Page 24 - Well Logging and Formation Evaluation

14 Well Logging and Formation Evaluation

client’s office. Therefore, a second program is usually sent to detail the
further logging requirements.

1.5 OPERATIONAL DECISIONS

While the logging program will aim to cover most eventualities during
the logging job, often instant decisions have to be made where it is not
possible to call everyone into a meeting and get the approval of all parties
concerned. Below are some of the things likely to happen and some con-
siderations in decision making.

1.5.1 Tool Failures

The standard procedure if a tool fails is to replace it with a backup and
carry on as before. In general, if the program has specified that a tool is
to be run over a specified interval (particularly a reservoir interval), then
it is best to ensure that good-quality data are acquired, even if it means
making additional runs. However, the following situations may arise:

• When logging with LWD and in the hole drilling near the end of a
section, it may be far more cost-effective, if the data are not critical, to
simply carry on drilling and reacquire the data during a check trip. In
some cases, the memory data may still be usable.
• If a wireline tool starts to act erratically, it may not justify rerunning
the tool, since the data may not be critical over the particular interval
and can sometimes be corrected by postprocessing the raw tool data at
the surface.
• If an advanced tool fails, it sometimes happens that no backup tool is
available on the rig. The choices then are to either try and repair the
tool, arrange a backup tool to be sent from another location, or replace
the tool with an earlier version with less capability. Most commonly,
the latter option is chosen.
• In the event of failures occurring with potential safety implications
(e.g., explosive charges going off accidentally), it is normal for opera-
tions to be suspended until a full investigation has been carried out to
establish the cause.

1.5.2 Stuck Tools

Fairly regularly during logging operations, tools get stuck in the hole,
either temporarily or permanently. There are often indications of bad hole