CA1113233A

CA1113233A - Polymer solutions for use in oil recovery

Info

Publication number: CA1113233A
Application number: CA330,440A
Authority: CA
Inventors: Gary W. Pace; Trevor J. Holding (Sec 33(4)
Original assignee: Tate & Lyle Patent Holdings Ltd
Current assignee: Tate & Lyle Patent Holdings Ltd
Priority date: 1978-06-23
Filing date: 1979-06-22
Publication date: 1981-12-01
Also published as: SU1215624A3; MX153483A; FR2429236A1; FR2429236B1; NO150695B; US4265673A; NO792092L; NO150695C

Abstract

ABSTRACT

A process for preparing a polymer solution for use in "polymer based"
oil recovery, comprises adding to water the polymer in the form of an aqueous solution, a dry particulate solid or a suspension in a non-aqueous liquid, and also adding a complexing agent for multivalent ions and, where the solution so formed does not already contain an alkali metal salt, subsequently incorporating therein an alkali metal salt. The polymer may be a substantially cell-free microbial polysac-charide gume, a polyacrylamide or a cellulose derivative.

Description

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l~L~1ER soLurIoNs FOR U~E lN OIL RECOVEI~Y

The present invel1tion relates to a process for preparing a solution of a polymer for use in oil recovery, and more particularly to the use of complexing agents in such processes.

Typically, oil is recovered frc~ underground reservoir deposits by 5 a series of operating procedures. A new borehole generally gives a limited quantity of oil as a result of the liberation of internal pressure in the borehole. When this pressure has gone, it becomes necessary to pump out further amounts of the oil from the oil-bearing formation with the help of mechanical devices. However, these procedures 10 recover only about 25% of the total oil present in the deposit, and a large portion of the oil remains trapped within the pores of the formation.

A further increase in oil recovery can be achieved by means of the so-called "secondary recovery". In one method, water is pumped down 15 a borehole or a number of boreholes and a part of the trapped oil is displaced frc~ the porous stone or other kind of formation, and the displacecl oil is collected through the surrounding boreholes.
However, water displacement still leaves about 55 to 60% oE the available oil trapped in the formation, primarily because water possesses a 20 very low viscosity in relation to crude oil and displays the tendency to follow the path of least resistance so that it finds its way through the rock and leaves large pockets of oil untouched.

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A number of methods have been developed in recent years for the recovery o further quantities of oil from these deposits by the use of so-called "mobility regulating solutions". Such solutions increase the displacement of the oil by decreasing the mobility o~ the displacing fluid so that 5 !t can permeate ~he rock more throughly. Of interest are those recovery processes which use polymer displacement with a polysaccharide, such as xanthan gum, or polyacrylamide serving to increase the viscosity of the displacing fluid.

The present invention is concerned with the use of viscous polymer 10 solutions to enhance oil recovery from oil-bearing formations. More specifically, the invention is concerned with improving the injectability of such solutions, as will be considered below.

Before so doing, it is first necessary to point out that the invention relates to "simple" polymer solutions, "simple" being used in the 5 sense that we are not concerned with the use of polymer solutions containing surfactants. It is known to recover oil using water to which a surfactant has been added to lower surface tension between the injected solution and the oil to be recovered. In a variation a polymer such as a polyacrylamide, cellulose ether or polysaccharide 20 is incorporated in the surfactant solution. This variation is the subject, for example, of U.S.A. Patent No. 4049054 and as mentioned in the specification of that U.S.A. Patent it is possible further - to add additives such as builders. Typical builders include sodium tripolyphosphate, a chelating agent, to cooperate with the surfactant 25 to increase its detergent power.

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3Z~3 l~en injectin(3 ~x~l~ner solutiorls, particularly xanthan gum solutions, one drawback ~lich is often cncounter~l is a tendency ~or the solutions to block the oil-bearing formations into which they are injected.
It is generally held that this blocking ter,dency may arise fram several 5 causes which include the presence of residual solids in the solution and a propensity to precipitate or form gels when injected into alkaline formations.

