Triq Almurk , Leiming Li , Jun Hong Ng , Hishm Nsr-El-Din ,Mohmmed AlKhldi

a Texas A&M University, USA

b Aramco Services Company, Saudi Arabia

c Saudi Aramco EXPEC ARC, Saudi Arabia

ABSTRACT Current interest in deep, low-permeability formations (＜10 md) demands accelerated development of high-temperature hydraulic fracturing technologies.Conventional guar systems break down above 300 °F and require higher polymer loadings to maintain thermal stability.However, higher polymer loadings generate more residue and damage to the proppant pack and the formation.To resolve these problems, a variety of high-temperature stabilizers are added to enhance the thermal stability of these fracturing fluids at temperatures above 300 °F.The focus of this work is to: (1) identify those additives that best enhance temperature stability of fracturing fluids and (2) study the rheological influence of incorporating these additives on the fracturing fluid systems.

Keywords:Oxygen scavenger Crosslinker Crosslinker delay Shear tolerance High temperature Synthetic polymer CMHPG Buffers

1.Introduction

Hydraulic fracturing fluids are pumped at high pressures to break down the rock and carry proppant inside the generated fractures.With the recent expansion of exploration technologies,many potential wells are located in high-temperature formations.As part of the fracturing fluids, polymers, crosslinkers, and other chemicals are added to the solution to generate the required elastic and viscous properties to carry the proppant downhole.For example, carboxymethyl hydroxypropyl guar (CMHPG) is a negatively charged polysaccharide-based biopolymer that is commonly used.CMHPG is produced by treating guar with both propylene oxide and chloroacetic acid, Fig.1 [1,2].Guar is derivatized to CMHPG to reduce impurities, increase pH tolerance, and improve temperature stability.These properties enable CMHPG to work as an excellent polymer for fracturing fluids in a variety of formations.

At high temperatures (＞300°F), polymers, especially polysaccharides, thermally degrade, resulting in a loss of viscosity,which ultimately causes early proppant screen-out and failure in treatments.Therefore, measures must be taken to prevent the thermal degradation of the polymer.This loss in viscosity results from thermal degradation that occurs when polymer chains or the 3D crosslinked polymer structures break down.Thermal stability in polymers depends on the crosslinking bonds between the polymer and the crosslinker, and the bonds between the monomers in the backbone of the polymer chain.

In the case of polysaccharides,the polymer backbone weakness resides in the glycosidic bonds linking mannose structures.Polysaccharide glycosidic (acetal) linkages are susceptible to degradation through hydrolysis [3-5].At high temperatures, oxygen, free radicals, and protons accelerate the degradation of these bonds[6-10].Some cases,such as the presence of hydrogen bonds,have shown an increase in the resistance of polymers to thermal oxidation by changing the conformation of polymers in solution[6].

For synthetic polyacrylamide-based polymers, specialty monomers such as 2-acrylamido-2-methylpropane sulfonic acid(AMPS)can be added to the chain to improve its thermal stability, salt resistance, and shear tolerance [11].Several researchers have investigated this monomer using different terpolymers by incorporating AMPS at a concentration between 0 and 80%.Results published in the literature show that a concentration of 60%AMPS worked well in a polymer structure containing 0.5% acrylic acid,whereas a concentration of 80% AMPS performed worse [12].In some cases, depending on the other monomer choices, certain concentrations of AMPS did not produce a crosslinkable polymer structure.These results show that the concentration of AMPS has to be properly optimized in the polymer structure prior to any application.The addition of AMPS was also shown to enhance salt tolerance due to the position of the sulfonated molecule being a few atoms away from the polymer backbone structure, thus shielding the acrylic acid monomers from cations and giving them more freedom to crosslink with the intended crosslinker.Researchers have further shown the ability to use AMPS-containing polymers in solutions of 2 wt% KCl where the similar polymer will readily precipitate in the absence of AMPS [13,14].In addition, AMPS has been proven to provide delayed crosslinking behavior in water shut off treatment designs giving the fluid more time to go deep in the formation before crosslinking[15].The AMPS monomer also shows exemplary temperature-enhancement capabilities and shear tolerance by providing stiffness to the polymer structure due to the short branch generated by adding this monomer.The influence of AMPS was also studied over a range of temperatures up to 450°F with the addition of high-temperature stabilizers and exhibited a stable performance at these conditions [16-19].

