STANDARD HANDBOOK OF PETROLEUM & NATURAL GAS ...

MWD and LWD 961
One steel diaphragm is exposed to the internal pressure, the other is exposed
to the external pressure. Four gages are normally used. Two of them are sensitive
to pressure and temperature, and two are sensitive to the temperature. A
Wheatstone bridge is used for detection of the pressure.
Downhole Shocks Measurements. An accelerometer in the MWD telemetry
tool measures transverse accelerations, or shocks, that may be damaging for the
bottomhole assemblies. When acceleration exceeds a certain threshold, the event
is signaled to the surface as being a shock. These events versus time or depth
are displayed as shock count. This information is used as a warning against
excessive downhole vibrations and to alert the driller to change the rpm or
weight on the bit [106].
A simple circuit has been designed to count the number of shocks that the
tool experiences above a preset “g” level. The transverse shocks are measured
in the range of 2 to 1,000 Hz in excess of the preset level. The level is adjustable
and defaults at 25 g’s (when no preset level is specified).
Downhole shock measurements are used to:
send alarms of excessive downhole vibration in real-time so that action can
be taken to reduce damage to the MWD tools, drill bits, and bottomhole
assemblies;
reduce costly trips to replace damaged equipment;
improve drilling rate by eliminating counter-productive BHA vibration motion.
Downhole Flowrate Measurement. Anadrill’s basic MWD tool can be set up
to monitor the alternator voltage being produced by the mud flowing across
the MWD turbine downhole. By comparing this voltage to the standpipe pressure
and the pump stroke rate, the surface system shows that a washout in the drill
string is occurring much quicker than with conventional methods [106].
The downhole flowrate monitoring and washout detection system is used to
avoid potential twist-offs from extensive drill string washouts;
determine if the washout is above or below the MWD tool, thus saving rig
time when searching for the failure.
Safety Parameters
One area where MWD would be most useful is drilling safety and, particularly,
early gas kick detection and monitoring. Conventional kick monitoring is based
on pit gain measurements and all other available surface indication such as
drilling rate break, injection pressure variation, etc.
Using the probable detection threshold achievable and a gas kick model
applied to a typical 10,000-ft drill hole, an early alarm provided by MWD systems
decreases significantly the amount of gas to be circulated as compared to using
conventional methods of kick detection.
Dissolved Gas. Gas which enters the borehole when penetrating a high pressure
zone may not dissolve immediately in the mud. The free gas considered here is
the gas entering the borehole minus the dissolved gas. Table 4-128 indicates
the maximum volume of dissolved gas at bottomhole conditions expressed in
percent of annulus mud volume. Thus, when entering a high pressure permeable
formation this much gas will dissolve first before free gas appears in the mud.