000 ppm of the total oil recovery formulation.

The total dissolved solids content (TDS, measured according to ASTM D5907) of water which can be injected together with the surfactant can vary widely and depends on the availability of the various water sources.

Generally, the TDS of the water to be injected will be of from 1000 to 50,000 ppm, more specifically of from 5,000 to 50,000 ppm. The expression "ppm" indicates parts per million by weight on total weight amount present. In order to prevent scaling, it is generally preferred to use water having a limited amount of divalent ions. If such water is not readily available, it can be obtained by so-called softening processes. Most preferably, the water has an ionic strength of 0.15 M or less, or from 0.02 M to 0.125 M, or from 0.0 3M to 0.1 M. Ionic strength, as used herein, is defined by the equation

I=½*∑ ₁₌ i ⁿ C i Z i where I is the ionic strength, c is the molar

concentration of ion i, z is the valency of ion i, and n is the number of ions in the measured solution. Such water including its preparation is described in WO-A- 2014/0041856.

The mixture recovered in step (a) of the process of the present invention contains oil, water and surfactant but can contain other compounds as well depending on the compounds present in the formation and the components present in the original oil recovery formulation.

Step (b) requires the addition of salt. The salt can be in any form such as a solid or as an aqueous solution. Generally, an aqueous solution is preferred as it mixes better with the mixture of oil and water and often is more readily available. The salt is thought to cause the surfactant to become part of the oil phase. The aqueous salt solution preferably has a TDS of at least 5000 ppm, more specifically at least 10,000 ppm, more specifically at least 20,000 ppm, more specifically at least 40,000 ppm, more specifically at least 50,000 ppm. The upper limit generally will be the amount of salt which can be dissolved in the fluid at the operating conditions. The aqueous salt solution used in step (b) has a higher molar concentration of salt than the water present in the mixture of surfactant, water and oil obtained in step (a) .

From an efficiency point of view, it is preferred to use in step (b) an aqueous salt solution which has been used or produced in the recovery process of the present invention. A solution which is especially suitable is a reject stream of desalination or a softening process.

These streams can be produced in removing salt or

multivalent ions in order to produce water of sufficiently low TDS and/or ionic strength. Alternatively, water from another field, aquifer water or sea water can be used provided that the aqueous salt solution used in step (b) is more saline than the water present in the mixture of surfactant, water and oil obtained in step (a) .

If the mixture is an emulsion, phase separation also is referred to as breaking of the emulsion. In specific circumstances, it can be preferred to add demulsifiers to the separation step (b) to further aid in breaking the emulsion. Suitable demulsifiers may be selected from the group consisting of amyl resins; butyl resins; nonyl resins; acid- or base-catalyzed phenol-formaldehyde resins; phenol-acrylate anhydride polyglycol resins;

urethanes; polyamines; polyesteramines ; sulfonates; di- epoxides; polyols; esters and polyol esters including triol fatty acid esters, triol adipate esters, and triol fumarate esters; ethoxylated and/or propoxylated compounds of amyl resins, butylresins, nonylresins, acid- or base- catalyzed phenol-formaldehyde resins, fatty acids, polyamines, di-epoxides, and polyols; and combinations thereof which may be dispersed in a carrier solvent selected from the group consisting of xylene, toluene, heavy aromatic naphtha, isopropanol, methanol, 2- ethoxyhexanol, diesel, and combinations thereof.

The separation of step (b) can be enhanced by

electrostatic methods. Preferably, the separation of step (b) is carried out in separation tanks, which may comprise heating elements. In a "continuous feed" operation, the mixture and the aqueous salt solution are fed into a separation tank, and substantially dewatered oil and substantially oil-free water phases are removed from the tank, wherein the rate of delivery of crude oil and aqueous salt solution and the rate of removal of the oil and water phases are essentially in equilibrium. In the traditional "batch" gravity-style separation process, the oil containing mixture and aqueous salt solution are added to the tank and mixed, optionally heated to accelerate the separation process, and permitted to stand and settle until the separation is sufficiently complete that the aqueous phase and the oil phase can be separately removed.

Other methods are sometimes used in conjunction with the above separation processes such as large-scale centrifuge separators .

In step (c) , the oil phase containing surfactant is washed with water. The water can be an aqueous solution or it can be water per se. It depends on the circumstances what aqueous solution or water to use. If an aqueous solution is used, it will generally have a TDS of from 0 to 30,000 ppm, more preferably at most 10,000 ppm, more preferably at most 5,000 ppm, more preferably at most 1,000 ppm and most preferably at most 500 ppm. The water can be obtained from any suitable natural source or can have been obtained by treating, purifying, softening or desalinating the source water in order to reduce the contaminants and/or salt content.