The most common high-temperature stabilizers in the oil and gas industry are oxygen scavenging compounds.Oxygen scavengers such as chalcogen heterocyclic compounds protect the polymer at high temperatures from oxidation.They do so by donating electrons to reduce molecular oxygen to the-2 oxidation state[20].Examples of oxygen scavengers include methanol, sodium thiosulfate, and hydrosulfite.Care must be taken in selecting the appropriate oxygen scavenger, as the by-products formed by the scavenger must not interfere with the viscosity of the polymer.For this reason,oxygen scavengers should be tested with the treatment fluid to assess compatibility on a case-by-case basis.This is typically done using routine jar tests, rheometer, and fracture conductivity or coreflooding measurements [21,22].

Fig.1.Derivatization of guar to CMHPG.

Fig.2.Synthetic polymer composition (AA-AM-AMPS).

Buffers can also influence the thermal stability of polymers used in fracturing fluids.LaGrone [23] shows a positive correlation between basicity and the viscosity of biopolymer solutions at high temperatures.At low pH,acid-catalyzed hydrolysis of the glycosidic bonds in polysaccharides occurs, causing a loss in viscosity.However,using high-pH fluids can result in formation damage and can negatively impact and limit the choice and subsequent performance of crosslinkers.Examples of buffers include amines, hydroxide, carbonate/bicarbonate solutions, and acetic acid/acetate solutions [24].

Conventional fracturing fluids use borate-based crosslinkers due to their low cost, simplicity, and re-healing characteristic.Borate crosslinked systems may not perform well at high temperatures because of the changes in pH and the associated reduction of the concentration of monoborate ions(MBI)in solution[25].Strong buffers are typically required to resolve this issue by maintaining a pH ＞9 at these high temperatures.On the other hand, metallic crosslinkers form stronger bonds that are stable over a wider range of conditions.Metallic crosslinkers are cheap, highly reactive with commonly used polymers, work well at pH ranges of 3-12, are stable at higher temperatures up to 450°F, can handle high salt concentrations, and have been successfully tested with produced waters [26-29].However, metallic crosslinkers can be incompatible with other polymeric additives such as polymeric scale inhibitors and must be tested thoroughly [30].They should be avoided when enzyme breakers are used because they can denature the enzymes [31,32].Metallic crosslinkers are known to be shear sensitive,the crosslinking bonds can break due to mechanical shear, and the crosslinking bonds are irreparable once broken[29,33].

To increase the resistance of the crosslinking bonds to the shear rate at high temperatures, the crosslinking reaction time with the polymer can be delayed.Delaying the crosslinking reaction reduces the number of crosslinked bonds formed initially and thus reduces the number of bonds that are irreversibly broken [34].Crosslinker delayers can be incorporated through the manufacturing process of metallic crosslinkers by choosing the appropriate ligand combination [24,35-38].By selecting the appropriate ligand-to-metal complex, the reaction rate of the metal crosslinkers can be controlled.In addition to the choice of crosslinker ligands,external additives such as polyols can be used to delay crosslinking.Polyols such as sorbitol, fructose, and gluconic acid, and their related derivatives, act as high-temperature stabilizers by undergoing preferential crosslinking in place of the polymer [39-41].

Related advances in hydraulic fracturing include the ability to produce acceptable rheological properties with alternative water sources such as seawater and produced water [30,42,43].Other examples of innovation in this field include incorporating nanoparticles and nanocrosslinkers in fracturing fluids to enhance properties such as high-temperature stability and reducing polymer loading [27,28,44].However, there is still room for improvement in the area of high-temperature fracturing fluid application.This work quantifies the high-temperature limitations of conventional fracturing fluids and proposes multiple ways to resolve these issues by using high-temperature stabilizer additives such as slowreacting crosslinkers, crosslinker delayers, oxygen scavengers, and buffers.The work compares the performance of these additives to realize the impact and the possible combinations to develop more efficient high-temperature fracturing fluid solutions.

Fig.3.Viscosity at 40 lb/1000 gal CMHPG fracturing fluid, 4 gpt crosslinker, pH 5,200-400 °F.

