Page 190 - Practical Well Planning and Drilling Manual

Section 2 revised 11/00/bc 1/17/01 12:04 PM Page 166

[ ] Well Programming
2.3.2

rate times and depths so that shots can be matched on the film with
the survey depths. This slows down the trip out since the string must
be held still for longer than it generally takes to rack a stand to ensure
that a survey is taken.
Gyro multishots produce a similar strip of film that has to be
matched to times and depths. The advantage of a GMS is that it can be
run inside steel, either in casing before drilling out or inside the drill-
string (e.g., after tagging the cement plugs and before drilling out).
The main disadvantage of a GMS is that the quality control checks
must be done properly to ensure that the gyro is correctly aligned at
the surface, drift checks are done regularly to track the rate of drift,
and the alignment is again checked on surface. All this takes time, so
a GMS generally takes more time than a MMS. However, the GMS is
often run in casing on wireline while flanging up the BOP and so the
actual time impact may not be too much.
If a wireline unit is used for running a multishot survey, the depth
counter must be accurate and the wire in good condition. A faulty
depth counter will throw out your whole survey without you knowing
about it. This should be part of the quality control checks that are wit-
nessed by the drilling supervisor.
Even in a vertical well it is important to have a reasonably accurate
picture of the wellpath. Apart from anything else, if disaster strikes and
a blowout results, a relief well can then be drilled to allow the blowing
well to be killed. I advise that a MMS or GMS be routinely run in the
casing set before the first section which may encounter hydrocarbons,
to provide a definitive wellbore position at that point.
Wireline deployed surface readout gyro (SRG). When kicking off
the well in close proximity to steel interference in the Earth’s magnet-
ic field, a gyro tool can be run on wireline to give a constant surface
readout of directional information. Normally a working stand is made
up with a special gooseneck on the top, which allows mud to be
pumped down while the top of the gooseneck seals on the wire. A mud
motor is used for the kickoff so the working stand is not rotated.
The working stand is made up to the drillstring, the SRG is run on
wireline until it lands in the UBHO. The seal is pumped closed and the
mud pumps are kicked in. Drilling continues with a full surface data
readout until the stand is drilled down. Once the stand is drilled down,
it is pulled back, the pumps are stopped, and the SRG pulled back to
inside the working stand. The stand is broken and racked back in the

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