NO303010B1 - Coating inhibitor composition, coating formation inhibition composition, and method of inhibiting coating formation in an aqueous fluid present in or produced in an underground formation - Google Patents

Description

Background for the invention.

The present invention relates to a coating formation inhibitor, a mixture for inhibiting coating formation, as well as a method for inhibiting coating formation in an aqueous fluid which is present in or which is produced in an underground formation.

More specifically, the invention relates to a relatively high molecular weight polyvinyl sulfonate and a method for inhibiting coating formation, especially of inorganic sulfate such as barium sulfate, in an underground formation using an aqueous solution that has the relatively high molecular weight polyvinyl sulfonate dissolved in it.

Precipitation of inorganic salts, such as calcium carbonate and calcium, barium and strontium sulfate, as scale formation is a persistent and common problem encountered in many field operations for the recovery of hydrocarbons from underground formations. Mixing of immiscible aqueous fluids during field operations, especially Enhanced Oil Recovery (EOR) operations that involve water flooding or water drift, leads to coating and deposition in the formation and in production equipment and piping systems. Two or more aqueous fluids are incompatible if each fluid contains specific ions that form a precipitate and precipitate as a coating when the two or more aqueous fluids are mixed together. Typically, formation water or the brine present in a reservoir will contain barium, calcium and/or strontium ions, while water injected into the underground formation during EOR operations will contain sulfate ions. E.g. offshore operations may include injecting large volumes of seawater containing a relatively high concentration of sulfate ions into an underground formation that has brine containing relatively high concentrations of barium, calcium and strontium. When mixing the aqueous fluids in situ, precipitation of barium, calcium or strontium sulphate will occur in the formation and in production equipment and/or pipe systems below or on the surface. Mixing of immiscible aqueous fluids usually takes place in the near production wellbore environment in a subterranean formation.

Injection of carbon dioxide into an underground hydrocarbon-bearing formation as an EOR method leads to the absorption of carbon dioxide in the adhesion water present in the formation. In addition, some underground formation salt solutions, such as those found in the North Sea, can naturally contain a relatively high concentration of carbon dioxide. As the pressure decreases, e.g. during production, carbon dioxide will be driven off to the gas phase thereby increasing the pH of the aqueous fluids and allowing the formation of calcium carbonate coatings predominantly in the close production borehole environment in the formation and in production equipment and/or piping systems on or below the surface.

Conventional removal of coatings that form in an underground formation and in production equipment and piping systems on and below the surface is costly and ineffective. Coating removal by repeated injection of a chemical agent is relatively expensive. Coatings have therefore been removed using various mechanical devices, such as powerful jets and/or cavitation jets. As the dead time in connection with extracting production pipes and cleaning such pipes on the surface is expensive, especially at sea, the wells are cleaned down the hole after the well has been killed. Such mechanical cleaning is time-consuming, relatively ineffective, and potentially dangerous where a radioactive fallout, e.g. radium sulphate, is present in the coating to be removed.

Fouling inhibition has been claimed to be an easier, and therefore more preferable, way to effectively reduce fouling. Conventional commercial scale inhibitors consist primarily of polyelectrolytes, such as polycarboxylates or polyphosphonates. The effectiveness of such polyelectrolyte coating formation inhibitors, however, depends to a significant extent on the degree of ionization of these inhibitors at the pH value of the formation water. At relatively low pH values, e.g. equal to or lower than approx. 6.0, the effectiveness of a conventional polyelectrolyte scale inhibitor to inhibit the scale formation of barium, calcium or strontium sulfate will decrease significantly. Also, conventional polyelectrolyte scale inhibitors used to inhibit inorganic sulfate scale formation will dissolve calcium carbonate scale and thereby increase the calcium ion concentration, causing unwanted precipitation of the conventional polyelectrolyte scale inhibitors. There is thus a need for a scale formation inhibitor for use in underground formations which effectively inhibits the formation of scale, especially inorganic sulfate such as barium sulfate, in a relatively low pH environment, such as a pH of about 6.0 and below.

