**Abstract**

The materials and chemistry used to manufacture hydraulic fracture fluids are often confus‐ ing and difficult for the practicing hydraulic fracturing engineer to understand and opti‐ mize. Many times the failure of a particular fracturing treatment is blamed on the fluid because that is a major unknown from the design engineer's viewpoint. Many of the compo‐ nents and processes used to manufacture the fluid are held proprietary by the service com‐ pany which adds to the confusion and misunderstanding. This paper makes an attempt to describe the components used in fracturing fluids at a level that the practicing frac engineer can understand and use. The paper is intended as a companion paper to the Fracturing Flu‐ ids design paper which describes how to use the fluids and viscosity generated by the fluids to design a fracturing treatment.

### **1. Introduction**

#### **1.1. Water**

The water used for hydraulic fracturing is a critical component of the fluid. It must be carefully quality controlled as describe in the Quality Control Chapter. Typically the wa‐ ter is filtered to 50μ (microns) for propped fracturing treatments and to 2μ for frac and pack treatments. Fresh water is normally used but there are gelling agents available for seawater. The main disadvantage of seawater is the presence of Sulfate which can inter‐ act with connate reservoir water causing sulphate scales to form and provides a sulfur source for Sulfate reducing bacteria. The use of post frac flowback water is becoming

common especially for slickwater fracs. When flowback water is used to manufacture crosslinked gels care must be taken because the water may contain residual breaker.

**3.1. Polyacrylic Acid (PAAc)**

**3.2. Polyacrylamide (PAAm)**

PAAc which is a non-toxic synthetic high molecular weight polymer of acrylic acid. The material is sold as either a white solid or as a 50% active dispersion of the solid in mineral oil which makes it easy to disperse and solublize in water. The molecule is very sensitive to divalent cationic ions (cations) such as Ca, Mg, Fe etc. and will quickly precipitate if used in hard water. Other uses for PAAc include adsorbents for disposable diapers, ion exchange

Fracturing Fluid Components http://dx.doi.org/10.5772/56422 27

PAAm is formed from acrylamide subunits. It is non-toxic however unpolymerized acryla‐ mide is a neurotoxin and if the PAAm is not properly manufactured it can contain some unpolymerized acrylamide. As a solid PAAM is slower to hydrate than PAAc but is less sensitive to divalent cations. It is typically delivered to the field as a 50% active suspension of PAAM emulsified in mineral oil. The PAAm polymer is quite difficult to break and is used to gel 15% HCl so is damaging to the reservoir rock and proppant pack when used. When used in Slickwater fracturing Carman and Cawiezel [19] have reported successful breaker optimiza‐ tion for the material. Other uses for PAAm include flocculants for wastewater treatment and

PHPAis the most common friction reducer available. It is made by reacting sodium acrylate with acrylamide so that approximately 30 % of the acrylamide groups are in the hydrolyzed form. This improves the solubility in water, makes the polymer more compatible with cationic minerals and and is commonly marketed as a 50% active dispersion in mineral oil. Because it is widely used in industry as a flocculant for water and paper manufacture it is the least

AMPS is chemically structured so that the molecule is less susceptible to precipitation by cationic mineral salts which may be present in hard water or to high temperatures. It is also stable at a wide range of pH so that it is functional in energized fluids that contain CO2. The Sulfonate character of the polymer also makes it active as a scale inhibitor. It is typically marketed as a 50% active emulsion. Other uses for AMPS include electrocardiogram gels,

These materials are added to the fracturing fluid to increase the viscosity. This increases the fracture width so it can accept higher concentrations of proppant, reduces the fluid loss to improve fluid efficiency, improves proppant transport and reduces the friction pressure. The

resins, adhesives and as thickeners' for pharmaceuticals, cosmetics and paints.

papermaking, as a soil conditioner and for making soft contact lens.

plasticizers for concrete and as coagulants in water treatment processes.

**3.3. Partially Hydrolyzed Polyacylamide (PHPA)**

expensive FR and therefore the most widely used.

