Porosity Determination Using Optimization of Inelastic and Capture Count Rates in Downhole Logging

A system comprising:

• a downhole tool having one or more gamma detectors;
• processing circuitry coupled to the downhole tool, the processing circuitry being operable to perform a method comprising:
• detecting inelastic gammas and capture gammas using the detectors for a specified borehole depth of the formation;
• calculating an inelastic count rate by:
• summing inelastic and capture counts during an ON cycle of the downhole tool;
• calculating an estimated capture count rate that occurs during an OFF cycle of the downhole tool;
• and
• subtracting the estimated capture count rate from the summed inelastic and capture counts, thereby calculating the inelastic count rate;
• calculating a true capture count rate by summing inelastic and capture counts during the OFF cycle of the downhole tool;
• calculating a ratio of the inelastic count rate to the true capture count rate;
• and
• using the ratio to determine a porosity of the formation.

A non-transitory computer readable medium comprising instructions which, when executed by at least one processor, causes the processor to perform a method comprising:

• detecting inelastic gammas and capture gammas using one or more gamma detectors for a specified borehole depth of a downhole formation, the gamma detectors forming part of a downhole tool;
• calculating an inelastic count rate by:
• summing inelastic and capture counts during an ON cycle of the downhole tool;
• calculating an estimated capture count rate that occurs during an OFF cycle of the downhole tool;
• and
• subtracting the estimated capture count rate from the summed inelastic and capture counts, thereby calculating the inelastic count rate;
• calculating a true capture count rate by summing inelastic and capture counts during the OFF cycle of the downhole tool;
• calculating a ratio of the inelastic count rate to the true capture count rate;
• and
• using the ratio to determine a porosity of the formation.