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in water. In general, the high-swelling 
clays are desirable and are added to the mud for 
viscosity and filtration control. The low-swelling 
clays enter the mud as cuttings and cavings, and are 
referred to as contaminants or drilled solids. 
2.3.1.1 Commercial Clays. Commercial clays used in 
drilling fluids are graded according to their ability to 
increase the viscosity of water. The yield of a clay is 
defined as the number of barrels of mud that can be 
• 
DRILLING FLUIDS 
produced using I ton of clay if the mud has an ap-
parent viscosity of 15 cp when measured in a 
rotational viscometer at 600 rpm. The most common 
commercial clay mined for use in drilling fluids is 
called Wyoming bentonite. It has a yield of about 100 
bbl/ton when used with pure water. A less expensive 
commercial clay called high-yield clay has a yield of 
about 45 bbl/ton. It is not uncommon for native 
clays to yield less than 10 bbl/ton. A comparison of 
mud viscosities obtained from various concentrations 
of Wyoming bentonite, high-yield clay, and an 
example native clay in pure water is shown in Fig. 
2.15. Note that regardless of clay type, once suf-
ficient clay has been added to obtain a 15-cp mud, 
the mud viscosity increases rapidly with further 
increases in clay content. 
Wyoming bentonite is composed primarily of 
sodium montmorillonite. The name montmorillonite 
originally was applied to a mineral found near 
Montmorillon, France. The term now is reserved 
usually for hydrous aluminum silicates ap-
proximately represented by the formula: 4 Si02 · 
Al203 · H 20 + water; but with some of the 
aluminum cations, AI3 + , being replaced by 
mafnesium cations, Mg2+. This replacement of 
AI + by Mg2 + causes the montmorillonite structure 
to have an excess of electrons. This negative charge is 
satisfied by loosely held cations from the associated 
water. The name sodium montmorillonite refers to a 
clay mineral in which the loosely held cation is the 
Na + ion. 
API and the European Oil Companies Materials 
Assn. (OCMA) have set certain specifications for 
bentonites that are acceptable for use in drilling 
fluids. These specifications are listed in Table 2.8. 
A model representation of the structure of sodium 
montmorillonite is shown in Fig. 2.16. 4 A central 
alumina octahedral sheet has silica tetrahedral sheets 
on either side. These sheetlike structures are stacked 
with water and the loosely held cations between 
them. Polar molecules such as water can enter 
between the unit layers and increase the interlayer 
spacing. This is the mechanism through which 
montmorillonite hydrates or swells. A 
photomicrograph of montmorillonite particles in 
water is shown in Fig. 2.17. 4 Note the platelike 
character of the particles. 
In addition to the substitution of Mg2 + for AI3 + 
in the montmorillonite lattice, many other sub-
stitutions are possible. Thus, the name mont-
morillonite often is used as a group name including 
many specific mineral structures. However, in recent 
years, the name smectite has become widely accepted 
as the group name, and the term montmorillonite has 
been reserved for the predominantly aluminous 
member of the group shown in Fig. 2.16. This 
naming convention has been adopted in this text. 
The salinity of the water greatly affects the ability 
of the commercial smectite clays to hydrate. A 
fibrous clay mineral called attapulgite can be used 
when the water salinity is too great for use of the 
smectite clays. The name attapulgite originally was 
applied to a clay mineral found near Attapulgus, 
GA. Attapulgite is approximately represented by the 
55 
TABLE 2.8-SPECIFICATIONS FOR BENTONITE 
Specified Values 
API OCMA 
Minimum yield 
Maximum moisture 
Wet-screen analysis 
91 bbl/ton 16m 3 tonne 
(residue on No. 200 sieve) 
Maximum API water loss 
22.5 lbm/bbl H2 0 
26.3 lbm/bbl H2 0 
Minimum yield point 
(22.5 lbm/bbl H 2 0) 
Minimum dial reading, 600 rpm 
(22.5 lbm/bbl H20) 
10 WI% 10 wt% 
2.5 WI% 2.5 WI% 
15.0 mL 
3x plastic 
viscosity 
30 
15 mL 
E x,·t>anqeo!Jie Cations 
rHrO 
Fig. 2.16-Structure of sodium montmorillonite. 4 
56 
Fig. 2.17-Transmission electron micrograph of 
montmorillonite. 4 
formula: (OH2 ) 4 (OH)z Mg5Si 80 20 · 4Hz0, but 
with some pairs of the magnesium cations, 2Mg2+, 
being replaced by a single trivalent cation. A 
photomicrograph of attapulgite in water is shown in 
Fig. 2.18. The ability of attapulgite to build viscosity 
is thought to be due to interaction between the at-
tapulgite fibers rather than hydration of water 
molecules. A longer period of agitation is required to 
build viscosity with attapulgite than with smectite 
clays. However, with continued agitation, viscosity 
APPLIED DRILLING ENGINEERING 
decreases eventually are observed due to mechanical 
breakage of the long fibers. This can be offset 
through the periodic addition of a new attapulgite 
material to the system. 
The clay mineral sepiolite, a magnesium silicate 
with a fibrous texture, has been proposed as a high-
temperature substitute for attapulgite. The idealized 
formula can be written Si 12 Mg 80 3z · nHzO. X-ray 
diffraction techniques and scanning electron micro-
scope studies have established that the crystalline 
structure of this mineral is stable at temperatures up 
to 800°F. Slurries prepared from sepiolite exhibit 
favorable rheological properties over a wide range of 
temperatures. 
2.3.1.2 Low-Swelling Clays. As formations are 
drilled, many different minerals enter the mud 
system and are dispersed throughout the mud by 
mechanical crushing and chemical hydration. 
Various types of low-swelling clays enter the mud, 
which contributes to the total cation exchange 
capacity of the mud. These clays are very similar to 
montmorillonite in that they have alumina oc-
tahedral sheets and silica tetrahedral sheets (Fig. 
2.19). The major difference in such clays is the 
presence of different ions within the lattice of the 
sheets that were introduced during clay deposition. 
2.3.2 Cation Exchange in Smectite Clays. The 
smectite clays have the ability to exchange readily the 
loosely held cations located between the sheetlike 
structures for other cations present in the aqueous 
solution. A well-known application of the ion ex-
change reaction is the softening of water. Ion ex-
change reactions in drilling fluids are important 
because the ability of the clay particles to hydrate 
depends greatly on the loosely held cations present. 
The ability of one cation to replace another depends 
on the nature of the cations and their relative con-
centrations. The common cations will replace each 
other when present in the same concentration in this 
order: 
Fig. 2.18-Transmission electron micrographs of attapulgite (left) and sepiolite (right). 4 
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DRILLING FLUIDS 
However, this order can be changed by increasing the 
concentration of the weaker cation present. Many 
organic compounds also will adsorb between the 
sheetlike clay structures. As discussed in Section 
2.12, the adsorption of methylene blue is the stan-
dard test for the cation exchange capacity of the 
mud. 
2.3.3 Effect of Montmorillonite and Drilled Solids 
on Drilling Fluids Density. Solids in the drilling 
fluid cause an increase in density as well as viscosity. 
Since the specific gravity of all clays is near 2.6, the 
density of a clay/water mixture of a given viscosity 
depends on the yield of the clay used. A clay with a 
high yield must be used if a mud having a density 
near water is desired. If a mud having a higher 
density is desired, a clay with a lower yield can be 
used. In many cases, a natural buildup of low yield 
drilled solids in the mud as drilling progresses 
provides the desired fluid density. Also, API barite, a 
dense, inert