A recent LWD density log in an exploration well showed excessive abrasive metal loss on the density measurement stabilizer. Towards the end of the drilling run it was noticed that the bottom quadrant density correction (delta rho) was slowly moving from values normalized on zero to a more positive number of about 0.15 g/cm3. Measurements of the density stabilizer diameters performed after the logging run showed the diameter had been reduced by abrasion by approximately 0.2 inch along the entire length of the stabilizer. Therefore, the compensated density measurement was logically questioned.
A post-job calibration showed a significant difference from the pre-job calibration, as expected. What was unexpected was that the compensated density computed from the pre- and post-job calibrations compared favorably at the end of the well, but not at the beginning of the well. This implies that the density correction algorithms derived during characterization will compensate for metal loss but not for metal gain. Monte Carlo N-Particle (MCNP) modeling is used to review this finding and investigate a method to define the amount of metal loss that can be tolerated before compensated density measurement inaccuracies exceed specifications.
In order to compute an accurate photoelectric effect (PEF) and caliper that are derived from the individual short and long detector densities, the pre- and post-job calibrations need to be utilized for processing the data. A new methodology of blending the pre- and post-job calibrations as a function of metal loss was developed to accurately reprocess the density count rate data over the entire drilled interval. The final compensated density measurement from this reprocessing compared favorably to the original compensated density measurement (with only the pre drilling calibration in effect). This blending process resulted in valid single detector and compensated density data over the entire interval confirmed by independent measurements.