REMOVAL OF CHELATED IRON FROM PRODUCED WATER

Abstract

Method of decomposing high molecular weight polymer downhole to prevent chelation of iron by residual high molecular weight polymer thereby producing flowback without iron contamination as chelated iron. A secondary method is also described to treat iron chelated produced water with oxidants at surface conditions, utilizing aluminum electrolytes, specifically low basicity polyaluminum chloride, to either co-precipitate residual polymer and bound iron, or to substitute chelated iron with aluminum in the polymer-metal complex, resulting in liberating of iron to enable neutral pH oxidation and removal by precipitation, coagulation, flocculation and physical separation. The produced water with removed iron can be then stored or re-used for other oilfield applications.

Claims

1. A method of cleaning produced water from a reservoir, said method comprising: a) testing a water produced from a hydrocarbon reservoir (produced water) to confirm that iron is chelated by a high molecular weight (MW) polymer in said produced water, and if so treating said produced water having polymer chelated iron at a temperature of 55? C. or greater with an amount of an oxidant for a period of time sufficient to convert said polymer to polymer remnants that no longer chelate iron and producing free iron; b) oxidizing said free iron by neutral pH oxidation; and c) removing said polymer remnants and said iron to produce cleaned produced water.

2. The method of claim 1, wherein said oxidant is selected from the group consisting of H.sub.2O.sub.2, sodium persulfate, ammonium persulfate, sodium perborate, or combinations thereof.

3. The method of claim 1, wherein said oxidant is sodium- or ammonium-persulfate.

4. The method of claim 1, wherein step b is performed downhole.

5. The method of claim 1, wherein step b is performed at the surface.

6. The method of claim 1, wherein said high molecular polymer is selected from the group consisting of acrylamido-methylpropane sulfonate polymer (AMPS), polyacrylamide (PAM), polyacrylic acid (PAA) and partially hydrolyzed polyacrylamide (PHPA), or copolymers thereof.

7. The method of claim 1, wherein said high MW polymer is AMPS or copolymers thereof.

8. The method of claim 1, further comprising step b2, repeating said testing step a) to confirm that at least 80% of said high MW polymer is converted to polymer remnants before proceeding to step c, and if less than 80% is converted, then increasing said period of time, said temperature, said amount, or combinations thereof.

9. The method of claim 1, wherein said oxidant is sodium- or ammonium-persulfate, said polymer is AMPS, and said temperature is at least 70? C.

10. The method of claim 1, wherein said removal step c) methodology is selected from one or more of precipitation, flotation, centrifugation, decantation, filtration, or combinations thereof.

11. A method of cleaning produced water, comprising: a) treating a produced water having iron-polymer chelates with an aluminum compound at surface conditions to free iron; b) oxidizing said free iron by neutral pH oxidation; and c) removal of said oxidized iron to produce cleaned produced water.

12. The method of claim 11, wherein said aluminum compound is selected from the group consisting of polyaluminum chloride, aluminum chlorohydrate, and polyaluminum sulfate.

13. The method of claim 11, wherein said aluminum compound is polyaluminum chloride, and the pH is maintained at pH 7.0+/?0.25.

14. The method of claim 11, wherein said polymer in iron-polymer chelates is selected from the group consisting of acrylamido-methylpropane sulfonate polymer (AMPS), polyacrylamide (PAM), polyacrylic acid (PAA) and partially hydrolyzed polyacrylamide (PHPA), or copolymers thereof.

15. The method of claim 11, wherein said polymer in iron-polymer chelates is AMPS or copolymers thereof.

16. The method of claim 11, further comprising a pre-testing step i) to confirm that said produced water has iron-polymer chelates.

17. The method of claim 11, further comprising a post-testing step b2) to confirm that said at least 80% of iron is free iron.

18. The method of claim 11, wherein said removal step c) methodology is selected from one or more of precipitation, flotation, centrifugation, decantation, filtration, or combinations thereof.

19. A method of hydraulic fracturing, comprising: a) testing a polymer suitable for use in hydraulic fracturing to determine said polymer will chelate iron; b) testing an oxidant for said polymer to determine if it will degrade said polymer at reservoir temperatures to form polymer fragments that will not chelate iron; c) fracturing a reservoir with a hydraulic fracturing fluid comprising water, said polymer and said oxidant; d) waiting a period of time to degrade said polymer to polymer fragments that no longer chelate iron; e) producing oil and water from said reservoir and separating said produced oil from produced water; f) optionally testing said produced water to confirm that iron is free iron, and if a portion of said iron remains chelated, adding more oxidant to further degrade said polymer; g) treating said produced water to oxidize free iron by neutral pH oxidation; and h) removal of said oxidized free iron.

20. The method of claim 19, wherein said removal step h) methodology is selected from one or more of precipitation, flotation, centrifugation, decantation, filtration, or combinations thereof.