It is therefore an object of the present invention to provide a new scale formation inhibitor which will effectively inhibit the formation of scale, especially inorganic sulfate such as barium sulfate, in aqueous fluids present in and/or produced from an underground formation.

It is another object according to the present invention to provide a scale formation inhibitor which does not dissolve carbonate scale to any particular extent when injected into an underground formation via a borehole in fluid communication therewith.

It is a further object of the present invention to provide a method for inhibiting scale formation, in particular inorganic sulfate such as barium sulfate, from fluids with a pH equal to or lower than 6.0 which are present in and/or which are produced from an underground formation .

Summary of the invention

With the present invention, a coating formation inhibitor is provided which is characterized by the fact that it comprises a polyvinyl sulphonate with a molecular weight of from 10,500 to 30,000.

With the present invention, a mixture is also provided which is characterized by the fact that it comprises an aqueous solution containing dissolved a scale formation inhibitor according to the invention in an amount from 0.4 to 25 vol%, preferably from 2 to 20 vol%, more preferably from 5 to 10 vol%, of the solution.

The present invention also provides* a method for inhibiting coating formation in an aqueous fluid that is present in or produced in an underground formation, and the method is characterized by the fact that the aqueous fluid is brought into contact with a coating formation inhibitor according to the invention or a mixture according to the invention, in that an amount of the coating formation inhibitor or of the mixture is used so that the polyvinyl sulphonate constitutes from 0.4 to 25 vol%, preferably from 2 to 20 vol%, most preferably from 5 to 10 vol%, of the solution.

The present invention thus provides a method for inhibiting scale formation, in particular inorganic sulfate scale, from aqueous fluids present in and/or produced from an underground formation by bringing such fluids into contact with an aqueous solution that has dissolved in it a new, relatively high molecular weight polyvinyl sulphonate. The polyvinyl sulfonate has a molecular weight of from 9,000 to 30,000, preferably from 10,500 to 25,000, and more preferably from 12,000 to 20,000. Preferably, the polyvinyl sulfonate has a polydispersity of less than 2.0. The usual fluid is brought into contact with the aqueous solution which has dissolved polyvinyl sulphonate in it, e.g. by means of a press treatment via a production borehole in fluid communication with the formation. The polyvinyl sulphonate according to the present invention is dissolved in an aqueous solution in an amount of from 0.4 to 25 vol% of the total solution. The pH of the aqueous fluid is equal to or less than 6.0.

Brief description of the drawings

The accompanying drawings, which are incorporated into and form part of the description, illustrate the embodiments according to the present invention and serve, together with the description, to explain the principles of the invention. In the drawings are: fig. 1 is a diagram illustrating the percent barium efficiency, i.e. barium sulfate coating inhibition, of polyvinyl sulfonate at a pH of 4 and 6 as a function of polyvinyl sulfonate molecular weight;

fig. 2 a diagram illustrating the calorimetric titration curves for a non-inhibited and several polyvinylsulfonatin-inhibited sulphate salt solutions, titrated with barium salt solution,

fig. 3 a diagram indicating the change in heat of reaction for each of the many polyvinylsulfonate-inhibited sulfate-

salt solutions that have been titrated with barium salt solution; and

fig. 4 a diagram of the barium efficiency in percent, i.e. the barium sulfate coating inhibition, for polyvinylsulfonates of different molecular weight at a pH of 4 and 6 as a function of the polyvinylsulfonate concentration.

Detailed description of the preferred designs

The present invention relates to a scale formation inhibitor and a scale formation inhibitor mixture comprising an aqueous solution which has dissolved in it a relatively high molecular weight polyvinyl sulphonate, and a method which uses this scale formation inhibitor mixture to effectively inhibit the formation of scale, especially inorganic sulphate scale, from aqueous fluids which is present in an underground formation and piping systems and/or equipment for hydrocarbon production on or below the surface. As used throughout this description, the term "molecular weight" shall refer to the polyvinyl sulfonate's weight average molecular weight. The weight average molecular weight is determined from experiments in which each molecule or each chain makes its contribution to the measured result. The stated weight average molecular weight in this specification was determined using a size exclusion chromatographic determination of molecular weights using a column packed with a polymeric gel. As also used throughout this description, the term "polydispersity" shall refer to the weight average molecular weight of the polyvinyl sulfonate divided by the number average molecular weight of the polyvinyl sulfonate. The number average molecular weight is calculated by dividing the sum of the individual molecular weight values by the number of molecules.

