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20 Part I Liquid Drilling Systems

2.2.1 Drilling Mud Classifications
Many different types of drilling mud are used in the industry. Their behaviors
are very different in the drilling circulation systems. Pressure losses in flow
conduits (drill string and annulus, in our case) are due to the resistance to
flow. For a fluid particle flowing along the wall of a flow conduit, this resis-
tance is the friction force from the wall. The friction force acts on the particle
in the opposite direction of flow and tries to slow down the flow velocity of
the particle. In the same way, the fluid particle along the wall exerts a friction
force to the particle next to it. The friction forces, or flow resistances,
between the fluid particle and the wall, and among fluid particles, depend on
the fluid properties and the differences between the flow velocities of the
particles. The study of this phenomenon of flow resistance is called rheology.
Rheology is basically the study of the flow or deformation of matter. It
generally describes flow or deformation in terms of shear rate and shear
stress. Shear rate is defined as the flow velocity gradient in the direction per-
pendicular to the flow direction. The higher the shear rate, the higher the
friction between the flowing particles. The friction between particles is
measured by the shear force per unit area of shearing layer, or shear stress.
Fluids are classified according to the different categories in rheologi-
cal studies on the basis of their flow behaviors. Figure 2.1 shows five

250

b
200
e d
Shear Stress 150 c a

100

50

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5
Shear Rate
Figure 2.1 Different types of fluids found in drilling operations.