21. A method of hydraulic fracturing, comprising: a) fracturing a reservoir with a hydraulic fracturing fluid comprising water and a polymer that can form iron-polymer chelates; b) producing oil and water from said reservoir and separating said produced oil from produced water; c) treating said produced water having iron-polymer chelates with an aluminum compound to form free iron; d) treating said produced water to oxidize free iron by neutral pH oxidation; and e) removal of said oxidized free iron.

22. The method of claim 11, wherein said aluminum compound is selected from the group consisting of polyaluminum chloride, aluminum chlorohydrate, and polyaluminum sulfate.

23. A method of cleaning produced water, comprising: a) treating a produced water that has been determined to have iron-polymer chelates with either an aluminum compound at surface conditions to free iron or an oxidant at a temperature of at least 55? C. to free iron; b) oxidizing said free iron by neutral pH oxidation; and c) removal of said oxidized iron to produce cleaned produced water.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0075] FIG. 1 Reaction of iron to form ferric oxide and ferric hydroxide.

[0076] FIG. 2. Hydraulic fracturing fluids.

[0077] FIG. 3. Some HVFR polymers.

[0078] FIG. 4. Iron-PAA chelate structure.

[0079] FIG. 5. Iron floc suspended in produced water.

[0080] FIG. 6. Plot of total iron concentration versus time showing degradation in iron treatability associated with flowback of wells fractured with AMPS copolymer and treated produced water.

[0081] FIG. 7. Plot of pH versus time compared with plot of residual H.sub.2O.sub.2 versus time.

[0082] FIG. 8. HPLC trace of produced water from various wells indicating the presence of high MW polymers.

[0083] FIG. 9A. Bottle test result for A well after treatment for iron.

[0084] FIG. 9B. Bottle test result for B well after treatment for iron.

[0085] FIG. 10A. Bottle test result for produced water (sample C) at varying pH after addition of aluminum sulfate for coagulation.

[0086] FIG. 10B HPLC trace indicating high MW polymer presence.

[0087] FIG. 10C Aluminum dosage v. cation concentration.

[0088] FIG. 11A-B. HPLC trace for thermal testing of produced water with AMPS co-polymer with no added breaker (sample C).

[0089] FIG. 12A. Bottle test result for produced water with 1 L/m.sup.3 AMPS co-polymer, and FIG. 12B with 3 L/m.sup.3 AMPS co-polymer without added breaker.

[0090] FIG. 13A-B. HPLC trace for thermal testing of produced water with AMPS co-polymer with 3.8 g/m.sup.3 of sodium perborate breaker.

[0091] FIG. 14A. Bottle test result for produced water with 1 L/m.sup.3 AMPS co-polymer and FIG. 14B with 3 L/m.sup.3 AMPS co-polymer with sodium perborate breaker.

[0092] FIG. 15A-B. HPLC trace for thermal testing of produced water with AMPS co-polymer with 3.8 g/m.sup.3 of ammonium persulfate breaker.

[0093] FIG. 16A. Bottle test result for produced water with 1 L/m.sup.3 AMPS co-polymer and FIG. 16B with 3 L/m.sup.3 AMPS co-polymer with ammonium persulfate breaker.

[0094] FIG. 17A-B. HPLC trace for thermal testing of produced water with AMPS co-polymer with 0.76 g/m.sup.3 of H.sub.2O.sub.2 breaker.

[0095] FIG. 18A. Bottle test result for produced water with 1 L/m.sup.3 AMPS co-polymer and FIG. 18B with 3 L/m.sup.3 AMPS co-polymer with H.sub.2O.sub.2 breaker.

[0096] FIG. 19. A typical benchtop mini flowloop set-up.

[0097] FIG. 20. Mini flowloop test result.

[0098] FIG. 21. Flowloop test resulttemperature ramp.

DETAILED DESCRIPTION

[0099] The presence of high MW polymer in the produced water from a North American unconventional play was measured by high performance liquid chromatography (HPLC) size exclusion (SE) chromatography. Wells hydraulically fractured using typical anionic polyacrylamide friction reducers and fresh water did not show evidence of high MW polymer presence in produced water. However, wells hydraulically fractured using produced water or mixed fresh and produced water with salt tolerant AMPS copolymer were found to contain elevated concentrations of high MW polymer residue, even several years after initial flowback. Iron in the produced water is readily chelated by residual AMPS, making removal of iron from the produced water challenging and in turn causing produced water storage and re-use difficult.

[0100] To understand iron-chelate problem in produced water, it was first necessary to ascertain that iron was indeed chelating with polymers from the produced water. All experiments were conducted at benchtop for proof of concept.

[0101] Produced water samples from wells at an unconventional play were treated in the lab using pH adjustment (sodium hydroxide pH neutralization) and chemical oxidation using hydrogen peroxide (H.sub.2O.sub.2), and then filtered with a 0.45-micron Millipore filter. The filtrate liquid was visually observed for particles and iron and dissolved iron was measured using inductively coupled plasma (ICP) optical emissions spectroscopy (OES).

