**7. Buffers**

Buffers adjust the pH of the base fluid so that dispersion, hydration and crosslinking of the fracturing fluid polymers can be engineered. Because some buffers dissolve slowly they can be used to delay crosslinking for a set period of time to reduce friction in the tubing. Typically this delay time is adjusted so that crosslinking occurs at about 2/3 of the pipe time i.e. when the fluid is about 2/3 of the way to the top perforation. The use of the proper buffer package also improves the high temperature capability of Borate crosslinked fluids and reduces the amount of polymer need to get good viscosity[17]. Table 1 shows some commonly used buffer components.


**Table 1.** Commonly Used Fracturing Fluid pH buffering chemicals.

#### **8. Surfactants/Mutual solvents**

Surfactants/Mutual solvents are added to the fracturing fluids to reduce the surface tension of the fracturing fluid to improve fluid recovery and compatibility between the fracturing fluid and the formation matrix or formation fluids. Lab tests are used to determine the type and loading level of surfactant to be used. The primary goals of a surfactant are to leave the rock surface water wet, act as an emulsion preventer or as a defoamer and reduce the surface tension. In very dry gas wells the water in the frac fluid can shift the relative permeability to the water side and form a "water block" that reduces gas flow. An example of how a proper formulation can reduce this effect is shown in Figure 16. Typically if a well is producing any 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 mutual solvents.

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

#### **9. Biocides/Bactericides**

**6.4. Viscosity stabilizers**

42 Effective and Sustainable Hydraulic Fracturing

same time.

**7. Buffers**

components.

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

Buffers adjust the pH of the base fluid so that dispersion, hydration and crosslinking of the fracturing fluid polymers can be engineered. Because some buffers dissolve slowly they can be used to delay crosslinking for a set period of time to reduce friction in the tubing. Typically this delay time is adjusted so that crosslinking occurs at about 2/3 of the pipe time i.e. when the fluid is about 2/3 of the way to the top perforation. The use of the proper buffer package also improves the high temperature capability of Borate crosslinked fluids and reduces the amount of polymer need to get good viscosity[17]. Table 1 shows some commonly used buffer

> Sodium Bicarbonate Formic Acid Sodium Carbonate Fumaric Acid Sodium Hydroxide Hydrochloric Acid Monosodium Phosphate Magnesium Oxide

Surfactants/Mutual solvents are added to the fracturing fluids to reduce the surface tension of the fracturing fluid to improve fluid recovery and compatibility between the fracturing fluid and the formation matrix or formation fluids. Lab tests are used to determine the type and loading level of surfactant to be used. The primary goals of a surfactant are to leave the rock surface water wet, act as an emulsion preventer or as a defoamer and reduce the surface tension. In very dry gas wells the water in the frac fluid can shift the relative permeability to the water side and form a "water block" that reduces gas flow. An example of how a proper formulation can reduce this effect is shown in Figure 16. Typically if a well is producing any

**Table 1.** Commonly Used Fracturing Fluid pH buffering chemicals.

**8. Surfactants/Mutual solvents**

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 of bactericides to provide protection.
