Resource-intensive openhole characterization
To evaluate a complex shale gas reservoir in Pennsylvania, USA, an
operator had run a full suite of openhole logs in the 8 3/4-in borehole,
including triple-combo, nuclear magnetic resonance (NMR), and an advanced
spectroscopy tool. The high-definition spectroscopy data was critical in
quantifying the complex mineralogy, including the spectroscopy dry-weight TOC
as a key input for determining the kerogen volume for evaluating reservoir
quality (RQ). Having both density and NMR (Track 7) was necessary to compute
the gas volume and total porosity because they have contrasting responses to
kerogen and gas.
The operator wanted to learn if the accurate, detailed interpretation
provided by an openhole logging program could be achieved with cased hole
logging, which would streamline well construction and reduce the risk posed by
wellbore stability in the shale reservoir.
Stand-alone cased hole formation evaluation
Schlumberger recommended new Pulsar multifunction spectroscopy service,
which integrates a high-performance pulsed neutron generator with multiple
state-of-the-art detectors in a single 1.72-in diameter tool for cased hole
logging. In addition to providing petrophysical volumetric interpretation with
the capabilities and quality of openhole logging to cased well environments,
Pulsar service's measurement technology can be operated in different pulsed
neutron logging modes for monitoring consistency with previously obtained
conventional cased hole services.
Pulsar service also introduces the FNXS measurement, which makes it
possible to reliably differentiate gas-filled porosity from tight formations
and quantify the gas volume.
Detailed, accurate volumetric interpretation from a single slim cased
To test Pulsar service's performance in evaluating the complex shale gas
formation, the cased well (5 1/2-in 23-lbm/ft casing) was logged prior to
completion. The borehole was filled with freshwater. Three separate passes of
Pulsar service were made at 300 ft/h in hybrid GSH-lithology mode, which
simultaneously acquires data for gas, sigma, and hydrocarbon index (GSH) in
addition to elemental spectroscopy including TOC and the carbon/oxygen ratio.
The data were stacked, and a stand-alone volumetric interpretation was
conducted using sigma, thermal neutron porosity (TPHI), FNXS, and spectroscopy
data, all from Pulsar service. At this relatively slow logging speed, Pulsar
service's spectroscopy data, including dry-weight TOC, has very good precision
and compares favorably with the openhole data from the larger-diameter advanced
A stand-alone cased hole interpretation was performed next using all the
Pulsar service data in a weighted linear solver with standard end-point values.
Again, the interpreted volume compares quite favorably, including the gas
volumes and total porosity, even though no openhole logs were used in the
interpretation, as would be required if a conventional pulsed neutron logging
tool had been run.