Page 191 - Practical Well Planning and Drilling Manual

Section 2 revised 11/00/bc 1/17/01 12:04 PM Page 167

Directional Planning [ ]
2.3.2

derrick, a stand of pipe made up and run in. Again the working stand
is picked up and the process is repeated.
Alternatively a side entry sub can be placed higher up in the drill-
string and the wire passes from inside the string to the outside, where
it follows the annulus up to the surface. The major disadvantage is that
if the BOP has to be closed, you now have a wire in the way.
Measurement and logging while drilling (MWD and LWD) tools.
MWD tools started to become commonly used in the early to mid-
1980s. At that time you had to have two engineers on the rig just to
run the tool and it was generally not too reliable. Modern MWD tools
are normally run by the directional driller and can usually stay for
hundreds of hours downhole.
Most MWD tools send signals to the surface using pressure pulses
in the mud stream travelling inside the drillstring. A very sensitive
pressure transducer on the standpipe detects these pressure pulses and
converts them into data, which can be presented to the driller. Real-
time (delayed by a few seconds to a minute) data on tool inclination,
azimuth, and toolface azimuth is displayed on the drillfloor allowing
corrections to be made quickly as required.
There are several methods of generating pressure pulses. Negative
pulses are caused by opening a small valve, momentarily allowing mud
to bypass the rest of the BHA and bit. Positive pulses are caused by
actuating some kind of restriction inside the BHA, which causes a pos-
itive pressure wave to travel up the drillstring. A carrier wave can also
be generated that gives a continuously cycling pressure variation, and
this carrier wave can then be modulated to give a data stream to sur-
face in the order of 5 to 12 Hz.
More recent developments combine MWD and LWD tools to give
real-time readouts of directional data, sonic, resistivity, gamma ray, and
other formation parameters while drilling. The LWD package general-
ly transmits its data electronically to the MWD transmitter package
and this then transmits all the data up the hole. The modulated carri-
er wave transmission method allows a higher rate of transmission for
large amounts of data.
LWD tools transmitting real-time formation data to the surface
allow the drillers to actively steer the BHA within the reservoir. For
instance, say you wanted to drill a high angle well in a sandstone reser-
voir. The plan is to drill 3-4 m below the shale cap rock. By running a
gamma ray or resistivity tool, you would know when you approached

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