**3.4. AcrylamidoMethylPropane Sulfonate (AMPS)**

**4. Gelling agents**

#### **2. Clay control agents**

KCl or an organic clay stabilizer is added to the base fluid to prevent the water from interacting with the reservoir mineralogy. KCl is typically added at a concentration of 2% but can be added at concentrations as high as 8% depending on laboratory testing results. Most testing on the commercially available organic clay stabilizers, which are typically some form of Quaternary Amine compound, has found them to be ineffective at the normal concentrations recommend‐ ed. KCl is unique in its ability to stabilize clays and is much more effective than other inorganic salts such as NaCl, CaCl2 etc.

#### **3. Friction Reducers (FR)**

These materials are added to water to manufacture what is called "slickwater". They are added to reduce the friction generated as the fluid is pumped down the well tubulars. FR's are typically added to the frac fluid at a concentration of 0.25 to 2 gal/1000 gal. Figure 1 shows a comparison of the friction when pumping water, FR "Slickwater" and Guar "Waterfrac". There are several forms of FR which are also shown in Figure 1. They are:

**Figure 1.** Chemical Structure of various Friction Reduction (FR) agents and a comparison of friction pressure for water containing only 2% KCl vs. water containing 2% KCl and 2 gallons per 1000 gallons (FR) and 10# Guar pumped down 4 ½" 11.5# 4" ID casing.

#### **3.1. Polyacrylic Acid (PAAc)**

common especially for slickwater fracs. When flowback water is used to manufacture crosslinked gels care must be taken because the water may contain residual breaker.

KCl or an organic clay stabilizer is added to the base fluid to prevent the water from interacting with the reservoir mineralogy. KCl is typically added at a concentration of 2% but can be added at concentrations as high as 8% depending on laboratory testing results. Most testing on the commercially available organic clay stabilizers, which are typically some form of Quaternary Amine compound, has found them to be ineffective at the normal concentrations recommend‐ ed. KCl is unique in its ability to stabilize clays and is much more effective than other inorganic

These materials are added to water to manufacture what is called "slickwater". They are added to reduce the friction generated as the fluid is pumped down the well tubulars. FR's are typically added to the frac fluid at a concentration of 0.25 to 2 gal/1000 gal. Figure 1 shows a comparison of the friction when pumping water, FR "Slickwater" and Guar "Waterfrac". There

**Figure 1.** Chemical Structure of various Friction Reduction (FR) agents and a comparison of friction pressure for water containing only 2% KCl vs. water containing 2% KCl and 2 gallons per 1000 gallons (FR) and 10# Guar pumped down

are several forms of FR which are also shown in Figure 1. They are:

**2. Clay control agents**

26 Effective and Sustainable Hydraulic Fracturing

salts such as NaCl, CaCl2 etc.

**3. Friction Reducers (FR)**

4 ½" 11.5# 4" ID casing.

PAAc which is a non-toxic synthetic high molecular weight polymer of acrylic acid. The material is sold as either a white solid or as a 50% active dispersion of the solid in mineral oil which makes it easy to disperse and solublize in water. The molecule is very sensitive to divalent cationic ions (cations) such as Ca, Mg, Fe etc. and will quickly precipitate if used in hard water. Other uses for PAAc include adsorbents for disposable diapers, ion exchange resins, adhesives and as thickeners' for pharmaceuticals, cosmetics and paints.

#### **3.2. Polyacrylamide (PAAm)**

PAAm is formed from acrylamide subunits. It is non-toxic however unpolymerized acryla‐ mide is a neurotoxin and if the PAAm is not properly manufactured it can contain some unpolymerized acrylamide. As a solid PAAM is slower to hydrate than PAAc but is less sensitive to divalent cations. It is typically delivered to the field as a 50% active suspension of PAAM emulsified in mineral oil. The PAAm polymer is quite difficult to break and is used to gel 15% HCl so is damaging to the reservoir rock and proppant pack when used. When used in Slickwater fracturing Carman and Cawiezel [19] have reported successful breaker optimiza‐ tion for the material. Other uses for PAAm include flocculants for wastewater treatment and papermaking, as a soil conditioner and for making soft contact lens.

