STANDARD HANDBOOK OF PETROLEUM & NATURAL GAS ...

where vw = volume percent water in 96
v, = volume percent oil in %
vs = volume percent solids in %
Sm = specific gravity of the mud
Drilling Muds and Completion Fluids 655
(Oil is assumed to have a specific gravity of 0.8.)
For freshwater muds, a rough measure of the relative amounts of barite and
clay in the solids can be made by using Table 4-45. As both suspended and
dissolved solids are retained in the retort for muds containing substantial
quantities of salt, corrections are made for the salt [23].
PH. Two methods for measuring the pH of drilling mud have been used: (1) a
modified colorimetric method using paper test strips, and (2) the electrometric
method using a glass electrode. The paper strip test may not be reliable if the
salt concentration of the sample is too high. The electrometric method is subject
to error in solutions containing high concentrations of sodium ions, unless a
special glass electrode is used, or unless suitable correction factors are applied
if an ordinary electrode is used. In addition, a temperature correction is required
for the electrometric method of measuring pH.
The paper strips used in the colorimetric method are impregnated with such
dyes that the color of the test paper is dependent upon the pH of the medium
in which the paper is placed. A standard color chart is supplied for comparison
with the test strip. Test papers are available in a wide range type, which permits
estimating pH to 0.5 units, and in narrow range papers, with which the pH can
be estimated to 0.2 units.
The glass electrode pH meter consists of a glass electrode system, an electronic
amplifier, and a meter calibrated in pH units. The electrode system is composed
of (1) the glass electrode, a thin walled bulb made of special glass within which
is sealed a suitable electrolyte and an electrode; and (2) the reference electrode,
which is a saturated calomel cell. Electrical connection with the mud is established
through a saturated solution of potassium chloride contained in a tube
surrounding the calomel cell. The electrical potential generated in the glass
electrode system by the hydrogen ions in the drilling mud is amplified and
operates the calibrated pH meter.
Table 4-45
Relative Amounts of Barite and Clay in Solids
Specific Gravity Barite, Percent Clay, Percent
of Solids by Weight by Weight
2.6 0 100
2.8 18 82
3.0 34 66
3.2 48 52
3.4 60 40
3.6 71 29
3.8 81 19
4.0 89 11
4.3 100 0