STANDARD HANDBOOK OF PETROLEUM & NATURAL GAS ...

658 Drilling and Well Completions
Filtration. The API filtration test for oil-base muds usually gives an all-oil filtrate.
The test may not indicate downhole filtration, especially in viscous oils. The
alternative high-temperature-high-pressure (HT-HP) filtration test will generally
indicate a pending mud problem by amount of fluid loss or water in the filtrate.
The instruments for the HT-HP filtration test consist essentially of a controlled
pressure source, a cell designed to withstand a working pressure of at least
1000 psi, a system for heating the cell, and a suitable frame to hold the cell
and the heating system. For filtration tests at temperatures above 200"F, a
pressurized collection cell is attached to the delivery tube. The filter cell is
equipped with a thermometer well, oil-resistant gaskets, and a support for the
filter paper (Whatman No. 50 or the equivalent). A valve on the filtrate delivery
tube controls flow from the cell. A nonhazardous gas such as nitrogen or carbon
dioxide should be used for the pressure source.
The test is usually performed at a temperature of 300'F and a pressure of
600 psi over a 30-min period. When other temperatures, pressures, or times are
used, their values should be reported together with test results. If the cake
compressibility is desired, the test should be repeated with pressures of 200 psi
on the filter cell and 100 psi back pressure on the collection cell.
Electrical Stability of Emulsions. The electrical stability test indicates the stability
of emulsions of water in oil. The emulsion tester consists of a reliable circuit using
a source of variable AC current (or DC current in portable units) connected to strip
electrodes. The voltage imposed across the electrodes can be increased until a
predetermined amount of current flows through the mud emulsion-breakdown point.
Relative stability is indicated as the voltage at the breakdown point.
Sand Content. Sand content measurement is the same as for water-base muds
except that diesel oil instead of water should be used for dilution.
Liquids and Solids Content. Oil, water, and solids volume percent is determined
by retort analysis as in a water-base mud. More time is required to get a
complete distillation of an oil mud than of a water mud. Then the corrected water
phase volume, the volume percent of low gravity solids, and the oil-water ratio can
be calculated; the chart in Figure 4-108 can be used for the calculations [24].
Example. Find the volume fraction of brine, the low gravity solids content, the
adjusted mud weight, and the oil-to-water ratio from the test data below (use
Figure 4-107).
Mud weight (specific weight) = 15.7 lb/gal
Volume % water (retort) = 20%
Volume % oil (retort) = 45%
Strong silver nitrate = 4.3 ml
(1 ml equivalent to 0.01 g C1)
Step 1. To determine the percent by weight of calcium or of sodium chloride
in the internal phase, locate the intersection of the line drawn horizontally from
the cm' of strong silver nitrate required to titrate 1 cms of whole mud with the
line projected vertically from the volume percent of fresh water by retort.
Percent by weight brine in internal phase:
Strong silver nitrate = 4.3 ml
Volume % water (retort) = 20%
Read 25% by weight brine in internal phase