[0102] FIG. 6 shows a plot of total iron concentration in produced water versus time for produced water entering a central processing facility. During the initial time-period, the central processing facility was treating produced water from wells that were hydraulically fractured using an anionic polyacrylamide friction reducer, sodium perborate breaker and fresh surface water. During this time, it was observed that the produced water was readily treatable for iron removal by neutral pH chemical oxidation, as seen from the iron concentration levels of <5 mg/L for treated produced water between January and July 2020.

[0103] In August 2020, however, flowback commenced from a new wellpad, for which a mix of fresh and treated produced water had been utilized for hydraulic fracturing. The stages fractured using treated produced water made use of 2-acrylamido-2-methylpropane sulfonic acid (AMPS) copolymer friction reducer at up to 3 L/m.sup.3 loading, with no oxidant breaker employed. Following flowback of this second multiwell pad, the total iron concentration in the treated produced water began to increase as shown in the plot between August 2020 to March 2021. The iron concentration for this period can be seen at about 20 mg/L, stabilizing to be equal to the influent iron concentration.

[0104] The pH trace of the produced water after chemical oxidation using H.sub.2O.sub.2 is shown in FIG. 7 and compared with residual H.sub.2O.sub.2 in solution after oxidation. After the addition of H.sub.2O.sub.2, the pH of the solution should reduce as Fe(OH).sub.3 precipitates. Data between February till August 2020 showed this reduction of pH trend, but once the influent produced water contained flowback water from the wells fractured with AMPS copolymer friction reducer, the pH of solution remained basic, indicating that the H.sub.2O.sub.2 was not being consumed for oxidation, as would be expected by the ?50 mg/L residual H.sub.2O.sub.2 in solution.

[0105] Produced water from wells that were hydraulically fractured using AMPS co-polymer without the use of an oxidant breaker were found to contain a high concentration of dissolved iron (>10 mg/L) post treatment. The clarity of the supernatant water was poor. The iron particles passed through the 0.45-micron filter, thus no visible iron oxide/hydroxide floc was formed.

[0106] Next the raw produced water that was used to formulate the water samples described above were analyzed using SE-HPLC. The analysis indicated distinct differences between the samples that were treatable for iron removal and those that were not, as shown in FIG. 8. The treatable samples contained a clear chromatogram response at 8 to 10 minutes column residence time that could be attributed to typical inorganic cations and anions in solution, with no peaks at lower residence time that could be attributed to higher MW organic compounds.

[0107] In contrast, analysis of the samples that could not be readily treated for iron removal using chemical oxidation showed a HPLC detector response peak at around 6 to 7 minutes residence time, attributed to presence of high molecular weight (MW) polymers in the order of about 3 to 5 million Daltons. Based on the propensity for AMPS to bind with iron and the stability of the salt tolerant friction reducers, the root cause of iron removal issues is directly related to excess friction reducer polymer fragments present in the produced water.

[0108] Efficiency of oxidant breakers in downhole degradation of AMPS co-polymer was also evaluated at reservoir temperature (70? C. in this case). Control experiment with no breaker in produced water was tested along with addition of oxidant breakers like persulfate, perborate, stabilized H.sub.2O.sub.2, etc. that were dosed in at 1.9-11.4 g/m.sup.3 of total solution volume.

[0109] To study the fate of iron in the hydraulic fracturing fluid-friction reducer matrix, ferric chloride was dosed to each sample solution (60 mg/L Fe equivalent) after a hold time of 5 days at 70? C. Weekly analysis indicated substantial change to polymer residue MW after the samples were spiked with ferric chloride. The polymer breakdown was rapid compared to samples that were only treated with breakers and not spiked with ferric chloride. Visual observation also showed that the samples spiked with ferric chloride had varying degrees of polymer flocculation. The addition of ferric chloride as a sudden rapid dose may have artificially accelerated polymer degradation beyond what would be observed in the reservoir naturally. Therefore, in future work it was added more slowly.

[0110] Following four weeks of hold time at 70? C., with three of those weeks after dosing of ferric chloride, the fluid in each jar was treated for iron removal by adjusting the pH to neutral and adding hydrogen peroxide at 100 mg/L. The supernatant water was filtered through a 0.45-micron membrane filtration, before analysis by ICP OES for dissolved iron concentration. The results show that ammonium persulfate breaker was the most effective for iron removal by neutral pH chemical oxidation, with <5 mg/L residual iron present after treatment even with up to 3 L/m.sup.3 AMPS copolymer initial fluid loading.

[0111] A novel combination of static breaker testing, and rock-water interaction testing was also conducted to test the action of oxidant breakers on AMPS co-polymer under downhole degradation conditions. The purpose of adopting this combination tests was to simulate dissolution of iron and other ions from rock (in place of ferric chloride dosing), intended to better balance the kinetics of polymer degradation by oxidation and cation/anion dissolution. The combination testing was conducted to optimize oxidant breaker dosage for varying AMPS co-polymer loadings and to provide a customized chemical package to minimize chemical cost and ensuring AMPS co-polymer degradation to avoid iron chelation in produced water.