The washing is preferably carried out by thoroughly mixing the oil and water and subsequently allowing the water phase and oil phase to separate as described above.

Depending on the amount of contaminants present in the oil phase and the purity required, the washing can be repeated most preferably of from 1 to 10 times. The volume ratio of oil phase to wash water preferably is of from 0.5 to 20.

The wash water which is used in step (c) can be used in various ways . A preferred option is to in ect the wash water into the formation and thereby contribute to the surfactant flood for recovering oil.

Another option is to treat the wash water obtained in step (c) in a further step (d) with an ion exchange resin to obtain wash water having reduced surfactant content. This allows the treated wash water to be used again in step (c) . An anion exchange resin is to be used for anionic surfactant, and a cation exchange resin is to be used for cation surfactant.

It can be desirable to regenerate the ion exchange resin used for removing the surfactant. An advantageous method is to (e) treat the used ion exchange resin with a salt solution to obtain a regenerated ion exchange resin and a concentrated surfactant-containing regenerating solution, and (f) inject concentrated surfactant- containing regenerating solution obtained in step (e) into the formation. The concentrated surfactant-containing regeneration solution obtained in step (e) preferably is combined with injection water before being injected into the formation in step (f) .

The oil recovered by the process of the present invention will generally contain at most 5 %wt, most preferably of from 0.001 to 3 %wt of surfactant and at most 5 %wt, most preferably of from 0.001 to 3 %wt of water .

Embodiments of the process as described herein are shown in Figure 1 and Figure 2.

Figure 1 shows a process scheme involving the steps of treating an oil containing mixture with an aqueous salt solution and separating the treated mixture into an oil phase and an aqueous phase followed by 2 subsequent wash steps and injecting the wash water into the formation optionally after further surfactant is added.

Figure 2 shows a process scheme involving the steps of treating an oil containing mixture with an aqueous salt solution and separating the treated mixture into an oil phase and an aqueous phase followed by a wash step and treating the wash water with an ion exchange resin. Fluid obtained during regeneration can be injected again into the formation optionally after further water is added.

Specific embodiments of the invention are shown herein by way of example only. The invention is susceptible to various modifications. It should be understood that the process schemes shown are not intended to limit the invention to the particular process disclosed but on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the scope of the invention as defined by the appended claims.

In the process scheme of Figure 1, a mixture of oil, water and surfactant is recovered from reservoir 101 and sent via line 1 to a treating unit 102 for contacting the mixture with an aqueous salt solution added via line 2. The mixture is mixed intensely in unit 102 and separated into an aqueous phase which is removed via line 4 and an oil phase which is sent via line 3 to wash unit 103. The aqueous phase removed via line 4 is thought to contain a reduced amount of contaminants such as polymers and surfactants. In wash unit 103, the oil phase is thoroughly contacted with wash water added via line 5. The oil phase is separated from the wash water and sent via line 6 to further wash unit 104 where the oil phase is once more thoroughly contacted with wash water added via line 8. The crude oil obtained has reduced surfactant content and is removed from the process via line 9. The wash water used in unit 103 is combined via line 7 with wash water used in unit 104. The combined wash water streams are sent via line 10 back to the formation optionally after further surfactant is added via line 11.

In the process scheme of Figure 2, a mixture of oil, water and surfactant is recovered from reservoir 201 and sent via line 21 to a treating unit 202 for contacting the mixture with an aqueous salt solution added via line 22. The mixture is mixed intensely in unit 202 and separated into an aqueous phase which is removed via line 24 and an oil phase which is sent via line 23 to wash unit 203. In wash unit 203, the oil phase is thoroughly contacted with wash water added via line 25. The crude oil obtained has reduced surfactant content and is removed from the process via line 26. The wash water used in unit 203 is sent via line 27 to ion exchange unit 204. The wash water removed via line 28 is low in contaminants and can be used for many purposes such as fresh wash water for use in wash unit 203. The ion exchange resin becomes loaded with surfactant during use and has to be regenerated at certain time intervals. In order to ensure continuous operation, several ion exchange units can be operated in parallel. In such operation, one or more units are taken out of operation for regeneration while regenerated units are taken into normal operation again. Alternatively, several ion exchange resins are operated in series. This allows regeneration of a particular unit while the remaining units in the series ensure that wash water can continue to be treated. During regeneration, the ion exchange resin of unit 204 is contacted with a suitable salt solution added via line 29. The used regeneration fluid has a high concentration of surfactant and is especially suitable to be used again in oil recovery. Therefore, it is preferred to send the used regeneration solution via line 30 back to the formation for injection. Preferably, additional injection water is added to the concentrated surfactant fluid in line 30 via line 31.