TECHNOLOGY UPDATE

18 JPT • OCTOBER 2017

For more than 40 years, the indus-try has used pulsed neutron logging to determine hydrocarbon and water sat-urations behind casing for reservoir management. Multiphase saturation measurements over time are useful for tracking reservoir depletion, planning workover and enhanced recovery strat-egies, and diagnosing production prob-lems such as water influx and injection fluid breakthrough. Cased-hole logs also serve as a contingency when openhole logs cannot be run or are not considered for reservoir characterization.

Although the cased-hole measure-ment suite has been greatly improved over many tool generations, the intrin-sic physical measurements remained unchanged, which meant that opera-tors could not obtain a complete pic-ture of the rock and fluids behind cas-ing. Input from openhole logs was required from a porosity or bulk den-sity measurement for combination with the neutron porosity. Absent this input, primary formation evalua-tion in cased wellbores can be ambig-uous. An additional challenge with cased-hole logging is correctly com-pensating for the effects of borehole fluids and the presence of comple- tion hardware.

Next-Generation LogsTo meet the need for accurate surveil-lance in cased holes, Schlumberger recently introduced the Pulsar multi-function spectroscopy system. The sys-tem builds on innovative technologies originated by the company to provide the first complete cased-hole forma-tion evaluation and reservoir saturation monitoring capability with openhole-equivalent measurements.

This next generation in pulsed neu-tron logging integrates multiple detec-tors and a high-output pulsed neu-tron generator (PNG) to significantly improve measurement precision, data acquisition accuracy, and logging speed. The measurements are comple-mented by powerful algorithms that compensate for variation in the bore-hole fluids and completion in deliver-ing robust, representative answers in complex conditions.

The PNG and four detectors are housed in a 1.72-in.-outside-diameter (OD) tool that is designed for through-tubing access and logging through most completion restrictions.

The detector adjacent to the PNG is the compact neutron monitor, which is primarily sensitive to fast neu-trons to provide accurate and pre-cise output measurement. There are three scintillation gamma-ray detec-tors for near, far, and deep detec-tion. The near and far detectors use cerium-doped lanthanum bromide (LaBr3:Ce) scintillators, and the deep detector, farthest spaced from the PNG, has an yttrium aluminum  perov- skite scintillator.

The three gamma-ray detectors are coupled to high-temperature-rated photo multiplier tubes, and their puls-es  are counted with specialized elec-tronics matched to the high rate and resolution of the LaBr3:Ce scintillators.

Detector resolution is only mini-mally degraded at high temperatures to 350°F, which avoids the need for a conventional flask that would increase tool OD and limit downhole operating time. The housing is corrosion resistant and NACE-MR0175-compliant, which enables deployment in corrosive well

environments such as those with hydro-gen sulfide or carbon dioxide.

The engineered architecture of this new advanced tool and the use of self-compensating algorithms provide a wealth of information in a single log-ging run: traditional cased-hole mea-surements; an expanded suite of ele-ments, including total organic carbon (TOC); and the new fast neutron cross-section (FNXS) measurement.

Unlike the hydrogen index (HI) mea-surement that dominates convention-al cased-hole logging, FNXS is direct-ly sensitive to the volume of gas in the formation to differentiate and quantify gas-filled porosity from liquid-filled and tight zones without any openhole input. It has a functionality similar to the bulk density log, although it is not a cased-hole density measurement. The combi-nation of sigma, HI, and FNXS measure-ments computes complex multiphase fluid saturations without an externally supplied porosity curve.

The high-fidelity determination of mineralogy by the new tool revolution-izes gamma-ray spectroscopy by direct-ly measuring the majority of the ele-ments that constitute the Earth’s crust. The multifunction spectroscopy system enables the acquisition of quantitative elemental concentrations downhole at reservoir pressures and temperatures in both open and cased holes.

