Geology Reference
In-Depth Information
We give estimates for typical H values. Consider a typical thirty-feet
MWD collar and assume a pulser is positioned at the very top. If the drillbit lies
at its bottom (this is almost never the case, since resistivity subs or drilling
motors are found adjacent to the bit), and the sound speed is 3,000 ft/sec, then
the collar travel time is 30/3,000 or 0.01 sec. In general, thus, H >> 0.02 sec.
4.5.5
Run 1. Long, low data rate pulse.
Figure 4.5b
. Long, low data rate pulse.
C CASE 1
A = 0.8
R = 100.0
SIGNAL = A*(TANH(R*(T-0.1))-TANH(R*(T-0.35)))/2.
For our Method 4-5 figures, black, red, green and blue curves are plotted,
respectively, from bottom to top. In this run, the time delay between pulser and
drillbit is 0.01 sec, the same as the sampling time. The pulse width is about 0.25
sec. The black curve is the positive over-pressure created ahead of the valve
when the poppet valve or the siren closes. At the same time, a negative under-
pressure at the valve propagates down toward the bit and reflects upward
without a change in sign. This negative pressure is shown flipped over in the
red curve so it can be compared with the black one more easily. Green is the
actual pressure ahead of the pulser transmitted to the surface and shows positive
and negative bumps. Most of the signal has been lost due to destructive wave
interference, and what is worse, the single black pulse is replaced by two smaller
green pulses. It is this green double-pulse that is detected uphole. If the green
data is available, the blue curve shows how the black curve is recovered
successfully using the solution to Equation 4.5g.
4.5.6 Run 2. Higher data rate, faster valve action.
The time delay between pulser and bit is 0.01 sec, same as the sampling
time. We consider a much higher data rate represented by a narrower pulse as
shown in Figure 4.5c. As before, we have very good signal recovery. The pulse
width (black curve) is about 0.05 sec, for a data rate of about 20 bits/sec.
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