#### **3.3. Partially Hydrolyzed Polyacylamide (PHPA)**

PHPAis the most common friction reducer available. It is made by reacting sodium acrylate with acrylamide so that approximately 30 % of the acrylamide groups are in the hydrolyzed form. This improves the solubility in water, makes the polymer more compatible with cationic minerals and and is commonly marketed as a 50% active dispersion in mineral oil. Because it is widely used in industry as a flocculant for water and paper manufacture it is the least expensive FR and therefore the most widely used.

#### **3.4. AcrylamidoMethylPropane Sulfonate (AMPS)**

AMPS is chemically structured so that the molecule is less susceptible to precipitation by cationic mineral salts which may be present in hard water or to high temperatures. It is also stable at a wide range of pH so that it is functional in energized fluids that contain CO2. The Sulfonate character of the polymer also makes it active as a scale inhibitor. It is typically marketed as a 50% active emulsion. Other uses for AMPS include electrocardiogram gels, plasticizers for concrete and as coagulants in water treatment processes.

### **4. Gelling agents**

These materials are added to the fracturing fluid to increase the viscosity. This increases the fracture width so it can accept higher concentrations of proppant, reduces the fluid loss to improve fluid efficiency, improves proppant transport and reduces the friction pressure. The chemical structure of some gelling agents also allow for crosslinking. The viscosity of a gelling agent in solution is a function of its molecular weight. The viscosity increases with increasing chain length and concentration. Figure 2 shows how this occurs. For slick water the polymer concentration should be below the Critical Overlap Concentration C\*, for crosslinked gels the ideal range is between the C\* and the Critical Entanglement Concentration C\*\*. When the concentration exceeds the C\*\*a process call sineresis occurs in which the gel is over-crosslinked and water is "squeezed" out of the gel matrix. As water is removed from the polymer mixture as fluid loss occurs in the fracture the concentration of polymer increases dramatically causing damage to the proppant conductivity.

through the cis-hydroxyl functionality shown in Red and easily broken through the acetyl linkages shown in Blue. When Guar is broken it leave a 6 to 10% insoluble residue. To reduce this insoluble residue, improve the high temperature stability and improve the crosslinking performance in low pH fluids such as CO2 the molecular structure of guar is chemically modified with Propylene Oxide to form HPG and with Monochloric Acetic Acid to form CMG

Fracturing Fluid Components http://dx.doi.org/10.5772/56422 29

When using Guar or its derivatives the fluid loss control mechanism is "wall-building – i.e. C-III" in that when the base fluid leaks off the polymer is deposited on the rock face forming a filter cake. The initial leakoff is quite rapid and is called "Spurt". Once a filter cake forms the leak-off becomes a function of the square root of time as described in the companion paper on

When mixing dry powered Guar, care must be taken to avoid "fisheyes by adjusting the pH of the base water to above 7 and using a high energy mixer to allow proper dispersion. Once the polymer is dispersed the pH is adjusted to just below 6 to allow hydration. Most modern commercially packaged powered Guar systems contain a buffer package that automatically adjusts the pH of the water as the powder is added to prevent fisheyes. When packaged systems are hydrated the pH of the base water needs to be near neutral and a high energy mixer used. Care must also be taken when using very cold water (<60°F) because the rate of solution for the buffer packages can be affected. Guar emulsified in mineral oil as a 50% active

or CMHPG. The chemical process is shown in Figure 6.

Fracturing Fluids.

**Figure 3.** Guar

material is also commonly used.

**Figure 2.** Intrinsic Viscosity of a Solution as a Function of the Polymer Concentration

#### **4.1. Guar**

Guar and its derivatives HydroxyPropyl Guar (HPG), CarboxyMethyl Guar (CMG) and CarboxyMethylHydroxyPropyl Guar (CMHPG) are the most common gelling agents used for fracturing. As shown in Figure 3 Guar [Cyamopsis tetragonoloba] is a natural glactomannan gum of the Legume family which is mostly grown in India. Beckwith[1] provides a very nice summary of guar and reports that in 2012 the industry used about 25,000 tons of guar a month at a wholesale cost of \$1,723 US/100 kg (\$7.83/lb).

