A NEW CASED-HOLE 2 1/8-IN. MULTI-DETECTOR PULSED-NEUTRON TOOL: THEORY AND CHARACTERIZATION

Transcription

1 A NEW CASED-HOLE 2 1/8-IN. MULTI-DETECTOR PULSED-NEUTRON TOOL: THEORY AND CHARACTERIZATION Jianxing Chen, Larry Jacobson, Weijun Guo; Halliburton Copyright 2015, held jointly by the Society of Petrophysicists and Well Log Analysts (SPWLA) and the submitting authors. This paper was prepared for presentation at the SPWLA 56th Annual Logging Symposium held in Long Beach, California, USA, July 18-22, ABSTRACT Pulsed-neutron logging has advanced in recent years for formation evaluation in cased-hole. A number of recent publications from operators and service companies revealed new field applications of multidetector pulsed-neutron tools (MDPNT). The addition of a long-spacing detector was demonstrated with significantly improved sensitivity for formation gas saturation. A new multi-detector pulsed-neutron tool has been recently developed. The new tool is 2 1/8 in. and employs BGO detectors. This paper documents the measurement physics using the long-spacing detector. With an extensive laboratory and modeling database, the gas response has also been characterized for a variety of casing/borehole environments. Three-phase saturation (Sw, So, and Sg) determination usually requires two runs with different tool modes: a capture mode pass to obtain formation Sigma (SGF for Sw)) and SATG (for Sg) and an inelastic mode pass to obtain a quality C/O measurement (for So). With some modest degradation in saturation accuracy, it is now possible with the RMT3D to obtain all three measurements with a single tool mode if logging time is a critical factor. The benefits and trade-offs will be documented. APPLICATION BACKGROUND For mature reservoirs, cased-hole petrophysics provides essential information for reservoir surveillance and monitoring for recovery optimization. The cased-hole environment imposes limitations on technical options for a viable logging program to support reservoir management decisions. The size of tubing/casing requires logging tools with smaller outer diameters (OD) than those for openhole ones. Careful pre-job planning and thorough understanding of well/field production history are necessary to maximize measurement sensitivity and optimize interpretation uncertainties. The need for sensing the formation behind electrically conductive steel casing and cement barriers in the annulus between the casing and the formation limits the measurement sensitivity of electromagnetic (EM) sondes, as well as requiring measurements with deeper radial depth-of-investigation than neutron/density porosity tools. The pulsed-neutron (PN) tools have been commercially available for several decades to provide sensitive measurements for cased-hole petrophysics on saturation monitoring (Schmidt 1967; Syed 1991; Badruzzaman 2007; Zett 2012), water-flow surveillance (Jacobson 1994; Schnorr 1995), and gravel-pack evaluation (Olesen 1989; Jacobson 2005). The advent of exploring and developing unconventional assets has been leading to the recent commercialization of cased-hole PN measurements for quantitative formation gas saturation evaluation (Trcka 2006; Ansari 2009; Guo 2010; Guo 2012; Zett 2012). The addition of a long-spacing detector provides optimal gas measurement sensitivity with the multi-detector pulsed-neutron tool (MDPNT). Also, the additional 3rd detector enabled cased-hole porosity measurement sensitivity for rocks of 30% or higher hydrogen index (Jacobson 2013). The measurement dependence on formation salinity is also much reduced compared to the traditional thermal-neutron capture cross section (Sigma) analysis (Guo 2012). These technical details will be further reviewed in the measurement physics section. A literature survey was performed on the OnePetro publication repository for pulsed-neutron papers. Figure 1 presents the paper count for 5-year groups. Since the conceptual design and early physics database establishment in the 1960s, the number of publications seems to track the technology evolvement through the early 1990s. For those 20 years between 1985 and 2005, the publication numbers hold very steady. The significant increase 1

