Technical Paper: An Innovative Slim Pulsed Neutron Logging Tool

Society: SPWLA
Paper Number: SPWLA-2015
Presentation Date: 2015


A new-generation slim pulsed neutron logging tool has been developed to deliver reliable answers for formation evaluation and reservoir monitoring in conditions where existing technologies struggle, including temperatures up to 175°C. The tool introduces a stand-alone fast neutron measurement that is independent of neutron porosity and sigma formation properties but is highly sensitive to variations in gas volume while insensitive to variations in water volume. Additionally, the tool provides high-resolution spectroscopy as well as self-compensated sigma and neutron porosity measurements in a wide range of environmental conditions.

The improvements in hardware over previous generation tools include a high-output pulsed neutron generator (PNG), a compact neutron monitor (CNM), two lanthanum bromide (LaBr3) gamma ray detectors, and an yttrium aluminum perovskite (YAP) gamma ray detector at a longer spacing from the source. The diamond-based CNM accurately measures the neutron output of the PNG and normalizes the count rate of the YAP detector, which makes the stand-alone gas measurement possible. The high-neutron output and a fast acquisition system improve the precision of the measurements, enabling faster logging speeds. The PNG pulsing scheme is designed to optimize the gas, sigma, and neutron porosity measurements in terms of both accuracy and precision. The LaBr3 detectors have a fast response time, excellent energy resolution, and minimal temperature degradation, which enhance the capture and inelastic spectroscopy performance, particularly at high temperatures.

Several log examples demonstrate how the measurements from this tool improve formation evaluation and reservoir monitoring in the complex cased-hole environment, where slim pulsed neutron tools are often deployed. In a tight gas reservoir, stand-alone formation evaluation was achieved with gas, sigma, neutron porosity, and spectroscopy measurements logged in cased hole in a single pass. The gas measurement differentiates productive gasbearing intervals from very low porosity intervals. This was not possible previously by using just sigma or gamma ray detector ratios. The spectroscopy measurement clearly differentiates sandstone and limestone, even at a logging speed of 1,000 ft/hr in this example. Another example from a heavy oil steamflood in California shows how spectroscopy is used to quantify oil saturation, as validated by core, and compares the improved carbon/oxygen ratio precision to that of previous technologies. Total organic carbon is computed from the combined inelastic and capture spectroscopy, which was not feasible with the previous-generation slim tools. The response of the gas measurement in steam zones is also demonstrated.

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