STANDARD HANDBOOK OF PETROLEUM & NATURAL GAS ...

964 Drilling and Well Completions
Table 4-128 shows maximum dissolved gas concentrations in drilling muds at
the bottom of the hole. Figure 4-264 shows the variation of the acoustic velocity
for two water-base muds and two oil-base muds of 9 and 18 lb/gal at pressures
of 5,000 and 10,000 psi.
A sharp velocity decrease is seen for the water-base muds. Assuming a
threshold detection of 500 ft/s, the alarm could be given for 0.5% of free gas
or 1.1 to 1.4% of total gas (dissolved and free).
The oil-base muds having no free gas behave differently and the 5OO-ft/s
threshold is not reached before approximately 5% of gas is dissolved. Then the
velocity decrease is almost as fast as with the water-base mud.
Mud Specific Weight. The water-base mud specific weight can be calculated readily
using Equation 4-199. The oil-base mud specific weight requires the use of tables.
The variations are shown in Figure 4-265 for the same 9- and 18-lb/gal muds.
Specific weight-wise, the muds behave in a similar manner. Assuming that a
density measurement with the gradiomanometer can be made accurately, the
specific weight threshold would be 0.15 lb/gal. The gas content of the mud would
be 2 to 5% according mainly to the density, the greater sensitivity being for
the heavier mud.
Mud Resistivity. The mud resistivity can be measured only with the water-base
muds. It is measured easily with a small microlog-type sensor embedded in the
outer wall of the drill collar. Assuming the free gas is dispersed in small bubbles
in the mud, the resistivity of the gas cut mud is
Gas In mud, YO of volume
Figure 4-264. Acoustic velocity in the annulus as a function of the gas
content in the mud. (Courtesy Petroleum Engineer lnternational [96].)