Typical residual solids which may be present in xanthan gum or other heteropolysaccharide solutions include whole bacterial cells or cell 10 debris arising from the fermentation process conventionally used for produciny the heteropolysaccharide. W. German Offenlegungsschrift No. 2734364 discusses this aspect of the use of xanthan gum and discloses a fermentation method by which a gum practically free from insoluble material of greater than 3 microns can be obtained.

15 In particular, the Offenlegungsschrift lists five main factors which make this result possible. The factors all relate to careful control of the fermentation conditions, and as the third such factor it is specified that hard water with high co~centrations of calcium ions is not used in the preparation of the fermentation medium. To this 20 end, a chelating agent such as ethylenediamine tetraacetic acid or ; preferably citric acid is added to sequester excess calciu~ and prevent precipitation of calcium ions as an insoluble phosphate. When taken with the other measures outlined in the specification it is then said to be possible to obtain a product with substantially no insoluble 25material of size above about 3 microns. In summary, the chelating agent is thus added before or during fermentation.

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In i~s analysis of ~ rtinent prior art, the O~el-le~lmgsschrift 2,734,364 mentions that U.S.A. Patent No. 3,853,771 solves the blocking problem of whole er~entation broths by a process for dissolving or dispersing cellular ~croorganisms which ccmprises bringing this material (i.e.
5 whole fermentation broth) into contact with an aqueous dispersing solution containing a surface active agent, a chelating agent, and an alkali metal hydroxide. rrhe surface active agent acts to disperse the outer walls of the microorganism cells and the chelating agent acts to disperse the inner walls thereof, while the hydroxide encourages 10 these dispersing effects. The U.S.A. Patent is thus considered to disclose a variation in which chelating agent along with other oomponents is added after fermentation to the broth containing the desired poly-saccharide along with residual solids such as cellular microorganisms.

Reference to the U.S. Patent itself, however, shcws that it is not 15 concerned with the blocking problem of whole fermentation broths resulting from production of heteropolysaccharide gums. Instead it is concerned with dispersing bacteria such as Desulfovibrio desulfuricans which naturally grcw in oil-containing formations. The dispersing solution is injected at the well-head as part of an oil recovery process based on the injection of water. Insofar as one is concerned with the use 20 of complexing agents in the preparation of solutions of polymers for use in oil recovery, the actual U.S.A. Patent 3853771 is of less relevance than the discussion thereof to be found in W. German Offenlegungsschrift 2734364.

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~egarding the propensity for xanthan gums to precipitate or form gels, 25 the currently accepted theory is that aggregation is primarily brought .
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~ 2 ~ 3 _ 5 about by di- or ~rivalent ions present in tlle solut.~on, the aggregate ~lymer then Lo~ning a precipitate or a ge1. ~. Lipton in his paper ~PE S~ prepar~d for the ~9th ~nnual Fall Meeting of the ~ociety o~E ~etroleum Engillecrs of AIl~r~l October 6 - 9 1974, discusses this 5 phenomenon and others which affect injectabi~.ity of biopolymers, and mentions that gel formation i.s generally encountered at above E*l 9.
Gelation or precipitation under alkaline conditions in the presence of multivalent ions has also been put forward by ll.J. Hill et al (~PE
4748, Improved Oil Recovery Sytt~osium, Tulsa 1974) to explain a failure 10 to obtain reproduciable experimental results with xanthan gum solutions, but no evidence was found for gel formation with polyacrylamide solutions such as are used for oil recovery.

We have ncw found that the injectability of a solution of a polymer at acid or approximately neutral pH can be improved by the inclusion 15 of a complexing agent for complexing multivalent ions. This finding applies generally to the viscous poly.mer solutions employed for "polymer-based" oil recovery (as defined belc~), and not just to xanthan gurn - solutions.

According to the present invention we provide a process for preparing a solution of a polymer for use in "polymer-based" oil recovery.
By "polymer-based oil-recovery", we mean a process such as described above which involves injection of a simple polymer solution and which : does not involve the use of surfactants with the polymer. It therefore inherently follows that the present process is one in which surfactant 25 is not added to the polymer solution.
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Accorcling to Lhe pre~sent inventioll, there is providcd a process for preparing a poly~ler solution for use in "~ ner based" oil recovery, which comprises adding to ~ater the polyr~r in the form of an aqueous solution, a ~ry particulate solid or a suspension in a non-aqueous 5 liquid, and also adding a complexing agent for multivalent ions and, where the solution so fonned does not already contain an alkali metal salt, subsequently incorporating therein an alkali metal salt.

