Page 14 - Well Logging and Formation Evaluation

4 Well Logging and Formation Evaluation

LWD tools present a complication for drilling, as well as additional
expense. However, their use may be justified when:

• Real-time information is required for operational reasons, such as
steering a well (e.g., a horizontal trajectory) in a particular formation
or picking of formation tops, coring points, and/or casing setting depths
• Acquiring data prior to the hole washing out or invasion occurring
• Safeguarding information if there is a risk of losing the hole
• The trajectory is such as to make wireline acquisition difficult (e.g., in
horizontal wells)

LWD data may be stored downhole in the tools memory and retrieved
when the tool is brought to the surface and/or transmitted as pulses in the
mud column in real time while drilling. In a typical operation, both modes
will be used, with the memory data superseding the pulsed data once the
tool is retrieved. However, factors that might limit the ability to fully use
both sets of data are:

• Drilling mode: Data may be pulsed only if the drillstring is having mud
pumped through it.
• Battery life: Depending on the tools in the string, tools may work in
memory mode only between 40 and 90 hours.
• Memory size: Most LWD tools have a memory size limited to a few
megabytes. Once the memory is full, the data will start to be overwrit-
ten. Depending on how many parameters are being recorded, the
memory may become full within 20–120 hours.
• Tool failure: It is not uncommon for a fault to develop in the tool
such that the pulse data and/or memory data are not transmissible/
recordable.

Some of the data recorded may be usable only if the toolstring is rotat-
ing while drilling, which may not always be the case if a steerable mud
motor is being used. In these situations, the petrophysicist may need to
request drilling to reacquire data over particular intervals while in
reaming/rotating mode. This may also be required if the rate of penetra-
tion (ROP) has been so high as to affect the accuracy of statistically based
tools (e.g., density/neutron) or the sampling interval for tools working on
a fixed time sampling increment.
Another important consideration with LWD tools is how close to the
bit they may be placed in the drilling string. While the petrophysicist will
obviously want the tools as close to the bit as possible, there may be