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While Drilling Tool - MWD Tool. The data is transmitted through the mud
column in the drillstring, to surface. At surface the signal is decoded and presented
to the driller in an appropriate format. The transmission system is known as mud
pulse telemetry and does not involve any wireline operations.
Commercial MWD systems were first introduced in the North Sea in 1978 as a more
cost effective method of taking directional surveys. To take a directional survey
using conventional wireline methods may take 1-2 hours. Using an MWD system
a survey takes less than 4 minutes. Although MWD operations are more expensive
than wireline surveying an operating company can save valuable rig time, which is
usually more significant in terms of cost.
More recently MWD companies havedeveloped more complicated tools which
will provide not only directional information and drilling parameters (e.g. torque,
WOB) but also geological data (e.g. gamma ray, resistivity logs). The latter tools are
generally referred to as Logging While Drilling - LWD Tools. As more sensors are
added the transmission system must be improved and so MWD tools are becoming
more sophisticated. Great improvements have been made over the past few years
and MWD tools are now becoming a standard tool for drilling operations.
2. MWD SYSTEMS
All MWD systems have certain basic similarities (Figure 1)
- a downhole system which consists of a power source, sensors, transmitter
and control system.
- a telemetry channel (mud column) through which pulses are sent to surface.
- a surface system which detects pulses, decodes the signal and presents
results (numerical display, geological log, etc.).
The main difference between the 3 MWD systems currently available is the method
by which the information is transmitted to surface. All three systems encode the
data to be transmitted into a binary code and transmitting this data as a series of
pressure pulses up the inside of the drillstring. The process of coding and decoding
the data will be described below. The only difference between the systems is the
way in which the pressure pulses are generated (Figure 2).