**5. Crosslinkers**

Crosslinkers are used to increase the molecular weight of the polymer by crosslinking the polymer backbone into a 3D structure as shown in Figure 11. This increases the base viscosity of the linear gel from less than 50 cps into the 100's or 1000's of cps range. This crosslinking also increases the elasticity and proppant transport capability of the fluid.

For guar and CMHEC based gels, Boron and several metals including Titanium and Zirconium are used as crosslinkers. In addition to these materials Iron, Chromium and Aluminum will crosslink guar but are not commonly used. Iron is a major contaminant for fracturing fluids and is one of the metals that must be carefully controlled during the QC process to prevent premature crosslinking. Each crosslinker has a unique reaction requirement and behavior.

**Figure 9.** Chemical Structure of Hydroxyethyl Cellulose (HEC) and Carboxy Methyl Hydroxy Ethyl Cellulose (CMHEC)

#### **5.1. Borate**

are greater concerns regarding personnel safety and environmental impact, as compared to

**Figure 8.** Chemical Structure of CarboxyMethyl Guar (CMG) and CarboxyMethylHydroxy Propyl Guar (CMHPG)

Crosslinkers are used to increase the molecular weight of the polymer by crosslinking the polymer backbone into a 3D structure as shown in Figure 11. This increases the base viscosity of the linear gel from less than 50 cps into the 100's or 1000's of cps range. This crosslinking

For guar and CMHEC based gels, Boron and several metals including Titanium and Zirconium are used as crosslinkers. In addition to these materials Iron, Chromium and Aluminum will crosslink guar but are not commonly used. Iron is a major contaminant for fracturing fluids and is one of the metals that must be carefully controlled during the QC process to prevent premature crosslinking. Each crosslinker has a unique reaction requirement and behavior.

also increases the elasticity and proppant transport capability of the fluid.

most water-fluids.

34 Effective and Sustainable Hydraulic Fracturing

**5. Crosslinkers**

Borate in the form of Boric Acid, slowly soluble salts of Ca and Mg and Organic Borate complexes is, by far, the most common crosslinker in use today. Borate crosslinked fracturing fluids can be applied across a wide range of treating conditions and are resistant to shear degradation. Figure 11 shows diagrammatically how the borate complexes with Guar. As the figure shows the Borate source forms a tetrahedral form of the borate ion when the pH of the base fluid is above about 8.2. These borate ions complexes with the hydroxyl functionality on the polymer causing a 3 dimensional network to be formed which tremendously increases the molecular weight and viscosity. Once this mechanism is understood several things become apparent.

**1.** The crosslinking is a function of pH which means it can be formed or reversed simply by adjusting the pH. Borate crosslinked fluids are manufactured in the field by mixing the base polymer in water at a pH above 7, adjusting the pH to below 6 and adding in the borate crosslinker and any other additives. During pumping a buffer, usually caustic, is added at the blender which brings the pH above 8 and the crosslink is formed. This also means the process can be reversed simply by dropping the pH below 8 with acid. Cement is a particularly troublesome contaminant when proppant transports are used to also transport cement because the cement raises the pH to 14 which causes premature crosslinking.

**Figure 10.** Structure of a Viscoelastic Surfactant Thickener


bers of +4 so they form a strong covalent rigid bond with the polymers cis hydroxyls as shown in Figure 12. The various complexing agents allow the crosslinker to become ac‐ tive under a range of time, temperature and pH conditions. Titanium and Zirconium crosslinked fluids can be manufactured that are stable at pH levels from 3.5 to 10.5 and up to temperatures of 350°F. When compared to Borate crosslinked fluids metallic cross‐

Fracturing Fluid Components http://dx.doi.org/10.5772/56422 37

**1.** The metallic crosslink is a strong covalent bond which makes the crosslink susceptible to high shear rates. Once the bond is broken it will not heal as a Borate crosslink will. To prevent shear degradation the crosslink time should always be delayed to about 2/3 of the

**2.** Because it takes time for the metal to interact with the polymer the crosslink time can be delayed. The type of ligand used to complex the metal controls the delay time. Sometimes

**3.** Metallic crosslinked polymer systems can be built that cover a broad range of pH conditions so they can be used in CO2 based fracturing fluids. They are also much more

it is quite difficult to achieve any delay particularly at a pH < 5.

linked fluids have several advantages/disadvantages.

**Figure 11.** Crosslinking Mechanism of Borate onto Guar

pipe time.

stable at high temperatures.

**4.** Any polymer which has hydroxyls in the cis position can be crosslinked with Borate. These include Guar and all of its derivatives and CMHECellulose.

