GB2213175A - Inhibiting the precipitation of sparingly soluble salts - Google Patents

Abstract

The precipitation of sparingly soluble salts eg barium sulphate in aqueous media eg in water in underground formations in secondary oil recovery is inhibited by the presence of poly vinyl sulphonic acid.

Description

THRESHOLD TREATMENT' The present invention relates to threshold treatment of acidic aqueous media to inhibit the precipitation, or modify the crystal habit, of sparingly soluble salts of, e.g. alkaline earth metals, such as their carbonates and sulphates. It is applicable to the inhibition of fouling in a wide variety of industrial processes. It is particularly applicable to the inhibition of fouling of oil wells and oil bearing formations due to the precipitation of barium sulphate, especially during secondary oil recovery.

The use of threshold treatment agents to prevent or inhibit the deposition of scale in boilers and pipelines is well known. Threshold agents differ from chelating agents in that the former do not require to be in stoichiometric proportions based on the solute and are often effective at low concentrations, e.g. in the range 0.5 to 200 ppm, while the latter always require to be present in stoichiometric proportions based on the solute.

One use that has been suggested for threshold agents is the treatment of oil wells to prevent fouling of the well, for instance during secondary recovery.

Secondary recovery entails the injection of water and/or gas into the oil bearing strata to displace the oil. Fouling of the well has been attributed to crystallisation of alkaline earth salts. For example, during secondary recovery, this may occur due to interaction between alkaline earth metal ions, especially barium, in the formation water, and sulphate and/or carbonate ions in the injection water.

In many cases attempts to prevent fouling of the well and formation by injecting known threshold treatment additives have proved ineffective.

Methods available to help restore production are inconvenient and very expensive in lost production. Examples are reboring the well, reperforating the production string with explosives and acidising the well to dissolve carbonates.

We believe the ineffectiveness of known threshold treatment agents in preventing the blockages in many oil wells may be due to high acidity in the formation water preventing threshold inhibition of crystallisation. We have now discovered that a polyvinyl sulphonic acid and its salts and acid-hydrolysable precursors (hereinafter collectively referred to as PVSA) can provide effective threshold activity at low pH, are especially effective at inhibiting crystallisation of barium sulphate, and are effective in controlling, inhibiting or clearing fouling of oil wells during secondary oil recovery.

Our invention provides a method for inhibiting the precipitation or deposition from an aqueous solution of sparingly soluble salts, e.g. alkaline earth metal salts formed by the interaction of a solution containing alkaline earth metal ions with a solution containing carbonate and/or sulphate ions, under acidic conditions eg at a pH of less than 7, preferably e.g. less than 5, which comprises providing therein a polyvinyl sulphonic acid, preferably having a molecular weight between 200 and 500,000, or a water-soluble salt or acid-hydrolysable precursor thereof.

According to a particular embodiment the invention provides a method for inhibiting or preventing the deposition of alkaline earth metal carbonates and/or sulphates during or after primary or secondary oil recovery from water in pipes, vessels or other process equipment or in underground formations which comprises introducing threshold concentrations of PVSA into said water.

The PVSA (also referred to as polyethylene sulphonic acid) is at least a dimer and preferably has a molecular weight between 300 and 400,000, more preferably 500 to 250,000, most preferably 750 to 100,000, especially 1,000 to 50,000, e.g. 1,500 to 30,000 though 500 to 50,000 is often valuable. For certain purposes, molecular weights in the range 500-to 1,000 are preferred. More usually 2,000 to 20,000 e.g. 5,000 to 15,000 is employed but in some instances we may prefer to use molecular weights in the range 50,000 to 500,000.

Monomeric ethylene sulphonic acid is not effective and polymers with a molecular weight of more than about 1 million may tend to flocculate, rather than inhibit crystal growth.

The polyvinyl sulphonic acid polymer may contain minor amounts eg up to 30 or 50 molar % such as 5-20% molar % (based on total monomers) of structural units derived from other comonomers eg vinylic ones, such as vinyl chloride, styrene, vinyl pyrrolidone, vinyl acetate, vinyl alcohol, and/or acrylic and/or methacrylic acids and/or their salts. The polymer may be a block random or graft copolymer or any other type of copolymer. The polymer may contain other substituent groups, on the backbone carbon atoms, each being, e.g.

alkyl groups, e.g. methyl, ethyl, propyl etc., aryl, e.g. phenyl, halogen, e.g. chlorine, or other, e.g. sulphonate, sulphate, carboxyl, phosphonate or nitro. However preferably the polymer is based on monomer units substantially all of which are vinyl sulphonic acid (or vinyl sulphonate) units.

The polymer is preferably used as its sodium salt or as the acid, although other alkali metal salts such as lithium or potassium, ammonium salts or mono, di or tri alkyl eg lower C1 6 alkyl amine salts, such as methyl amine, dimethylamine, trimethylamine, ethyl amine, monoethanolamine, diethanolamine or triethanolamine salts may be used.

