BAKER HUGHES - Drilling Fluids Reference Manual

Baker Hughes Drilling Fluids
Solubility of Covalent Compounds in Water
Covalent compounds are generally soluble, or miscible, if they are polar. If polar groups are
present, they tend to interact more strongly with water molecules than with each other. Thus the
molecules split apart and the substance dissolves. There are no ions present, but each molecule is
separated from its companions and the substance is said to molecularly dispersed. Examples of
this type of compound include sugar, alcohol, and starch.
Non polar covalent compounds are generally insoluble in water, as there are no strong attractive
forces between them and the water molecules. Thus diesel oil is insoluble in water.
Solubility of Gases in Water
This is an important topic for drilling fluids, as small quantities of dissolved oxygen, carbon
dioxide or hydrogen sulfide can have a large effect in making the fluid more corrosive. Hydrogen
sulfide is also extremely poisonous and represents a real hazard to rig personnel. Carbon dioxide
in solution can also alter the interaction between clay particles in a fluid and thus change the
fluids rheological properties. All of these gases are soluble in water, particularly at high pressure,
and efforts are generally made to exclude them as much as possible.
Factors Affecting Solubility
The maximum amount of a substance that can be dissolved under any given condition is called its
solubility. This figure depends on the nature of the solute and solvent, and on the amount and
type of other solutes already in solution. Most solubility’s increase when the temperature
increases, although gases are an exception to this. The solubility of a gas decreases when the
temperature rises, and increases when the pressure increases. The pressure effect is more
important and gases are generally more soluble when a drilling fluid is at the bottom of a well
than when it is on the surface.
Other ions in solution have a very important influence. When a mixture of salts is present, the
most soluble one suppresses the solubility of the other components. Sodium chloride and
potassium chloride are much less soluble in concentrated magnesium chloride solutions.
Reactions in Solution
If two compounds which are soluble in water can react together to form an insoluble compound,
then they will do so. Thus calcium chloride and sodium carbonate are both soluble in water on
their own, but if they are mixed together, insoluble calcium carbonate precipitates out of solution.
The reaction is written as follows:
Na CO + CaCl → CaCO 2NaCl
2 3
2
3
+
Sodium chloride, being soluble, is left in solution. The reaction occurs because both the
reactants, being ionic, are split up into their constituent ions in solution. When mixed together,
the solution does not contain sodium carbonate and calcium chloride as such, but rather sodium,
calcium, carbonate and chloride ions, and it is impossible to say which cation is associated with
which anion.
This type of reaction occurs very commonly in drilling fluid chemistry. It is the basis of many
analytical tests and chemical treatments. Sodium carbonate is used, as above, to “treat out”
soluble calcium, in order to maintain the correct chemical properties of the fluid. Silver nitrate
(AgNO 3 ) which is soluble is used to test for chloride ions, due to the formation of insoluble silver
chloride (AgCl).
Baker Hughes Drilling Fluids
Reference Manual
Revised 2006 3-15