[0112] The basic testing procedure was: [0113] a. An oxidant breaker was added to the hydraulic fracturing fluid at ambient surface conditions. [0114] b. Samples were removed at regular intervals to analyze the inorganic composition by ICP OES and the polymer molecular weight distribution by SEC-HPLC. [0115] c. At the conclusion of the experiment, once the rate of polymer MW changes had significantly slowed, the supernatant fluid was separated from solids, and treated by pH neutralization with sodium hydroxide, followed by oxidation using hydrogen peroxide. [0116] d. The iron floc was removed by mechanical methods, leaving clear produced water, with low dissolved iron (<5 mg/L). [0117] e. In some variations to this method, additives such as sand, rock (drill cuttings), hydrochloric acid for pH adjustment, chemical oxygen scavenger or nitrogen for oxygen stripping were added to mimic downhole conditions and mitigate oxidation of ferrous iron in the aqueous solution.

[0118] When treating produced water at surface conditions (e.g., room temperature) containing iron and residual AMPS copolymer, addition of oxidants such as hydrogen peroxide and Fenton's reagent, was found to be ineffective, and a significant amount of soluble/complexed iron was left in solution.

[0119] However, for surface ambient condition applications, aluminum compounds, such as aluminum polyelectrolytes, aluminum salts, including aluminum sulfate, and polyaluminum coagulants, including polyaluminum chloride (PAC) or aluminum chlorohydrate, were found to be effective in removing iron or at least making iron accessible for removal by oxidation at neutral pH. The decomposed polymer and iron floc were removed by one or more industrially accepted mechanical treatment methods, such as precipitation, centrifugation, filtration, decantation, flotation, sedimentation or combinations thereof. Thus, this treatment led to effective removal of polymer residue and iron in the form of iron floc from produced water.

[0120] For tests to degrade high MW AMPS copolymers into smaller fragments to prevent iron chelation in produced water at formation temperatures, the tests were conducted at 70? C. Oxidants tested for their ability to degrade AMPs copolymer at reservoir conditions included sodium perborate, stabilized hydrogen peroxide, ammonium persulfate and sodium persulfate. Breakers may also include sodium perborate tetrahydrate and solubility increasing boron complex or ester forming compounds; enzymatic breakers such as hydrolases (if the enzymes are sufficiently stable in PW); inorganic peroxide particles, such as calcium peroxide or magnesium peroxide, and the like.

[0121] Under the reservoir testing conditions, sodium or ammonium persulfate were found to be effective for reducing residual polymer MW, and thus preventing or reversing iron chelation with polymers. These oxidants were found to be effective for polymer decomposition at temperatures of 55 to about 75? C. or higher. Effectiveness was found to be dependent on the presence of dissolved iron.

[0122] The above experiments are discussed in more detail in the following examples:

1: Produced Water Treatment at Surface Conditions

[0123] Produced water treatment for removal of dissolved iron on benchtop is carried out as described below: [0124] a. To a 100 mL solution of produced water, 50% NaOH solution was added to achieve a target pH of 7-7.5. [0125] b. H.sub.2O.sub.2 addition was carried out with a solution of 3 wt. % H.sub.2O.sub.2 to achieve residual H.sub.2O.sub.2 of 25 mg/L in solution. [0126] c. After 5 minutes reaction time, the solution was filtered to achieve clear supernatant liquid. HPLC trace of the supernatant liquid was carried out to determine the residual polymer MW, with ICP OES used to measure for residual dissolved/chelated iron.

[0127] Samples where high MW polymer was detected were difficult to treat for iron removal by oxidation, indicating that the polymers had to be broken down into smaller fragments to successfully remove iron from the solution. The presence of iron that can be passed through a 0.45-micron pore size filter despite the fluid conditions being near neutral pH with an excess of hydrogen peroxide oxidant present indicates that the iron is chelated.

[0128] In general, commercially available sodium perborate tetrahydrate, sodium persulfate, ammonium persulfate or stabilized hydrogen peroxide or any other polymer breaker may be used for downhole polymer degradation. A commercially available AMPS copolymer was used for all experiments described herein.

[0129] Table 1 shows the result of produced water treatability and iron removal of two samples, referred to from this point on as A well and B well, after treatment with pH neutralization and addition of oxidant H.sub.2O.sub.2. HPLC was used to determine the presence of high molecular weight polymers in the samples after oxidant treatment. With the presence of high MW polymer in the sample (B well), removal of iron from solution was poorer (12.566 mg/L residual Fe) compared to the sample that contained low MW polymers (A well), which showed significant removal of iron (0.007 mg/L residual Fe).

[0130] FIG. 9A shows the bottle test with a clear supernatant and precipitated iron floc for A well PW test, FIG. 9B shows the bottle test result with hazy supernatant for the B well.