After harvesting the seed coat and germ are removed to form what is called a Guar Split. This Guar Split is ground to form guar powder. This process is shown diagrammatically in Figure 4. The chemical structure of guar (See Figure 5) is unique in that it can be readily crosslinked

#### **Figure 3.** Guar

chemical structure of some gelling agents also allow for crosslinking. The viscosity of a gelling agent in solution is a function of its molecular weight. The viscosity increases with increasing chain length and concentration. Figure 2 shows how this occurs. For slick water the polymer concentration should be below the Critical Overlap Concentration C\*, for crosslinked gels the ideal range is between the C\* and the Critical Entanglement Concentration C\*\*. When the concentration exceeds the C\*\*a process call sineresis occurs in which the gel is over-crosslinked and water is "squeezed" out of the gel matrix. As water is removed from the polymer mixture as fluid loss occurs in the fracture the concentration of polymer increases dramatically causing

damage to the proppant conductivity.

28 Effective and Sustainable Hydraulic Fracturing

**4.1. Guar**

**Figure 2.** Intrinsic Viscosity of a Solution as a Function of the Polymer Concentration

at a wholesale cost of \$1,723 US/100 kg (\$7.83/lb).

Guar and its derivatives HydroxyPropyl Guar (HPG), CarboxyMethyl Guar (CMG) and CarboxyMethylHydroxyPropyl Guar (CMHPG) are the most common gelling agents used for fracturing. As shown in Figure 3 Guar [Cyamopsis tetragonoloba] is a natural glactomannan gum of the Legume family which is mostly grown in India. Beckwith[1] provides a very nice summary of guar and reports that in 2012 the industry used about 25,000 tons of guar a month

After harvesting the seed coat and germ are removed to form what is called a Guar Split. This Guar Split is ground to form guar powder. This process is shown diagrammatically in Figure 4. The chemical structure of guar (See Figure 5) is unique in that it can be readily crosslinked through the cis-hydroxyl functionality shown in Red and easily broken through the acetyl linkages shown in Blue. When Guar is broken it leave a 6 to 10% insoluble residue. To reduce this insoluble residue, improve the high temperature stability and improve the crosslinking performance in low pH fluids such as CO2 the molecular structure of guar is chemically modified with Propylene Oxide to form HPG and with Monochloric Acetic Acid to form CMG or CMHPG. The chemical process is shown in Figure 6.

When using Guar or its derivatives the fluid loss control mechanism is "wall-building – i.e. C-III" in that when the base fluid leaks off the polymer is deposited on the rock face forming a filter cake. The initial leakoff is quite rapid and is called "Spurt". Once a filter cake forms the leak-off becomes a function of the square root of time as described in the companion paper on Fracturing Fluids.

When mixing dry powered Guar, care must be taken to avoid "fisheyes by adjusting the pH of the base water to above 7 and using a high energy mixer to allow proper dispersion. Once the polymer is dispersed the pH is adjusted to just below 6 to allow hydration. Most modern commercially packaged powered Guar systems contain a buffer package that automatically adjusts the pH of the water as the powder is added to prevent fisheyes. When packaged systems are hydrated the pH of the base water needs to be near neutral and a high energy mixer used. Care must also be taken when using very cold water (<60°F) because the rate of solution for the buffer packages can be affected. Guar emulsified in mineral oil as a 50% active material is also commonly used.

