WO1990004159A1

WO1990004159A1 - Method for monitoring polyacrylic scale inhibitor content

Info

Publication number: WO1990004159A1
Application number: PCT/US1989/004563
Authority: WO
Inventors: John Hen
Original assignee: Mobil Oil Corporation
Priority date: 1988-10-14
Filing date: 1989-10-11
Publication date: 1990-04-19
Also published as: GB2245360A; GB9105089D0; GB2245360B; NO911363D0; NO911363L; NO302675B1

Abstract

The concentration of a polyacrylic acid in a solution containing polyvalent cations having a valence of greater than or equal to 3, e.g., Fe (III), which interfere in the conventional quantitative analysis of polyacrylic acid scale-inhibitors, is determined by initially adjusting pH of the solution to a value in the range of 0.5 to less than 2, and then quantitatively measuring the polyacrylic acid content in a conventional manner.

Description

"METH00FORMONITORINGPOLYACRYLICSCALEINHIBITORCONTENT"

This invention relates to a method for monitoring polyacrylic scale inhibitor content in the presence of an interfering polyvalent cation. Scale deposits frequently occur in the production of water, oil and gas from subterranean formations and can result in plugging of well bores, well casing perforations and tubing strings, as well as sticking of downhole safety valves, downhole pumps and other downhole and surface equipment and lines. Scale deposits can occur as a result of mixing of incompatible waters in the well which produce precipitates, or as a result of temperature and pressure changes in the produced waters during production. Generally, incompatible waters occur in waterflooding, such as when injected sea water mixes with formation water in the borehole during water breakthrough. The more common concern is scale deposited due to changes in supersaturation or solubility of minerals in the formation or produced waters caused by pressure and temperature changes, or changes in other physical and chemical parameters, such as gas composition and ratio of gas/oil/water. Scale may also be formed from corrosion of metal equipment used in the subterranean oil and gas production. Scale formation is also a problem in aqueous systems used in cooling towers, boilers and the like. Precipitation frequently encountered as scale includes calcium carbonate, calcium sulfate, barium sul ate, magnesium carbonate, magnesium sulfate, and strontium sulfate.

Scale formation can be reduced by the introduction of inhibitors into the formation. Various inhibitors are known, including a widely used class of materials which are carboxylated polymers. Typically, these are polymers and copolymers of acrylic or methacrylic acids, commonly referred to as polyacrylic acids. As disclosed by Rothman in U.S. Patent 4,514,504, it is desirable to have a method for monitoring the polyacrylic acid content of aqueous systems to know whether additional quantities of polyacrylic acid need to be added to maintain the optimum levels necessary to reduce scale formation. The method disclosed in Rothman involves initially lowering the pH of the system to 2-3, but this leads to a problem where the aqueous system contains ions having a valence equal to or greater than 3, such as Fe (III), Cr (III) and Al (III), since the latter form complexes with polyacrylic acid at such pH values. An object of the present invention is therefore to obviate or alleviate this problem.

Accordingly, this invention resides in a method for determining the concentration of polyacrylic acid in a solution containing a polyvalent cation which has a valence of at least 3, and is capable of reacting with the polyacrylic acid, comprising:

(a) adjusting the pH of the solution to a value of 0.5 to less than 2; and then

(b) measuring the concentration of the polyacrylic acid in the solution. in the accompanying drawings:

Figure 1 is a plot of the degree of ionization as a function of pH for polyacrylic acid of molecular weight of 1000 in a 1 molar (1M) aqueous solution of NaCl.

Figure 2 is a plot of the degree of ionization as a function of pH for phosphino-polyacrylic acid (MW = 1200) in 1M aqueous solution of NaCl.

Figure 3 contains plots of absorbance vs. content of phosphino-polyacrylic acid inhibitor in the absence of Fe (III), and in the presence of 25 ppm Fe (III) at a pH of 2.5. Figure 4 is a plot of absorbance as a function of content of polyacrylic acid inhibitor in the presence of Fe (III) obtained at pH of 1.0.

Figure 5 is a plot of absorbance as a function of content of phosphino-polyacrylic acid inhibitor in the presence of Cr (III) and Al (III) obtained at pH of 1.0.