TABLE-US-00002 TABLE 1 Residual Fe after treatment with oxidant H.sub.2O.sub.2 High MW H.sub.2O.sub.2 Residual polymer Dosage dissolved Fe Clear Well present (mg/L) (mg/L) Supernatant A No 100 0.007 Yes B Yes 100 12.566 No

[0131] To test alternative methods for removal of iron chelated by high MW polymer, Fenton's reagent catalyst was added with oxidant. The catalyst, containing complexed iron, is used to catalyze hydrogen peroxide degradation and free radical formation. The free radical hydroxide and hydroperoxyl formed by catalytical degradation of hydrogen peroxide are powerful oxidants, having a significantly higher oxidation potential than hydrogen peroxide alone. This was found to be ineffective for pH ranging from 3-8 and improved slightly in performance at pH 8, but higher pH can cause calcite scale and thus, this application is not preferred. Addition of ferric chloride coupled with H.sub.2O.sub.2 was also found to be ineffective for polymer degradation at all ranges of pH. Oxidation of the residual high MW polymer for the purpose of enabling removal of iron by oxidation and precipitation at surface conditions was found to be ineffective at surface ambient conditions.

[0132] In bench testing, aluminum sulfate dosing to AMPS copolymer contaminated iron rich produced water was also tried (FIG. 10A) and found effective for removal of dissolved/chelated iron at high dosages. However, extremely high doses of aluminum sulfate were required, rendering it less practical due in part to the excessively high solids generation and depression of the aqueous pH. At pH of 7.3, significant precipitation of solids was observed, and the supernatant was not clear even after prolonged sedimentation.

[0133] Additional testing using aluminum polyelectrolytes in place of aluminum sulfate was performed. Aluminum coagulants of varying basicity were tested from <10% to almost 50% basicity. There was a strong correlation between coagulant basicity and residual iron concentration following bench scale treatment, whereby each coagulant was dosed to produced water at ranges from zero to 1000 ppmv, followed by pH neutralization, hydrogen peroxide dosing and flocculation of particles using anionic polyacrylamide flocculant.

[0134] The effect of low basicity aluminum coagulant dosing was further studied by varying the amount of 9.95% basicity aluminum coagulant, monitoring the residual friction reducer concentration and molecular weight along with residual chelated iron concentration. The results are summarized in Table 2:

TABLE-US-00003 TABLE 2 Varying dosage of 9.95% basicity aluminum coagulant Supernatant Dosage of pH Dosage of taken aluminum after 1N NaOH to Dosage of Residual Test from Jar pH coagulant coagulant get to pH peroxide Dosage of final Iron Polymer Jar # initial (ppm) dosage 7.5 pH (ppmv) flocculant pH (mg/L) concentration 1 4 7.393 0 NA NA 7.393 80 10 7.393 8.32 same as before 2 4 7.303 300 5.371 0.2 7.532 80 10 7.532 7.74 half 3 4 7.396 600 4.663 0.5 7.48 80 10 7.51 N.D. less than 100k MW

[0135] The residual polymer concentration and relative molecular weight is shown in FIG. 10B.

[0136] Furthermore, the residual dissolved aluminum concentration (passing through 0.45-micron filter) was also tracked under varying coagulant dosage, shown in FIG. 10C. Under the condition of 20 mg/l Al.sup.3+ dosage, the dissolved aluminum concentration is observed to significantly exceed the aluminum saturation concentration under neutral pH conditions, indicating that aluminum is chelating with residual AMPS co-polymer. Furthermore, once the break point aluminum dosage is achieved, the residual chelated iron and aluminum concentrations both decrease to near zero, indicating charge neutralization of the residual AMPS co-polymer may have occurred, with either co-precipitation of the AMPS co-polymer and aluminum, or co-precipitation of the polymer and aluminum and release of complexed iron.

2: Friction Reducer Polymer Thermal Degradation

[0137] As alternative to chemical treatment of AMPS co-polymer contaminated produced water at surface conditions using aluminum coagulant dosing, thermal degradation of high MW polymer without the use of any breaker was evaluated at reservoir temperature. For these reactions, the following procedure was followed: [0138] a. Produced water from A well was treated (as described in experiment 1) to remove dissolved iron from it. After treatment, <5 mg/L dissolved iron produced water sample was obtained. [0139] b. 1 L/m.sup.3 AMPS co-polymer was added to 1000 mL of produced water stripped of iron in one beaker and 3 L/m.sup.3 AMPS co-polymer was added to another 1000 mL beaker of iron-stripped produced water. [0140] c. These solutions were held in a water bath at 70? C. for 25 days. [0141] d. After five days, both beakers were dosed with ferric chloride, equivalent to 60 mg/L as Fe. [0142] e. HPLC SEC was conducted for the produced water sample periodically at 5 days, 12 days, 17 days, and 25 days to map the reduction of high MW at the end of 25 days. [0143] f. The final produced water solution was treated for iron removal (described in experiment 1) to measure the amount of dissolved iron at the end of 25 days attributable to the reduction of high MW polymers, thus preventing iron chelation, and causing easy floc formation and removal of iron.