It may be desirable to use an acid hydrolysable derivative or precursor of polyvinyl sulphonic acid such as an ester or preferably a partial ester, e.g. a methyl, ethyl, propyl, benzyl, or isopropyl ester, an amide, an acid chloride or any other derivative or precursor of polyvinyl sulphonic acid which is hydrolysable to the latter in the presence of dilute aqueous acid, in order to provide a slow and/or controlled release of the PVSA into the formation or injection water.

The PVSA may also be encapsulated or microencapsulated in a slow dissolving material, or absorbed in a slow release glass in order to obtain the same effect.

As a result of the method of the invention the solution susceptible to precipitation of salts contains the polymer. The polymer may be added directly to the solution eg a supersaturated solution or may be added to one solution which when combined with another solution produces the solution susceptible to precipitation; examples of the latter are the addition to injection water eg sea water prior to its introduction into an underground formation where it mixes with the underground water to produce the solution susceptible to precipitation.

The PVSA is preferably introduced directly to the site of deposition, or potential deposition by dissolving or suspending in any injection water eg sea water or recycled formation water by separate injection at or near the borehole, or by back washing the well with aqueous PVSA under pressure to force the solution into the formation.

The invention is of particular value with inhibition of barium sulphate scale in water containing barium obtained from certain underground formations. The inhibition may be of production of insoluble solids or of deposition of those solids in an adherent form eg as scale, such as by modifying the crystal growth of the solids.

We prefer to maintain at least threshold concentrations of PVSA in the water system, eg the aqueous medium susceptible to precipitation of the sparingly soluble salts, of at least 0.lppm eg up to 5000 ppm but usually at least Ippm and up to about 1000 ppm, preferably Sppm to 200ppm, e.g. 10ppm to 100ppm, especially 20ppm to 50ppm. Usually at least an effective amount of the polymer is used in order to inhibit or prevent the deposition of the salts. Inhibition is observed of the deposition of the sulphates and carbonates of calcium, strontium and barium at pHs below 7, in particular below 5, often below 4.3 eg below 4, such as below 3 especially 2 - 5 such as 2.5 - 4.3.

The medium susceptible to scaling contains ions of sparingly soluble salts such as sulphates and/or carbonates of divalent metals eg alkaline earth metals, transition metals, lead and zinc.

The concentrations of alkaline earth metals in the medium susceptible to scale may be 1-10,000 ppm eg 100-5,000 ppm, for example with the concentration of barium, if present, being 10-3000 eg 100-1000 ppm and with the total concentration of calcium and magnesium, if present, being 10-10000 eg 100-1000 ppm. The concentration of sulphate ions ppm in the medium susceptible to scale may be 1-10,000 ppm eg 100-5000 ppm especially 100-1000 ppm.

The method of the invention may be used in connection with inhibition of solids deposition in aqueous media other than those mentioned above. Other Examples are in the beneficiation of ores eg involving froth flotation (eg of tin ores mica or felspar), acid leaching or acid pretreatment of ores or solvent extraction purification (eg of transition or heavy metals eg chromium, niobium, copper or rare earth metals). It may also be useful in inhibition of lead sulphate deposition eg in lead acid accumulators and in the production of food and fruit juice products involving acidic media susceptible to scaling.

The invention is illustrated by the following examples: Example 1 1. WATER COMPOSITIONS ION CONCENTRATION (PPM) FORMATION WATER SEAWATER SODIUM 25,990 11,200 POTASSIUM 1,050 370 CALCIUM 320 400 MAGNESIUM 43 1,400 BARIUM 765 STRONTIUM 33 CHLORIDE 40,440 19,750 SULPHATE 13 2,650 2. TEST METHOD 2.1 The synthetic formation water and seawater were prepared according to the compositions given above.

2.2. The formation water was filtered through 0.45 micron filter.

2.3 50 ml of seawater and 200 mi of formation water were measured out.

2.4 An amount of inhibitor solution was added to the portion of formation water to provide a concentration of 25 ppm of active inhibitor ingredient therein.

2.5 Both of the portions of test water were adjusted to the desired pH using 0.5 normal sodium hydroxide or hydrochloric acid.

2.6 Both portions of test water were heated to 820C.

2.7 The portions of test water were mixed and kept at 820C for 1 hour.

2.8 The mixture was filtered through a 0.45 micron filter.

2.9 The filtered solids were dried at 1050C and weighed.

2.10 A blank was performed with no inhibitor present and the percentage inhibition calculated from 100% - (the , of filtered solids in the presence of inhibitor to those in its absence).

