High-Resolution Density Logging Using a Three Detector Device

Abstract

Abstract Significant improvements in both the resolution and the precision of formation density logging tools are possible through the integration of a new "backscatter" gamma-ray detector into the traditional dual-spacing device. The spacing and the geometry of the new measurement permit intrinsic high resolution logging while much higher count rates provide increased precision. Backscattering or single Compton Scattering in the formation complements standard attenuation logging by exhibiting a positive correlation between count rates and density. Statistical variations at higher densities are thus reduced. The reactions occur at short distances from the source dictating very short source-detector spacings and very high count rates. Such count rates have required adoption of a new crystal with attendant shielding and new count-rate electronics capable of digital treatment of the pulses. The backscatter detector is integrated into a skid that also carries more conventional short- and long-spacing detectors. The energy spectra from the three detectors are binned into energy windows for data analysis. A nonlinear response model is established for each window count rate and the set of equations formed is solved for five unknowns—the formation density and photoelectric factor together with the mudcake density, thickness and photoelectric factor. This approach permits the optimal treatment of measurement errors and provides the best possible estimation of formation density and photoelectric factor available from the acquired measurements. The processing algorithms provide outputs at both standard and high resolution. Field log examples show significant improvements in density precision and vertical resolution, and a dramatic improvement in the quality of the photoelectric factor (Pef) log.