According to the present invention, an aqueous solution that has a relatively high molecular weight polyvinyl sulfonate dissolved in it is injected into the underground formation via a production well in fluid communication with it, in order to effectively inhibit the coating formation in aqueous fluids that are present in the formation and/or in production piping systems and/or equipment on or below the surface, when the well is brought back into operation. Applicants have discovered that the use of a polyvinyl sulfonate having a molecular weight of from 9,000 to 30,000 leads to an unexpected improvement in the inhibition of coatings, especially inorganic sulfate coatings such as barium sulfate, from these aqueous fluids. The polyvinyl sulfonate according to the present invention has a molecular weight of from 9,000 to 30,000, more preferably from 10,500 to 25,000, and most preferably from 12,000 to 20,000. Preferably, the polyvinyl sulfonate according to the present invention has a polydispersity of less than 2.0. The polyvinyl sulphonate according to the present invention is thermally stable. The polyvinyl sulfonate coating inhibitor of the present invention is prepared by polymerizing a commercially available aqueous solution of vinyl sulfonic acid sodium salt, such as that manufactured by Air Products & Chemicals, Inc., Aldrich Chemical Co., Inc. or Farbwerke Hoechst AG. For example, 25 and 30% by weight aqueous solutions of vinyl sulfonic acid sodium salt are commercially available from Air Products & Chemicals, Inc. and Farbwerke Hoechst AG, respectively. As received, the vinyl sulfonic acid sodium salt solution may contain a polymerization inhibitor, e.g. 100 ppm methyl ether hydroquinone. In such cases, the particular polymerization inhibitor used can be extracted by passing the monomer, i.e., the vinyl sulfonic acid sodium salt, slowly through a column packed with a suitable resin as will be apparent to one skilled in the art. The monomer solution is flowed through for approx. 1 hour with nitrogen at room temperature while the solution is stirred to remove oxygen from it. A suitable catalyst, such as ammonium persulfate, is also flushed with the nitrogen, and then added to the monomer solution at room temperature. The resulting solution is stirred continuously at room temperature for a period sufficient to allow maximum conversion of the monomer to polyvinyl sulfonate. If the resulting polymerization solution contains harmful by-products of the sulfonation, such as sulfate ions or hydroxyethyl sulfonate, polyvinyl sulfonate can be separated from these harmful by-products of the sulfonation by any suitable method that will be apparent to the skilled artisan. Preferably, this separation is carried out by adding methanol to effect a liquid/liquid separation of polyvinyl sulphonate from the solution containing harmful sulphonation by-products. This method is described in an assigned, concurrent, pending patent application entitled "Process for Recovering and Purifying a High Molecular Weight Sulfonate" to Richard T. Barthorpe which was filed concurrently herewith.

The polyvinyl sulfonate scale inhibitor of the present invention is incorporated into an aqueous solution in an amount that effectively inhibits scale formation in aqueous fluids present in and/or produced from a subterranean formation. The aqueous solution which has the polyvinyl sulfonate coating inhibitor according to the present invention dissolved in it can be brought into contact with aqueous fluids present in the underground formation and/or in production piping systems and/or equipment on and/or below the surface, on any suitable means known to those skilled in the art, such as metering into an aqueous fluid present in a production wellbore through a small diameter pipe, e.g. a 1/4 to 1 inch, by injection through a gas lift valve, or by introducing encapsulated polyvinyl sulfonate into a production wellbore. However, it is preferred to press the aqueous solution having polyvinyl sulfonate dissolved in it into an underground formation. According to a press-in technique, an aqueous solution of the polyvinyl sulphonate according to the present invention is injected into an underground formation via a production borehole in fluid communication therewith, and can be followed by a flushing of e.g. a salt solution containing a relatively low amount of sulphate ions, e.g. a salt solution that is compatible with the formation fluids. The production borehole can be stopped for a suitable period, e.g. 0 to 24 hours, and then put into production again. The polyvinyl sulfonate is absorbed within the formation matrix during the stop period, and is then desorbed over a period of time in the aqueous fluids present in and produced from the formation to effectively inhibit scale formation, especially inorganic sulfate scale such as barium sulfate scale. The polyvinyl sulfonate according to the present invention should be incorporated into an aqueous solution to be pressed into an underground formation in an amount of from 0.4 to 25% by volume, more preferably in an amount of from 2 to 20% by volume, and most preferably in a amount of from 5 to 10% by volume of the solution to effectively inhibit coating formation after desorption in aqueous fluids present in and produced from the formation. After the intrusion treatment, aqueous fluids produced from the underground formation will be analyzed for inhibitor concentration to ensure that an appropriate concentration of inhibitor is present in produced fluids to effectively inhibit scale formation and determine the need for subsequent intrusion treatments.

