**1. Introduction**

The selection of a proper fracturing fluid is all about choices. It begins with choosing the pad volume where one must consider what and how much pad is required to create the desired fracture geometry. This is followed by choosing how much viscosity the fluid needs to have to:


**•** Control fluid loss. In cases where a gel filter cake cannot form the fracturing fluid viscosity (i.e. CI) may be the main mechanism for fluid loss control.

reduces the 19% error in width to only a 9.5% error in fluid volume requirements. While such an error is not desirable it does illustrate that precise viscosity data is not a requirement for treatment design which is fortunate since the measurement of the viscosity of fracturing fluids is such a difficult task. This complexity combined with multiple methods for testing and

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There are several types of fracturing fluids and a wide and confusing range of fluid additives.

The fracturing fluids that were used in the first experimental treatments were composed of gasoline gelled with Palm Oil and crosslinked with Naphthenic Acid. This technology was developed during the Second World War and is commonly referred to as Nalpalm. Because of the hazards associated with this fluid and its relatively high cost work was done to develop safer fluids where the base fluid was water. The vast majority of fracturing fluids used today use water as the base fluid. Generally, the components that make up crosslinked fracturing fluids include a polymer, buffer, gel stabilizer or breaker and a crosslinker. Each of these components is critical to the development of the desired fracturing fluid properties. The role of polymers in fracturing fluids is to provide fracture width, to suspend proppants, to help provide fracture width, to help control fluid loss to the formation, and to reduce friction pressure in the tubular goods. Guar gum and cellulosic derivatives are the most common types of polymers used in fracturing fluids. The first patent (US Patent 3058909) on guar crosslinked

reporting viscosity data makes the selection of precise values virtually impossible.

The types of fluids include:

**•** Water based fluids

**•** Oil based fluids

**•** Energized fluids

**•** Acid Fluids

**•** Gelling agents

**•** Crosslinkers

**•** Bactericides

**2. History**

**•** Breakers

**•** Multi-phase emulsions

The additives include:

**•** Fluid loss additives

**•** Clay control Additives.

**•** Surfactants and Non-emulsifing agents

This choice system continues when it comes to selecting the appropriate fluid system for a propped or acid frac treatment. The considerations include:


In summary an ideal fracturing fluid would be one that would have an easily measured controllable viscosity, controllable fluid loss characteristics, would not damage the fracture or interact with the formation fluid, would be completely harmless and inert and cost less the \$4.00 US/ gallon. Unfortunately this is currently not possible so compromises have to be made. Typically cost is the driving force and chooses are made which can be disastrous to the PI of the well.

Of these factors the fluid viscosity is the major fluid related parameter for fracture design. However, **how much viscosity needed is often overrestimated**. Excessive viscosity increases costs, raises treating pressure which may cause undesired height growth, and can reduce fracture conductivity since many of the chemicals used to increase viscosity leave residue which damages the proppant permeability.

The need for a precise value of viscosity is also over engineered. This can be seen from the basic equations where treating pressure, and thus fracture width, is proportional to viscosity raised to the ¼ power (for a Newtonian fluid).

 $p\_{net} \propto \frac{E^{\*3\psi}}{H} \text{L}\mu\text{QL}$   $\mathbf{J}^{1/4} \star P\_{Tip}$ 

Thus a 100% error in viscosity results in an error of about 19% in calculating fracture width. This error would, of course, lead to an error in the fluid volume requirements for a particular job. However, further assuming that 1/2 of the fracturing fluid leaks off to the formation reduces the 19% error in width to only a 9.5% error in fluid volume requirements. While such an error is not desirable it does illustrate that precise viscosity data is not a requirement for treatment design which is fortunate since the measurement of the viscosity of fracturing fluids is such a difficult task. This complexity combined with multiple methods for testing and reporting viscosity data makes the selection of precise values virtually impossible.

