BAKER HUGHES - Drilling Fluids Reference Manual

Water Based Drilling Fluids
These types of reactions also exert a strong influence on solubility. If a reaction can occur
between two components, then their solubility’s will be interdependent. For example, the
solubility of CMC (carboxymethylcellulose), a polymer commonly used in drilling fluids),
depends on the concentration of calcium ions in the mud. If this exceeds a certain value, then the
CMC precipitates out as an insoluble calcium salt.
Intimate Mixtures of Solids, Liquids and Gases
One characteristic of solutions is that they are homogenous mixtures. This implies that the solute
should be split up into its constituent ions or molecules and that these should be dispersed
uniformly throughout the solvent. Many insoluble substances apparently form homogenous
mixtures with a liquid, but are not dissolved, as the mixture consists of very small particles of the
substance dispersed throughout the liquid. Such a mixture is said to consist of two phases, as it
contains two physically distinct parts. In contrast, a solution of sodium chloride in water is only
one phase, as no physical distinction can be made between the sodium chloride and the water.
The case of a solid mixed with a liquid or gas is called a suspension or dispersion. All drilling
fluids are suspensions. If the particle size of the solid phase is fine enough, the solid stays in
suspension and does not separate out under gravity. Thus a typical water-based drilling fluid
consists of bentonite (a clay mineral) and barite (BaSO 4 ) suspended in water. Neither substance
is dissolved, since both are insoluble and are not split up into ions or molecules.
A liquid mixed into a different liquid with which it is immiscible is known as an emulsion. This
is illustrated in Figure 3-10, for the case of water emulsified into oil. The water is dispersed in
the form of fine droplets throughout the oil. The oil is known as the continuous or external phase.
The water is called the dispersed, or internal phase. This example is the basis of the invert
emulsion drilling fluid. These “invert” systems also contain calcium chloride dissolved in the
water phase, and solids dispersed in the oil phase, giving an intimate mixture of three separate
phases.
OIL
WATER
Figure 3-10 Invert Emulsion Water Droplets in a Continuous Oil Phase
A gas mixed into a liquid is called foam. Foams have been used as drilling fluids and consist of
air dispersed in the form of bubbles throughout a continuous liquid phase (usually water).
In a water-based drilling fluid, certain chemicals cause foaming by increasing surface tension
stabilizing air/water mixtures. This is to be avoided as it leads to a decrease in mud weight and
makes estimation of mud volumes very difficult.
Dispersibility of Solids in Water
Some solids are much more readily dispersed in water than others and the reason is again due to
the polar character of water. Ionic compounds that are insoluble tend to disperse easily since
water is attracted to the surfaces of the particles, which are charged, in much the same way as
hydration of ions occurs for soluble compounds. This attraction is commonly known as water
Reference Manual
Baker Hughes Drilling Fluids
3-16 Revised 2006