BAKER HUGHES - Drilling Fluids Reference Manual

Oil / Synthetic Fluids
As the concentration of ions increases, so does the concentration of water molecules tied up in
hydration. As the concentration of water molecules tied up in hydration increases, the concentration of
free water molecules (not involved in hydration) decreases. The availability of the water molecules in
a solution to hydrate additional solutes can be quantified by the “activity” scale. All the water
molecules in freshwater are available for hydration and the activity (Aw) is defined as “1”. As salt, or
any substance that hydrates, is added, the concentration of hydrated ions or molecules increases and
the availability of the brine molecules in the solution to hydrate any additional material decreases the
activity (Aw).
If two solutions of different Aw are poured together, diffusional mixing will result in a uniform Aw
intermediately between the two. If two solutions of different Aw are separated by a semi-permeable
membrane, an unusual situation occurs, as shown in Figure 5-15. A semi-permeable membrane is
defined as a thin membrane which will allow water molecules to freely pass, but not salt ions.
Water molecules in the solution of higher Aw will diffuse into the solution with lower Aw, as before,
but not the ions. If the chamber with the solution of lower Aw is sealed, then the water diffusing
through the membrane increases the pressure in the chamber. This is defined as osmotic pressure
(Figure 5-15)
Semi-permeable membranes are created between emulsified water droplets in water in oil emulsions
and between emulsified water droplets and the formation drilled (see Figure 5-16). The layer of
surfactant-rich oil will allow water molecules to pass, but not salt. Unless the Aw values are equal, an
osmotic imbalance, or potential, will exist and water will migrate.
As the concentration of salt in the aqueous phase increases and the Aw decreases, less water is
available to solubilize and hydrate other materials. Lime, for example, is less soluble in CaCl 2
solutions than in water.
Figure 5-15
Osmosis
Baker Hughes Drilling Fluids
Reference Manual
5-20 Revised 2006