**Author details**

**8. Proppant fall rates**

22 Effective and Sustainable Hydraulic Fracturing

Fall Rate = V (ft/sec) = 1.66x10<sup>5</sup>

D = the average proppant diameter in feet

= the apparent viscosity of the fluid in Cps

50 cps after four hours at reservoir temperature.

measured viscosity or (1.63\*50 = 81 cps).

apparent viscosity of (4.3\*324 = 1393 cps).

Where:

μf

The rate of fall for proppant is normally calculated using Stoke's Law which can be written as:

Stokes's Law is generally not valid for Reynolds numbers much in excess of unity15 or for hindered settling due to proppant clustering in static fluids16. For crosslinked fluid the actual fall rate may be much less than Stokes Law. Hannah and Harrington17 present lab data that shows that proppant in crosslinked fluids falls at a rate which is reduced by about 80% when compared to non-crosslinked linear gels with the same apparent viscosity. The rate of proppant fall in foams and emulsions is also much less than would be indicated by using the apparent viscosity in Stoke's Law18. Another factor affecting proppant fall is the particle concentration which increases slurry viscosity (Figure 11). This retards or hinders the proppant fall because of clustered settling16 in static fluids. Finally the slurry flowing down a fracture is generally much lower that the shear rate of 170 or 511 sec-1 used to report the fluid apparent viscosity. When all of these factors are put together they can significantly affect the viscosity. To provide an example consider a crosslinked gel which has a reference apparent viscosity at 170 sec-1 of

**1.** Shear Rate Correction – If the fluid has an n' of 0.6 and the shear rate in the fracture is 50 sec-1, the effective apparent viscosity in the fracture would be (170/50)1-n' times the

**2.** Slurry Correction – If the slurry enters the fracture at a concentration of 1 PPG (pounds of sand per liquid gallon) and concentrates to 10 PPG after four because of fluid loss, the average concentration of 5 PPG gives a viscosity multiple of 2 from Figure 11. This would

**3.** Fall Rate Correction – Harrington and Hannah17 state that for a crosslinked fluid the rate of fall is reduced by up to 80%. For this example assume that the fall rate is reduced by

**4.** Temperature Correction – The fluid enters the fracture at a relatively low temperature and thus a higher viscosity. If the fluid viscosity reduces by a factor 10 over the 4 hour exposure time (down to the originally referenced 50 cps) with a log viscosity versus time relation‐ ship (typical for most crosslinked fluids) the average fluid viscosity over the four hour period would be a factor of 4.3 times the final viscosity. This gives an effective average

give an effective average apparent viscosity of (2\*81 = 162 cps).

50%. This effectively doubles the viscosity to (2\*162 = 324 cps).

/μf SGprop– SGfluid

D2

SG prop = the specific gravity of the proppant (i.e. 2.65 for sand)

SGfluid = the specific gravity of the fluid (i.e. 1 for water)

Carl Montgomery

NSI Technologies, Tulsa, Oklahoma , USA

### **References**


[5] Economides, M. J, & Nolte, K. G. Reservoir Stimulation- Third Edition", John Wiley and Sons, LTD, 0-47149-192-6(2000).

**Chapter 2**

**Fracturing Fluid**

**Components**

Carl Montgomery

**Abstract**

http://dx.doi.org/10.5772/56422

to design a fracturing treatment.

**1. Introduction**

**1.1. Water**

Additional information is available at the end of the chapter

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

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

and reproduction in any medium, provided the original work is properly cited.

© 2013 Montgomery; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2013 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution,


**Chapter 2**
