Design, Calibration, Characterization, and Field Experience of New High-Temperature, Azimuthal, and Spectral Gamma Ray Logging-While-Drilling Tools

Abstract

Abstract Two new gamma ray tools have been developed, one provides high temperature total gamma ray measurements, and another delivers spectral gamma ray measurements while drilling. The high-temperature gamma ray tool can operate at temperatures up to 356°F (180°C) and survive to 392°F (200°C). It uses multiple two-detector banks of Geiger Muller Tubes. Five banks are used in the 4.75" tool and eight banks are used in the 6.75" and 8" tools. The tubes are arranged symmetrically to provide azimuthal measurements while rotating and/or sliding. The new spectral azimuthal gamma ray tool (SAGR) employs three high-efficiency scintillation detectors symmetrically arranged around the center bore of the drill collar. This design provides a superior total gamma ray measurement and spectral capability that has equivalent accuracy to a wireline spectral gamma ray tool. Calibration of LWD gamma ray tools has always been problematic because the main gamma ray calibration facility at the University of Houston has a smaller borehole than the collar size of the majority of LWD tools. A procedure was developed to calibrate LWD gamma ray tools when the collar size exceeded 4.75" and could not be conventionally lowered into the API pit. Measurements were made with a 4.75" LWD tool in the University of Houston gamma ray formation to obtain an absolute reference standard. The tool was then used to create two secondary standards with larger boreholes to be able to calibrate larger-size tools. Comprehensive computer modeling was performed to validate the API measurements and develop environmental corrections. The new spectral gamma ray sensor provides real-time K, U, and Th measurements. The spectral data of the three detectors are first aligned to a standard spectrum to compensate for gain changes due to temperature and electronics drift. The count rates are then summed and a full-spectrum least squares fit to K, U, and Th reference standards is applied to determine the K, U, and Th contents of the formation. The measurement is calibrated to the KUTh API standards at the University of Houston. Details of the calibration, processing, and correction algorithms are presented. Field examples are presented to illustrate the applications, accuracy, and repeatability of the measurements.