Page 209 - Well Logging and Formation Evaluation

Well Deviation, Surveying, and Geosteering 199

Well path
0.5 km
10 m

Target horizon

OWC

Figure 13.3.1 Example of Geosteered Well

available bandwidth (of the mud pulse telemetry system). Therefore,
careful consideration should be given to which tools are most effective in
determining whether or not one is in the target formation as opposed to
above or below it.
Bear in mind that the density/neutron tools require the toolstring to be
rotating for meaningful data to be obtained, and when changing the well
course it is often necessary to slide the toolstring using a turbine and bent-
sub. Resistivity data are generally more reliable, since they are not a
statistical type of measurement. The LWD-GR devices can generally be
placed closer to the bit and may be sufficient in many cases for deter-
mining whether one is exiting a target formation from above or below.
If a long bit run is planned, battery life may be an issue (typical battery
life is 50–100 hours), as may the downhole memory in which data,
assumedly, are being recorded, which may become full after a certain
number of hours. It is generally recommended to always record the data
in a downhole memory in addition to pulsing to surface. To avoid making
additional runs with pipe-conveyed logging at TD, it may be considered
worthwhile to include tools in the toolstring set to only record downhole
and not pulse to surface.
When permeability or presence of fractures is a particular issue, there
may be a requirement for tools (such as NMR [nuclear magnetic reso-
nance], pressure testing, or sonic) that are not available from all the con-
tractors. Data that are missing or of poor quality may be reacquired during