Preferably the solution prepared by the present process is obtained from a preconcentrate of the polymer and the complexing a3ent which 10 can then be diluted or dissolved up on site to give the in~ection solution. As is conventional when making up polymer solutions for use in oil recovery, the solution may be based on an alkali metal salt solution such as a solution of sodium chloride or sodium sulphate.
When a salt solution is used, and for some reason which we are unable 15 to explain, we find that better injectability is obtained by first ccmbining the polymer and complexing agent than when adding the complexing agent to a prepared dilute solution of the polymer made up in the salt solution. We also find that solutions prepared using the present process have better optical clarity than those prepared without the 20 inclusion of complexing agent.

; Where the polymer is a microbial polysaccharide, an aqueous solution thereof is conveniently formed by cultivating a polysaccharide-producing strain of a microorganism in a nutrient medium and subjecting the broth so formed to treabnent by a process to remove residual solids 25 e.g. of size greater than 3 microns. Typical processes include filtration, centrifugation ar~ en~.~ne treatment ar~ are well kn-~wn in the microbial polysaccharide field.

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ALternatively, any solid polymer can be added to water to fonn the solution. The solid may be a dry powder or granulated material or, as in the case of xanthan, may be formulated as a suspension in a non-aqueous liquid such as alcohol or liquid paraffin. This dry powder or suspension may conveniently also contain the complexing agent.

A combination of the polyrner and the ccmpiexing agent, either as a dry powder or granulated material or as a suspension in a non-aqueous liquid constitutes a further feature of this invention. In the case of a microbial polysaccharide gurn, such as xanthan, the co~bination may conveniently be prepared by adding the complexing agent, especially sodium hexametaphosphate, to a polysaccharide-containing broth and co-precipitatir.g the ccmbined polymer and canplexing agent.

The addition of the c~nplexing agent may be effected either ~ust before the microbial cells are removed by filtration, centrifugation etc., or afte~w~rds. The cc-~recipitation may be obtained by a~dition of a precipitant for the polymer which also precipitates the complexing agent, for example an alcohol such as isopropanol.

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Alternatively the combination may be prepared by simple mixing of the two isolate~ ingredients~

It is difficult to rationalise the present invention on the basis oE the current theories discussed above regardin~ aggregation upon injection oE xanthan gum into alkaline formations; evidently some co~plexing effect occurs but it is not clear why benefits should be ~ .,~,. . .
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.; :' , . ~ , .:; ;, ' : . `- ' , - 7a ~ Z~3 o~tained for example with hydrox~ethyl cellulose solutions for injection at: non-~lkaline pH. Equally it is ~ifficult to suggest a reason why the order of mixing should be important.

Especially when the injection solution is to contain a salt such as sodium chloride or sulphate, we find that better results are obtained by fonming an initial solution of the polymer and the complexing agent and then adding the salt. If the order is reversed and the salt added before the complexing agent then the injectability is typically intermediate between that of the conventional solution without complexing agent and that of the solution prepared in the preferred order. That the order should be significant is particularly surprising since the action of the complexing agent should be unaffected by the presence of monovalent salts such as sodium chloride or sulphate.

It is however, readily apparent that the present effect is not the same as obtains for the prior art discussed above since the process now provided involves the use of polymer solutions free from residual solids and to which no surfactant is added.

The process according to the present invention can be contrasted fr that described in U.X. patent ~o.1,546,560 where xanthan gum is rehydrated in fresh water containing no complexing agent, before addition of , salt. We find that addition of the ccmplexing agent gives a distinct ' advanta3e over the use of fresh water alone, the filtration rate being increased by a third or m~re.
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Ex~nples of the ccmplexing agent which rnay be used include sodium hexalnetaphosphate such as is available under the trade mark "Calgon", ethylenediamine tetraacetic acid (EDTA) and salts o~ EDrA. The amount o complexing agent employed is not critical, though a preferred range for the concentration in the final injection solution is 0.01 to 2.0%
by weight.

