Page 256 - Drilling Technology in Nontechnical Language

Chapter 10 – EVALUATION 247

The logging unit may have self-contained satellite communications,
or it may use the rig communication system to transmit log results back
to the operating company. Sometimes, important decisions must be made
as soon as possible after logging, and the rig may have to wait for those
decisions to be made before operations continue. With total daily operating
costs that can exceed six figures in some cases, clearly the time taken to

make these decisions must be minimized.
On an exploration well where the hole is drilled solely to obtain
information, the logging program for each hole section (and especially in
the reservoir) will be extensive. Money invested in logs at this stage allows
better decisions to be made when designing and drilling subsequent wells

in the field. The logs will also allow any zones bearing hydrocarbons to be

identified and to quantify some basic reservoir properties, such as porosity
and permeability. The major objective is to identify whether producible
hydrocarbons are present in commercially viable quantities. If so, further
wells will normally be drilled to extend knowledge of the reservoir before
wells are drilled to produce the hydrocarbons. These further wells are
called appraisal wells.

If the hole condition is good (stable in-gauge wellbore and no particular
hole problems), it is possible to run a log on wireline at inclinations up to
50°. If the hole is badly enlarged in places, has potential sticking problems,
or has a higher inclination, other methods of deploying the logging tools
have to be considered. These can add greatly to the cost of obtaining logs.

Logging using drillpipe or coiled tubing

If the wellbore passes through some problem sections, but the reservoir
is loggable with standard techniques, drillpipe that is open at the lower end

can be run to the point where the hole is in good shape (fig. 10–10). Special
slim (small-diameter) logging tools can be run on wireline through the
drillpipe, exiting at the bottom of the pipe and then logging conventionally.
In this case, the drillpipe merely acts as a conduit to get the logging tools
past the problem areas. Many wells build up to a high inclination but then
drop angle back towards vertical through the reservoir. If the maximum
inclination is too high for the tools to slide down the drillpipe, mud can be
pumped in at the top to push the tools along the pipe.

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