Page 307 - Practical Well Planning and Drilling Manual

Section 2 revised 11/00/bc 1/17/01 12:04 PM Page 283

Formation Evaluation [ ]
2.8.1

directional performance), bedding plane dips (directional), and may
produce acceptable directional surveys.
Pressure measurements and formation fluid samples. Used to deter-
mine wellbore stability, well control, reducing reservoir damage, and/or
cementing problems due to reaction of fluids with mud and cement.
Lithology and porosity determination. Determines bit selection, well-
bore stability, mud and cement programs, drilling practices, and casing
seat selection.
Sidewall core samples. Used for shale mineralogy studies; and is
especially useful if mechanical rather than explosive sidewall cores
have been taken. If an MSCT or CST is programmed, try to get one or
two samples from each distinct shale lithology.
Gamma ray (GR). Used to determine bit selection, correlation, and
wellbore stability. The gamma ray spectrometry tool (GRST) provides
details of clay mineralogy and allows good well-to-well correlation.
Seismic tools. Detects the presence of faults. Walkaway survey can
look ahead of the bit. Also used for overpressure prediction and bed-
ding planes (directional performance).
This provides a general picture of types of logging tools and the
general applications they have in well planning. It is important to study
what tools are actually available and to recognize how they may apply
to well planning. Wildcat exploration wells will be more thoroughly
logged than development wells. In an exploration well, the objective is
to gather as much information as possible to improve performance in
the subsequent wells, primarily by optimizing bit selection and mud
design. On development wells the logs run will have more to do with
addressing known problem areas or addressing specific areas of interest.
Electric logs may be run on wireline in open hole or casing.
Logging while drilling is self-explanatory; it allows the driller to have
a real-time picture of the formations at the bottom before much reac-
tion has taken place with the mud. Some tools offer the ability to look
ahead of the bit (seismic while drilling) or to make real-time decisions
on casing points or geosteering in horizontal wells, e.g., resistivity at
the bit (RAB). Logging while drilling may be done in memory mode as
well as surface readout; if memory mode only is used then of course
real-time decision making is not possible.
Electric logs can also be run on drillpipe or coiled tubing if diffi-
cult hole conditions make wireline logging impossible. These tech-
niques make logging much more expensive than on wireline.

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