STANDARD HANDBOOK OF PETROLEUM & NATURAL GAS ...

962 Drilling and Well Completions
Table 4-1 28
Maximum Dissolved Gas Content of Drilling Muds
Density Pressure Gas Volume’ % of InJected
Mud (Iblgal) (PSI) (scflSTB) Mud Volume*
Water base 9 4,680 16 0.9
Water base 18 9,360 13.7 0.6
Oil base 9 4,680 760 38
Oil base 18 9,360 7,200 77
Conditions:
Depth = 10,000 ff
Temperature = 150°F
Filtrate salinity = 20 kppm
Gas density (air = 1 .O) = 0.7
Oil S.G. = 0.83 (39”API)
Brine density = 1 .I 75 g/cm3
Brine-oil ratio = 16234
“At bubble point in the mixture
Note that from Table 4-128 the very large volumes that can dissolve in oilbase
muds. For the water-base muds, 0.6 to 0.9% of gas will dissolve and not
appreciably change the density or compressibility of the mud. It will be difficult
to detect these low concentrations with downhole physical measurements. Free gas
will be easily detected as shown hereafter. For the oil-base muds we will assume
no free gas is present at bottomhole and the mud properties are changed only
due to the dissolved gas. The detection will be more difficult than with free gas.
Bottomhole Gas Detection. Many techniques could be used for bottomhole gas
detection:
mud acoustic velocity
mud acoustic attenuation
mud specific weight
mud resistivity
mud temperature
annulus noise-level
Figure 4-263 shows the various sensors that could, schematically, be installed
in the annulus. Cuttings, turbulent flow, vibration and shock may render some
measurements difficult. We shall study those that can be related easily to gas content.
Mud Acoustic Velocity. Acoustic velocity can be accurately predicted. The
measurements could be made over a short distance in the annulus of the order
of 1 to 2 ft. The “free” mud formula can be used. This is
(4- 197)
where V = acoustic wave velocity in ft
K = gas cut mud compressibility in psi-’
y = gas cut mud specific weight in lb/ft3