The method according to the present invention

can be used to inhibit fouling, particularly inorganic sulfate fouling such as barium sulfate fouling, from aqueous fluids containing and/or produced from any subterranean formation in which immiscible aqueous fluids may

mix, e.g. during an EOR operation, and/or in which the aqueous fluids present in the formation contain a relatively high concentration of carbon dioxide. Preferably, the polyvinyl sulfonate scale inhibitor and method of the present invention are used to effectively inhibit scale from aqueous fluids present in or produced from a subterranean formation having a pH equal to or less than 6.0, and more preferably equal to or less than 4, 0. The method according to the present invention can be used at many different underground formation temperatures and mineralogies.

The following examples demonstrate the practice and utility of the present invention.

Example 1

A 25% by weight solution of vinyl sulfonic acid sodium salt manufactured by Aldrich Chemical Co., Inc. is passed through a resin column to remove methyl ether hydroquinone which is used as a polymerization inhibitor for the vinyl sulfonic acid sodium salt monomer during transport. 1000 g of vinyl sulfonic acid sodium salt is filled into a reaction bottle and purged with nitrogen at room temperature for 1 hour with stirring to remove oxygen. Ammonium persulphate is added to 25 ml of nitrogen-flushed distilled water in an amount which gives an ammonium persulphate concentration of 0.1 g/ml. The mixture is then heated to 50°C. 3.0 ml of the resulting ammonium persulfate solution is injected into the monomer solution present in the reaction flask, and the resulting solution is stirred and allowed to polymerize at 50°C with a nitrogen purge for 24 hours. The reaction leads to 100% conversion of the monomer to a polyvinyl sulfonate which is determined to have a molecular weight of approx. 9109 and polydispersity of approx. 1.79.

Example 2

The polymerization reaction set forth in Example 1 is repeated, except that the vinyl sulfonic acid sodium salt that is filled in the reactor is reduced to 100 g, that the concentration of ammonium persulfate present in the reaction solution is increased to 0.50 mg/ml, that the reaction temperature is reduced to room temperature, i.e. 22.2°C, and that the reaction time is reduced to 8 hours. This polymerization reaction leads to 53.2% conversion of monomer to a polyvinyl sulfonate which is determined to have a molecular weight of approx. 13565 and a polydispersity of approx. 2.0.

Example 3

The polymerization reaction presented in example 2 is repeated, except that the concentration of ammonium persulfate catalyst used in the reaction solution is increased to 1.0 mg/ml and that the reaction time is extended to three days. This polymerization reaction leads to a 78% conversion of monomer to a polyvinylsulfonate which is determined to have a molecular weight of approx. 15,542 and a polydispersity of approx. 1.99.

Example 4

The polymerization reaction in example 2 is repeated, except that the concentration of ammonium persulfate catalyst used in the reaction solution is reduced to 0.25 mg/ml and that the reaction time is extended to 24 hours. This polymerization reaction leads to a 68% conversion of monomer to a polyvinylsulfonate which is determined to have a molecular weight of approx. 16,269 and a polydispersity of approx. 1.70.

