A New Small-Diameter, High-Performance Reservoir Monitoring Tool

Abstract

Abstract This paper discusses a new small-diameter, high performance pulsed-neutron spectrometry tool that has been introduced recently for reservoir monitoring applications. Basic measurement principles are explained, and important operating parameters are presented. Field examples from Western Venezuela and South Texas illustrate multiple applications of the new device and compare the performance of the new instrument with a larger pulsed-neutron spectrometry tool. The new 2-1/8-inch tool is designed for logging through 27/8-inch or larger tubing. The tool is slightly larger than traditional 1-11/16-inch through-tubing tools. This design accommodates larger detectors, thereby improving the performance of the carbon-oxygen (C/O) measurement. Two bismuth germanate detectors yield high gamma ray count rates with good spectral resolution. To optimize C/O measurements, one mode of tool operation interlaces 5ms of background measurements with 20ms of 10-kHz neutron pulses. The tool records 256-channel spectra from each detector: during the neutron bursts (for analyzing inelastic events),during the interpulse period (for analyzing capture events), andduring the background period (for analyzing activation and background events). The tool also provides a simultaneous neutron capture cross section (sigma) measurement. Carbon-oxygen and calcium-silicon (Ca/Si) response characteristics for saturation and lithology analysis are derived from laboratory measurements and are illustrated in the customary fan-chart format. In a second mode of operation, the tool optimizes the sigma measurement and provides high-quality neutron capture spectra for quantitative lithology determination.