2.Experimental studies

2.1.Materials

CMHPG polymer powder and three zirconium crosslinkers with varying crosslinking reaction rates (slow: zirconium lactate and propylene glycol,medium:zirconium lactate and triethanolamine,and fast:zirconium lactate;all at 5.5-6 wt%ZrO2)were provided by a service company.The rheological behavior of these crosslinkers has been studied in detail [45].The slow-reacting crosslinker was used for the majority of the tests in this work.The medium- and fast-reacting crosslinkers were only used for comparison in the crosslinker choice experiments.The synthetic polyacrylamidebased polymer was provided in emulsion form (30 wt% active)and was used as received.The synthetic polymer is composed of acrylamide (AM), acrylic acid (AA), and AMPS (Fig.2).Sodium thiosulfate pentahydrate and the crosslinking delay additives(sugar alcohol derivatives) were provided by a chemical company and used as received.Tetraethylenepentamine (TEPA) and acetic acid were provided at 99 wt% purity and used as received.Acetic acid and sodium acetate buffer was provided at 30 wt% and was used as received.Houston tap water (＜500 ppm) was used to mix all the systems.

Fig.4.Viscosity at 40 lb/1000 gal CMHPG fracturing fluid, 2 gpt crosslinker, pH 5-10,300 °F (Almubarak et al.[46]).

Fig.5.Viscosity at 15, 20, 30, and 40 lb/ 1000 gal CMHPG, 4 gpt crosslinker, pH 5,300 °F.

Fig.6.Viscosity at 40 lb/1000 gal to evaluate sodium thiosulfate HT stabilizer, 4 gpt crosslinker, pH 5, 300 °F.

Fig.7.Viscosity at 40 lb/1000 gal fracturing fluid, 4 gpt crosslinker, pH 5, 300 °F.

2.2.Fluid preparation

Fig.8.Viscosity at 15,20,30,and 40 lb/1000 gal synthetic polymer,4 gpt crosslinker,pH 5, 300 °F.

Fig.9.Viscosity at 40 lb/ 1000 gal 1:2 and 2:1 (CMHPG: Synthetic) fracturing fluid, 4 gpt crosslinker, pH 5, 300 °F.

Fig.10.Viscosity at 40 lb/1000 gal 1:1 (CMHPG: Synthetic) fracturing fluid, 5 gpt crosslinker, pH 5, 300-400 °F.

The fracturing fluids were prepared, mixed, and tested within 12 h; for that reason, no biocide was used.The 40 lb/1000 gal fracturing fluid was prepared by adding 3.84 g of CMHPG powder to 800 ml of tap water.The solution was mixed for 20 min at 800 RPM to achieve full hydration.For the experiment where synthetic polymer was incorporated,the fluids were prepared following the procedure in Almubarak et al.[46].After preparing the base gel,the fluid was transferred to the blender,and external high-temperature additives were added as needed and mixed thoroughly for 5 min at 200-400 RPM.The pH of the solution was adjusted by adding an appropriate amount of TEPA for the pH 10 tests,and acetic acid for the pH 5 tests.Zirconium crosslinkers were added last and mixed thoroughly for 30 s.

Fig.11.Viscosity at 40 lb/1000 gal 1:2 (CMHPG: Synthetic) fracturing fluid, 5 gpt crosslinker, pH 5, 300-400 °F.

Fig.12.Viscosity at 40 lb/1000 gal 1:1 and 1:2 (CMHPG: Synthetic) fracturing fluid, 5 gpt crosslinker, pH 5, 330 °F.

Fig.13.Viscosity at 40 lb/1000 gal 1:1 and 1:2 (CMHPG: Synthetic) fracturing fluid, 5 gpt crosslinker, pH 5, 350 °F.

2.3.HP/HT rheometer

An HP/HT rheometer was used to measure the apparent viscosity of the fracturing fluids at 200-400°F.The rheometer utilized R1/B5 bob and rotor combination,which requires a sample volume of 52 cm3.The rheometer uses an electric jacket for heating; a temperature sensor is mounted on the stator/bob to control sample temperature.A pressure of 350-500 psi was applied with nitrogen gas to prevent the boiling of the sample.

Fig.14.Viscosity at 30 lb/1000 gal 1:2 (CMHPG: Synthetic) fracturing fluid, 6 gpt crosslinker, pH 5, 300-400 °F.

Fig.15.Zirconium crosslinker type comparison on 30 lb/1000 gal 1:2 (CMHPG: Synthetic) fracturing fluid viscosity, equivalent Zr to 6 gpt crosslinker, pH 5, 370 °F.

Fig.16.Zirconium crosslinker type comparison on 30 lb/1000 gal 1:2 (CMHPG: synthetic) fracturing fluid viscosity, equivalent Zr to 6 gpt crosslinker, pH 5, 400 °F.

