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62 Part I Liquid Drilling Systems

3.2.1 Extreme Mud Properties
Mud properties that influence the type of pump include mud weight
(density) and rheological properties. For Newtonian fluids, viscosity is the
only parameter describing fluid rheological characteristics. Plastic viscosity
and yield point are the two parameters used to describe the rheological
characteristics of Bingham plastic fluids. The consistency and the flow
behavior indexes are the two parameters that are utilized to characterize
Power Law fluids, also called pseudoplastic fluids in other industries. The
consistency index, the flow behavior index, and yield strength are the
three parameters that are employed to characterize Herschel-Bulkley
fluids. All of these fluid properties can be measured using state-of-the-art
instruments used in the oil and gas industry.
In hole cleaning, the properties of the mud affect the settling velocity
of drill cuttings in the annulus. To ensure that drilling operations are
done safely, the expected ranges of mud properties should be found from
mud programs and listed against the hole depths with different borehole
geometries. The extreme values in the ranges of properties will be used
for estimating the cuttings settling velocity and thus the minimum
required mud flow rate from the mud pump.

3.2.2 Extreme Annular Geometry
The minimum required mud flow rate from the mud pump is equal to
the minimum required mud velocity times the maximum possible cross-
sectional area of annular space during drilling. Therefore, the information
of borehole geometry should be known for selecting mud pumps to drill
the wells. Figure 3.1 shows a typical borehole geometry diagram. Drill
pipe sizes and the extreme mud properties should be marked in the dia-
gram at each level of open hole sizes.

3.2.3 The Minimum Required Flow Rate
The minimum required mud flow rate demanded by the borehole geo-
metry from the mud pump is estimated based on the minimum required
mud velocity, which should be higher than the drill cuttings slip velocity.
The criterion for the minimum required mud velocity was described in
Chapter 2. For mud pump selection, we consider the minimum mud
flow rate required for drilling the hole sections of extreme geometries.