Page 210 - Well Logging and Formation Evaluation

200 Well Logging and Formation Evaluation

a round trip, either once TD is reached or at some other point during
pulling out of or running in the hole. Such decisions are typically made
in conjunction with the drillers. It is recommended at the start of a
geosteered well to set up a strict and rigorous system of naming data files
transmitted from the rig to the office, so that there is no confusion as to
whether data are pulsed or memory, and whether or not they were acquired
during drilling or tripping. Distinctions also need to be made between
up/down data and data for which depths have been corrected to be con-
sistent with previous runs or known casing shoes, etc.
Tool failures during geosteering are a common occurrence. It is
recommended to keep accurate records of serial numbers of tools used in
the hole and to check that regular calibration and maintenance are being
performed. It can significantly add to the cost of a well if a toolstring has
to be brought back to surface due to tool failure in the critical section of
a well, and in some cases may even result in the well being lost if the
openhole time becomes too great.
For a geosteered well to be successful, there needs to be good com-
munication between the petrophysicist, wellsite geologist, office geolo-
gist, and drilling department. The wellsite geologist, particularly if he has
a good knowledge of the field, is usually in the best position to know
which formation the well is in, but he needs the support of the petro-
physicist to interpret the real-time formation evaluation data. The neces-
sary course of action that these two decide upon needs to be fed back to
the drillers so that the well trajectory is optimized.
The depth offset between the up and down readings of tools can be used
to establish whether the trajectory is veering structurally deeper or shal-
lower. This is done as follows. Consider that one is drilling a low-GR
sand, bounded above and below by high-GR shales. In the event that one
exits the sand into the structurally shallower shale, one would expect that
the up reading on the GR would respond before the down reading; simi-
larly, if the wellbore exits to the structurally deeper shale, the down log
would respond first. The offset between up and down readings, together
with knowledge of the borehole size, can yield an estimate of the relative
dip between the borehole and formation. Consider the example in Figure
13.3.2.
If the borehole diameter is d and the offset between the up and down
readings is t (measured in similar depth units as d), then the relative angle
(q) between the borehole and the formation is given by:

q= arctan (dt ). (13.3.1)