Page 452 - Practical Well Planning and Drilling Manual

Section 3 revised 11/00/bc 1/17/01 12:00 PM Page 428

[ ] Practical Wellsite Operations
3.7.6

3.7.6. Totco Single Shot Survey Tool

The Totco tool is much simpler than the Magnetic Single Shot
(MSS). No monel collars are needed. A paper disk is pierced by two
pins showing inclination but not azimuth. The tool is loaded by
opening the angle unit, placing one of the marked paper disks inside
(compatible with the angle unit) and making the unit up again. The
timer is set and it can be run on wireline or dropped prior to pulling
out of the hole.
After recovering the paper disk, it can be examined under the mag-
nifying examiner. The two pinholes should both show the same angle,
180° apart. There are two to act as a quality check of the survey; and as
long as they both show the same angle, the survey should be reliable.
The Totco is simpler and more reliable than the MSS. However, the
Totco does not give azimuth information.

3.7.7. Gyro Multishot Surveys

A Gyro survey will sometimes be run in casing while nippling up
the BOPs. It will be run on wireline by a dedicated unit with a winch
operator and a survey technician.
The gyro is set up on surface by reference to a fixed point, usually
some distance away, that has a known bearing from the rotary table.
When the gyro is recovered to surface, it will again be compared to the
reference point to see how far the gyro has drifted in that time.
Quality checks of the gyro drift should be taken by stopping in the
wellhead for 5 minutes before leaving and when arriving back at sur-
face, then for a 5-minute stop after every 15 minutes surveying. The
camera takes pictures on to a strip of film and by carefully recording
times and depths on surface, each picture can be related to a depth. By
stopping for 5-minute checks after every 15 minutes, the accuracy of
the gyro can be verified and the correction applied to the photograph-
ic survey results. Surveys pulling out should match surveys running in
at the same depths.
Check that the depth counter on the unit is in good condition.
Record the holdup depth and check that the tool reads zero again when
back at surface with the survey. The accuracy of the depth counter is
crucial to a good survey, and yet this often seems to be forgotten.

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