Polyacrylic acid inhibitors which can be quantitatively analyzed in accordance with this invention include all homopolymers of alpha, beta-ethylenically unsaturated acid monomers, such as ⁵ acrylic acid or methacrylic acid; diacids, such as maleic acid (or maleic anhydride), itaconic acid, fumaric acid, mesoconic acid, citraconic acid; and monoesters of diacids with alkanols having 1-8 carbon atoms. The polyacrylic acid inhibitors may also be copolymers of unsaturated acid monomers, with any monomer ¹⁰ copolymerizable therewith, such as olefinic monomers with: (a) non-polar groups, e.g., styrene; (b) polar funtional groups, e.g., vinylacetate, vinyl chloride, vinyl alcohol, acrylate ester, vinylpyridine, vinyl pyrrolidone, acrylamide or acrylamide derivatives; and (c) ionic functional groups, e.g., styrenesulfonic ¹⁵ acid, 2-acrylamido-2-methylpropanesulfonic acid (AMPS), vinylsulfonic acid or vinylphosphonic acid. The copolymers are preferably copolymers of at least 50% by weight of acrylic acid, methacrylic acid or maleic anhydride and 50% or less by weight of the aforementioned different copolymerizable monomer. The ²⁰ polyacrylic acid inhibitor includes modifications of the polymers described above, such as phosphino-polyacrylic acid sold under the tradenames "Belsperse 161" or "Belasol S-29" by Ciba Geigy. The preferred polyacrylic acid inhibitor is phosphino-polyacrylic acid. The polyacrylic acid inhibitor which is quantitatively ²⁵ analyzed in accordance with this invention is in the form of an aqueous solution containing polyvalent cations of a valence of at least 3. Such cations are typically produced in situ in subteranean formations by, for example, corrosion of metal equipment or from minerals, such as siderite. «. ³⁰ The step of adjusting the pH of the solution which is essential to this invention is conducted in any conventional * manner. Usually, the pH of the sample to be analyzed is treated with an acid because the field-generated samples, e.g., from offshore oil exploration, usually have pH higher than 2.0, e.g., generally, a pH of between 5.0 and 9.0. The acid is any conventionally-known acid, e.g., hydrochloric, sulfuric or nitric acid, most preferably hydrochloric acid. The sample is contacted with a sufficient amount of the acid to lower pH thereof to a value of 0.5 to less than 2.0, preferably 0.5 to 1.9, more preferably 0.6 to 1.5 and most preferably about 0.9 to 1.2. As will be apparent to those skilled in the art, if the sample has pH lower than that of the desired value, it should be contacted with a suitable conventionally-known alkaline agent to adjust the pH thereof to within the range specified above. If particulates and oil are present after the pH of the sample has been adjusted to the desired range specified above, the sample can be filtered or the particulates and the oil separated by any conventional means.

The polyacrylic acid content analysis subsequent to the pH-adjustment step is conducted using a method similar to that described by Rothman in U.S. Patent 4,514,504. Broadly, the analysis steps comprise adsorbing the sample at a pH of 0.5 to less than 2.0 on a non-polar adsorbent, such as a non-polar, bonded phase silica gel or a rigid, macroreticular styrene-divinylbenzene polymer; desorbing the polyacrylic acid from the adsorbent with a displacement fluid, such as methanol or an aqueous sodium hydroxide; and determining the carboxyl content of the polyacrylic acid by the iron-thiocyanate method or by complexing with a cationic surfactant. The iron-thiocyanate method is based on the formation of a colorless complex between iron (III) and polyacrylic acids while the complex formed between iron (III) and thiocyanate ions is red. Therefore, a decrease in the color of iron-thiocyanate complex, upon complexing of iron with polyacrylic acids, is directly proportional to the polyacrylic acid concentration. After the iron (III) has been allowed to complex with the polyacrylic acids to form the colorless complex, potassium thiocyanate is added as an aqueous solution to the complex. The thiocyanate (SCN-) ions will react with non-complexed iron (III) ions to form the red iron-thiocyanate complex. Therefore, the color of the resulting solution can be correlated with the quantity of iron (III) and thiocyanate ion added to arrive at the concentration of the polyacrylic acids. The color of the resulting solution, in terms of the percentage of light transmitted therethrough, is an accurate measure of the polyacrylic acid concentration.

