BAKER HUGHES - Drilling Fluids Reference Manual

Baker Hughes Drilling Fluids
Therefore, g Cl¯ in diluted filtrate = 19.1/1000 x 0.0282 x 35.5 (since atomic weight of Cl¯ is
35.5)
0.0282 x 35.5 = 1.0, which is why this strength of AgNO 3 is used as a standard solution.
Calculating the concentration of Cl¯ in the solution, we have:
mg Cl¯ = (19.1/1000 x 0.282 x 35.5) x 10 x 100 = 19.1
Therefore, The 10 ml sample of x 100 diluted filtrate contains 19.1 mg Cl –
The concentration in the diluted filtrate sample is thus (19.1 x 1000)/10 = 1919 mg/l and in the
filtrate thus 1919 x 100 = 191,000 mg/l
This corresponds roughly to the saturation value for sodium chloride at 20ºC.
Other Methods of Analysis
Very often it is found that there is no suitable indicator to determine the end point of a reaction.
If this is the case, other methods must be used.
A common solution to this problem is to add an excess of a standard solution to a known volume
of the filtrate and precipitate the ion under test as an insoluble compound. Since excess of the
reagent is added, all of the ions are precipitated and the amount of precipitate is thus proportional
to the amount of the ion present in the filtrate. The amount of precipitate can be measured in
three ways:
1. “Gravimetric” Analysis. The precipitate is dried and weighed as accurately as possible.
This method is rarely used on a drilling rig because of the need for expensive and delicate
equipment and the time needed for the test.
2. Centrifugation. The precipitate is centrifuged under standard conditions of speed and
time and the floc volume in the centrifuge tube is measured. This is proportional to the
weight of precipitate. The test is not particularly accurate and is comparative in nature
rather than absolute. It is used quite commonly to measure potassium ion concentration
and the concentration of certain polymers, as great accuracy is not usually required.
3. “Turbidimetric” Analysis. This method is used for dilute solutions and depends on an
estimate of the cloudiness (or turbidity) caused when the precipitate forms. The
estimation can be done instrumentally using a photo-electric cell or it can be done by eye.
It is again a comparative test and when the eye is used as a detecting “instrument’, there is
no pretense of accuracy. This method is used to indicate the presence of the sulfate ion in
drilling fluids by reaction with barium chloride solution.
BaCl 2 + SO 4 2– → BaSO 4 ↓ + 2Cl –
cloudy
precipitate
Two other techniques of analysis are occasionally used for drilling fluids.
1. Flame Photometer. This is used for an accurate determination of sodium, potassium and
lithium ions and depends on a measurement of the intensity of the light of a well defined
wavelength that is emitted by these ions when they are heated. The instrument is quite
delicate and is also fairly expensive.
2. Colorimetric Analysis. This may be used to determine the quantities of silicon, iron or
aluminum. These ions form colored complexes with certain chemicals. The intensity of
color developed is proportional to the concentration of the complex and to the
concentration of the ion under test.
Baker Hughes Drilling Fluids
Reference Manual
Revised 2006 3-29