BAKER HUGHES - Drilling Fluids Reference Manual

Hydraulics
term “viscosity” is confused with the term “rheology”. A more detailed analysis of the term
“rheology” follows.
Rheology
Rheology is defined as physics of the flow and the deformation of matter. Rheology and the
associated annular hydraulics (Chapter 9) relate directly to borehole stability and how effectively the
borehole is cleaned. An understanding of rheology is essential if wellsite engineering of the drilling
fluid is to cost effectively complement the objective of drilling the well. Rheology and hydraulics of
drilling fluids are not exact sciences, but are based upon mathematical models that closely describe
the rheology and hydraulics of the fluid and do not conform exactly to any of the models.
Consequently, different methods are used to calculate rheology and hydraulic parameters.
Fluid Deformation
Rheology is the study of the deformation of all forms of matter. The deformation of a fluid can
simply be described by two parallel plates separated by some distance as shown in Figure 1-1.
Figure 1-1
Deformation of a Fluid by Simple Shear
Shear Stress
An applied force (F), acting over an area (A), causes the layers to slide past one another. However,
there is a resistance, or frictional drag, force that opposes the movement of these plates. This
resistance or drag force is called shear stress ( τ ). In equation form,
τ
=
F
-----
A
with shear stress having typical units of lb f /100 ft 2 .
Additionally, the fluid layers move past each other easier than between a pipe wall and fluid layer.
Therefore, we can consider a very thin layer of fluid next to the pipe wall as stationary.
Shear Rate
The difference in the velocities between two layers of fluid divided by the distance between the two
layers is called the shear rate ( γ ). In equation form,
γ
velocity difference
= -----------------------------------------------
distan
ce
With typical units of
or, reciprocal seconds.
ft/sec 1
------------ = ------ =
ft sec
se c 1
Reference Manual
Baker Hughes Drilling Fluids
1-6 Revised 2006