#### **5.2. Titanium and zirconium**

Titanium and zirconium crosslinkers were originally developed for manufacturing explo‐ sive gels[14]. Because Borate crosslinked systems were limited to temperatures below 250°F and pH's above 8 metallic crosslinked fluids were developed to broaden that range. The crosslinkers are manufactured in the form of a metal ligand or chelant using various complexing agents including TEA (Triethanol Amine), LA (Lactic Acid) and AA (Acetylacetone) [15]. When the chelant complex is exposed to water the metal becomes active and crosslinking can occur. Once exposed to water the ionic metal starts to oxidize and if left will become inactive. Both Zirconium and Titanium have coordination num‐

**Figure 11.** Crosslinking Mechanism of Borate onto Guar

**2.** The optimum borate crosslinker efficiency is at a pH of about 10.5.

quickly build the crosslink again once the shear is dropped.

include Guar and all of its derivatives and CMHECellulose.

**5.2. Titanium and zirconium**

36 Effective and Sustainable Hydraulic Fracturing

**Figure 10.** Structure of a Viscoelastic Surfactant Thickener

**3.** Because the crosslink is in equilibrium it can be broken by shear in the tubing and will

**4.** Any polymer which has hydroxyls in the cis position can be crosslinked with Borate. These

Titanium and zirconium crosslinkers were originally developed for manufacturing explo‐ sive gels[14]. Because Borate crosslinked systems were limited to temperatures below 250°F and pH's above 8 metallic crosslinked fluids were developed to broaden that range. The crosslinkers are manufactured in the form of a metal ligand or chelant using various complexing agents including TEA (Triethanol Amine), LA (Lactic Acid) and AA (Acetylacetone) [15]. When the chelant complex is exposed to water the metal becomes active and crosslinking can occur. Once exposed to water the ionic metal starts to oxidize and if left will become inactive. Both Zirconium and Titanium have coordination num‐ bers of +4 so they form a strong covalent rigid bond with the polymers cis hydroxyls as shown in Figure 12. The various complexing agents allow the crosslinker to become ac‐ tive under a range of time, temperature and pH conditions. Titanium and Zirconium crosslinked fluids can be manufactured that are stable at pH levels from 3.5 to 10.5 and up to temperatures of 350°F. When compared to Borate crosslinked fluids metallic cross‐ linked fluids have several advantages/disadvantages.


on fluid temperature which varies with time. The three general types of breakers are Oxidizers,

Fracturing Fluid Components http://dx.doi.org/10.5772/56422 39

Oxidizer breakers include Ammonium persulfate, Sodium persulfate, and Calcium and Magnesium peroxides. They work by cleaving the acetyl linkages in the polymer backbone as shown in Figure 13[6]. Ammonium persufate [(NH4)2S2O8] and Sodium persulfate (Na2S2O8) are very strong oxidizers which forms a free Oxygen radical when the temperature exceeds 125°F. These free radicals attach the backbone of the polymer strand and break it down into its constitutive sugars. If left in the fracture these residual sugars will cook and form insoluble precipitates resulting in conductivity damage[7]. This is the reason flow back of the fractured well is suggested as soon as the fracture is known to be closed. Both Calcium and Magnesium peroxide (CaO2 and MgO2) release Oxygen when they come in contact with water. The breaking action is controlled by the solution rate of the peroxide into the water. They are not affected by temperature as much at the persulfates and are used for low temperature appli‐ cations. The free radical oxidation is not specific to the polymer backbones and the materials will spend on any available free radical acceptor such as a gel stabilizer. All of these materials are strong oxidizing agents and will produce a very active fire when exposed to organic material. They are used in industry for applications such as a water disinfectant, bleach and

The main disadvantage of oxidizing breakers is both how well they work and how fast they work is a function of the amount of chemical added. Figure 14 shows that a concentration of 0.5 lb/1000 gal of persulfate breaker will break the polymer viscosity back to the viscosity of water but will damage the proppant pack so that only 20% of the original conductivity remains. If we want to get the maximum retained permeability we need to go to concentrations of 10 to 12 lb/1000 gallons which will break the fluid viscosity instantly. To counteract this and retard the release of the persulfate encapsulated breakers were developed. There are two types of encapsulated breakers available. The release rate of the breaker in the first type is controlled by hydrostatic pressure, elevated temperatures and the pH of the fracturing fluid[10]. The second method of release is by crushing the capsule coating as the fracture closes. Because these encapsulated breakers require conditions similar to those in the fracture i.e. closure or hydrostatic pressure they are difficult to test for QC purposes in the field and to date no field

Acids such as HCl or Acetic acid will attach the polymer back bone and break the gel similar to oxidizing breakers but they are much less selective and can cause considerable amount of insoluble material to be formed. They are generally used to try and clean fractures that are believed to be damaged by a job where sufficient breaker was not used or the gel is believed to not be broken. They also work by reversing the crosslink in Borate crosslinked systems. They are typically used after a job has been completed and placement becomes the main issue.

test has been developed to quantify their activity in the field.

