Development, Characterization, and Field Test Experience of a New Slim Logging-While-Drilling Spectral Gamma Ray Tool

Abstract

Abstract Today’s fast-paced development of petroleum resources depends on an efficient and accurate evaluation of both clastic and unconventional reservoirs. A new high-performance, slim logging-while-drilling (LWD) natural gamma ray spectroscopy tool has been developed to assist real-time petrophysical evaluations of net-to-gross for conventional reservoirs and to identify "sweet spots" for completion for unconventional reservoirs. Additionally, its azimuthal sensitivity can help position the well in lateral operations. This new tool provides wireline quality formation thorium (Th), uranium (U), and potassium (K) elemental concentrations in real time that can quantify clay content, identify clay minerals, and estimate total organic content. Further, real-time processing provides a color display derived from a Briggs color cube rendition of relative elemental contributions that can be correlated with stratigraphic features in the field. This first-of-its-kind, real-time feature is output at a high sampling rate for the full azimuth of the borehole and should be a useful aid in geosteering applications where the goal is to maintain the borehole within a target formation or to follow a known stratigraphic feature. Calibration and characterization of the tool were performed using newly developed Monte Carlo modeling techniques superior to previously used laboratory techniques while maintaining direct links to industry standards at the API Gamma Ray Calibration and K-U-Th Logging Calibration Facilities at the University of Houston. These techniques were developed because the borehole at the API Gamma Ray Calibration Facility cannot accept the 5.25-in. collar diameter, and the potassium formation at the API K-U-Th Logging Calibration Facility is not reliable. The instrument is fully characterized for operations in barite-, hematite-, or formate-weighted water-based mud systems as well as barite- or hematite-weighted oil-based muds. Further, corrections for borehole potassium can be applied in real time. A novel mechanical design enables the sensor to operate at temperature up to 329°F and borehole pressure up to 25,000 psi while minimizing the attenuation of formation gamma rays entering the detector and maintaining good azimuthal sensitivity. The tool uses a robust, constrained, weighted least-squares (WLS) analysis to derive elemental concentrations from measured pulse-height gamma ray spectra. Proprietary spectral processing algorithms regulate the detector gain without the use of an additional radioactive reference source and compensate for variations of the detector’s energy resolution caused by operating conditions within the borehole that change over time. A general description of the tool together with its operational specifications, details of the computer models used to calibrate and characterize its responses, and example logs from early field trials are within this paper.