2 in papers in the last 10 years may likely be attributed to expanded field application and technology advancements. Fig. 2 Average elastic collision numbers to thermalize high-energy source neutron. The red line is for a 14 MeV pulsed-neutron source. The blue line represents the average source energy for a tool with an AmBe chemical neutron source (4 MeV). Fig. 1 Number of publications for pulsed-neutron logging from the 1960s to 2014 in 5-year groups. MEASUREMENT PHYSICS The source neutron cloud generated by a 14 MeV source goes through a series of scattering events stochastically. Within tens of microseconds, inelastic reactions and elastic scattering slow the neutron energy dramatically while high-energy gamma rays are generated from inelastic neutron scattering. The energy levels for inelastic gamma-rays are characteristic to each element. As the neutron energy decreases, elastic scattering becomes more and more dominant to the neutron slow-down process. The lighter the target atom, the more energy neutrons tend to lose after the elastic scattering events. The average number for elastic scattering events to thermalize a source neutron with 14 MeV and 4 MeV are plotted in Figure 2 (Ellis 2007). As the neutron energy decreases, the likelihood of radioactive capture increases. After the neutron source turns off, the total gamma ray counts (mostly capture gamma-ray) decrease as a function of time and formation capture cross section. Laboratory PN tool prototypes became available as early as the 1960s (Tittman, 1960). Early tools provided formation capture cross section Sigma logs (Youmans 1964; Dewan 1973). This was based on the observation that the inverse of the decay constant ( the capture cross section) for the gamma flux after a pulse of fast neutrons was related to formation parameters of interest: where log is the measured value, is the porosity, and fl and ma are the pore fluid and matrix neutron capture cross sections, respectively. Further, fl could be partitioned between water and oil: where Sw is the relative saturation of water, and w and o are the capture cross sections for water and oil, respectively. Combining these two equations and solving for Sw leads to: A few years after the introduction of the PN capture tool, the development of C/O logging using PN technology solved the problem of saturation determination in freshwater reservoirs (Lock and Hoyer 1974; Schultz and Smith 1974) although somewhat less robustly. In this case, the C/O measurement in a sandstone reservoir was a direct measure of oil saturation (a similar equation exists for limestone):

3 where hc is the hydrocarbon density, and Yc/Yo is the measured ratio of the carbon and oxygen yield. The odd numerical coefficients in this equation arise from the relative atomic densities for carbon and oxygen in the materials in the reservoir. Service providers use several different methods for extracting Yc/Yo from the spectrometric measurements, which are not described further in this paper. In recent decades, the need to determine gas saturation has become increasingly important (Badruzzaman 2007) and several schemes have been explored to determine it. Equation 2 can be expanded to include a gas term: where Sg and g is the gas saturation and capture cross section, respectively. Plugging this into Equation 1 above and solving for Sg: The primary problem in using only PNC logging to obtain Sg is that the term in the numerator of Equation 6 contains So, which would have to be provided from some other source. For situations in which the formation water is fresh ( w ~ o), there is little or no contrast between o and w (the fourth term goes to zero), and then: been applied as qualitative indicators for gas zones. An 4 algorithm using these ratios is constructed to match 4 known gas saturation, based on a great deal of field data and lab and/or modeling measurements, with a strong emphasis on the latter because high pressure gas cannot readily be used in lab formations. Accurate computer modeling and modern computer speed are essential in making these step changes from qualitative indicators to quantitative estimates. Unfortunately, these ratios involve two gamma detectors at different source-todetector spacing. The measurements are sensitive to both hydrogen index and bulk density (thereby, rock lithology). Without fundamental improvements in tool physics, the lithology effect has been reported to be strong on gas saturation interpretation (Inanc et al. 2009) A new, previously reported technique for direct gas saturation determination was based solely on countrates from a single gamma detector for a 1 11/16-in. cased-hole PNT. With the