Acid and Enzymes.

pickling agents for metals.

**6.2. Acids**

**6.1. Oxidizer**

**Figure 12.** Crosslinking Mechanism for Metallic (Ti+4 and+4 Zr) Crosslinkers onto Guar


#### **6. Breakers**

Breakers are added to the fracturing fluid to reduce the molecular weight of the various polymers used. This reduces the viscosity and facilitates the blowback of residual polymer which allows for cleanup of the proppant pack. The inappropriate use or ineffective breakers can cause significant damage in the proppant pack and a reduced PI. Ideally these materials would be totally inactive during the treatment and then instantly "spring to action" when pumping stops, rapidly breaking the fluid back to a low viscosity preparing the fracture and formation for flow. This is very difficult to achieve as the breaker activity is very dependent on fluid temperature which varies with time. The three general types of breakers are Oxidizers, Acid and Enzymes.

#### **6.1. Oxidizer**

Oxidizer breakers include Ammonium persulfate, Sodium persulfate, and Calcium and Magnesium peroxides. They work by cleaving the acetyl linkages in the polymer backbone as shown in Figure 13[6]. Ammonium persufate [(NH4)2S2O8] and Sodium persulfate (Na2S2O8) are very strong oxidizers which forms a free Oxygen radical when the temperature exceeds 125°F. These free radicals attach the backbone of the polymer strand and break it down into its constitutive sugars. If left in the fracture these residual sugars will cook and form insoluble precipitates resulting in conductivity damage[7]. This is the reason flow back of the fractured well is suggested as soon as the fracture is known to be closed. Both Calcium and Magnesium peroxide (CaO2 and MgO2) release Oxygen when they come in contact with water. The breaking action is controlled by the solution rate of the peroxide into the water. They are not affected by temperature as much at the persulfates and are used for low temperature appli‐ cations. The free radical oxidation is not specific to the polymer backbones and the materials will spend on any available free radical acceptor such as a gel stabilizer. All of these materials are strong oxidizing agents and will produce a very active fire when exposed to organic material. They are used in industry for applications such as a water disinfectant, bleach and pickling agents for metals.

The main disadvantage of oxidizing breakers is both how well they work and how fast they work is a function of the amount of chemical added. Figure 14 shows that a concentration of 0.5 lb/1000 gal of persulfate breaker will break the polymer viscosity back to the viscosity of water but will damage the proppant pack so that only 20% of the original conductivity remains. If we want to get the maximum retained permeability we need to go to concentrations of 10 to 12 lb/1000 gallons which will break the fluid viscosity instantly. To counteract this and retard the release of the persulfate encapsulated breakers were developed. There are two types of encapsulated breakers available. The release rate of the breaker in the first type is controlled by hydrostatic pressure, elevated temperatures and the pH of the fracturing fluid[10]. The second method of release is by crushing the capsule coating as the fracture closes. Because these encapsulated breakers require conditions similar to those in the fracture i.e. closure or hydrostatic pressure they are difficult to test for QC purposes in the field and to date no field test has been developed to quantify their activity in the field.

#### **6.2. Acids**

**4.** Because of the permanent nature of the metallic crosslink, the molecular weight of the broken gel residue is much greater than that formed from linear or borate crosslinked gels.

**5.** Any polymer which has hydroxyls in the cis position can be crosslinked with Metallic crosslinkers. These include Guar and all of its derivatives and CMHECellulose.

Breakers are added to the fracturing fluid to reduce the molecular weight of the various polymers used. This reduces the viscosity and facilitates the blowback of residual polymer which allows for cleanup of the proppant pack. The inappropriate use or ineffective breakers can cause significant damage in the proppant pack and a reduced PI. Ideally these materials would be totally inactive during the treatment and then instantly "spring to action" when pumping stops, rapidly breaking the fluid back to a low viscosity preparing the fracture and formation for flow. This is very difficult to achieve as the breaker activity is very dependent

This causes a greater degree of proppant pack damage and conductivity loss.

