BAKER HUGHES - Drilling Fluids Reference Manual

Baker Hughes Drilling Fluids
attraction of water to doubly or triply charged metal ions is so strong that many of these metals
form salts which contain chemically bound water, known as water of crystallization. Calcium
and magnesium salts such as MgCl 2··6H 2 O or CaCl 2·2H 2 O, are common examples. If the water is
driven off by heating, the resulting anhydrous salts absorb water readily from the atmosphere so
that the hydrous salt reforms rapidly. Even in the hydrous form, the salts given above absorb
water and if left standing in the open for long enough they will eventually absorb enough water to
dissolve completely. The group 1A metals, being singly charged, are much less hydrated that the
group IIA metals, such as calcium and magnesium.
An important break occurs between sodium and potassium. The Na + ion of radius 0.98Å, is
hydrated, but the K + ion, of radius 1.33Å is not hydrated, since the slight difference in size leads
to the potassium ion having a lower electric field than the sodium ion. This means that the K + ion
is effectively smaller than the Na + ion as it is not dragging around a shell of water molecules on
its back. This is very important in determining the way in which these two ions react in solution
with clay surfaces and also explains why strong potassium brines have less viscosity than strong
sodium brines. This also explains why potassium form of clays (Illite) is more stable and is the
basis of the potassium inhibited drilling fluids. Anions are not hydrated because they are much
larger in size than cations. Thus the electric field around anions is not very strong. Temporary
attraction of water molecules occurs, but no permanent structuring of the water molecules around
the anion exists.
Chemical Calculations
An important aspect of drilling fluid engineering involves a careful chemical analysis of the fluid,
as certain chemical characteristics can substantially affect the performance. Also, calculations
have to be performed to determine treatment levels. This section deals with both these aspects.
Concentration of Solutions
The concentration of a solution is a measure of how much of a particular substance is dissolved in
a certain volume or weight of solution. There are various ways of expressing this.
1. Weight of solute per volume of solution (w/v). This is normally expressed in grams/liter
(g/l), kilograms/meter3 (kg/m3), milligrams/liter (mg/l) or pounds per barrel (lb/bbl).
2. Weight of solute per weight of solution (w/w). This is normally expressed as a
percentage (%). Thus a 10% solution = 100g solute/kg solution. In lower concentrations
it is expressed as parts per million (ppm). Thus, 1 ppm = 1 mg solute/kg solution and 1%
= 10,000 ppm. Parts per thousand (ppt) is occasionally used as a unit, especially for sea
water analysis. 1 ppt = 1,000 ppm = 0.1%
3. Volume of solute per volume of solution (v/v). This is often used to describe mixtures,
rather than true solutions, and is the unit used for retort analysis of drilling fluids.
Thus 1% (v/v) = 1 m 3 of dispersed phase/100 m 3.
4. Molarity (M). This is used mainly for laboratory analytical reagents. A one molar
solution (1M) contains one mole (molecular weight in grams) dissolved in one liter of
solution.
5. Normality (N). Again, this is used mainly for laboratory reagents. A one normal solution
(1N) contains one mole of solute divided by its valency in one liter of solution.
Baker Hughes Drilling Fluids
Reference Manual
Revised 2006 3-23