STANDARD HANDBOOK OF PETROLEUM & NATURAL GAS ...

MWD and LWD 941
where P(x) = pressure wave amplitude at distance x in psi
P(0) = pressure wave amplitude at distance 0 in psi
q = kinematic viscosity in ft2/s (1 cSt x 1.075 x
w = angular frequency in rad/s (0 = 2x0
with f = frequency in Hz
di = pipe internal diameter in ft
V = wave velocity in ft/s
= 1 ft2/s)
Figure 4-252a and b gives the pressure wave amplitude versus the distance
for various typical muds.
Electromagnetic Transmission Systems. One system uses a low-frequency
antenna built in the drill collars. This system is a two-way electromagnetic
arrangement allowing communication from bottom to surface for data transmission
and from surface to bottom to activate or modify the tool mode. At any
time the sequence of the transmitted parameters, as well as the transmission
rate, can be modified. The tool is battery powered and can work without mud
circulation. The principle of the system is shown in Figure 4-253. The receiver
is connected between the pipe string and an electrode away from the rig for
the bottom to surface mode. This system can be used on- or off-shore. Two tools
are available: the directional tool, which transmits inclination, azimuth, gravity
tool face or magnetic tool face, magnetic field inclination and intensity; and
the formation evaluation tool, which measures gamma ray and resistivity. The
formation evaluation data are stored downhole in a memory that can be
interrogated from the surface or transferred to a computer when pulling out.
Figure 4-254a gives the attenuation per kilometer as a function of frequency
for an average formation resistivity of 10 and 1 Q m.
PULSE AMPLITUDE, psi
2 5 10 20 50 100
0
PULSE AMPLITUDE, psi
5 10 20 50 1
0
5,000
5,000
a=
10,000 i
.z
10,000 I-
L
n
15,000
20,000
20,000
Figure 4-252. Wave amplitude variation as a function of distance in waterbase
mud and in oil-base mud: (a) mud weight, 9 Ib/gal; (b) mud weight,
17.9 Ib/galt. (Courtesy Petroleum Engineer International [108].)