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Chapter 7 – DRILLING FLUIDS 165

Mud physical properties

Density. Primary control of downhole pressures is obtained with
a mud of such density as to exert a greater hydrostatic pressure on the
formation than exists within formation pores.
The lower safe limit of mud density is calculated by the density to
balance formation pore pressure, plus a small additional amount as a
safety margin.

Some formations require a minimum hydrostatic pressure to keep them
stable. When a hole is drilled through a rock in the ground, the stresses
in the surrounding rock will tend to push the rock into the hole. If mud
hydrostatic pressure is kept high enough, it pushes back against the rock
and so supports it. The required density gradient is likely to be something
greater than that required to maintain well control.
The upper safe limit of mud density will be given by one of several
factors:
1. Losses or formation breakdown may be induced if the hydrostatic
pressure plus circulating pressure losses exceed formation
strength. Circulating pressure losses refer to the pressure
necessary to force fluid to flow along a pipe or annulus. The

pressure at the bottom of the well while circulating equals mud
hydrostatic pressure plus the pressure required to force the
mud to flow up the annulus. This extra pressure imposed while

circulating along the open hole can be enough to fracture weak
formations in some cases.
2. A high mud density will give a reduced MAASP. (MAASP was
explained in chapter 3 in the discussion of drilling out the casing
and testing the formation strength.) This means that the well is
less able to withstand a kick than with a lower mud density.
3. Rate of penetration is generally reduced with higher weights due
to chip hold down (as explained in chapter 6).
4. Sticking in the hole becomes more likely at higher mud densities.
5. Some shales contain tiny fractures. When mud or filtrate is

forced into the fractures, this lubricates the fracture faces. It also
changes the stress regime in the near-wellbore zone. The wellbore
will become unstable, and chunks of the shale will fall off into

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