**Figure 4.** The process of manufacturing Guar Powder

#### **4.2. HydroxyEthyl Cellulose (HEC)**

HEC and CarboxyMethylHydroxyEthyl Cellulose (CMHEC) are derivatives of cellulose which is the most common organic compound on Earth. About 33% of all plant matter is a cellusosan organic compound with the formula (C6H10O5)n, a polysaccharide consisting of a linear chain of several hundred to over ten thousand linked glucose units. As with Guar, Cellulose can be reacted with Propylene Oxide and/or Monochloric Acetic Acid to produce HEC or CMHEC. The chemical makeup of HEC and CMHEC is shown in Figure 9. The base cellulose used to make HEC and CMHEC comes mainly from cotton which is 90% cellulose. HEC and CMHEC are non-toxic and hypoallergenic and are widely used as a viscosifer and emulsion stabilizer in ice cream, K-Y Jelly, toothpaste, cosmetics, laxatives, diet pills, water-based paints, textile sizing and paper.

above 60 to 80 lb of polymer/1000 gallons of water it becomes difficult to mix. Because the hydroxyls in HEC are in the trans- position (See Figure 9) it cannot be crosslinked and can only be used as a linear gel. The addition of the Carboxy Methyl group in CMHEC provides a crosslinking site so it can be crosslinked using the same mechanisms described for Guar.

Fracturing Fluid Components http://dx.doi.org/10.5772/56422 31

VES are polymer free aqueous based fracturing fluids that generate their viscosity through the association of surfactant molecules (Figure 10). As the concentration of surfactant is increased the molecules reach a point where they form aggregates called micelles where the hydrophobic tails form the core of the aggregate and the hydrophilic heads are in contact with the sur‐ rounding aqueous liquid. This occurs at a point called the Critical Micelle Concentration (CMC). As the concentration of micelles increase they become entangled with one another at C\* as shown in Figure 10. Typically this point is at about 4 to 6% by weight of surfactant. Anionic, cationic and zwitterionic surfactants are used to formulate VES fluids. The main advantage of these fluids is that they are non-damaging to the fracture conductivity. Fluid loss is "Viscosity- Controlled – i.e. C-II" which make the fluids particularly appropriate for Frac and Pack applications. Breaking is accomplished by overflushing with a Mutual Solvent, using an encapsulated electrolyte or by dilution. The main disadvantage these fluids have is their strong surfactant base which makes them incompatible with many reservoir fluids. The

**4.3. ViscoElastic Surfactant (VES)**

**Figure 5.** The chemical structure of Guar

Because HEC and CMHEC is 100% soluble in water and contain very little insoluble residue they are used where conductivity is the main driver for design. This is in applications such as gravel packing and Frac/Packing. The fluid loss mechanism is "Viscosity- Controlled – i.e. C-II". To control fluid loss the polymers are used to produce very viscous linear gels. However

**Figure 5.** The chemical structure of Guar

above 60 to 80 lb of polymer/1000 gallons of water it becomes difficult to mix. Because the hydroxyls in HEC are in the trans- position (See Figure 9) it cannot be crosslinked and can only be used as a linear gel. The addition of the Carboxy Methyl group in CMHEC provides a crosslinking site so it can be crosslinked using the same mechanisms described for Guar.

#### **4.3. ViscoElastic Surfactant (VES)**

**4.2. HydroxyEthyl Cellulose (HEC)**

30 Effective and Sustainable Hydraulic Fracturing

**Figure 4.** The process of manufacturing Guar Powder

sizing and paper.

HEC and CarboxyMethylHydroxyEthyl Cellulose (CMHEC) are derivatives of cellulose which is the most common organic compound on Earth. About 33% of all plant matter is a cellusosan organic compound with the formula (C6H10O5)n, a polysaccharide consisting of a linear chain of several hundred to over ten thousand linked glucose units. As with Guar, Cellulose can be reacted with Propylene Oxide and/or Monochloric Acetic Acid to produce HEC or CMHEC. The chemical makeup of HEC and CMHEC is shown in Figure 9. The base cellulose used to make HEC and CMHEC comes mainly from cotton which is 90% cellulose. HEC and CMHEC are non-toxic and hypoallergenic and are widely used as a viscosifer and emulsion stabilizer in ice cream, K-Y Jelly, toothpaste, cosmetics, laxatives, diet pills, water-based paints, textile

