STANDARD HANDBOOK OF PETROLEUM & NATURAL GAS ...

Drilling Muds and Completion Fluids 657
can enter the mud when anhydrite (CaSO,) or gypsum (CaS0,.2H20) formations
are drilled. Cement also contains calcium and can contaminate the mud. The
total hardness is determined by titration with a standard (0.02 N) Versenate
(EDTA) solution. The standard Versenate solution contains Sodium Versenate,
an organic compound capable of forming a chelate with Ca2+ and Mg2+.
The hardness test sometimes is performed on the mud as well as the mud
filtrate. The mud hardness indicates the amount of calcium suspended in the
mud as well as the calcium in solution. This test usually is made on gypsumtreated
muds to indicate the amount of excess CaSO, present in suspension. To
perform the hardness test on mud, a small sample of mud is first diluted to 50
times its original volume with distilled water so that any undissolved calcium
or magnesium compounds can go into solution. The mixture then is filtered through
hardened filter paper to obtain a clear filtrate. The total hardness of this filtrate
then is obtained using the same procedure used for the filtrate from the lowtemperature
low-pressure API filter press apparatus.
Methylene Blue. Frequently, it is desirable to know the cation exchange capacity
of the drilling fluid. To some extent, this value can be correlated to the
bentonite content of the mud.
The test is only qualitative because organic material and some other clays
present in the mud also will absorb methylene blue. The mud sample usually is
treated with hydrogen peroxide to oxidize most of the organic material. The
cation exchange capacity is reported in milliequivalent weights (meq) of methylene
blue per 100 ml of mud. The methylene blue solution used for titration is
usually 0.01 N, so that the cation exchange capacity is numerically equal to the
cubic centimeters of methylene blue solution per cubic centimeter of sample
required to reach an endpoint. If other adsorptive materials are not present in
significant quantities, the montmorillonite content of the mud in pounds per
barrel is five times the cation exchange capacity.
The methylene blue test can also be used to determine cation exchange
capacity of clays and shales. In the test a weighed amount of clay is dispersed
into water by a high-speed stirrer. Titration is carried out as for drilling muds,
except that hydrogen peroxide is not added. The cation exchange capacity of
clays is expressed as milliequivalents of methylene blue per 100 g of clay.
Oil-Base Muds [23-251
Specific Weight. Mud weight of oil muds is measured with a mud balance. The
result obtained has the same significance as in water-base mud.
Viscosity. The measurement procedure for API funnel viscosity is the same as for
water-base muds. Since temperature affects the viscosity, API procedure recommends
that the mud temperature should always be recorded along with the viscosity.
Plastic Viscosity and Yield Point. Plastic viscosity and yield point measurements
are obtained from a direct indicating viscometer. Due to the temperature
effect on the flow properties of oil-base mud, the testing procedure is modified.
The mud sample in the container is placed into a cup heater [23]. The heated
viscometer cup provides flow property data under atmospheric pressure and
bottomhole temperature.
Gel Strength. The gel strength of oil-base muds is measured with a direct
indicating viscometer exactly like that of water-base muds.