A new logging-while-drilling (LWD) tool that combines traditional
measurements of gamma ray, propagation resistivity, gamma-gamma density, and
thermal-neutron porosity with measurements unique to the LWD arena, including
neutron capture spectroscopy and capture cross section, opens up new
opportunities for formation evaluation on LWD.
The compact design of the new-generation LWD tool greatly increases the
likelihood that measurements will be made before the onset of significant
invasion. The colocation of resistivity- and neutron-based sensors also means
that key measurements are being made at the same depth at the same time and on
a similar volume of the formation. These features ensure that all measurements
are essentially seeing the same amount of invasion, thus removing a major
complication in conventional LWD interpretation.
A new-generation LWD tool has been developed that integrates measurements of
gamma ray, propagation resistivity, gamma-gamma density, and thermal-neutron
porosity with additional measurements unique to the LWD arena, including
neutron capture spectroscopy and measurement of formation capture cross section
(Weller et al. 2005). The EcoScope multifunction LWD service integrates all of
these measurements in a single collar optimized to:
- Minimize measurement distance to bit.
- Improve real-time data-transmission rates.
- Improve service reliability.
- Minimize use of chemical nuclear sources.
The new-generation LWD tool is currently available in a 6¾-in. collar size,
with a total length of 26 ft (Fig. 1). The tool is rated to operate at up to
20,000 psi and 300°F. Other collar sizes are also under development to allow
the tool to be deployed in a wider variety of hole sizes.
Array propagation resistivity measurements and neutron-based measurements,
including neutron porosity, neutron capture spectroscopy, and formation capture
cross section, are colocated in the top half of the tool, with the highest
measure point being less than 16 ft above the bottom of the tool.
The conventional stabilized gamma-gamma density measurement is located
within 8 ft of the bottom of the tool; this provides average and quadrant
values and 16-sector images of bulk density and photoelectric factor.
Stabilizers are available to support use in hole sizes ranging from
approximately 8 to 10 in. Future developments of the tool will extend this
range. Adjacent to the stabilizer are two diametrically opposed ultrasonic
standoff sensors. These provide a 16-sector ultrasonic borehole image when the
tool is rotating and allow for a caliper measurement even when sliding.
Finally, an azimuthal gamma ray measurement device, capable of producing
16-sector gamma ray images as well as quadrant and average gamma ray
measurements, is located only 5 ft above the bottom of the tool. The gamma ray
detector uses a very large sodium iodide crystal to achieve high count rates,
and owing to a focused design, the measurement has good azimuthal sensitivity;
this enables gamma ray images to be acquired.
Efforts to address the four objectives described above have led to the
design of a tool with unprecedented application for formation evaluation. We
will discuss here the application of the rich data set produced by this tool
for the evaluation of lithology and mineralogy, porosity, and fluid
Evaluation of these properties using a similar suite of measurements
acquired while using wireline-logging tools has been presented previously
(Herron et al. 2002). However, the suite of measurements available with this
new tool, and the fact that many of the measurements are colocated, has allowed
this methodology to be extended and enhanced. In addition, recent enhancements
in data-transmission rates allow all of these measurements to be accessed in
real time, thus bringing the possibility to use advanced formation-evaluation
results to make while-drilling decisions.
© 2007. Society of Petroleum Engineers
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- Original manuscript received:
14 July 2005
- Meeting paper published:
9 October 2005
- Revised manuscript received:
30 October 2005
- Manuscript approved:
24 February 2007
- Version of record:
20 April 2007