A new-generation slim pulsed neutron logging toolhas been developed to deliver reliable answers forformation evaluation and reservoir monitoring inconditions where existing technologies struggle,including temperatures up to 175°C. The toolintroduces a stand-alone fast neutron measurementthat is independent of neutron porosity and sigmaformation properties but is highly sensitive tovariations in gas volume while insensitive tovariations in water volume. Additionally, the toolprovides high-resolution spectroscopy as well asself-compensated sigma and neutron porositymeasurements in a wide range of environmentalconditions.The improvements in hardware over previousgenerationtools include a high-output pulsedneutron generator (PNG), a compact neutronmonitor (CNM), two lanthanum bromide (LaBr_3)gamma ray detectors, and an yttrium aluminumperovskite (YAP) gamma ray detector at a longerspacing from the source. The diamond-based CNMaccurately measures the neutron output of the PNGand normalizes the count rate of the YAP detector,which makes the stand-alone gas measurementpossible. The high-neutron output and a fastacquisition system improve the precision of themeasurements, enabling faster logging speeds. ThePNG pulsing scheme is designed to optimize thegas, sigma, and neutron porosity measurements interms of both accuracy and precision. The LaBr_3detectors have a fast response time, excellentenergy resolution, and minimal temperaturedegradation, which enhance the capture andinelastic spectroscopy performance, particularly athigh temperatures.Several log examples demonstrate how themeasurements from this tool improve formationevaluation and reservoir monitoring in the complexcased-hole environment, where slim pulsed neutrontools are often deployed. In a tight gas reservoir,stand-alone formation evaluation was achievedwith gas, sigma, neutron porosity, and spectroscopymeasurements logged in cased hole in a single pass.The gas measurement differentiates productive gasbearingintervals from very low porosity intervals.This was not possible previously by using justsigma or gamma ray detector ratios. Thespectroscopy measurement clearly differentiatessandstone and limestone, even at a logging speed of1,000 ft/hr in this example. Another example froma heavy oil steamflood in California shows howspectroscopy is used to quantify oil saturation, asvalidated by core, and compares the improvedcarbon/oxygen ratio precision to that of previoustechnologies. Total organic carbon is computedfrom the combined inelastic and capturespectroscopy, which was not feasible with theprevious-generation slim tools. The response of thegas measurement in steam zones is alsodemonstrated.
展开▼
