Page 34 - Well Logging and Formation Evaluation

24 Well Logging and Formation Evaluation

Gas detection analysis

Gas detectors work by passing air drawn from where the mud reaches
the surface (the bell nipple) over a hot detector filament. This combusts
the gas, raising the temperature and lowering the resistance of the fila-
ment. At high voltages all the combustible gases burn, whereas at lower
temperatures only the lighter components burn. By recording the change
in resistance at different voltages, the relative proportions of the various
components may be estimated.
A gas chromatograph may also be used to further differentiate the
various hydrocarbon components. Particularly for the detection of
poisonous gases, such as H 2S, Drager tubes may be used on the rig
floor.

1.8 TESTING/PRODUCTION ISSUES

At the end of the logging, a decision will have to be made as to whether
casing should be run or not. If the well results are not as expected, there
may be an immediate decision required to either sidetrack or abandon the
well. Therefore, a quick but accurate interpretation of the data, not always
made using a computer, is of primary importance.
If the decision is made to test or complete the well, the petrophysicist
will also be required to pick the perforation intervals. A few points to bear
in mind here are that when picking the intervals from a log it is impor-
tant to specify the exact log being used as a depth reference. Since depths
on field prints are sometimes adjusted to tie in with previous runs when
the final prints are made, confusion can occur. The safest thing is to
include a photocopy of the reference log, with the intervals to be per-
forated marked on the log, along with any program passed to the rig.
The correct procedure for ensuring that the well is perforated “on
depth” with wireline operations is as follows:

1. Initially it is necessary to tie the depths of casing collars, as measured
in the well using a CCL, with the reference openhole GR log. This is
done by making a run in the hole with a GR/CCL tool and comparing
the depth with the openhole reference log on which the perforated
interval has been marked.
2. The print of the GR is then adjusted so that the depths of the casing
collars are on depth with the openhole GR.