**Figure 12.** Crosslinking Mechanism for Metallic (Ti+4 and+4 Zr) Crosslinkers onto Guar

**6. Breakers**

38 Effective and Sustainable Hydraulic Fracturing

Acids such as HCl or Acetic acid will attach the polymer back bone and break the gel similar to oxidizing breakers but they are much less selective and can cause considerable amount of insoluble material to be formed. They are generally used to try and clean fractures that are believed to be damaged by a job where sufficient breaker was not used or the gel is believed to not be broken. They also work by reversing the crosslink in Borate crosslinked systems. They are typically used after a job has been completed and placement becomes the main issue.

**Figure 13.** Oxidative Breakers and their action on Guar

#### **6.3. Enzymes**

Enzymes are protein molecules that act as organic catalysts that attach and digest the polymer at specific sites along the polymer backbone. Because they are catalysts they are not "used up" during the breaking process and persist until there is no polymer present to digest. Typical enzymes that are used include hemicellulase, cellulose, amylase and pectinase. These enzymes are susceptible to thermal degradation and denaturing when exposed to very high or very low pH so are limited to mild temperatures below 150°F (66°C) and fluid pH's between 4 and 9. Recent work by Brannon and Tjon-Joe-pin have developed proprietary GLSE (Guar Linkage Specific Enzymes) that are reported to work at temperatures more than 300°F[8]. Figure 15 shows diagrammatically how enzymes work and the degradation of the molecular weight of HPG with time as it is digested by Hemicellulase.

**Figure 15.** Degradation of Guar by Hemicellulase Enzymes

**Figure 14.** Gel Cleanup vs Breaker Loading (after [9])

Fracturing Fluid Components http://dx.doi.org/10.5772/56422 41

**Figure 14.** Gel Cleanup vs Breaker Loading (after [9])

**6.3. Enzymes**

Enzymes are protein molecules that act as organic catalysts that attach and digest the polymer

at specific sites along the polymer backbone. Because they are catalysts they are not "used up"

during the breaking process and persist until there is no polymer present to digest. Typical

enzymes that are used include hemicellulase, cellulose, amylase and pectinase. These enzymes

are susceptible to thermal degradation and denaturing when exposed to very high or very low

pH so are limited to mild temperatures below 150°F (66°C) and fluid pH's between 4 and 9.

Recent work by Brannon and Tjon-Joe-pin have developed proprietary GLSE (Guar Linkage

Specific Enzymes) that are reported to work at temperatures more than 300°F[8]. Figure 15

shows diagrammatically how enzymes work and the degradation of the molecular weight of

HPG with time as it is digested by Hemicellulase.

**Figure 13.** Oxidative Breakers and their action on Guar

40 Effective and Sustainable Hydraulic Fracturing

**Figure 15.** Degradation of Guar by Hemicellulase Enzymes

#### **6.4. Viscosity stabilizers**

Viscosity stabilizers are added to the fracturing fluids to reduce the loss of viscosity at high reservoir temperatures. The two most common stabilizers are methanol (used at 5 to 10% of the fluid volume) and Sodium thiosulfate[16]. These materials will extend the temperature range of guar based fluids to over 350°F. Thiosulfate is the more effective of the two and is less hazardous to handle. These materials act as free radical scavengers that are present in the base water. An example would be free oxygen. Without the stabilizers these free radicals can naturally oxidize the polymer as described in the breakers section. Because breakers are free radical generators and these materials are free radical scavengers they should not be run at the same time.

water this "water block" effect is minimal. EGMBE (ethylene glycol monobutyl ether) used at 10 gal/1000 and BGMBE (butylene glycol monobutyl ether) used at 5 gal/1000 are common

Fracturing Fluid Components http://dx.doi.org/10.5772/56422 43

Biocides/Bactericides are added to minimize the enzymatic attack of the polymers used to gel the fracturing fluid by aerobic bacteria present in the base water. If not controlled the growth of micro-organisms will quickly degrade the polymer to a non-functional level. In addition biocides and bactericides are added to fracturing fluids to prevent the introduction of anae‐ robic sulfate reducing bacteria (SRB) into the reservoir. These bacteria can "sour" a well and produce corrosive hydrogen sulfide gas. They can also produce a black, slimy "biofilm" in wells that produce water which will block production. Quaternary amines, amides, aldehydes and Chlorine dioxide are effective biocides used in the industry[12]. The use of ultraviolet (UV) light as a disinfectant for fracturing water is also used[18]. A good functional bactericide not only kills the bacteria but also inactivates the enzymes that the bacteria release. Bacteria also mutate so can become resistant to a particular bactericide if used continuously i.e. use a variety

**Figure 16.** Residual Permeability to Dry Gas of a 0.5 md Berea Sandstone Core

**9. Biocides/Bactericides**

of bactericides to provide protection.

mutual solvents.
