STANDARD HANDBOOK OF PETROLEUM & NATURAL GAS ...

938 Drilling and Well Completions
Surface Detection of the Mud Pressure Signals. The pulse or wave amplitude
varies largely according to depth, frequency, mud type and pulse generator
device. A typical mud surface pulse amplitude is 1 bar (14.5 psi). In a sine wave
transmission the surface amplitude may go as low as 0.1 bar (1.5 psi) rms. The
pump noise must be lowered to minimum by the use of properly adjusted
dampeners and triplex instead of duplex pumps, the pulse amplitude being about
twice as large for the duplex pumps. The pump noise amplitude varies from
0.1 to 10 or more bars (1.5 to 145 psi) with dominant frequencies ranging from
2 to 10 Hz. The rotation speed of the pumps may have to be changed so the
noise frequency does not interfere with the measurements. The pressure sensors
are generally of the AC-type, which sense only the pressure variations. A
common sensor is of the piezoelectric type with a crystal transducer. Generally
a built-in constant current follower amplifier converts the signal to a low
impedance voltage. A typical sensitivity is 5 V per 1,000 psi (70 bars) with a
maximum constant pressure of 10,000 psi (700 bars). The filtering can be done
with digital filters or Fourier transform analyzers. For a Fourier transform
processing, the signal must be properly analog-filtered and then digitized. Two
pressure transducers can be used at different locations on the standpipe, as
shown in Figure 4-250, to take advantage of the phase shift that is opposite for
pump noise and downhole signal. Sophisticated digital cross correlation techniques
can then be used.
Downhole Recording. Most MWD service companies offer the possibility of
recording the data versus time downhole. The memories available may reach
several megabytes, allowing the recording of many parameter values during many
hours. This information is particularly valuable when the mud pulse link breaks
down. The data can be dumped in a computer, during the following drillpipe trip.
Figure 4-250. Surface pressure transducers location for pump noise elimination.