Description


The Multi-Sensor Spectral Gamma Ray Tool (MGT) was developed by the Lamont-Doherty Borehole Research Group to improve the vertical resolution of natural gamma-ray logs. This is achieved in the MGT by using an array of short detector modules with approximately 2-ft spacing. Each module comprises a small 2"x4" NaI detector, a programmable 256-channel amplitude analyzer, and an Am241 calibration source. The spectral data are later recalculated to determine the concentration of potassium, thorium, and uranium radioisotopes or their equivalents. The spectral data from individual modules are sampled 4 times per second and stacked in real time based on the logging speed. This approach increases vertical resolution by a factor of 3-4 over conventional tools while preserving comparable counting efficiency and spectral resolution. The radius of investigation depends on several factors: hole size, mud density, formation bulk density (denser formations display a slightly lower radioactivity) and the energy of the gamma rays (a higher energy gamma ray can reach the detector from deeper in the formation).

The tool also includes an accelerometer channel to improve data stacking by the precise measurement of logging speed. A specialized telemetry system developed for the MGT allows it to be combined with Schlumberger toolstrings in a single logging operation, minimizing the required rig time to acquire the log.

Spectral gamma-ray logs provide one of the best means to investigate the mineralogy of thin-bedded sedimentary sequences, to correlate among different logging runs, and to compare logging data and core measurements. Increasing vertical resolution over currently available tools provides new opportunities for log analysis in reservoirs with rapidly changing lithology and for finer resolution of thin layering and in areas with low sedimentation rates. The added resolution provided by the MGT will be of particular use in paleoclimate studies.

The MGT response is affected by borehole size, mud weight, and by the presence of bentonite or KCl in the mud. In ODP boreholes KCl is sometimes added to the mud to stabilize freshwater clays which tend to swell and form bridges. This procedure takes place before logging operations start, and even though KCl is probably diluted by the time the tool reaches total depth, it can still affect the tool response. All of these effects are accounted for during the processing of the MGT data onshore.

The MGT is always run at the top of the Schlumberger toolstring. The downhole switch in the MGT telemetry module provides switching of the signal and power lines between the MGT and the Schlumberger logging system.

Stuck/lost tool information

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