There are several types of fracturing fluids and a wide and confusing range of fluid additives. The types of fluids include:


**•** Control fluid loss. In cases where a gel filter cake cannot form the fracturing fluid viscosity

This choice system continues when it comes to selecting the appropriate fluid system for a

**•** Environmentally Friendly – The composition of the fluid should be as "green" as possible. **•** Breaker – The fluid must "break" to a low viscosity so that it can flow back and allow clean-

**•** Cost Effective – The fluid must be economical and not drive the treatment cost to an

**•** Compatibility – The fluid must not interact and caused damage with the formation miner‐

**•** Clean-up – The fluid should not damage the fracture conductive of the fracture or, to prevent water blocks, change the relative permeability of the formation. This becomes very impor‐

**•** Easy to Mix – The fluid system must be easy to mix even under very adverse conditions. **•** Fluid Loss – The fluid need to help control fluid loss. An ideal fluid should have fluid loss

In summary an ideal fracturing fluid would be one that would have an easily measured controllable viscosity, controllable fluid loss characteristics, would not damage the fracture or interact with the formation fluid, would be completely harmless and inert and cost less the \$4.00 US/ gallon. Unfortunately this is currently not possible so compromises have to be made. Typically cost is the driving force and chooses are made which can be disastrous to the PI of

Of these factors the fluid viscosity is the major fluid related parameter for fracture design. However, **how much viscosity needed is often overrestimated**. Excessive viscosity increases costs, raises treating pressure which may cause undesired height growth, and can reduce fracture conductivity since many of the chemicals used to increase viscosity leave residue

The need for a precise value of viscosity is also over engineered. This can be seen from the basic equations where treating pressure, and thus fracture width, is proportional to viscosity

Thus a 100% error in viscosity results in an error of about 19% in calculating fracture width. This error would, of course, lead to an error in the fluid volume requirements for a particular job. However, further assuming that 1/2 of the fracturing fluid leaks off to the formation

(i.e. CI) may be the main mechanism for fluid loss control.

tant in low pressure wells or wells that produce very dry gas.

**•** Safe – The fluid should expose the on-site personnel to a minimal danger.

propped or acid frac treatment. The considerations include:

up of the fracture.

unacceptable level.

flexibility.

the well.

*pnet* <sup>∝</sup> *<sup>E</sup>*'

3/4

alogy and/or formation fluids.

4 Effective and Sustainable Hydraulic Fracturing

which damages the proppant permeability.

raised to the ¼ power (for a Newtonian fluid).

*<sup>H</sup> μQL* 1/4 <sup>+</sup> *PTip*


The additives include:


#### **2. History**

The fracturing fluids that were used in the first experimental treatments were composed of gasoline gelled with Palm Oil and crosslinked with Naphthenic Acid. This technology was developed during the Second World War and is commonly referred to as Nalpalm. Because of the hazards associated with this fluid and its relatively high cost work was done to develop safer fluids where the base fluid was water. The vast majority of fracturing fluids used today use water as the base fluid. Generally, the components that make up crosslinked fracturing fluids include a polymer, buffer, gel stabilizer or breaker and a crosslinker. Each of these components is critical to the development of the desired fracturing fluid properties. The role of polymers in fracturing fluids is to provide fracture width, to suspend proppants, to help provide fracture width, to help control fluid loss to the formation, and to reduce friction pressure in the tubular goods. Guar gum and cellulosic derivatives are the most common types of polymers used in fracturing fluids. The first patent (US Patent 3058909) on guar crosslinked by borate was issued to Loyd Kern with Sinclair (later ARCO) on October 16, 1962. Metal-based crosslinking agents developed by DuPont for plastic explosive applications were found to be useful for manufacturing fracturing fluids for high temperature applications2 . Cellulosic derivatives are residue-free and thus help minimize fracturing fluid damage to the formation and are widely used in Frac and Pack applications. The cellulosic derivatives are difficult to disperse because of their rapid rate of hydration. Guar gum and its derivatives are easily dispersed but produce some residue when broken. Strong oxidizing agents such as Sodium or Ammonium persulfate are added to the fracturing fluids to break the polymer as it reaches temperature. The first patent (US Patent 3163219) on borate gel breakers was issued to Tom Perkins, also with Sinclair, on December 29, 1964.