Because HEC and CMHEC is 100% soluble in water and contain very little insoluble residue they are used where conductivity is the main driver for design. This is in applications such as gravel packing and Frac/Packing. The fluid loss mechanism is "Viscosity- Controlled – i.e. C-II". To control fluid loss the polymers are used to produce very viscous linear gels. However

VES are polymer free aqueous based fracturing fluids that generate their viscosity through the association of surfactant molecules (Figure 10). As the concentration of surfactant is increased the molecules reach a point where they form aggregates called micelles where the hydrophobic tails form the core of the aggregate and the hydrophilic heads are in contact with the sur‐ rounding aqueous liquid. This occurs at a point called the Critical Micelle Concentration (CMC). As the concentration of micelles increase they become entangled with one another at C\* as shown in Figure 10. Typically this point is at about 4 to 6% by weight of surfactant. Anionic, cationic and zwitterionic surfactants are used to formulate VES fluids. The main advantage of these fluids is that they are non-damaging to the fracture conductivity. Fluid loss is "Viscosity- Controlled – i.e. C-II" which make the fluids particularly appropriate for Frac and Pack applications. Breaking is accomplished by overflushing with a Mutual Solvent, using an encapsulated electrolyte or by dilution. The main disadvantage these fluids have is their strong surfactant base which makes them incompatible with many reservoir fluids. The

**Figure 6.** The formulation of HPG, CMG and CMHPG from Guar

surfactants are so strong they have been known to upset even very high API condensate type hydrocarbons.

> case of polyemulsions and gelled Propane, pumping a flammable fluid. CO2 has an additional hazard in that it can cause dry ice plugs as pressure is reduced. These fluids are generally also

Fracturing Fluid Components http://dx.doi.org/10.5772/56422 33

Oil based fluids are used on water-sensitive formations that may experience significant damage from contact with water based fluids. The first frac fluid used to fracture a well used Palm Oil as the gelling agent, Naphthenic Acid as the crosslinker and gasoline at the base fluid. Today most crosslinked oil based fracturing fluids use an aluminum phosphate-ester chem‐ istry[5] that was originally developed to gel hydraulic oils. The aluminum phosphate-esters form a three dimensional structure similar to that described in the VES section. Because the aluminum will attract any polar species the presence of water in the base oil/crude will cause excess viscosity and will adversely affect the thermal stability of the fluid. Breaking of the fluid is accomplished by buffering the pH which causes the association between the base oil and the ester to break down. Although some crude oils have particulate which could build a filter cake, fluid loss is generally considered to be "Viscosity- Controlled – i.e. C-II". There are some disadvantages in using gelled oils. Gelling problems can occur when using high viscosity crude oils or crude oils which contain a lot of naturally occurring surfactants. When using refined oils such as diesel the cost is very high and the oil must be collected at the refinery before any additives such as pour point depressants, engine cleaning surfactants etc. are added. Also there

more expensive and the gases may not be available in remote areas.

**Figure 7.** Chemical Structure of HydroxyPropyl Guar (HPG)

**4.5. Oil based fluids**

#### **4.4. Foam/PolyEmulsions**

Foam/polyemulsions are fluids that are composed of a material that is not miscible with water. This could be Nitrogen, Carbon dioxide or a hydrocarbon such as Propane, diesel or conden‐ sate. These fluids are very clean, have very good fluid loss control, provide excellent proppant transport and break easily simply via gravity separation. PolyEmulsions are formed by emulsifying a hydrocarbon such as Condensate or Diesel with water such that the hydrocarbon is the external phase. The viscosity is controlled by varying the hydrocarbon/water ratio. Foams made with Nitrogen or Carbon dioxide is generally 65 to 80% (termed 65 to 80 quality) gas in a water carrying media which contains a surfactant based foaming agent. Sometimes N2 or CO2 are added at a lower concentration (20 to 30 quality) to form "Energized Fluids". This is done to reduce the amount of water placed on the formation and to provide additional energy to aid in load recover during the post-frac flow back period. Nitrogen can dissipate into the reservoir quite quickly so fluids energized with N2 should be flowed back as soon as the fracture is closed. CO2, under most conditions, is in a dense phase at static down hole conditions (prior to the well being placed on production), so is less susceptible to dissipation. CO2 does dissolve in crude oil so will act to reduce the crude viscosity which, again, improves cleanup and rapid recovery. When N2/CO2 are added is qualities greater than 90% the resulting mixture is termed a mist with a "0" viscosity. This quality is normally not used in fracturing. The main disadvantage of these fluids is safety i.e. pumping a gas at high pressure or in the