Example 5

The polymerization reaction in example 2 is repeated, with the exception that the concentration of ammonium persulfate catalyst used in the reaction solution is increased to 1.0 mg/ml, that a cocatalyst, triethanolamine, is also used at a concentration of 1.2% by weight in the reaction solution, and that the reaction time is extended to 24 hours. This polymerization reaction leads to a 22% conversion of monomer to a polyvinylsulfonate which is determined to have a molecular weight of approx. 18,887 and a polydispersity of approx. 1.82.

Example 6

The polymerization reaction in example 2 is repeated, with the exception that the concentration of ammonium persulfate catalyst used in the reaction solution is reduced to 0.25 mg/ml, and that the reaction time is extended to 5 days. This polymerization reaction leads to a 72% conversion of monomer to a polyvinylsulfonate which is determined to have a molecular weight of approx. 18,902 and a polydispersity of approx. 1.81.

Example 7

The polymerization reaction in example 2 is repeated, with the exception that a 37.5% by weight solution of vinyl sulfonic acid sodium salt is used instead of a 25% solution, that the concentration of ammonium persulfate catalyst present in the reaction solution is reduced to 0.25 mg/ml and that the reaction time is extended to 3 days. This polymerization procedure should result in a satisfactory conversion of monomer to a polyvinyl sulfonate determined to have a molecular weight of about 25,000 and a polydispersity of less than 2.0.

Example 8

The polymerization reaction in example 2 is repeated, with the exception that a 50% by weight solution of vinyl sulfonic acid sodium salt is used instead of a 25% solution, that the concentration of ammonium persulfate catalyst present in the reaction solution is reduced to 0.25 mg/ml, and that the reaction time is extended to 3 days. This polymerization procedure should result in satisfactory conversion of monomer to a polyvinyl sulfonate determined to have a molecular weight of 30,000 and a polydispersity of less than 2.0.

The preceding examples demonstrate the polymerization process to be used to obtain a polyvinyl sulfonate with a molecular weight of from 9,000 to 30,000, which can be used as an improved coating inhibitor, especially for barium sulfate coating, according to the present invention.

Example 9

The effectiveness of the polyvinyl sulfonates obtained by the methods demonstrated in Examples 1-6 above in inhibiting barium sulfate scale is determined using the following vial test procedures containing germs. The pH of a salt solution A that has 2.465 g CaCl2.2H20, 5.253 g MgCl2.6H20, 2.177 g BaCl2.2H20, 0.157 g SrCl2.6H20, 3.486 g KC1 and 55.626 g NaCl per liters, is adjusted to a pH of either 4 or 6, depending on the exact pH test being performed, by adding hydrochloric acid to lower the pH or sodium hydroxide to raise the pH. 100 ppm of the particular polyvinyl sulfonate obtained in Examples 1-6 above to be tested is added to 100 ml of salt solution B containing 60.155 g of NaCl and 1.597 g of Na 2 SO 4 pr. litres. The pH of the resulting salt mixture B is adjusted to either pH 4 or 6, depending on the test to be performed, by adding hydrochloric acid to lower the pH or sodium hydroxide to raise the pH. Subsequent mixing of equal volumes of salt solutions A and B simulates a salt solution of 80% formation or adhesion water and 20% seawater, which are representative of an aqueous formation fluid obtained when North Sea seawater is used as a driving fluid in an underground formation in the Brae field . The Brae field is located in the British sector of the North Sea. 0.01 g of barium sulphate, i.e. seed crystals, is added to a glass culture tube with a screw cap. 5.00 ml of saline solution B (containing inhibitor) is added to the glass tube using a volumetric pipette. 5.00 ml of salt solution A is added to the glass tube in a similar manner. The glass tubes are quickly closed again and inserted horizontally in a rack in an oil shaking bath that has been preheated to 80°C. The oil roasting bath is activated, and the glass tubes are mixed for 1 hour. This procedure is repeated for each sample to be tested. Each glass tube is removed from the shaking bath, and approx. 2 g of the resulting slurry is withdrawn from the tube in a syringe. The resulting slurry is filtered through a 0.2 µm syringe filter to remove solid barium sulfate, and approx. 2 g of the filtrate is weighed into a plastic tube containing 5.0 g of deionized water and 0.5 g of basic EDTA solution to chelate the barium contained in the filtrate. The remaining slurry is returned in the glass tube to the shaking bath, and mixed for a further 1 hour. The resulting slurry is analyzed again as above, and the filtered samples are analyzed for barium concentrations by ICAP analysis. The percentage barium efficiency of the polyvinyl sulfonate inhibitor is calculated by determining the amount of barium ion in the final solution, and dividing this amount by the amount of barium ion in the original solution of salt solution A + B, and multiplying by 100%. The results of this testing are shown in fig. 1.