The polymer incorporated in the solution may be any water-soluble polymer of use in oil recovery. Typical polymers include microbial polysaccharide gums, polyacrylamides and cellulose derivatives. The preferred microbial polysaccharide is xanthan. Other microbial poly saccharides include ~-1,3-glucan gums, e.g. that sold under the trade mark "Polytran" by Ceca, in Francer obtained fro~ the fungus Sclerotium glucanicum. Acrylamide polymers are well established in oil recovery processes.

As well as microbial polysaccharide gum and acrylamide Folymers, the invention can employ other polymers such as hydroxyethyl cellulose and related cellulosic materials. The amount of polymer used will depend on tne required viscosity for the injection solution. By way of example we mention that xanthan gum gives effective results when used at a concentration of about one gram per litre.

In the preferred process a preconcenteate of the complexing agent and the polymer is formed which can then be diluted. Xanthan gum is usually obtained as a fermentation broth containing 20 to 50, typically 30 g/l of the gum. In view of the effective concentration mentioned ~ - .

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2 . ., 3 at,ove o~: about 1 g/l in the injection sGlutio~ follows that xanthan g~ concentratcC; can conveniently be fonmulat~d as approx~nately 30x concelltrates by incor~orating in the broth 30 times the ultimately rc~uired collcentration of co~plexing a~ent a~er renoval from the broth of solids greater than 3 microns. For other polymers like considera-tions apply and ccnvenient concentrations for the preconcentrates can readily be ~orked out.

The present invention is illustrated by the following non-limiting examples (all ~ values are by weight).

Example l Xanthan fermentation broth (obtained from fermentation of Xanthomonas campestris) was diluted and centrifuged to remove cellular debris.
The following solutions were prepared (a) by diluting this solution with sodium chloride solution to give a solution containing 3% sodium chloride and O.l~ polymer (the control), (b) by diluting this solution with a solution of a complexing agent, sodium hexametaphosphate, and then adding solid sodium chloride to give a solution containing 0.25%
hexametaphosphate, 3% sodium chloride, and 0.1% polymer, the sodium chloride being added last.

The injectability of the solutions was then assessed by filtering the solutions at 20 psi and at room temperature through an Ap 200 Millipore prefilter and a 0.~ ~illipore filter and measuring the rate of filtration. The rate was detenmined after various volumes had been collected. The results are presented in Table l.
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'.. ,., - . :: , '" ` ' ' '' , -: ~ .: -_ 10 -Table 1 __ ___ _ _ Filtration Rate ml sec 1 __ __ Volur~ Through Solution (a) Solution (b) ~ilter (ml) ¦ Control 1.1 2.2 1.0 2.0 100 0.9 1.9 150 0.8 1.5 The l~illipore filter evaluation is based on that described by Lipton 10 (~E~ cit.), in which a ~lillipore filter is used as a convenient approximation for a porous rock.

Example 2 ~he follcwing solutions of hydroxyethyl cellulose (HEC) were prepared ) 0.2~ HEC in a 3% sodium chloride solution (control) (b) 0.2% HEC
15 in solution with 0.2% sodium hexametaphosphate plus 3% sodium chloride, the sodium chloride being added last. The filterabilities of these solutions were assessed using the method described in Example 1.
The results are presented in Table 2.

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Table 2 .
Filtration Rate (ml sec 1) _ _ Volume Through Solution (a) Solution (b) Filter (ml) Control .

100 1 1.4 200 ~.8 1.3 ;~ -320 0.~ 1.1 :

Example 3 10 The following solutions of a ~ -1,3-glucan gum produced by the fungus ~-Sclerotium glucanicum and available under the trade mark "Polytran"
fran Ceca, France were prepared (a) 0.2% Polytran in 3% sodium chloride solution (control), (b) 0.2~ Polytran in solution with 0.2~ sodium hexametaphosphate plus 3~ sodium chloride, the sodium chloride being added last. The filterabilities of these solutions were assessed using the method described in Example 1 and the results are presented ~ in Table 3 i - .
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- 12 ~ 3~ 3 Tab e 3 _ . ___ ___._ ___ ___ .__ _ Filtration l~te (ml sec . _. , _ _ _._ . . _ __ Volur~ Through Solution (a) Solution (b) ~'ilter (ml) Control . __ _ lO0 1.6 l.9 200 l.0 1.5 320 0.6 1.2 _ample 4 The effect of sodium hexametaphosphate on the optical clarity of pol~ner solutions was s.tudied. Solutions containing 1% polymer (xanthan or Polytran gum) and either 0% or 1.0% hexametaphosphate were prepared.
The optical clarity of the solutions was assessed using a colorimeter, the scale used being arbitrary.

