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 hole tool

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 spectroscopy tool.

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.

Where the openhole density and dry-weight TOC logs responded to a transition at ~X,550 ft from low porosity, low kerogen, and low gas volume to higher values below, Pulsar service’s FNXS and dry-weight TOC similarly respond to the low porosity–gas transition at that depth. The lack of change in the sigma and TPHI measurements shows that if conventional pulsed neutron cased hole logs had been run instead of Pulsar service, the transition would not have been obvious.

The operator was pleased that the comparison confirms that Pulsar service’s single-tool performance in cased hole is equivalent to a full suite of openhole logs. Having this reliable, accurate option for thoroughly evaluating complex unconventional reservoirs in cased hole will simplify future operations.