As indicated by these results, the use of a polyvinyl sulphonate with a molecular weight greater than approx. 9,000, lead to an unexpected increase in the percentage barium efficiency, and therefore inhibition of barium sulfate coating, at a pH of 4 and 6.

Example 10

A solution of 3 wt% sodium chloride and 0.15 M sodium sulfate ("sulfate salt solution") was titrated at 25°C with a solution of 3 wt% sodium chloride and 0.5 M barium chloride ("barium salt solution") to precipitate barium sulfate in compliance with the following turnover:

The barium salt solution was added as titrant to 50 ml of the sulphate salt solution containing 0.75 mmol sulphate at a rate of 0.00664 ml per minute. second for a total period of 290 seconds. A total of approx. 0.96 mmol of barium was added to the sulfate salt solution, resulting in an excess of 0.21 mmol of barium.

A model 458 isoperibol (adiabasic) calorimeter manufactured by Tronac Inc. was used to perform the titration. This calorimeter comprises a 600 rpm synchronous motor which drives a glass stirrer to provide thorough mixing. The results of this titration of sulfate solution with barium salt solution are illustrated in fig. 2 as calorimetric titration curve B. Line A represents the heat of stirring and the heat of dilution for sodium salts determined by adding the barium salt solution as set forth above to a 3% by weight sodium chloride solution. The total heat of reaction for the repeated titration is calculated by extrapolating the slope of the titration curve B for the first

100 seconds of the test, i.e. before addition of barium salt solution titrant, to time zero and extrapolate the slope of the titration curve B for the last 100 seconds, i.e. after all the barium salt solution titrant has been added to the sulphate solution, to time zero. The extrapolated readings are subtracted to give a total heat of reaction for this titration expressed in volts. A conversion factor of 0.491 calories per volts can be used to convert this heat of reaction into calories.

Using preparative gel permeation chromatography, five separate molecular weight fractions are isolated from a polyvinyl sulfonate having a molecular weight of 14,500 and collected as fractions having molecular weights as set forth below.

A premeasured amount of each polyvinyl sulfonate fraction is added to separate sulfate solutions to inhibit the formation of barium sulfate precipitation.

Each resulting inhibited sulfate salt solution is titrated with barium salt solution in the manner set forth above, and the resulting thermometric titration curves plotted in Figs. 2 and marked respectively as curves C-G. In the procedure set forth above with respect to titration B, the total heat of reaction for each titration C-G is calculated and subtracted from the total heat of reaction for titration B to give the change in heat of reaction which indicates the amount of reaction, and consequently the barium sulfate precipitation, which is inhibited of each polyvinyl sulfonate fraction. These results are set aside and duly referred to in fig. 3. As indicated by these results, the use of a polyvinylsulfonate with a molecular weight above 9,000 to 26,000 will lead to an unexpected reduction in the amount of reaction.

Example 11

The seed bottle test procedure set forth in Example 9 above was repeated at six different concentrations and at a pH of 4 and 6 for each of the polyvinyl sulfonates obtained by the procedures demonstrated in Examples 1-4 above. The results shown in fig. 4 shows that the concentration of polyvinyl sulfonate coating inhibitor according to the present invention in an aqueous fluid in which coating formation is to be inhibited should be at least 50 ppm, more preferably at least 75 ppm, and most preferably at least 100 ppm to lead to the unexpected increase in the percentage barium efficiency at a pH of 4 and 6 in accordance with the method according to the present invention. It is important to note that an aqueous solution containing the polyvinyl sulfonate scale inhibitor of the present invention will not dissolve calcium carbonate scale to any significant extent when injected into a subterranean formation via a production well. Accordingly, the presence of calcium carbonate coating in a production borehole or the near borehole environment in a formation will not adversely affect the effectiveness of the polyvinyl sulfonate coating inhibitor of the present invention in any detrimental manner. Moreover, it has been determined that the polyvinyl sulfonate coating inhibitor according to the present invention has a significantly greater degree of inhibition of barium sulfate coating on calcium carbonate germ than conventional polyphosphonate or polycarboxylate coating inhibitors.