1~ The results are presented in Table 4. .

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_ Colorimeter Reading .' _ . _ _ , .
:~ Polyrner 0% hexameta- 1.0% hexameta-. phosphate phosphate .
Xanthan * 0.75 0.45 Polytran 0.65 0.20 * bacterial cells r~noved by enzyme treabment.

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(a) 0.2% xanthan gum plus 3~ sodium chloride (control) (b) 0.2% xanthan gum plus 0.1 M EDrA (eth~lenediamine tetra-acetic acid) and 3~ sodium chloride, the sodiurn chloride added last.

The filterabilities of these solutions were assessed using the method described in ~xarnple 1, and the results are presented in Table 5.

Table 5 ' ' .

Filtration Rate (ml sec ) Volume through Solution (a) Solution (b) Filter (ml) (Control) : 15 110 0.7 1.0 180 0.3 0.7 360 0.1 0.4 , .
. Example 6 The effect of the order of addition of hexarnetaphosphate and salt to polymer solutions on the filterability of those solutions was studied.
The follc~ing solutions containi~ 0.1% xanthan, clarified by centrifugation, . '-i .

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. . , were prepar~i; (a) xanthcln gum w~s dissolv~d in 3% sodium ct~loride solution, (b) xanthan gum ~as dissolvc~d in 3?i sodium chloride solution and then sodium hex~netaphosphate was added to a final conc~ntration of 0.2%, (c) xantharl gum was dissolved in U.2% sodiurn hexametaphosphate 5 solution and thcn sodium chloride was added to a final concentration of 3~. The filterabilities of these solutions were assessed usng the method described in Example 1. The results are presented in Table 6. Fran the table one can appreciate the improved filter- ability (and thùs improved injectability) of solutions prepared using the ~ 10 present process; the beneficial effect obtained by adding the canplexing agent before the sodium chloride is especially marked.

Table 6 Filtration Rate ml sec .

Volume Through Solution (a) Solution (b) Solution (c) 15 Filter (ml) _ 2.3 3.3 4.5 120 1.7 3.0 4.5 160 ~.3 2.7 4.5 200 1.1 2.0 4.5 ' ~............. ~ ..
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- 1 5 ~ Z;~3 ~x npk-~ 7 The proc~dure o~ ~xample 1 was repeat~l, but using a co~rcially available polyacl~lc~nide of the type ~seful in oil recovery, kncwn as ~II ~liper-pol. Solutions were made up (a) containing 0.1% polyrner ar,d approxilnately 3~ sodiurn chloride and (b) also containing 0.25~
sodium hexametaphosphate (Calgon) added before the salt. The prefilter was Millipore AP 20 042 00~

The results are given in Table 7:

Table 7 . _ -1 .
Filtration Rate of Sec ~ .
10 Weight through Solution (a) Solution (b) filter (g) Control _ .
0.17 0.28 0.12 0.23 100 0.08 0.16 150 0.05 0.12 180 I O.O5 1 0.10 ;' ' .

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F,xa~le 8 A 1~ solution of xanthan gwm, clariEied by centrifugation, was prepared in a conc. (ca. 10%) solution of sodium hexametaphosphate. A similar l ~ solution was prepared as a control in fresh water (containing no hexametaphosphate). Both solutions were diluted with hard brine (2% NaCl, 0.2% CaC12) to give (a) 0.1~ polymer in hard brine; (b) 0.1~ polymer + 1% NaHMP in hard brine.

The filterabilities of these solutions were assessed as in Example 1 and the results are shown in Table 8.

