Page 32 - Environmental Control in Petroleum Engineering

Drilling and Production Operations 21

Water-based Drilling Fluids

Water is the most commonly used base for drilling fluids or muds.
Because it does not have the physical and chemical properties needed
to fulfill all of the requirements of a drilling mud, a number of additives
are used to alter its properties. During drilling, formation materials get
incorporated into the drilling fluid, further altering its composition and
properties. A typical elemental composition of common constituents
of water-based drilling muds is given in Table 2-1 (Deeley, 1990).
These constituents are discussed in more detail below.

Viscosity Control

One of the most important functions of a drilling fluid is to lift
cuttings from the bottom of the well to the surface where they can be
removed. Because cuttings are more dense than water, they will settle
downward through the water from gravitational forces. The settling
velocity is controlled primarily by the viscosity of the water and the
size of the cuttings. Because the viscosity of water is relatively low,
the settling velocity for most cuttings is high. To remove the cuttings
from the well using water only, a very high water velocity would be
required. To lower the settling velocity of cuttings and decrease the
corresponding mud circulation rate, viscosifiers are added to the water
to increase its viscosity.
The most commonly used viscosifier is a hydratable clay. Some
clays, like smectite, consist of molecular sheets with loosely held
+
cations between them, such as Na . If the clay is contacted with water
having a cation concentration that is lower than the equilibrium
concentration for the cation in the clay, the cation atom between the
sheets can be exchanged with water molecules. Because water mole-
cules are physically larger than most cations, the spacing between the
clay sheets expands and the clay swells (hydrates). During the mixing
and shearing that occurs as water is circulated through the well, these
clay sheets can separate, forming a suspension of very small solid
particles in the water. The viscosity of this suspension is significantly
higher than that of pure water and is more effective in lifting the larger
formation cuttings out of the well.
The most common clay used is Wyoming bentonite. This clay
is composed mostly of sodium montmorillonite, a variety of smectite.