The tool architecture greatly improves spectral carbon/oxygen ratio (C/O) mea-surement for a highly accurate satura-tion answer obtained at faster speeds. An alternative to using the C/O for com-puting oil saturation is combining the inelastic and capture elemental yields to compute the formation’s elemen-tal concentrations expressed in dry-

Tool Enables Complete Cased-Hole FormationEvaluation, Reservoir Saturation Modeling Rahul Grover, SPE, Schlumberger

Copyright 2017, Society of Petroleum Engineers. Reprinted from the Journal of Petroleum Technology with permission.

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weight percentages to produce quanti-tative lithology fractions and estimate the TOC. The quantitative elemental concentrations enable data interpreta-tion in terms of mineral fractions, ker-ogen content, and hydrocarbon type and content.

With TOC logging a reality for uncon-ventional resource plays such as shale gas, shale oil, and in-situ thermal pro-cessing of oil shale, operators can eas-ily move from one shale play to anoth-er while building tangible workflows to evaluate these challenging reservoirs, long before the results of core analysis are available.

Case StudyA US land well was drilled with an 8¾-in. bit and completed with 4½-in. casing. The difference between the hole and casing diameters resulted in the

cement thickness of the completion exceeding 2 in.

The shaly sand formation had alter-nating low-porosity, gas-filled zones and very-low-porosity zones. Although openhole logs were run, the operator needed greater insight into the forma-tion and its fluid contents.

The new spectroscopy system unique-ly provided the operator with a stand-alone petrophysical volumetric inter-pretation that incorporated robust, high-fidelity quantified mineralogy and lithology. No openhole logging data were necessary for complete, single-run formation evaluation.

In addition to obtaining highly accu-rate elemental concentrations, includ-ing TOC, the tool acquired tradition-al cased-hole sigma, porosity, and C/O measurements but at a higher resolution and significantly faster logging speed

than possible with conventional cased-hole logging tools.

The FNXS measurement reliably dif-ferentiated gas-filled porosity from liquid-filled porosity and tight zones. Logging the shaly sand with a sin-gle run revealed two zones of inter-est. How ever, the environmentally cor-rected FNXS curve showed that only the lower zone contained gas—unlike the very-low-porosity upper zone that conventional cased-hole logging would have also assumed to be gas- bearing (Fig. 1).

The standalone volumetric interpre-tation performed by means of a linear solver with the tool’s sigma, FNXS, and neutron-porosity measurements was validated by the previously obtained openhole logs.

With this single-run, single-tool sys-tem for logging cased wells, the opera-

Fig. 1—The new multifunction spectroscopy tool’s FNXS measurement was environmentally corrected for the large volume of light cement in the annulus to differentiate the gas-filled porosity zone at X,160–X,180 ft from the dry zone at X,270–X,330 ft. A conventional cased-hole pulsed neutron log, as approximated by the near/deep count ratio in Track 5, would read gas in the upper tight zone. Source: Schlumberger.

20 JPT • OCTOBER 2017

tor was able to streamline operations to a single log obtained in more stable cased wells.

ConclusionsThe advent of the new multifunc-tion spectroscopy system’s self-

compensated sigma and thermal neu-tron porosity brings new robustness to pulsed neutron measurements. In com-bination with introduction of the new FNXS measurement and an expand-ed set of high-quality spectroscopy elemental concentrations, more and

higher-quality information is being obtained from standalone pulsed neu-tron logging than previously possi-ble, while less external input and fewer assumptions are required.

An important modeling advantage is that these independent measure-ments follow linear mixing laws, so they can be entered into simultaneous equations to compute the various min-eral and fluid volumes, including oil, water, and gas fractions. From these fractions, multiphase saturations are obtained (Fig. 2).

Another advantage in using the new tool’s formation properties is that complex nuclear models are no longer required to interpret raw data, such as count ratios during or after the PNG bursts. Instead, the same method and interpretation software used to analyze openhole logs can be consistently applied to formation properties mea-sured through casing. JPT

Fig. 2—The evaluation workflow for the new multifunction spectroscopy system can be adapted to any situation to solve for fluid saturations and other important unknowns for the reservoir from the tool’s simultaneously acquired independent measurements. Source: Schlumberger.