Viscosity measurements were taken under different shear rates to simulate the flow of the fracturing fluid through production tubular, perforations, and inside the created fracture.ISO13503-1 schedule [47] was followed, where the shear rate schedule was set to 100 s-1with short shear ramp spikes between 25 and 100 s-1.The heater was preheated to 150°F before running the tests to ensure quick and consistent heating profiles.The fluid took 10-20 min to reach the testing temperature.

Fig.17.Crosslinker delayer influence on 30 lb/1000 gal 1:2 (CMHPG: Synthetic) fracturing fluid viscosity, 6 gpt crosslinker, pH 5, 370 °F.

3.Results and discussion

3.1.Fracturing fluid temperature limits

The viscosity measurements of 40 lb/1000 gal crosslinked CMHPG at 200-400°F and pH 5 is seen in Fig.3.The results show that CMHPG can hold a stable viscosity performance up to 250°F with no additives at this pH.At temperatures above 250°F, the performance of CMHPG-based fracturing fluid deteriorates due to thermal and shear degradation.The performance of CMHPG was also found to vary proportionally with pH(Fig.4).Fig.5 shows the influence of high temperature(300°F)on the crosslinked viscosity at 15, 20, 30, and 40 lb/1000 gal of CMHPG.

CMHPG contains carboxymethyl groups that have a pKa of 3.5-4 at 77°F and partially deprotonate at pH 5 [48,49].The amount of intermolecular hydrogen bonding at pH 5 is influenced by the carboxymethyl content, which is typically low in industrial CMHPG [50].High-temperature and low-intermolecular hydrogen bonding influence the structural orientation of the polymer [51].This orientation could allow easier access for oxygen to attack the backbone.An attack on the polymer backbone would cleave the polymer chain and cause significant viscosity loss.Additionally,the polymer chains vibrate more at high temperatures,which becomes more pronounced at low polymer concentrations and weak hydrogen bonding [52,53].This reduces viscosity and influences the thermal stability as well.

Fig.18.Crosslinker delayer influence on 30 lb/1000 gal 1:2 (CMHPG: Synthetic)fracturing fluid viscosity, 6 gpt crosslinker, pH 5, 400 °F.

Fig.19.Effect of oxygen scavenger on 30 lb/1000 gal 1:2 (CMHPG: Synthetic) fracturing fluid viscosity, 6 gpt crosslinker, pH 5, 370 °F.

3.2.Addition of sodium thiosulfate

Sodium thiosulfate is commonly used as a high-temperature stabilizer.Fig.6 shows the effect of various concentrations of sodium thiosulfate on the thermal stability of CMHPG at 300°F.At this temperature, the performance of CMHPG alone is similar to those containing sodium thiosulfate.40 ppt sodium thiosulfate was observed to have a negative impact on the solution.Sodium thiosulfate enhances the thermal stability of polymers by scavenging oxygen from the system, thereby preventing oxidation of the polymer backbone.However, sodium thiosulfate contains sodium ions that can screen the polymer negative charges.Under low pH and high-temperature conditions, it causes the collapse of the extended and deprotonated polymer conformation.Therefore, sodium thiosulfate does not improve rheological performance at these conditions [54].

Fig.20.Influence of oxygen scavenger on 30 lb/1000 gal 1:2 (CMHPG: Synthetic)fracturing fluid viscosity, 6 gpt crosslinker, pH 5, 400 °F.

Fig.21.pH buffer influence on 30 lb/1000 gal 1:2 (CMHPG:Synthetic)fracturing fluid viscosity, 6 gpt crosslinker, pH 5, 370 °F.

Fig.22.pH buffer influence on 30 lb/1000 gal 1:2(CMHPG:Synthetic)fracturing fluid viscosity, 6 gpt crosslinker, pH 5, 400 °F.

3.3.Addition of a synthetic polymer

Synthetic polymer was added to CMHPG in the fracturing fluid to enhance the properties of CMHPG[55].Fig.7 shows the viscosity results for the 40 lb/1000 gal crosslinked fracturing fluid viscosity.The addition of synthetic polymer increased the thermal stability of the fracturing fluid.The synthetic polymer increases the intermolecular hydrogen bonds, which would ultimately change the structural orientation of CMHPG in solution.Researchers proved the existence of hydrogen bonding between the two polymers through Fourier-transform infrared spectroscopy (FTIR) measurements [46].Even if the glycosidic bond in CMHPG is cleaved, the intermolecular associations can hold the structure in shape and maintain viscosity for a longer time [56].

