BAKER HUGHES - Drilling Fluids Reference Manual

Baker Hughes Drilling Fluids
wetting. Examples of solids which readily water wet are the rock forming silicate minerals,
particularly clays, and the insoluble ionic compounds, such as barite (BaSO 4 ). The process is
illustrated in Figure 3-11.
Water wetting generally involves the formation of a monomolecular layer of water chemically
adsorbed on to the surface of the particle. The arrangement of the water molecules depends on
the distribution of charges, or partial charges, on the surface of the particle. The faces of clay
minerals are normally negatively charged, thus attracting the hydrogen atoms in the water
molecule (as in Figure 3-11). However, the edges of clay minerals usually carry positive charges
and attract the oxygen atoms.
Adsorbed water molecules
H δ+ δ - δ δ -
O
O
O
H δ+ H δ+ H δ+ H δ+ H δ+
O
O O
O
Si Si Si
Clay
mineral
O
O
OH
Al
OH
O
O
OH
Al
OH
O
O
O
Si Si Si
O
O
O
Figure 3-11
Water Wetting of Clay Mineral Surfaces
When water wetting occurs, the adsorbed water molecules are still attracted to the other water
molecules in the liquid and thus the particles can be easily dispersed. This “solid-liquid”
attraction partly accounts for the viscosity increase observed when an easily dispersed solid, such
as bentonite is added to water. The other effect is the solid-solid interaction between the
bentonite particles.
Non-polar substances, as well as being insoluble in water, are difficult to disperse because there is
no force of attraction between the surfaces of the particle and the water molecules. The particles
are usually more attracted to each other and thus tend to stick together, inhibiting dispersion. A
dispersion of diesel oil in water is therefore unstable if no surfactants are present. The droplets of
diesel are more attracted to each other than to water and tend to coalesce, so that separation of the
two phases occurs, giving a layer of diesel floating on the surface of the water. A surfactant,
because it contains a polar and non-polar group on the same molecule, can concentrate between
the water and oil, and thus make the two phase’s compatible, and an emulsion stable.
Acids
An acid is a compound containing one or more hydrogen atoms, which can be replaced by a
metal, to form a salt. When acids are dissolved in water, they ionize to produce “hydrogen ions”.
Strictly, these have the formula H 3 O + , but are generally denoted simply as H + , The common
acids include hydrochloric acid (HCl), sulfuric acid (H 2 SO 4 ), and acetic acid (CH 3 COOH), which
is an organic acid.
The reactions with metals are illustrated by the following equations:
2 HCl + Fe → FeCl2
+ H
2
↑
Hydrochloric acid + iron → Iron (II) chloride + hydrogen
Baker Hughes Drilling Fluids
Reference Manual
Revised 2006 3-17