For more than 40 years, the industry has used pulsed neutron logging to determine hydrocarbon and water saturations behind casing for reservoir management. Multiphase saturation measurements over time are useful for tracking reservoir depletion, planning workover and enhanced recovery strategies, and diagnosing production problems 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 measurement suite has been greatly improved over many tool generations, the intrinsic physical measurements remained unchanged, which meant that operators could not obtain a complete picture of the rock and fluids behind casing. Input from openhole logs was required from a porosity or bulk density measurement for combination with the neutron porosity. Absent this input, primary formation evaluation in cased wellbores can be ambiguous. An additional challenge with cased-hole logging is correctly compensating for the effects of borehole fluids and the presence of completion hardware.
Next-Generation Logs
To meet the need for accurate surveillance in cased holes, Schlumberger recently introduced the Pulsar multifunction spectroscopy system. The system builds on innovative technologies originated by the company to provide the first complete cased-hole formation evaluation and reservoir saturation monitoring capability with openhole-equivalent measurements.
This next generation in pulsed neutron logging integrates multiple detectors and a high-output pulsed neutron generator (PNG) to significantly improve measurement precision, data acquisition accuracy, and logging speed. The measurements are complemented by powerful algorithms that compensate for variation in the borehole fluids and completion in delivering 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 neutrons to provide accurate and precise output measurement. There are three -scintillation gamma-ray detectors for near, far, and deep detection. 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 perovskite scintillator.
The three gamma-ray detectors are coupled to high-temperature-rated photo-multiplier tubes, and their pulses are counted with specialized electronics matched to the high rate and resolution of the LaBr3:Ce scintillators.
Detector resolution is only minimally 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 hydrogen 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 logging run: traditional cased-hole measurements; an expanded suite of elements, including total organic carbon (TOC); and the new fast neutron cross-section (FNXS) measurement.
Unlike the hydrogen index (HI) measurement that dominates conventional cased-hole logging, FNXS is directly 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 combination of sigma, HI, and FNXS measurements computes complex multiphase fluid saturations without an externally supplied porosity curve.
The high-fidelity determination of mineralogy by the new tool revolutionizes gamma-ray spectroscopy by directly measuring the majority of the elements 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) measurement for a highly accurate saturation answer obtained at faster speeds. An alternative to using the C/O for computing oil saturation is combining the inelastic and capture elemental yields to compute the formation’s elemental concentrations expressed in dry-weight percentages to produce quantitative lithology fractions and estimate the TOC. The quantitative elemental concentrations enable data interpretation in terms of mineral fractions, kerogen content, and hydrocarbon type and content.
With TOC logging a reality for unconventional resource plays such as shale gas, shale oil, and in-situ thermal processing of oil shale, operators can easily move from one shale play to another while building tangible workflows to evaluate these challenging reservoirs, long before the results of core analysis are available.
Case Study
A US land well was drilled with an 8?-in. bit and completed with 4?-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 alternating low-porosity, gas-filled zones and very-low-porosity zones. Although openhole logs were run, the operator needed greater insight into the formation and its fluid contents.
The new spectroscopy system uniquely provided the operator with a standalone petrophysical volumetric interpretation 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 accurate elemental concentrations, including TOC, the tool acquired traditional 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 differentiated gas-filled porosity from liquid-filled porosity and tight zones. Logging the shaly sand with a single run revealed two zones of interest. How-ever, the environmentally corrected 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.
The standalone volumetric interpretation 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 system for logging cased wells, the operator was able to streamline operations to a single log obtained in more stable cased wells.
Conclusions
The advent of the new multifunction spectroscopy system’s self-compensated sigma and thermal neutron porosity brings new robustness to pulsed neutron measurements. In combination with introduction of the new FNXS measurement and an expanded set of high-quality spectroscopy elemental concentrations, more and -higher-quality information is being obtained from standalone pulsed neutron logging than previously possible, while less external input and fewer assumptions are required.
An important modeling advantage is that these independent measurements follow linear mixing laws, so they can be entered into simultaneous equations to compute the various mineral and fluid volumes, including oil, water, and gas fractions. From these fractions, multiphase saturations are obtained.
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 measured through casing.
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