The acrylic acid monomer in the synthetic polymer has a pKa of 4-5 at 77°F [57,58], while the AMPS monomer has a pKa of 1-2[59,60].At pH 5 the polymer is sufficiently deprotonated.Even though the AMPS monomer is deprotonated,it does not contribute directly to the crosslinking reaction[11].These pH conditions allow for changes in the conformation of the polymer in solution,adding stiffness and thermal stability,as observed in Fig.8[61].Therefore,the presence of the synthetic polymer eliminates the need for additional temperature stabilizers in the fracturing fluid systems at 300°F.

3.3.1.Effect of CMHPG: synthetic polymer ratio

Fig.9 compares the thermal stability for CMHPG: Synthetic ratios between 2:1 and 1:2 fracturing fluid systems at 30 and 40 lb/1000 gal.A pronounced viscosity slope decline is seen when the ratio of CMHPG is dominant compared to when the synthetic polymer is dominant in the mixture.This decline results from the weak thermal stability of CMHPG at these conditions.The thermal stability of the fracturing fluid is improved when the synthetic polymer concentration is dominant in the mix.

3.3.2.Performance above 300°F

Different ratios of the fracturing fluid were tested at temperatures above 300°F to assess its thermal stability limits.Figs.10 and 11 show the measured viscosity of 40 lb/1000 gal 1:1 and 1:2(CMHPG: Synthetic) fracturing fluid at temperatures between 300 and 400°F.Both ratios show a good performance up to 350°F.Beyond 350°F,a significant decrease in rheological performance is observed.

Figs.12 and 13 compare the viscosity of 1:1 and 1:2 (CMHPG:Synthetic) fracturing fluid at 330 and 350°F, respectively.These results show that the 1:2 (CMHPG: Synthetic) fracturing fluid is more thermally stable than the 1:1 (CMHPG: Synthetic) ratio.

3.3.3.Polymer loading reduction above 300°F

To reduce the damage from polymers,a lower loading of a 30 lb/1000 gal fracturing fluid was tested.The 1:2 (CMHPG: Synthetic)ratio fluid was able to maintain good viscosity and,similar to the 40 lb/1000 gal,was thermally stable up to 350°F,as shown in Fig.14.

3.4.Other additives to enhance thermal stability

Polymer thermal stability in crosslinked fracturing fluids depends on two types of bonds[18]that include the following:1)The crosslinking bonds between the polymer and the crosslinker,and 2)The monomer to monomer bonds in the polymer backbone structure.

The thermal stability in the fracturing fluid can be enhanced by reducing the damage to these two main types of bonds.Many additives can be categorized as HT stabilizers; however,the different functions of these additives,at a proper combination,can be used to protect multiple bond types and, therefore, enhance the performance at temperatures above 350°F.

3.5.Crosslinkers

The crosslinking bond can be protected against shear to some degree by using a slow-reacting crosslinker.The reaction rate of crosslinkers can be controlled by using ligands that generally consist of lactate,triethanolamine,propylene glycol,etc.The order,type,and the number of ligands in the crosslinker composition can influence the crosslinking reaction rate and high viscosity generation.Figs.15 and 16 show the viscosity measurements using three types of crosslinkers that vary in the crosslinking reaction rate crosslinkers(slow,medium,and fast),measured at 370 and 400°F,respectively.The results show that a slow-reacting crosslinking will enhance thermal stability by minimizing shear degradation.

3.5.1.Crosslinking delay additives

To protect the crosslinking bond further, an external delay additive can be used.This additive contains similar functional groups to the fracturing fluid polymer and is engineered to favorably bond with the crosslinker.Delay additives would, therefore, restrict the amount of the crosslinker interacting with the polymer, and they would break or release the crosslinker to the fracturing fluid polymer over a longer period of time,to maintain a stable viscosity performance.Figs.17 and 18 show the viscosity measurements of the slow-releasing crosslinker base case compared to a slowreleasing crosslinker with the external crosslinking delay additive at 370 and 400°F,respectively.With the external crosslinking delay additive, the initial crosslinking is suppressed, resulting in lower initial fluid viscosity.This delay, in turn, places the crosslinking bonds under less shear damage and increases the long-term stability of the fluid’s viscosity.The results show that the external delay additive will enhance thermal stability performance by controlling the crosslinking reaction and minimizing shear degradation to a greater extent.The tested delay additive showed a slightly higher initial viscosity profile at 400 compared to 370°F, showing that the controlled release is weakened at higher temperatures.