[0144] Table 3 shows the result of the thermal degradation of polymers. It was observed that at higher AMPS co-polymer loading, more residual polymer was left in solution, resulting in more chelated iron presence, as expected. When the PW+AMPS co-polymer solution was treated for iron removal, 0 mg/L Fe was found post 4 weeks at 70? C. with 1 L/m.sup.3 AMPS co-polymer, whereas 10 mg/L of Fe was in solution after 4 weeks at 70? C. with 3 L/m.sup.3 AMPS co-polymer, indicating the AMPS co-polymer iron chelation was insignificant at low loading under ideal conditions. 98.3% MW reduction in polymer was observed in samples containing 1 L/m.sup.3 AMPS co-polymer at the end of 25 days, whereas 87.3% MW reduction was observed in samples containing 3 L/m.sup.3 AMPS co-polymer.

TABLE-US-00004 TABLE 3 Thermal Degradation of polymer (No breaker) Number Residual of days Residual friction chelated at test Temp. reducer loading iron No. Sample Composition condition (? C.) (L/m3) (mg/L) 1 PW + 1 L/m.sup.3 AMPS co-polymer 5 RT 0.98 2 PW + 1 L/m.sup.3 AMPS co-polymer 5 70 1.1 3 PW + 1 L/m.sup.3 AMPS co-polymer 12 70 0.82 4 PW + 1 L/m.sup.3 AMPS co-polymer 17 70 0.42 5 PW + 1 L/m.sup.3 AMPS co-polymer 25 70 0.34 0 mg/L 6 PW + 3 L/m.sup.3 AMPS co-polymer 5 70 2.96 7 PW + 3 L/m.sup.3 AMPS co-polymer 12 70 2.32 8 PW + 3 L/m.sup.3 AMPS co-polymer 17 70 2.02 9 PW + 3 L/m.sup.3 AMPS co-polymer 25 70 2.26 10 mg/L

[0145] HPLC trace of each of the samples 1-9 in Table 3 is presented in FIG. 11A-B. Bottle test results for the samples with 1 L/m.sup.3 and 3 L/m.sup.3 AMPS co-polymer samples are shown in FIG. 12A and FIG. 12B, respectively. Interestingly, immediately following dosage of ferric chloride, a significant change in residual polymer MW was observed for all experiments, indicating that the presence of iron in solution or sudden lowering of fluid pH may result in polymer MW reduction. Further testing performed by adding iron to solution by near neutral pH dissolution of steel in the absence of oxygen showed a similar reduction in polymer MW, indicating that presence of iron, rather than pH, is the key driver for the shift in polymer MW.

3: Friction Reducer Polymer Thermal Degradation (with Breaker)

[0146] Static breaker testing was also carried out. Produced water from A well was used after removal of all iron from it, in a procedure as described in experiment 2. The iron free produced water was dosed with AMPS copolymer and a variety of oxidant breaker chemistries to study the impact of oxidant addition on the reduction of polymer MW and ability to eliminate iron chelation. The breakers were chosen from sodium perborate, ammonium persulfate and stabilized 7% H.sub.2O.sub.2 solution. The breaker concentration was added at 0.06 to 0.144 kg/m.sup.3 of total volume of solution. This solution was held in a water bath at 70? C. for 25 days.

[0147] The addition of ferric chloride may artificially accelerate polymer degradation as shown in experiment 2, thus after the solutions were allowed to rest at 70? C. for the first 5 days, 60 mg/L of ferric chloride as Fe was slowly added. SEC-HPLC was conducted after 5 days (prior to adding any ferric chloride), 12 days, 17 days, and 25 days of the start of the experiment.

[0148] The data from the static tests with 0.1 kg/m.sup.3 sodium perborate breaker is presented in Table 4. 98.8% polymer MW reduction was observed in solutions with 1 L/m.sup.3 AMPS co-polymer samples. With 3 L/m.sup.3 AMPS co-polymer, 87.3% polymer MW reduction was observed.

TABLE-US-00005 TABLE 4 Thermal Degradation of polymer (With breaker - 0.1 kg/m.sup.3 of sodium perborate) Residual friction No. of days reducer Sam- at test Temp loading ple Composition condition (? C.) (L/m3) 1 PW + 1 L/m.sup.3 AMPS co-polymer 5 70 1.54 2 PW + 1 L/m.sup.3 AMPS co-polymer 12 70 0.39 3 PW + 1 L/m.sup.3 AMPS co-polymer 17 70 0.69 4 PW + 1 L/m.sup.3 AMPS co-polymer 25 70 0.56 5 PW + 3 L/m.sup.3 AMPS co-polymer 5 70 3.82 6 PW + 3 L/m.sup.3 AMPS co-polymer 12 70 0.59 7 PW + 3 L/m.sup.3 AMPS co-polymer 17 70 2.78 8 PW + 3 L/m.sup.3 AMPS co-polymer 25 70 3.43

[0149] FIG. 13A-B presents the HPLC trace for samples from Table 4 showing reduction of high MW polymer over time. In samples containing 1 L/m.sup.3 AMPS co-polymer, 0 mg/L Fe was observed after iron treatment at the end of 25 days. In samples containing 3 L/m.sup.3 AMPS co-polymer, 34 mg/L residual Fe was observed post treatment after 25 days. Polymer and iron precipitation and settling out at low polymer loading was more efficient. Excessive chelation of iron was found at high concentration of high MW polymers, as expected. Bottle test results for sample with 1 L/m.sup.3 AMPS co-polymer and 3 L/m.sup.3 AMPS co-polymer is shown in FIGS. 14A and 14B, respectively.