long-spacing detector of the MDPNT (Guo 2010; Guo 2012), the measurement sensitivity is optimized. This new technique has been tested in several North America tight gas reservoirs (Wyoming, Rocky Mountain, and South Texas). One recent commercial application was monitoring gas cap 6 expansion in mature oil reservoirs (Kwong 2013). This paper presents model, laboratory and log data for a new 2 1/8-in. tool that was recently developed. For completeness, the description for the new measurement physics is also included in this paper. SATG PHYSICS With a PN tool, 14 MeV source neutrons interact with the surrounding medium to produce gamma rays. These gamma counts are tallied by detector(s) in both time and energy domains. The measurement of gamma rays provides useful information about rock properties, such as hydrogen index, bulk density, and lithology. However, even if the formation water is saline, in principle, So could be determined by Equation 4 (i.e., C/O logging) and used in Equation 6. However, Badruzzaman et al. have raised issues with this; fundamentally, there are too many poorly known inputs. Recently, techniques for quantifying gas saturation directly were proposed and demonstrated successes (Trcka 2006; Ansari 2009; Guo 2010; Guo 2012; Zett 2012). These gas saturation estimates are based on calibrated gamma count ratios that have traditionally 3 The neutrons emitted by a neutron generator go through a chain of stochastic scattering events until they are captured. Two statistically dominant scattering events are inelastic scattering and elastic scattering. When these high-energy neutrons are scattered by the nuclei of heavier earth elements, such as oxygen, silicon, and calcium, an inelastic reaction may occur, as shown in Figure 3. The red circle in Figure 3 indicates the spherical virtual source for inelastic gamma rays. Some of these inelastic gamma rays are tallied in detectors with particular time and energy. A simplified gamma transmission efficiency model is characterized by

4 exponential attenuation as in Equation 8, in which NInel is the inelastic count rate, is formation density, µ is the formation mass attenuation coefficient, and LInel is the attenuation distance between the red circle and a single detector. Probabilistically, when high-energy neutrons scatter with a lighter earth element, such as hydrogen, the energy loss is large. Eventually, the neutron energy decreases to below one electron volt. These low energy neutrons have high likelihoods of being captured by formation nuclei. The blue circle in Figure 3 indicates the source size for capture gamma rays. Similarly, Equation 9 characterizes the exponential attenuation process for capture gammas. 8 environments. One of the most influential fluid properties is formation water salinity. Capture counts from a single gamma detector are sensitive to formation water salinity; however, inelastic counts are not affected as much. This leads to strong formation salinity dependence with RIC, regardless of detector spacing. Without additional technical improvements, the formation water salinity poses practical interpretation difficulties for reservoirs with unknown formation water salinity. Unknown formation water salinity is often the case with mature fields that have been water or steam flooded. Even with new well developments, accurate formation water salinity is required to minimize uncertainty. 9 The effects of the hydrogen index on NInel and Ncap are complex. The greater the hydrogen index, the smaller the source sizes, and therefore, the longer the attenuation distances. This causes both NInel and NCap values to decrease. However, the greater the hydrogen index, the smaller the formation density. This causes both NInel and Ncap values to increase. These two effects compete against one another as the hydrogen index varies from 0 (zero porosity rock) to 1 (water). As shown in Figure 3, the source-size difference between the red circle and the blue circle is a primary driving factor for the ratio between NInel and Ncap (RIC). As the detector-to-neutron-source distance increases with the long-spacing detector of MDPNT, the measurement sensitivity to hydrogen index is further enhanced, whereas rock lithology sensitivity is reduced. This statement is based on the observation that the average gamma transmission efficiency becomes similar, at a long SD spacing, between inelastic gamma and capture gamma during the