3.6.Oxygen scavengers

Dissolved oxygen in solution breaks down the polymer backbone bonds by undergoing several reactions that generate reactive radicals that can influence polymers in solution [62-65].In addition,the polysaccharide molecular structure is thermally weak and can be easily cleaved at the glycosidic bond [66].To protect the polymer backbone bonds, reducing dissolved oxygen becomes important.Thiosulfate is a typical additive used to reduce oxygen in the solution.Figs.19 and 20 show the results of adding a sodium thiosulfate oxygen scavenger at 370 and 400°F, respectively.

The use of sodium thiosulfate has some downsides such generating a notable concentration of H2S at temperatures greater than 350°F [67].More importantly, sodium thiosulfate will scavenge oxygen generated from the oxidizer breakers used and will interact with several other additives.These interactions have been observed to cause precipitation at 300°F[68].For that reason,the combination of thiosulfate high-temperature stabilizer and fracturing fluid additives must be thoroughly tested before field application.

3.7.pH buffers

At high temperatures, the pH of the system is reduced because of a shift in the equilibrium of water dissociation [69].The pH change can influence the performance of many additives,including the reaction rate of crosslinkers.In addition, as the pH drops, the concentration of H+in solution increases, and that can initiate hydrolysis reactions that would break the polymer backbone bonds and eventually reduces viscosity[70].High pH systems are typically used to overcome this issue.However, lower pH fracturing fluid systems are frequently desired to reduce damage due to clays in the formation [71,72], reduce hydroxide precipitation, and for compatible applications in acidic fracturing treatments.To maintain a stable viscosity at lower pH,a pH buffer can be used.Figs.21 and 22 show the improved measured viscosity results with an acetic acid/acetate buffer at 370 and 400°F, respectively.

4.Conclusions

The main bonds that can influence the thermal stability of fracturing fluids are the polymer backbone bonds and the crosslinker-to-polymer bonds.This work presents different chemical techniques to enhance thermal stability in fracturing fluids.The tested fracturing fluid’s thermal stability was enhanced by utilizing additives such as a synthetic polymer, slow-reacting crosslinkers,oxygen scavengers, crosslinking delay additives,and pH buffers.

The results of this research lead to the following conclusions:

(1) Adding synthetic polymer (AA-AM-AMPS) to CMHPG can generate a stable 30 lb/1000 gal fracturing fluids up to a temperature of 350°F.

(2) The crosslinker-to-polymer bond can be protected at temperatures ＞ 350°F by using a slow-reacting zirconium crosslinker.

(3) The crosslinker-to-polymer bond can be further protected at temperatures ＞350°F by adding an external crosslinking delay additive(sugar alcohol derivative).

(4) Sodium thiosulfate can enhance thermal stability at temperatures ＞350°F.

(5) Acetic acid/acetate pH buffer adds minimal thermal stability at temperatures ＞350°F.

(6) Sodium thiosulfate does not add thermal stability in a 40 lb/1000 gal pure CMHPG fracturing fluid system at pH ～5 and temperatures ≥300°F.

This work supports the combination of a slow-reacting crosslinker (Zr-lactate and propylene glycol) and an external crosslinking delay additive(sugar alcohol derivatives)to achieve optimal shear tolerance and higher temperature stability performance at temperatures above 300°F for all polymer-based fracturing fluids.

Although the addition of oxygen scavengers showed excellent thermal stability, caution should be exercised in the addition of sodium thiosulfate pentahydrate in combination with pH buffers such as acetic acid/sodium acetate at pH 5 conditions because of the salt sensitivity of some polymers.This work further supports the addition of sodium thiosulfate pentahydrate at a maximum concentration of 10 ppt for pH 5 and 40 ppt for pH 10 conditions.

Declaration of competing interests

The authors declare that they have no conflict of interests.

Acknowledgments

The authors thank Gia Alexander for editorial assistance in preparing this paper.

Nomenclature

AA Acrylic acid

AM Acrylamide

AMPS 2-Acrylamido-2-methylpropane sulfonic acid

CMHPG Carboxymethylhydroxypropyl guar

HP/HT High pressure high temperature

RPM Rotation per minute

Stable Fracturing Fluid Viscosity ≥100 cp at 100 s-1and tested temperature for 1.5 h

TEPA Tetraethylenepentamine