[0150] The static tests with breaker were repeated using 0.1 kg/m.sup.3 ammonium persulfate. The data for the tests with ammonium persulfate breaker is presented in Table 5. With low MW loading experiments, no iron was observed at the end of 4 weeks at 70? C. post treatment. No iron was found for samples containing higher loading of AMPS co-polymer polymers as well, indicating effective elimination of iron chelation at both 1 L/m.sup.3 and 3 L/m.sup.3 polymer loading. Ammonium persulfate was thus found to be more efficient for higher polymer loading for breakdown of polymer and removal of iron by chelation.

TABLE-US-00006 TABLE 5 Thermal Degradation of polymer (With breaker - 0.1 kg/m.sup.3 ammonium persulfate) Residual friction No. of days reducer Sam- at test Temp loading ple Composition condition (? C.) (L/m3) 1 PW + 1 L/m.sup.3 AMPS co-polymer 5 70 1.62 2 PW + 1 L/m.sup.3 AMPS co-polymer 12 70 0.07 3 PW + 1 L/m.sup.3 AMPS co-polymer 17 70 0.03 4 PW + 1 L/m.sup.3 AMPS co-polymer 25 70 0.13 5 PW + 3 L/m.sup.3 AMPS co-polymer 5 70 3.51 6 PW + 3 L/m.sup.3 AMPS co-polymer 12 70 0.18 7 PW + 3 L/m.sup.3 AMPS co-polymer 17 70 1.23 8 PW + 3 L/m.sup.3 AMPS co-polymer 25 70 1.35

[0151] The HPLC trace for samples in Table 5 is presented FIG. 15A-B for 1 L/m.sup.3 and 3 L/m.sup.3 AMPS co-polymer polymer loading, respectively. Bottle test result after iron treatment showing clear produced water for both 1 L/m.sup.3 and 3 L/m.sup.3 AMPS co-polymer polymer loading is shown in FIGS. 16A and 16B.

[0152] The static tests with breaker were also conducted with 0.2 L/m.sup.3 H.sub.2O.sub.2 solution, results of which are presented in Table 6 below. 95.7% reduction in MW was observed with 1 L/m.sup.3 HVFR loaded samples, and a MW reduction of 96.6% for 3 L/m.sup.3 HVFR loaded samples. The HPLC trace is shown in FIG. 17A-B. The PW after treatment was found to contain 0 mg/L dissolved iron after 25 days in samples with 1 L/m.sup.3 HVFR, and 33 mg/L chelated iron was found in samples with 3 L/m.sup.3 HVFR. At higher polymer loading, the performance of the H.sub.2O.sub.2 breaker was poor. The bottle tests also shown in FIGS. 18A and 18B display suspended solids in the PW with low and high polymer loading making the use of H.sub.2O.sub.2 breaker less preferred.

TABLE-US-00007 TABLE 6 Thermal Degradation of polymer (With breaker - 0.2 L/m.sup.3 H.sub.2O.sub.2) Residual friction No. of days reducer Sam- at test Temp loading ple Composition condition (? C.) (L/m3) 1 PW + 1 L/m.sup.3 AMPS co-polymer 5 70 1.63 2 PW + 1 L/m.sup.3 AMPS co-polymer 12 70 0.02 3 PW + 1 L/m.sup.3 AMPS co-polymer 17 70 0.27 4 PW + 1 L/m.sup.3 AMPS co-polymer 25 70 0.50 5 PW + 3 L/m.sup.3 AMPS co-polymer 5 70 3.70 6 PW + 3 L/m.sup.3 AMPS co-polymer 12 70 0.15 7 PW + 3 L/m.sup.3 AMPS co-polymer 17 70 2.73 8 PW + 3 L/m.sup.3 AMPS co-polymer 25 70 2.16

[0153] A comparison of residual iron present with different breakers used is shown in Table 7. From the tabulated result, it can be inferred that at low concentration of polymer, improved degradation of polymer was achieved with all breakers as well as in thermal degradation test without a breaker. Ammonium persulfate performed well in reducing high MW polymer at both low and high polymer loading, with clear supernatant after treatment. H.sub.2O.sub.2 was the least preferred breaker for this application, as the resulting supernatant after treatment for iron was hazy for both low and high MW polymer loading.