transport from the edge of capture area to the detector. Fig. 3 The neutron/gamma transport process. The red circle marks the virtual source size for inelastic gamma rays, whereas the blue circle marks that for capture gamma rays. To further support the particle physics theory, laboratory data for the 2 1/8-in. tool are presented in Figure 4 and Figure 5 for inelastic and capture counts respectively. The count rates for near and long spacing detectors are plotted in red and blue curves. Although the inelastic/capture ratio at a long-spacing detector optimizes the hydrogen index measurement with a much reduced lithology effect, this ratio is affected by other rock and fluid properties and logging 4

5 tools, whereas the RIC value changes dramatically. Fig. 4 Capture count rates for near and long detectors. It is noted that the long spacing detector has a much larger dynamic range as a function of porosity. Fig. 6 SATG processing partitions the capture gate into fast and slow components using a double-exponential fitting algorithm. Table 1 Data for formation water salinity effects on RIC, while SATG is not sensitive to this parameter. RIC SATG RIC SATG 1-11/ /16 2-1/8 2-1/8 33 pu sand w FW pu sand w 150 kppm SW Fig. 5 Inelastic count rates for near and long detectors. It is noted that the long spacing detector is of slightly larger dynamic range as a function of porosity. The overall inelastic dynamic range is not as large as that of the capture count rates in Figure 4. A customized processing algorithm was developed to overcome this potential limitation. As illustrated in Figure 6, the capture gate is partitioned into fast and slow components. The inelastic gate overlaps with generator neutron pulses, and is most sensitive to rock density. The ratio between the inelastic gate and the slow capture gate is the saturation gate (SATG). As shown in Table 1, SATG laboratory measurements show no difference between 150 kppm of formation water salinity change for both 1 11/16-in. and 2 1/8-in. 5 LOGGING TOOL OVERVIEW PN tools have evolved in the last five decades through practical oilfield applications. The measurements offer three distinctive features compared to other nuclear measurements including tools that measure natural radioactivity and other tools with chemical nuclear sources: 1. PN tools use a downhole mini-accelerator as a radiation source. The operation frequencies are tailored to specific applications. 2. The neutron source is mono-energetic and has very high penetrating power

6 (14 MeV). The deeper depth-ofinvestigation reduces adverse effects from borehole or near-wellbore complexity. 3. The source intensity is typically very high (above 108 neutrons per second). This improves the measurements precision at a reasonable logging speed. Three sizes of PNT tools are available for commercial services, 1 11/16 in., 2 1/8 in., and 3 5/8 in.. Tools of smaller size provide accurate capture measurements as well as the flexibility of logging through all tubing in oil and gas fields. The larger size tools received better customer acceptance on enhanced inelastic spectroscopy applications. For the capture operation mode (SIG), the generator is pulsed with 25 ms frames, within each 25 ms, 20 ms is pulsing (on/off) and 5 ms is not pulsing at all for background measurements. For the 20 ms pulsing frame, 16 cycles of 1250 s pulse repeat. For the 1250 s pulse, 80 s is the neutron generator on time. For the inelastic operation mode (CO), the generator is pulsed with 25 ms frames, within each 25 ms, 20 ms is pulsing (on/off) and 5 ms is not pulsing at all for background measurements. For the 20 ms pulsing frame, 200 cycles of 100 s pulse repeat. For the 100 s pulse, 30 s is gen on time. A new fast pulsing operation mode (FP) was recently developed to provide SATG and carbon/oxygen ratio in the same logging pass while maintaining the good measurement precisions. Figure 7 shows the timing schematics for all three modes. Fig. 7 Time schematics for the operation modes of a pulsed-neutron generator. A number of curves in the logging database assess the quality of the primary measurements and stability of the tool operation. Figure 8 shows a sample quality log from the Halliburton test well. In this example, the tool was operated in the FP mode (useful for obtaining both sigma and C/O logging in a single pass) at 3 fpm. Similar plots obtain for the near and long spaced detectors and other