**Figure 7.** Chemical Structure of HydroxyPropyl Guar (HPG)

case of polyemulsions and gelled Propane, pumping a flammable fluid. CO2 has an additional hazard in that it can cause dry ice plugs as pressure is reduced. These fluids are generally also more expensive and the gases may not be available in remote areas.

#### **4.5. Oil based fluids**

surfactants are so strong they have been known to upset even very high API condensate type

Foam/polyemulsions are fluids that are composed of a material that is not miscible with water. This could be Nitrogen, Carbon dioxide or a hydrocarbon such as Propane, diesel or conden‐ sate. These fluids are very clean, have very good fluid loss control, provide excellent proppant transport and break easily simply via gravity separation. PolyEmulsions are formed by emulsifying a hydrocarbon such as Condensate or Diesel with water such that the hydrocarbon is the external phase. The viscosity is controlled by varying the hydrocarbon/water ratio. Foams made with Nitrogen or Carbon dioxide is generally 65 to 80% (termed 65 to 80 quality) gas in a water carrying media which contains a surfactant based foaming agent. Sometimes N2 or CO2 are added at a lower concentration (20 to 30 quality) to form "Energized Fluids". This is done to reduce the amount of water placed on the formation and to provide additional energy to aid in load recover during the post-frac flow back period. Nitrogen can dissipate into the reservoir quite quickly so fluids energized with N2 should be flowed back as soon as the fracture is closed. CO2, under most conditions, is in a dense phase at static down hole conditions (prior to the well being placed on production), so is less susceptible to dissipation. CO2 does dissolve in crude oil so will act to reduce the crude viscosity which, again, improves cleanup and rapid recovery. When N2/CO2 are added is qualities greater than 90% the resulting mixture is termed a mist with a "0" viscosity. This quality is normally not used in fracturing. The main disadvantage of these fluids is safety i.e. pumping a gas at high pressure or in the

hydrocarbons.

**4.4. Foam/PolyEmulsions**

32 Effective and Sustainable Hydraulic Fracturing

**Figure 6.** The formulation of HPG, CMG and CMHPG from Guar

Oil based fluids are used on water-sensitive formations that may experience significant damage from contact with water based fluids. The first frac fluid used to fracture a well used Palm Oil as the gelling agent, Naphthenic Acid as the crosslinker and gasoline at the base fluid. Today most crosslinked oil based fracturing fluids use an aluminum phosphate-ester chem‐ istry[5] that was originally developed to gel hydraulic oils. The aluminum phosphate-esters form a three dimensional structure similar to that described in the VES section. Because the aluminum will attract any polar species the presence of water in the base oil/crude will cause excess viscosity and will adversely affect the thermal stability of the fluid. Breaking of the fluid is accomplished by buffering the pH which causes the association between the base oil and the ester to break down. Although some crude oils have particulate which could build a filter cake, fluid loss is generally considered to be "Viscosity- Controlled – i.e. C-II". There are some disadvantages in using gelled oils. Gelling problems can occur when using high viscosity crude oils or crude oils which contain a lot of naturally occurring surfactants. When using refined oils such as diesel the cost is very high and the oil must be collected at the refinery before any additives such as pour point depressants, engine cleaning surfactants etc. are added. Also there

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

are greater concerns regarding personnel safety and environmental impact, as compared to most water-fluids.

**5.1. Borate**

apparent.

crosslinking.

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

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

Fracturing Fluid Components http://dx.doi.org/10.5772/56422 35

**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