**Chemical Name CAS Number Chemical Purpose Product**

007647-01-0 Removes acid soluble minerals and weakens the

000111-30-8 Eliminates bacteria in the water to prevent frac

055566-30-8 Eliminates bacteria in the water to prevent frac

007727-54-0 Breaks the polymer that is used to create the

1335-26-8 Delays the breakdown of the fracturing fluid gelling

1309-48-4 Delays the cross linking of the fracturing fluid

fracturing fluid

agent

gelling agent

rock to allow lower fracture iniciation pressures.

polymer premature breakdown and well souring

polymer premature breakdown and well souring

007647-14-5 Product Stabilizer Breaker NR

10043-52-4 Product Stabilizer and Freeze Protection Buffer NR

67-48-1 Prevents clays from swelling or migrating Clay Stabilizer 5\*

012125-02-9 Clay Stabilizer – Compatible with Mud Acid Clay Stabilizer 4\*,9\*\*

007447-40-7 Prevents clays from swelling or migrating Clay Stabilizer 5\*,5\*\*

000075-57-0 Prevents clays from swelling or migrating Clay Stabilizer 3\*,6\*\*

007647-14-5 NR

000067-63-0 Winterizing agent Winterizing

000067-56-1 Winterizing agent Winterizing

000075-07-0 Prevents the corrosion of the pipe Corrosion

000064-18-6 pH adjustment pH adjustment 4\*.8\*\*

63393-96-4 Clay Control Agents Biocides and

Hydrochloric Acid

Glutaraldehyde C5H8O2

Quaternary Ammonium Chloride Compounds

Tetrakis Hydroxymethyl-Phosphonium Sulfate C8H24O8P2.SO4

Ammonium Persulfate

(NH4)2S2O8

NaCl

MgO2

MgO

CaCl2

NH4Cl

KCl

chloride (CH3)4NCl

NaCl

Sodium Chloride

Magnesium Peroxide

Magnesium Oxide

Calcium Chloride

Choline Chloride [HOCH2CH2N+(CH3)3]C

Potassium chloride

Sodium Chloride

Isopropanol CH3CH(OH)CH3

Methanol CH3OH

Formic Acid HCOOH

Acetaldehyde CH3CHO

Tetramethyl ammonium

Ammonium Chloride

HCl

**Function**

Acid 4\*,8\*\*

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Biocide 3\*,6\*\*

Clay Stabilizers

Biocide NR

Breaker 4\*,5\*\*

Breaker 5\*\*

Buffer 4\*

agent and Surface Tension Reduction

agent

Inhibitor

3\*\*

3\*, 3\*\*

4\*,3\*\*

Hazard Rating1

Fracturing Fluids

7

3\*\*

Buffers are used in conjunction with polymers so that the optimal pH for polymer hydration can be attained. When the optimal pH is reached, the maximal viscosity yield from the polymer is obtained. The most common example of fracturing fluid buffers is a weak-acid/weak-base blend, whose ratios can be adjusted so that the desired ph is reached. Some of these buffers dissolve slowly allowing the crosslinking reaction to be delayed.

Gel stabilizers are added to polymer solutions to inhibit chemical degradation. Examples of gel stabilizers used in fracturing fluids include methanol, TriEthanol Amine (TEA) and various inorganic sulfur compounds. Other stabilizers are useful in inhibiting the chemical degrada‐ tion process, but many interfere with the mechanism of crosslinking. The TEA and sulfur containing stabilizers possess an advantage over methanol, which is flammable, toxic, expensive and can cause poisoning of reactor tower catalists.

There has been a huge volume of work done on fracturing fluids and their components. If a search is done on One Petro (http://www.onepetro.org) using "Fracturing Fluids" as the search item over 15,000 hits will result. Just using one of the main gelling agents used to manufacture water based fracturing fluid "Guar" results in over 400 hits. There are several good references 3,4,5,6 that discuss the current state of the art for fracturing fluids if the reader is interested in a more in depth study of fracturing fluids.