modes. Track 1 shows the far sigma borehole (SGBF) and the average fit error per time bin for the dual decay model to the background and deadtime corrected decay curve. The value of this quantity (and most others discussed here) will vary as the countrate, logging speed, and depth filtering. A value ~5 is a good fit. Track 2 shows far formation sigma (SGFF), capture count-rate (FCAP), and SGFU (SGFF statistical uncertainty to one standard deviation). For the countrates observed, this uncertainty was running 0.1 to 0.2cu, a very small repeatability error. The windows method C/O ratio (COIF), windows method lithology ratio (LIRF), far inelastic count-rate (ITCF), and statistical uncertainty to one standard deviation (STUF) in the same units as COIF are shown in track 3. STUF is running ~.005, again a very small statistical repeatability error. The downhole tool provides spectral gain stabilization using the prominent Fe line at 7.6 MeV. No offset correction is provided by the downhole tool. The integration time for this stabilization is in excess of 60 seconds. Track 4 shows the hydrogen peak control value (HPLF) and iron ratio value (FERF) computed at the surface; however, only using the depth filter integration time (~12 sec at 3fpm, ~2 sec at 15 fpm). If the offset is near zero, the HPLF should be running ~1.0, and if the downhole gain stabilization is correct, the FERF should be ~2.8. A good log will show both averaging around these values over the entire log. If minor gain errors occur, or a gain offset is suspected, a new gain and offset can be calculated and applied by the surface software. 6

7 The borehole effect is illustrated in Figure 10. The shape of interpretation fan chart is significantly different for two completion scenarios with different borehole/completion sizes. Figure 11 shows that borehole fluid salinity also drives the SATG value at 0 porosity up when the borehole salinity increases. For a practical field log interpretation, a local normalization would be necessary to align the water line with a wet zone to remove borehole effects from the saturation analysis. Fig. 8 A section of test well showing several primary log curves and the quality curves pertaining to them. This log was run in the FP mode at 3 fpm with a 3 ¼ foot triangular depth filter applied. SATG CHARACTERIZATION EXAMPLES For a pragmatic understanding of the SATG measurements, an extensive modeling program was carried out with the following parameter ranges: borehole size from 6 in. to 12 1/4 in., casing size from 4 1/2 in. to 9 7/8 in., formation porosity from 0 to 40 pu, and formation salinity from 0 kppm to 150 kppm. The modeling data showed that the SATG measurement showed little formation water salinity dependence. Figure 9 shows the modeling data for 0 ppm and 150 kppm formation water salinity, the interpretation fans between low and high formation salinity are similar. Fig. 10 Comparing the interpretation fan for small and large boreholes. Fig. 11 Comparing the interpretation fan for a 6BHS with FW (solid) or SW (Dashed). LOG EXAMPLES Figure 12 shows two passes in the test well in the SIG mode. Each was run at 15 fpm and uses a 3 ¼-ft triangular depth filter. The two SGFF passes in track 2 overlay very closely (<.2 cu). The SATG passes in track 3 (obtained from the long-spaced detector) overlay to within the predicted uncertainty (±0.015). Fig. 9 Comparing the interpretation fan for low and high formation water salinity. 7 Figure 13 shows three passes in the CO mode. One pass was a down log, the other two up logs. These runs were

8 at 3 fpm and used the 3 ¼-ft triangular depth filter. The SGFF is a single exponential fit to the decay curve instead of the usual dual exponential fit used for the SIG mode. In spite of the low count-rate for the decay curve, the SGFF repeats are reasonable because the logging speed is a factor of 5X slower than for the SIG mode. The SGFF for the down-log was slightly poorer (statistically) due to the large oxygen activation background that was removed from the decay-curve. One of the up logs was terminated prematurely (at 880 ft). There is very little C/O variability in this well (no oil or coal zones). The C/O (COIF) repeatability is satisfactory considering the physical weakness of the carbon signal, but this stack of single passes illustrates the virtue of averaging multiple passes (usually three) to provide a statistically meaningful saturation