Table 8 Filtration Rate ml sec 1 r Volume through Solution (a) Solution (b) filter (n~L) COr.trol .__ . _ 0.71 0.94 100 0.46 0.64 150 0.3~ 0.48 180 0.29 0.39 ' .

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The procedure o~ Example 8 was repeated, but sea water (collected o~f the South Coast oE England and filtered through a 5~ Millipore filter) was used inste~d of hard brine. The results are shown in Table 9.

~able 9 Filtration Rate ml sec .

Volurne through Solution (a) Solution (b) filter (rnl) (Control~
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0.5 0.61 100 0.3 0.39 150 0.21 0.29 180 0.16 0.24 ., . , .

Exam~le 10 To a xanthan fe~rnentation broth (ca 2.5% xanthan) was added a conc.
solution of sodium hexametaphosphate to give a l:l wt. ratio of polymer to canplexing agent. A control solution was prepared by diluting the broth with an equal volurne of fresh water. Both solutions were centrifuged to rernove cells, and then isopropanol was added to ef~ect co-precipitation of xanthan gurn and the canplexing agent. The product was dried and milled. The following solutions were prepared:

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(a) Control: 0.01~ polymer in 3% NaCl (b) 0.2% Na&~P/polymer co-precipitate in 3% NaCl A

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Table 10 _ .
Filtration Rate (ml sec 1) _ ' .

Vol~ne through Solution (a) Solution (b) Filter (ml) (Control) _ _ _ l . ., 0.37 1.7 150 0.13 0.78 /`
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Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A process for preparing a polymer solution for use in "polymer based" oil recovery, which comprises (a) first mixing the polymer with a complexing agent for multivalent ions, and then (b) forming an aqueous solution containing in solution both the mixture and an alkali metal salt.

2 A process according to Claim 1, in which the polymer is a substantially cell-free microbial polysaccharide gum, a polyacrylamide or a cellulose derivative.

3 A process according to Claim 2, in which the polymer is xanthan gum, a .beta.-1,3-glucan gum, or hydroxyethylcellulose.

4 A process according to Claim 1, 2 or 3, in which the polymer is a microbial polysaccharide in the form of an aqueous solution formed by cultivating a polysaccharide-producing strain of a microorganism in a nutrient medium and subjecting the broth so formed to treatment by a process to remove residual solids.

5. A process according to Claim l, 2 or 3 in which the complexing agent is sodium hexametaphosphate, ethylenediamine tetraacctic acid or a salt of ethylene derivative tetraacetic acid.

6. A process according to claim 1, 2 or 3 in Which the alkali metal salt is sodium chloride or sodium sulphate.

7. A process according to, claim 1, 2 or 3, wherein the aqueous solution is formed by adding the alkali metal salt to an aqueous solution of the mixture.

8. A process according to claim 1, wherein the aqueous solution is formed by adding the mixture to an aqueous solution of the alkali metal salt.

9. A process according to Claim 8, in which the mixture of polymer and complexing agent is in the form of a particulate solid or a suspension.

10. A process according to Claim 8 or 9, in which the polymer is a microbial polysaccharide.

11. A process according to Claim 8 or 9 in which the polymer is xanthan gum,

12. A process according to claim 1, in which the com-plexing agent is mixed with an aqueous solution of polymer solu-tion before dilution, in an amount to provide the required con-centration in a diluted solution for oil recovery.

13. A process according to claim 12, in which the com-plexing agent is added in an amount to provide a concentration of 0.01 to 2.0% by weight in the diluted solution.

14. A composition for putting in to effect a process according to claim 1, which is a suspension of xanthan gum in a non-aqueous liquid together with a complexing agent for multivalent ions.

15. A composition for putting in to effect a process according to claim 1, which is a particulate solid comprising a mixture of a polymer for use in oil recovery, with a complexing agent for multivalent ions.

16. A composition for putting in to effect a process according to claim 1, which is a particulate solid comprising a mixture of a polymer for use in oil recovery, with a complexing agent for multivalent ions in which the polymer is microbial polysaccharide, the solid mixture having been co-precipitated from solution in a culture broth.

17. A composition for putting in to effect a process according to claim 1, which is a solid according to claim 16, in which the polysaccharide is xanthan.