Another issue that has recently come to the forefront of fracturing fluids is their threat to the environment through the contamination of the groundwater. George King put it very elegantly in his JPT article7 where he says "The use of horizontal wells and hydraulic fracturing is so effective that it has been called "disruptive". That is, it threatens the profitability and continued development of other energy sources, such as wind and solar, because it is much less expensive and far more reliable." The internal Apache article8 that George wrote has 204 references on the subject. Table 1,2,3 provides a summary of all the various chemicals used to make Hy‐ draulic Fracturing fluids along with a degree of hazard rating from both the US Department of Transportation and the European Union Poison Class rating. There certainly are several of these chemicals that one must take care with when handling at their full concentrations but when used to manufacture fracturing fluids the concentrations are very dilute and pose very low hazards.


by borate was issued to Loyd Kern with Sinclair (later ARCO) on October 16, 1962. Metal-based crosslinking agents developed by DuPont for plastic explosive applications were found to be

derivatives are residue-free and thus help minimize fracturing fluid damage to the formation and are widely used in Frac and Pack applications. The cellulosic derivatives are difficult to disperse because of their rapid rate of hydration. Guar gum and its derivatives are easily dispersed but produce some residue when broken. Strong oxidizing agents such as Sodium or Ammonium persulfate are added to the fracturing fluids to break the polymer as it reaches temperature. The first patent (US Patent 3163219) on borate gel breakers was issued to Tom

Buffers are used in conjunction with polymers so that the optimal pH for polymer hydration can be attained. When the optimal pH is reached, the maximal viscosity yield from the polymer is obtained. The most common example of fracturing fluid buffers is a weak-acid/weak-base blend, whose ratios can be adjusted so that the desired ph is reached. Some of these buffers

Gel stabilizers are added to polymer solutions to inhibit chemical degradation. Examples of gel stabilizers used in fracturing fluids include methanol, TriEthanol Amine (TEA) and various inorganic sulfur compounds. Other stabilizers are useful in inhibiting the chemical degrada‐ tion process, but many interfere with the mechanism of crosslinking. The TEA and sulfur containing stabilizers possess an advantage over methanol, which is flammable, toxic,

There has been a huge volume of work done on fracturing fluids and their components. If a search is done on One Petro (http://www.onepetro.org) using "Fracturing Fluids" as the search item over 15,000 hits will result. Just using one of the main gelling agents used to manufacture water based fracturing fluid "Guar" results in over 400 hits. There are several good references 3,4,5,6 that discuss the current state of the art for fracturing fluids if the reader is interested in a

Another issue that has recently come to the forefront of fracturing fluids is their threat to the environment through the contamination of the groundwater. George King put it very elegantly

effective that it has been called "disruptive". That is, it threatens the profitability and continued development of other energy sources, such as wind and solar, because it is much less expensive

the subject. Table 1,2,3 provides a summary of all the various chemicals used to make Hy‐ draulic Fracturing fluids along with a degree of hazard rating from both the US Department of Transportation and the European Union Poison Class rating. There certainly are several of these chemicals that one must take care with when handling at their full concentrations but when used to manufacture fracturing fluids the concentrations are very dilute and pose very

where he says "The use of horizontal wells and hydraulic fracturing is so

that George wrote has 204 references on

. Cellulosic

useful for manufacturing fracturing fluids for high temperature applications2

Perkins, also with Sinclair, on December 29, 1964.

6 Effective and Sustainable Hydraulic Fracturing

dissolve slowly allowing the crosslinking reaction to be delayed.

expensive and can cause poisoning of reactor tower catalists.

more in depth study of fracturing fluids.

and far more reliable." The internal Apache article8

in his JPT article7

low hazards.