Without wishing to be bound by any theory of operability, it is believed that the method of this invention can be successfully used to measure the polyacrylic acid concentration in a solution because at the pH range of 0.5 to less than 2.0, polyacrylic acid is substantially not ionized in an aqueous solution and therefore it does not react with polyvalent cations having a valence equal to or

+3 greater than 3, such as Fe , present in the solution. In contrast, at a pH greater than about 2.0, the polyacrylic acid is ionized and it reacts with such polyvalent cations. This is illustrated in Figures 1 and 2 which show that polyacrylic acid (Figure 1) and phosphino - polyacrylic acid (Figure 2) are about 1 - 2.5% ionized at a pH of about 2. The previously-known method of Rothman can be used to determine the polyacrylic acid concentration at a pH of 2 to 3.5 in the absence of polyvalent cations of a valence of at least 3.0, such as in a simple brine system devoid of such cations. However, in the presence of such polyvalent cations, such as Fe⁺ , at a pH of 2.0 to 3.5, preferably at about 2.5, as specified by Rothman, it is believed that the ionized polyacrylic acid readily reacts with the Fe⁺ polyvalent cations which, in aqueous solutions at these pH levels, are thought to exist as tetravalent complexes, e.g., see F.A. Cotton $ G. Wilkinson, Basic Inorganic Chemistry, p. 110, J.Wiley $ Sons, New York 1976. The product of the reaction is not adsorbed selectively on non-polar adsorbents.

In contrast, at a pH of 0.5 to 2.0, the polyacrylic acid is not ionized and therefore it is not likely to react with the polyvalent cations or complexes thereof. Additionally, at such low pH values, it is believed that the polyvalent ions, such as ferric ions, are present as divalent complexes, which are not likely to react with the non-ionized polyacrylic acids (see Cotton and Wilkinson, supra). Therefore, the polyacrylic acids remain free in the solution and can be selectively adsorbed on the non-polar adsorbent for quantitative analysis in accordance with this invention.

The invention is illustrated by the following non-limiting examples in which all parts are by weight unless otherwise specified.

COMPARATIVE EXAMPLE A Belsperse 161, a commercial phosphino-polyacrylic acid from

Ciba Geigy was used as the model polyelectrolyte inhibitor. A master brine solution containing the following salts was prepared.

de-ionized l-LO to 1 liter

The prior art procedure of Rothman was conducted on 30 cc samples of standard solutions (containing 2, 5, 6 and 10 pp of the phosphino-polyacrylic acid in the master brine solution) as follows. 30cc of a standard solution was adjusted and buffered to _PH of 2.5. A non-polar LC cartridge (SEP-PA C18, a cartridge product from Waters Associates based on bonding octadecylsilane to silica gel) was pre-conditioned by eluting with 5 cc of a 60% solution of methanol in water followed by 20 cc of water at pH 2.5. The adsorption was accomplished by attaching the top of the LC cartridge to the fitting of a 3-way syringe valve. One end of the valve was connected to a 30 cc Luer-Lock syringe while the other end was attached to a flexible tubing used to aspirate or to discard liquids. After pre-rinsing the syringe with 5 cc of a sample solution, 20 cc of a sample solution was aspirated and pumped through the LC cartridge over at least 15 seconds to adsorb the

⁵ inhibitor. The eluted solution was discarded. The syringe was rinsed with 5 cc of de-ionized water and then with 5 cc of eluant (60% methanol solution in deionized water). 15 cc of the eluant was then aspirated and pumped through the LC cartridge over at least 45 seconds. The eluant was diluted with de-ionized water to a final

¹⁰ weight of 25 grams and pH of the resulting solution was adjusted to 3.4. A color reaction was started with the diluted eluant by adding 1 cc of reagent 1 (which contains ferric ions to complex with the inhibitor), then waiting 5 minutes before adding 1 cc of reagent 2 (which contains potassium thiocyanate). After 5 more minutes, the

15 absorbance of the thus-treated sample was measured at 480 nm using de-ionized water as reference. The response of the different standard solutions of the phosphino-polyacrylic acid present in the diluted eluant towards a ferric-potassium thiocyanate color reaction was determined by measuring absorbance at the aforementioned 480

20 nm. When no polymer is present, the sample exhibits the most red color due to the ferri-thiocyanate complex. The presence of the polymer reduces the intensity of the red color. The extent of reduction of the red color is a measure of the polymer concentration.

