Abstract

A series of logging surveys using Schlumberger's Gamma-Ray Spectrometer Tool (GST*) was made in several wells which had long and complicated production histories. These surveys were run to be able to more accurately determine potentially productive zones which might exist in these wells.

The basic tool operating principle, which uses a pulsed neutron generator to induce gamma-ray production in the formation through fast neutron reactions and thermal capture, is presented. The tool uses a NaI-based spectrometer system to record the gamma-ray spectra emitted from the formation. Significant tool innovations are discussed, including the manner in which the spectra are processed in real-time to obtain the relative formation elemental concentrations. The corrections for count-rate effects, temperature induced resolution losses and spectral drifts are discussed.

The results of the GST surveys by P.T. Stanvac Indonesia and the accuracy of the tool in identifying remaining hydrocarbons are discussed and typical responses under a variety of reservoir conditions are reviewed. Interpretation models which link the processed elemental concentrations to the bulk properties of the oil bearing formation are presented. The effect of changing borehole environmental conditions on the models are described. The results are compared with known formation and production results.