3. RESULTS pH VALUE PERCENTAGE INHIBITION PVSA POLYACRYLIC INHIBITOR PHOSPHONATE INHIBITOR 6.0 57 90 5.0 65 89 67 4.0 54 0 0 3.0 45 0 0 2.5 51 0 0 In the table the PVSA was homo polyvinyl sulphonic acid (as a sodium salt thereof in 25% aqueous solution at pH 7) of Molecular Weight 3000 (by GPC), the polyacrylic acid was homopolyacrylic acid (as a sodium salt thereof in 25% aqueous solution at pH 7) of Molecular Weight - 5000 (by GPC) and the phosphonate was diethylene triamine penta (methylene phosphonate) (as a sodium salt thereof in aqueous solution).

4. COMMENTS The above results show that the activity of the conventional polyacrylate and phosphonate inhibitors is lost at low pH whereas the activity of PVSA is retained.

ExamPle 2.

Experiments were performed to assess the inhibition of blocking of a tube by deposited sparingly soluble salts in the presence or absence of scale inhibitors. The aqueous medium susceptible to deposition was prepared by mixing equal weights of Solutions 1 and 2 which were deionized water to which had been added the ions specified as in the following table:: Concentration (ppm)

ION SOLUTION 1 SOLUTION 2 SODIUM I 30,000 1 30,000 BARIUM j | 600 SULPHATE 1 630 1 CHLORIDE g 42,300 1 46,600 The Solutions 1 and 2, after adjustment of pH to 3, were pumped at 5 ml/minute through preheaters, then mixed at a T piece and the aqueous medium produced passed through a stainless steel capillary coil with an internal diameter of 1.4 mm and hence via a 7 micron in -line filter and then a cooler and back pressure regulator valve set at 100 psi (70, 000 kg/m2) to an outlet. The preheater, mixer, coil and filter were kept in an oven at 120CC.The pressure difference, if any, across the capillary and filter ie between the aqueous medium prior to entering the capillary coil and just prior to entering the cooler was measured by means of a pressure transducer and recorded.

The first experiment was performed with no inhibitor, whereupon the pressure differential reached 100 psi (70,000 Kg/m2) within 1 hour, and then repeated separately with each of the polyvinyl sulphonic acid inhibitor and the polyacrylic acid inhibitor used in Example 1.

In each case the inhibitors were added to Solution 1 in amount of 100 ppm of an aqueous 25% inhibitor solution to give the combined aqueous media containing 12.5 ppm active polymer inhibitor ingredient. The variation of pressure with time was as follows:

TIME HOURS 0 1 2 3 4 5 6 PVSA PRESSURE (psi) 0 0 0 0 0 1 2 (Kg/m) 0 0 0 0 0 700 1400 POLYACRYLATE PRESSURE (psi) 0 2 6 13 28 69 > 100 (Kg/m) 0 1400 4200 9100 19600 48300 > 70000 The PVSA is very much more effective at pH3.0 than the polyacrylate as an inhibitor of scale formulation.

EXAMPLE 3 The process of Example 2 was repeated at pH 4.5. with and without PVSA inhibitor. In the absence of the inhibitor the pressure reached 100 psi (70,000 Kg/m2) within 1 hour, while in its presence the pressure only reached 9 psi (6300 Kg/m2) even after 6 hours. The results with inhibitor were as follows:

TIME HOURS 0 1 2 3 4 5 6 PVSA PRESSURE (psi) 0 1 2 3 4 6 9 (Kg/m) 0 700 1400 2100 2800 4200 6300

Claims (10)

1. CLAIMS 1. A method for inhibiting the precipitation or deposition of a sparingly soluble salt from an aqueous solution under acidic conditions, which comprises providing therein a polymer of a vinyl sulphonic acid, or water soluble salt or acid hydrolysable precursor thereof.
2. 2. A method according to claim 1 wherein the sparingly soluble salt is at least one of an alkaline earth metal carbonate or sulphate.
3. 3. A method according to claim 2 wherein the salt comprises barium sulphate.
4. 4. A method according to claim 2 or 3 for inhibiting or preventing the deposition of an alkaline earth metal carbonate and/or sulphate from water in process equipment and/or underground formations during or after primary or secondary oil recovery which comprises introducing a threshold concentration of said polymer into said water.
5. 5. A method according to any one of the preceding claims in which the aqueous solution is at a pH less than 5.
6. 6. A method according to any one of the preceding claims wherein the polymer is polyvinyl sul phonic acid.
7. 7. A method according to any one of the preceding claims wherein the polymer has a molecular weight of 500 to 50,000.
8. 8. A method according to any one of the preceding claims wherein an amount of 1-1000 ppm of the polymer is present in the aqueous solution.
9. 9. A method according to any one of the preceding claims wherein the aqueous solution is at a pH less than 4.3.
10. 10. A method according to claim 9 wherein the aqueous solution is at pH 2.5-4.3.