TABLE-US-00008 TABLE 7 Thermal Degradation of polymer - Comparison of breakers post treatment after 25 days at 70? C. Fe conc MW (mg/L) Breaker Reduction post Sample Composition used (%) treatment Supernatant 1 PW + 1 L/m.sup.3 AMPS co-polymer No 97.3 0 Clear 2 PW + 1 L/m.sup.3 AMPS co-polymer Sodium 98.8 0 Clear perborate 3 PW + 1 L/m.sup.3 AMPS co-polymer Ammonium 99.5 0 Clear persulfate 4 PW + 1 L/m.sup.3 AMPS co-polymer H.sub.2O.sub.2 95.7 0 Hazy 5 PW + 3 L/m.sup.3 AMPS co-polymer No 87.3 10 Hazy 6 PW + 3 L/m.sup.3 AMPS co-polymer Sodium 87.3 34 Hazy perborate 7 PW + 3 L/m.sup.3 AMPS co-polymer Ammonium 96.1 0 Clear persulfate 8 PW + 3 L/m.sup.3 AMPS co-polymer H.sub.2O.sub.2 96.6 33 Hazy

4: Friction Flow Loop Testing

[0154] Friction flow loop testing was performed to assess the impact of oxidant addition on polymer friction reduction performance from ambient temperature to downhole conditions. This testing is necessary as excessive polymer degradation at surface conditions may result in poor friction reduction, increasing wellhead pressures and limiting fluid rates during hydraulic fracturing.

[0155] Ideally, the breaker would be inactive until the hydraulic fracturing fluid has flowed through the wellbore and placed the proppant into the rock fractures. Sodium perborate and sodium persulfate breakers were used along with AMPS co-polymer friction reducer to simulate formation conditions in terms of friction and shear provided during downhole fluid movement. A benchtop miniloop instrument with 0.18-inch inner diameter loop was used, with an initial high shear rate held for 12 minutes to represent fluid flowing down the wellbore, and a further 10 minutes at lower shear rate to represent fluid flow through rock fractures. A typical benchtop miniloop is shown in FIG. 19. The parameters used were:

[0156] Total cycle time: 22 min [0157] i. 0-12 min at 7.0 L/min, shear rate 12,470 sec.sup.?1 [0158] ii. 12-22 min at 3.5 L/min, shear rate 6,182 sec.sup.?1

[0159] The following polymer mixtures were tested in the miniloop with treated produced water from the A well: [0160] i. 1 L/m.sup.3 AMPS co-polymer [0161] ii. 1 L/m.sup.3 AMPS co-polymer+0.05 kg/m.sup.3 sodium perborate [0162] iii. 1 L/m.sup.3 AMPS co-polymer+0.1 kg/m.sup.3 sodium perborate [0163] iv. 1 L/m.sup.3 AMPS co-polymer+0.05 kg/m.sup.3 sodium persulfate [0164] v. 1 L/m.sup.3 AMPS co-polymer+0.1 kg/m.sup.3 sodium persulfate

[0165] The flow loop result is shown in FIG. 20. The results indicated that the performance of the AMPS co-polymer with sodium perborate breaker at 0.05 and 0.1 kg/m.sup.3 with 1 L/m.sup.3 AMPS co-polymer showed steady rapid drop within a minute of the start of the flow loop test. With sodium persulfate breaker, the performance decline in the friction reducer was slower and remained insignificant until the fluid temperature reached 25? C.

[0166] Another test was performed using sodium persulfate oxidant, with a temperature ramp to study the impact of fluid warming as it exchanges heat with the higher temperature rock downhole. The results are shown in FIG. 21. No significant difference in performance of the FR was observed at 0.05 and 0.1 kg/m.sup.3 dose rates of sodium persulfate.

[0167] In summary, oxidant addition was found to be effective for polymer degradation at reservoir conditions (?70? C.) in the presence of iron. Persulfate type oxidants were found to be most effective for polymer degradation, whilst preventing degradation of friction reduction performance at low temperatures (<45? C.) expected during high friction loss flow down the wellbore and proppant placement. The use of oxidant breaker for completions is not new, however its use to target water treatability and prevention of iron chelation is novel, as are the tests to confirm both chelation and/or elimination of chelation on sufficient treatment.

[0168] Furthermore, the use of aluminum compounds for surface treatment of produced water contaminated with residual high molecular weight anionic polymer friction reducer is a novel, feasible and effective approach for treating this problem.

[0169] The examples herein are intended to be illustrative only, and not unduly limit the scope of the appended claims. Although the present disclosure and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the scope of the disclosure as defined in the claims.

[0170] The following references are incorporated by reference in their entirety for all purposes: [0171] U.S. Ser. No. 11/072,546 Decomplexation of chelated hardness at high pH. [0172] Sharma, R. R.; Bjornen K. Systematic Approach for developing a fit-for-purpose treatment for produced water reuse in hydraulic fracturing. (2018) Paper presented at the Unconventional Resources Technology Conference, Houston. [0173] Walsh, J. M.; Bansal, K. M. A reassessment of the role of colloidal iron in produced water from shale operationsA review of field data and literature (Part A). (2021) Oil and Gas Facilities, Journal of Petroleum Technology, Sep. 20, 2021. [0174] Carman, P. S.; Cawiezel, K. Successful breaker optimization for polyacrylamide friction reducers used in slickwater fracturing. (2007) Paper presented at the SPE Hydraulic Fracturing Technology Conference, College Station, Texas, U.S.A. Paper Number: SPE-106162-MS. [0175] Product Brochure: Nouryon (Unknown date). New high-performing scale inhibitor and dispersing agent with low free monomer levels.