determination. The LIRF repeatability is better since the Si and Ca signals are stronger in the inelastic spectrum. Additionally, the individual Si and Ca windows have higher count rates. These newer pulsed-neutron logging tools afford opportunities to include additional pulsing schemes other than the traditional C/O and Sigma modes. One such new mode introduced in the 2 1/8-in. two detector C/O tool was a pulsing mode that interrupts the fast pulsing more frequently to insert a decay curve measurement. This pulsing scheme was illustrated above (Figure 7). This scheme provides a much better simultaneous dual exponential SGFF measurement without any significant reduction in C/O sensitivity or precision. In part this arises because the modest reduction in actual decay-curve counts is compensated by the slower logging speed required for the C/O measurement. Figure 14 illustrates this schemes merits. The SGFF, COIF, and LIRF from the FP mode are compared to SGFF from the SIG mode and COIF and LIRF from the CO mode. There are slight offset differences in the SGFF, COIF, and LIRF between the two modes but this comparison shows that the sensitivity and repeatability are preserved. Because of the slower logging speed required by the CO mode the precision of the SGFF is better in the FP mode than the SIG. The C/O repeatability is even slightly better than the CO mode. Thus, a single mode can be used to obtain both a quality SGFF and C/O measurement. At this point, the SATG measurement has not been adapted to this mode but there is little doubt that it should be successful. Fig. 12 Repeatability of SIG mode runs in Halliburton test well for SGBF, SGFF, and SATG. Logging speed was 15 fpm and a 3 ¼-ft triangular depth filter was applied. 8

9 Fig. 13 Repeatability of 3 CO mode runs. Logging speed was 3 fpm and a 3 ¼-ft triangular depth filter was applied. The down log had a significant oxygen activation background signal to remove from the decay curve resulting in a much larger uncertainty for that pass. Fig. 14 The plot compares the SGFF, COIF, and LIRF from the FP mode pass with SGFF from the SIG mode pass and COIF and LIRF from the CO mode pass. CONCLUSIONS AND DISCUSSION A brief history of PN tools and their application to reservoir fluid saturation monitoring shows the ongoing development and growing sophistication of this technology. The latest technology increment deals with improved gas saturation monitoring through the addition of a third, longer spaced detector to enhance the gas sensitivity of PN tools. Until recently, gas 9

10 saturation was determined by inference from oil and water PN saturation measurements. A review of the physics of the Neutron-gamma transport validates the notion of superior gas sensitivity and robustness at the more distant long spaced detector. Now all three pertinent saturations (oil, water, and gas) can be independently measured using a single PN tool. A review of a new 2 1/8-in. PN tool with a third longspaced detector shows excellent repeatability in the SIG mode for SGFF and SATG. Fan charts for SATG demonstrate excellent gas sensitivity up to 40 pu with little formation salinity sensitivity. While SATG must be corrected for BH size and BH fluid salinity variations, the gas sensitivity remains satisfactory. At present, runs in two different tool operational modes (two passes in the well) are required since water and gas saturation are basically capture (SIG) mode measurements and oil saturation is an inelastic (CO) mode measurement. The significant improvement in SGFF using a new operational mode that retains the usual C/O ratio sensitivity and statistical precision suggests a little additional work may lead to all three saturations being obtained using a single PN tool operational mode. ACKNOWLEDGMENTS The authors would like to acknowledge Halliburton management team for supporting this work. REFERENCES Ansari, R., Mekic, N., Chace, D.M., Rust, M., & Starr, M., 2009, Field applications of a new cased hole gas saturation measurement in tight gas reservoirs. SPWLA 50th Annual Symposium. Badruzzaman, A., Logan, J.P., Bean, C., Adeyemo, A.O., Zalan, T.A., Barnes, D., & Platt, C., 2007, Is accurate gas/steam determination behind pipe feasible with pulsed neutron measurements: SPE Asia Pacific Oil & Gas Conference and Exhibition. Dewan, J. T. (1969). Pulsed and continuous neutron well logging technique. Ellis, D.V., & Singer, J.M., 2007, Well logging