**Chemical Name CAS Number Chemical Purpose Product**

000151-21-3 Used to prevent the formation of emulsions in the reservoir and to improve fluid recovery

001310-73-2 Adjusts the pH of fluid to initiate the effectiveness of other components, such as crosslinkers

001310-58-3 Adjusts the pH of fluid to initiate the effectiveness of other components, such as crosslinkers

effectiveness of other components, such as

effectiveness of other components, such as

007446-81-3 Prevents scale deposits in the pipe or in the fracture Scale Inhibitor NR

000091-20-3 Carrier fluid for the active surfactant ingredients Surfactant 3\*,4\*\*

000111-76-2 Surface Tension Reduction for Fluid Recovery Surfactant 4\*, 6\*\*

000497-19-8 Adjusts the pH of fluid to maintains the

000584-08-7 Adjusts the pH of fluid to maintains the

Sodium Polycarboxylate N/A Prevents scale deposits in the pipe Scale Inhibitor Phosphonic Acid Salt N/A Prevents scale deposits in the pipe Scale Inhibitor

1 – Hazard Rating – An attempt was made to rate the hazard associated with each of the chemicals listed. The first number with the single \* is the Poison Hazard as defined by the EU/Swiss Poison Class while the second number with the double \*\* is the transportation Hazard as defined by the US Department of Transportation (DOT). If a NR is present in the box

**Table 1.** A summary of the various chemicals used to make Hydraulic Fracturing fluids along with a degree of hazard

rating. Modified from " www. http://fracfocus.org/chemical-use/what-chemicals-are-used"

*Class Lethal Dose (mg/kg)*

*1S* 0 to 5, also teratogenic or carcinogenis

*1* 0 to 5

*2* 5 to 50

crosslinkers

crosslinkers

no rating was found and the substance was normally non-hazardous.

Lauryl Sulfate and its Derivatives C12H25OSO2ONa

Sodium Hydroxide

Potassium Hydroxide

Sodium Carbonate

Potassium Carbonate

Sodium Acrylate and Copolymers of Acrylamide

NaOH

KOH

Na2CO3

K2CO3

C3H3O2. Na

Naphthalene C10H8

Ethylene glycol

monobutyl ether - EGMBE C4H9OCH2CH2OH

\* EU/Swiss Poison Class

**Function**

and Surfactants

Non-Emulsifier

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pH Adjusting Agent

pH Adjusting Agent

pH Adjusting Agent

pH Adjusting Agent

Hazard Rating1

Fracturing Fluids

9

4\*

4\*,8\*\*

2\*,8\*\*

5\*,5\*\*

4\*


1 – Hazard Rating – An attempt was made to rate the hazard associated with each of the chemicals listed. The first number with the single \* is the Poison Hazard as defined by the EU/Swiss Poison Class while the second number with the double \*\* is the transportation Hazard as defined by the US Department of Transportation (DOT). If a NR is present in the box no rating was found and the substance was normally non-hazardous.

\* EU/Swiss Poison Class

**Chemical Name CAS Number Chemical Purpose Product**

Chelated Zirconium Crosslinker for High Temperature or low pH Fluids Crosslinker

agent.

000067-56-1 Surface Tension Reduction and / or winterizing

009000-30-0 Thickens the water in order to suspend the proppant and reduce friction

proppant and reduce friction

Xanthan gum 11138-66-2 Thickens Acid in order to control fluid loss Gelling Agent NR

000064-19-7 Prevents precipitation of metal oxides and pH

control

crosslinkers

064742-47-8 Carrier fluid for gelling agents, friction reducers and

013709-94-9 Crosslinker for borate crosslinked fluids Crosslinker 3\*

001330-43-4 Crosslinker for borate crosslinked fluids Crosslinker 4\*

13343-35-3 Crosslinker for borate crosslinked fluids Crosslinker 4\*

7699-43-6 Inorganic Clay Stabilizer Clay Stabilizer 4\*

000107-21-1 Product stabilizer and / or winterizing agent. Winterizing

000064-17-5 Product stabilizer and / or winterizing agent. Fluid Recovery

009003-05-8 "Slicks" the water to minimize friction Friction

Thickens the water in order to suspend the

000077-92-9 Prevents precipitation of metal oxides Iron Control 5\*,8\*\*

000068-11-1 Prevents precipitation of metal oxides Iron Control 3\*,8\*\*

006381-77-7 Prevents precipitation of metal oxides Iron Control NR

102-71-6 Maintains fluid viscosity as temperature increases Fluid Stabilizer 5\*,3\*\*

