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Drilling 139

an important tool for solving all of these problems. How this is accomplished reflects the engineered
properties of the mud.

properTies of drillinG fluids
To satisfy the needs described above, a drilling fluid must be sufficiently fluid to readily flow through
the drill string, around the bit, and back to the surface (Figure 8.3) and yet be viscous enough to
keep the cuttings from settling back to the bottom of the hole. At the same time, it is important that
the mud form a mechanically intact, impermeable layer along the bore hole wall and, to the extent
possible, permeate at least a narrow zone of the wall rock. Substances that possess properties that
allow flow if mechanically perturbed but remain in a solid or gel-like form if undisturbed are called
thixotropic materials.
The most commonly used material for drilling mud is a formulation that consists of a water + clay
slurry into which various additives are mixed. Clay mixtures that have a high proportion of mont-
morillonite are favored. Such mixtures are commonly referred to as bentonite. Bentonite is a soft
rock that occurs in natural geological deposits in sedimentary environments. Bentonite is a mixture
of various clay minerals but its principal characteristic is its high proportion of montmorillonite.
Clay minerals are formed from molecular sheets composed of either silicon atoms linked to four
oxygen atoms (the tetrahedral sheets) or aluminum atoms linked to six oxygen atoms (the octahe-
dral sheets). The sheets are bound together by various cations (mainly potassium, sodium, calcium,
and magnesium atoms), and are stacked in different ways. Water molecules occupy spaces between
the sheets. Different clay minerals have different proportions of the cations, different orders in
which the tetrahedral and octahedral sheets are stacked and different amounts of water in their
structure.
The water content of montmorillonite clays has an important impact on the physical character-
istic of the mineral. Water is readily added to or removed from the montmorillonite structure via
the reaction

(Na, K,0.5 × Ca) (Al, Mg, Fe) [(Si, Al) O ] (OH) ∙ nH O < = >
20
2
4
0.7
4
8
(8.1)
(Na, K, 0.5 × Ca) (Al, Mg, Fe) [(Si, Al) O ] (OH) ∙ (n-1)H O + H O.
0.7
4
8
2
2
20
4
Montmorillonite that has the minimum amount of water is so-called dehydrated montmorillo-
nite, while hydrated montmorillonite holds a maximum number of water molecules in its structure.
The amount of water contained in the clay has a strong effect on the size of the clay molecule—the
more hydrated the clay, the larger is its molar volume. Montmorillonite clays are among the clays
that can hold the greatest amount of water in their structure in their fully hydrated state. As they
absorb water they visibly swell. Hydrated montmorillonites can expand 10–20 times their dehy-
drated volume.
This property, as well as their low shear strength, make them ideal as the primary component in
drilling muds—addition of a few percentage montmorillonite to water makes a slurry that can be
pumped yet still possess adequate viscosity to entrain cuttings while remaining fluid. Such solutions
settle into a gel-like state if undisturbed.
Added to this mixture is a material to increase the overall density of the mixture so that it can
displace any fluids that may enter the drilled well. Usually barite, a barium sulfate mineral with a
high density, is used for that purpose. Organic compounds, many of them unknown because they
are part of the proprietary information manufacturers hold regarding their formulations, are added
to improve flow, thermal stability, viscosity, or other properties important for the mud.
When this solution enters the pores of the wall rock, it is no longer subject to the perturbations
inherent in a pumped fluid. As a result, it becomes gel-like, acting as a fixed mechanical barrier
preventing exchange of fluids across the wall of the well and isolating the well from the local