for earth scientists. Guo, W., Dorffer, D., Roy, S., Jacobson, L.A., & Durbin, D., 2012, Uncertainty analysis for determining 10 petrophysical parameters with a multi-detector pulsed neutron tool in unconventional reservoirs. SPWLA 2012 Annual Symposium, Paper RRR. Guo, W., Jacobson, L.A., Truax, J., Dorffer, D., & Kwong, S., 2010, A new three-detector 1 11/16-inch pulsed neutron tool for unconventional reservoirs: SPWLA 51st Annual Logging Symposium (p. Paper JJ). Perth, Australia. Jacobson, L.A., Guo, W., Dorffer, D., & Kwong, S., 2013, cased-hole porosity measurements using pulsedneutron logging tools: SPWLA 54th Annual Logging Symposium (p. Paper UUU). Jacobson, L.A., & Truax, J.J., 2005, Carbon/oxygen logging in gravel packs: Petrophysics, 46(3), Kwong, K., Liu, Z., Morgan, K., & Guo, W., 2013, Case history: monitoring gas (CO 2 ) flood in a carbonate reservoir with a new slim multidetector pulsed neutron tool: SPE Enhanced Oil Recovery Conference (p. SPE ). Lock, G.A., & Hoyer, W.A., (n.d.). Carbon-oxygen (C/0 ) log : use and interpretation: Journal of Petroleum Technology, SPE Olesen, J., Hudson, T.E., & Carpenter, W.W., 1989, Gravel pack quality control by neutron activation logging: SPE Annual Technical Conference and Exhibition (p. SPE 19739). Schmidt, A.W., Tinch, D.H., Carpenter, B.N., & Hoyle, W.R., 1967, Computerized log analysis for efficient evaluation of gas wells and gas storage reservoirs: SPE Midwest Regional Meeting. Schnorr, D.R., 1995, Optimizing water injection profiles with oxygen activation logging: SPE Annual Technical Conference and Exhibition (p. SPE ). Schultz, W.E., Smith, H.D., & Harry, D., 1974, Laboratory and field evaluation of a carbon/oxygen (C/O) well logging system. Journal of Petroleum Technology, October, Syed, E.U., Salaita, G.N., Mccaffery, F.G., & Field, C.O., 1991, Determination of residual oil saturation from time-lapse pulsed neutron capture logs in a large sandstone reservoir: SPE

11 Tittman, J., & Nelligan, W.B., 1960, Laboratory studies of a pulsed neutron-source technique in well logging. SPE Conference (p. SPE1227G). Trcka, D., Gilchrist, A., Riley, S., Bruner, M., Esfandiari, T., Ly, T., 2006, Field trials of a new method for the measurement of formation gas using pulsed-neutron instrumentation: SPE Annual Technical Conference and Exhibition (p. Paper ), San Antonio, TX. Retrieved from spe pdf Wyatt, D.F., Jacobson, L.A., & Fox, P., 1994, Enhanced carbo-oxygen log interpretation using supplemental log curves: SPE Asia Pacific Oil & Gas Conference and Exhibition (p. SPE 28758). Youmans, A.H., Hopkinson, E.C., Bergan, R.A., & Oshry, H.I., 1964, Neutron lifetime, a new nuclear log: Journal of Petroleum Technology. Zett, A., Webster, M.M., Surles, D., Bp, C.C., Spain, D., & Colbert, C., 2012, Application of new generation multi detector pulsed neutron technology in petrophysical surveillance: SPWLA 53rd Annual Logging Symposium. ABOUT THE AUTHORS Jianxing Chen is a principal scientist working on pulsed neutron tools in the Halliburton Sensor Physics group. He started with petrophysics software development when he joined Halliburton in Previously he spent 13 years with Baker Hughes, where he worked on developing pulsed-neutron logging software and other wireline and MWD/LWD software. He holds masters degrees in computer science, civil engineering, and fluid mechanics. Larry Jacobson joined Halliburton in 1984 and is a member of the physics team. He holds a Ph.D (1969) in nuclear physics from the University 11 of Wisconsin. Before joining Halliburton, he spent 15 years with Schlumberger, where he specialized in pulsed-neutron logging and held several managerial positions. Jacobson holds fifteen patents and is the author of numerous technical papers discussing pulsed-neutron capture and spectrometry logging, cased-hole density modeling, and log-filtering techniques. He is a member of the American Physical Society, SPWLA, and SPE. Weijun Guo is the nuclear physics manager for Halliburton. Prior to this assignment, he has worked on a few nuclear logging tools including most recently, development of the cased-hole pulsedneutron tools. His technical interest covers drilling, formation evaluation, and reservoir surveillance technologies. He has authored dozens of technical papers and invented a dozen patents. In 2003, Weijun earned his PhD degree at North Carolina State University in Raleigh, NC. He is a member of SPWLA and SPE.