Hydrotreated Light Petroleum Distillate

8 Effective and Sustainable Hydraulic Fracturing

Potassium Metaborate

Triethanolamine (TEA) N(CH2CH2OH)3

Sodium Tetraborate

Zirconium oxychloride

Ethylene Glycol OCH2CH2OH

KBO2

Na2B4O7

ZrCl2O

Methanol CH3OH

Ethanol C2H5OH

Polyacrylamide (C3H5NO)n

Guar Gum and its derivatives HPG, CMHPG

HEC, CMHEC R(n)OCH2COONa

Citric Acid

Acetic Acid CH3COOH

Thioglycolic Acid HSCH2COOH

Sodium Erythorbate C6H7O6. Na

Derivatives of cellulose -

(HOOCCH2)2C(OH)COOH

9004-34-6 9004-32-4

Boric Acid H3BO3

**Function**

Agent

and Winterizing Agent

and Winterizing Agent

Reducer

Gelling Agents NR

Gelling Agents NR

Iron Control and pH Adjustment

Fluid Recovery

Carrier fluid and fluid loss control

Hazard Rating1

3\*\*

4\*

3\*,3\*\*

3\*\*

5\*

4\*,8\*\*

**Table 1.** A summary of the various chemicals used to make Hydraulic Fracturing fluids along with a degree of hazard rating. Modified from " www. http://fracfocus.org/chemical-use/what-chemicals-are-used"



for most of the chemicals used by industry. The database can be searched by either Chemical

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http://ull.chemistry.uakron.edu/erd/ - The Department of Chemistry at the University of Akron developed this website to provide a database composed of over 30,000 hazardous chemicals made up of information provided by a number of different published references.

http://www.epa.gov/chemfact/ - This United States Environmental Protection Agency website provides OPPT Chemical Fact Sheets on selected chemicals that may be present in the environment in an ASCII text or Adobe PDF format along with access to other EPA databases.

Table 4 provides a qualitative listing of the desirable and undesirable aspects of most fluid systems available today. As one studies the table it is interesting to note that there is "no magic bullet". The qualitative score is close to the same for each fluid and each fluid has its advantages and disadvantages. This means that the final decision is up to the design engineer as to what is best for his reservoir. The different types of fluid systems are outlined below. A description of all the different components used to manufacture the fluids is provided in Side Bar 1.

Name or CAS Number.

**3. Types of fracturing fluids**

**Table 4.** Qualitative Fluid Selection Chart

**Table 2.** A summary of the various chemicals used to make Hydraulic Fracturing fluids along with a degree of hazard rating. Modified from " www. http://fracfocus.org/chemical-use/what-chemicals-are-used"


**Table 3.** A summary of the various chemicals used to make Hydraulic Fracturing fluids along with a degree of hazard rating. Modified from " www. http://fracfocus.org/chemical-use/what-chemicals-are-used"

#### **Additional hazard identification resources**

http://fracfocus.org/welcome - The Ground Water Protection Council and the Interstate Oil and Gas Compact Commission developed this web site to provide public access to chemicals used in the hydraulic fracturing process and provides a record of the chemicals used in wells in a number of different stated in the United States. At the time of this writing the site had records on over 34,000 wells.

http://www.osha.gov/chemicaldata/ - This United States Department of Labor website proves a OSHA (Occupational Safety and Health Administration) Occupational Chemical Database for most of the chemicals used by industry. The database can be searched by either Chemical Name or CAS Number.

http://ull.chemistry.uakron.edu/erd/ - The Department of Chemistry at the University of Akron developed this website to provide a database composed of over 30,000 hazardous chemicals made up of information provided by a number of different published references.

http://www.epa.gov/chemfact/ - This United States Environmental Protection Agency website provides OPPT Chemical Fact Sheets on selected chemicals that may be present in the environment in an ASCII text or Adobe PDF format along with access to other EPA databases.
