STANDARD HANDBOOK OF PETROLEUM & NATURAL GAS ...

MWD and LWD 965
(4-200)
where Rgem = gas cut mud resistivity in R m
Rrn = gas free mud resistivity in R m
f, = volumetric gas content (fraction)
The variation is independent of the mud weight, pressure, or temperature,
but is sensitive to fluids other than gas, such as oil or saltwater. Figure 4-266
shows the resistivity variations for a 1-Rem mud. If we assume that a change
of 10% can be detected, then the alarm could be given again for a free gas or
oil volumetric concentration of 2 to 5%.
Mud Temperature. One can attempt to calculate the variation of the temperature
of the mud when it mixes with a gas stream cooled by expansion.
Calculations were made with a 500-gal/min mud flowrate, an expansion from
10,500 to 10,000 psi with an 18-lb/gal mud and also an expansion from 5,500
to 5,000 psi with a 9-lb/gal mud. The temperature decrease of the mud was a
few O F up to 50% gas by volume in the mud.
Temperature measurements do not seem to be good gas indicators.
Mud acoustic attenuation and annulus noise level are being investigated. It is
expected that attenuation would be very sensitive to free gas concentration.
18
16
- m
14
0
x
.- c.
v)
g 12
-0
-0
r' 10
- 9 Ib/gal, 5,000-psi water-base mud
--- 18 Ib/gal, 10,000-psi water-base mud
- -- 9 Ibigal, 5,000-psi oil-base mud
-----. 18 Iblaal. 10,00O-~si oil-base mud
8
6
Gas in mud, % of Volume
Figure 4-265. Bottornhole mud density in the annulus as a function of the
gas content of the mud. (Courtesy Petroleum Engineer lnternational [96].)