25 No Fe (III) or other trivalent cations were present in any of the aforementioned standard solutions. The analysis yielded a linear standard plot of absorbance as a function of the concentration (ppm) of the phosphino-polyacrylic acid, as shown in the lower plot in Figure 3.

_*

³⁰ COMPARATIVE EXAMPLE B

$ Samples containing trivalent ions were prepared by adding

25 ppm of Fe (III) to each of the standard solutions of the Comparative Example A. The procedure of Comparative Example A at pH 2.5 was conducted but it failed to detect the presence of any polymer (see the upper plot in Figure 3). This result suggests that Fe (III) forms a complex with the polymer, that the complex is positively charged and is not adsorbed on the non-polar adsorbent. Thus, the normal procedure is not capable of analyzing a carboxylated polyelectrolyte when a cation with a valence of 3, such as iron, is present.

EXAMPLE 1 This example illustrates the quantitative analysis of polyacrylic acid present in a solution which also contains trivalent ion, conducted according to this invention.

Standard solutions containing 0, 1, 3, 5 and 10 ppm of phosphino-polyacrylic acid in the master brine solution were prepared with 25 ppm of ferric ions in the manner of Comparative Example B. The key change implemented in this example, as compared to Comparative Examples A and B, was the adjustment of pH of each sample solution, prior to the LC separation, to a pH of 1.0 with 1.2 N hydrochloric acid. The LC separation and colorimetric analysis followed those described in Comparative Example A. At pH 1.0 for ⁰ the LC separation, a linear relationship was surprisingly found between ppm of phosphino-polyacrylic acid and absorbance at 480 nonameters, as shown in Figure 4. These results indicate that the LC separation was successful at pH 1.0 and that ferric ions did not interfere with the phosphino-polyacrylic acid absorption at pH 1.0, ^ as was the case in Comparative Example B at pH 2.5.

EXAMPLE 2 The LC separation at pH 1.0 of standard solutions similar to those of Example 1, but containing 10 ppm of Cr ⁺ or Al also yielded a linear relationship (Figure 5), indicating that at 0 this pH these trivalent ions also do not interfere with the selective adsorption or separation of the phosphino-polyacrylic acid.

Claims

CLAIMS :

1. A method for determining the concentration of polyacrylic acid in a solution containing a polyvalent cation which has a valence of at least 3, and is capable of reacting with the polyacrylic acid, comprising:

(a) adjusting the pH of the solution to a value of 0.5 to less than 2; and then

(b) measuring the concentration of the polyacrylic acid in the solution.

2. The method of Claim 1 wherein the pH of the solution is adjusted to a value of 0.5 to 1.9.

3. The method of Claim 2 wherein the pH of the solution is adjusted to a value of 0.6 to 1.5.

4. The method of Claim 3 wherein the pH of the solution is adjusted to a value of 0.9 to 1.2.

5. The method of Claim 1 wherein the polyvalent cation is selected from the group consisting of Fe , Cr and Al .

6. The method of Claim 1 wherein the polyacrylic acid is a modified or unmodified water-soluble polymer selected from homopolymers of acrylic acid, methacrylic acid, and maleic acid, and copolymers of at least 50% by weight of acrylic acid, methacrylic acid or maleic acid and 50% or less by weight of a different copolymerizable monomer.

7. The method of claim 1 wherein the measuring step (b) includes the further steps of:

(i) adsorbing the polyacrylic acid on a non-polar adsorbent; (ii) desorbing the polyacrylic acid from the adsorbent with a displacement fluid; and (iii) measuring the concentration of the polyacrylic acid desorbed from the adsorbent.