Page 35 - Well Logging and Formation Evaluation

Basics 25

3. When the perforating guns are run, they will be combined with only a
CCL log. On first running in the hole, the CCL on the perforating guns
will be off depth with the corrected GR/CCL.
4. Because of irregularities in the length of joints of casing, the CCL
acquired with the guns may be overlain on the GR/CCL and a unique
fit made. This enables the logger’s depths of the perforating guns to
be adjusted so that the CCL is on depth with the GR/CCL (which is
itself on depth with the reference openhole log). Obviously if all the
joints of casing were the same length, it would be possible to find a fit
when the toolstring was off depth by the length of one casing joint.
This problem may be avoided by running a radioactive pip tag as part
of the completion, which enables the CCL on the perforating gun to
be tied with certainty to the GR/CCL.
5. Once the gun has been fired, there may be indications on the surface,
such as changes in cable tension. After a few minutes, there may also
be indications of an increase in tubing head pressure.

The most appropriate guns, charges, phasing, and well conditions (fluid
type and drawdown) all need to be considered. Usually the contractor is
able to offer advice on this and should be involved in any meetings at
which the perforation procedures are to be determined. Wherever possi-
ble it is best that wells be perforated “under drawdown.” This means that
the pressure in the wellbore is lower than the formation pressure, which
ensures that the well is able to flow as soon as perforation has occurred
and avoids the risk of either completion fluid or debris blocking the
perforations.
I have seen many cases in which the petrophysicist has picked many
short intervals to be perforated, separated by only a foot or so. Since the
accuracy of depth correlation is never perfect, it is sometimes advisable
to shoot a continuous section, which includes parts that are not of the
reservoir. There has always been caution about perforating shales, in case
they result in “fines” being produced. I can only say that I have never
heard of this occurring in practice. In general I favor perforating as much
of the potentially producible interval as possible (a safe distance from any
water-bearing sands). If you look at the economics of a well, an additional
10bbl/day over the life of a well will result in a far greater economic
benefit than the additional cost of perforating an extra 10m. In some cases,
picking too short an interval may result in the well never even managing
to flow to surface when otherwise it could be an economic producer. It
may frequently occur, particularly in depleted reservoirs, that the well