Schlumberger

Case Study: Replacing an Entire Openhole Logging Program for Shale Gas Reservoir with One Cased Hole Tool

Pulsar multifunction spectroscopy service matches previous interpretation for unconventional reservoir, including TOC, without requiring openhole log input

Challenge: Improve the efficiency of unconventional reservoir evaluation, including determination of the complex mineralogy and kerogen content and quantification of the gas volume.

Solution: Run Pulsar multifunction spectroscopy service to measure elemental concentrations —including total organic carbon (TOC)— sigma, porosity, and fast neutron cross section (FNXS) for a complete, stand-alone cased hole interpretation from a single tool.

Result: Streamlined well construction and eliminated openhole logging risks associated with well instability prior to running casing after confirming that a single run of Pulsar service run of Pulsar service provides the same accuracy and breadth of information as an entire openhole logging program.

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 highperformance 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.


Download: Replacing an Entire Openhole Logging Program for Shale Gas Reservoir with One Cased Hole Tool (0.39 MB PDF)

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Pulsar multifunction spectroscopy service
Multifunction spectroscopy service pairs multiple detectors with a high-output pulsed neutron generator to provide stand-alone cased hole formation evaluation and reservoir monitoring with openhole logging quality. Visit Pulsar Spectroscopy Service page