Claims (18)

1. Coating formation inhibitor, characterized in that it comprises a polyvinyl sulphonate with a molecular weight of from 10,500 to 30,000. 2. Coating formation inhibitor according to claim 1, characterized in that the polyvinyl sulphonate has a molecular weight of from 10,500 to 25,000, preferably from 12,000 to 20,000. 3. Coating formation inhibitor according to claim 1 or 2, characterized in that the polyvinyl sulphonate has a polydispersity which is lower than 2.0. 4. Mixture for inhibiting coating formation, characterized in that it comprises an aqueous solution containing dissolved a coating formation inhibitor according to claims 1-3 in an amount from 0.4 to 25 vol%, preferably from 2 to 20 vol%, more preferably from 5 to 10 vol%, of the solution. 5. Mixture according to claim 4, characterized in that the aqueous solution is a salt solution. 6. Mixture according to claim 5, characterized in that the aqueous solution is a saline solution with a relatively low concentration of sulfate ions. 7. Method for inhibiting coating formation in an aqueous fluid that is present in or that is produced in an underground formation, characterized in that the aqueous fluid is brought into contact with a coating formation inhibitor according to claims 1-3 or a mixture according to claims 4-7, using an amount of the coating formation inhibitor or of the mixture so that the polyvinyl sulphonate amounts to from 0.4 to 25 vol%, preferably from 2 to 20 vol%, most preferably from 5 to 10 vol%, of the solution. 8. Method according to claim 7, characterized in that a fluid with a pH of no more than 6.0, preferably no more than 4.0, is used as aqueous fluid. 9. Method according to claim 7 or 8, characterized in that an aqueous solution containing the dissolved polyvinyl sulphonate is injected into the underground formation via a borehole which is in fluid communication with the underground formation, so that the polyvinyl sulphonate is absorbed into a matrix in the underground formation and the contact step finds place by the polyvinyl sulphonate being desorbed from the matrix and into the aqueous fluid. 10. Method according to claim 7 or 9, characterized in that the concentration of polyvinyl sulphonate in the aqueous fluid produced from the underground formation is determined. 11. Method according to claim 10, characterized by repeating the injection of the aqueous solution containing dissolved polyvinylsulfonate in the underground formation, when the concentration of polyvinylsulfonate in the aqueous fluid produced from the formation is lower than 50 ppm, preferably 75 ppm, most preferably 100 ppm. 12. Method according to claims 9-11, characterized in that further aqueous solution is injected into the underground formation via the borehole to replace the aqueous solution containing dissolved polyvinyl sulphonate. 13. Method according to claim 12, where the borehole is a production borehole, characterized in that the borehole is kept closed after the injection of the further aqueous solution. 14. Method according to claims 7-10, characterized in that the aqueous fluid contains dissolved carbon dioxide. 15. Method according to claim 9, characterized in that a salt solution with a relatively low concentration of sulfate ions is used as the aqueous solution. 16. Method according to claims 7 - 14, characterized in that harmful sulfonation by-products are removed from the aqueous solution, before this is brought into contact with the aqueous fluid. 17. Method according to claim 7, characterized in that the contacting step is carried out in that an aqueous solution containing the dissolved polyvinyl sulphonate is passed through a small-diameter pipe placed inside a borehole that is in fluid communication with the underground formation and into the aqueous fluid that is present in the borehole. 18. Method according to claim 7, characterized in that a polyvinyl sulphonate is used which is encapsulated, and that this is injected into a borehole which is in fluid communication with the underground formation.