Measurement of formation porosity using a single gamma ray detector
A pulsed neutron source is used in a porosity logging tool with a single gamma ray detector coated with a neutron absorbing material. By using a ratio of the spectral peak associated with hydrogen to a spectral peak associated with the neutron absorbing material, the formation porosity is estimated.
1. A method of evaluating an earth formation, the method comprising:
obtaining an indication of a porosity of the earth formation using gamma rays measured by a single gamma ray detector, where the earth formation has been irradiated by a neutron source and the single gamma ray detector is coated with a neutron absorbing material.
2. The method of claim 1 further comprising using, for the neutron absorbing material, B 10 .
3. The method of claim 1 wherein using the measured gamma rays further comprises:
(i) estimating a spectrum of the measured gamma rays;
(ii) using a first peak associated with the neutron absorbing material and using a second peak associated with hydrogen nuclei.
4. The method of claim 1 further comprising using results of a plurality of calibration measurements.
5. The method of claim 4 further comprising making the plurality of calibration measurements over a plurality of lithologies, a plurality of porosities, and a plurality of well bore diameters.
6. An apparatus configured to evaluate a subterranean formation from within a wellbore, comprising:
(a) a pulsed neutron source configured to be conveyed in the wellbore and irradiate the earth formation;
(b) a single gamma ray detector coated with a neutron absorbing material and configured to measure gamma rays resulting from the irradiation; and
(c) a processor configured to:
use the measured gamma rays for obtaining an indication of a porosity of the earth formation.
7. The apparatus of claim 6 wherein the neutron absorbing material further comprises B 10 .
8. The apparatus of claim 6 wherein the processor is further configured to use the measured gamma rays by:
(i) estimating a spectrum of the measured gamma rays;
(ii) using a first peak associated with the neutron absorbing material and using a second peak associated with hydrogen nuclei.
9. The apparatus of claim 6 wherein the processor is further configured to use results of a plurality of calibration measurements.
10. The apparatus of claim 9 wherein the plurality of calibration measurements are obtained over a plurality of lithologies, a plurality of porosities, and a plurality of well bore diameters.
11. A non-transitory computer-readable medium product having thereon instructions that when read by a processor cause the processor to execute a method, the method comprising:
obtaining an indication of a porosity of an earth formation using gamma rays measured by a single gamma ray detector, where the earth formation has been irradiated by a neutron source and the single gamma ray detector is coated with a neutron absorbing material.
12. The non-transitory computer-readable medium product of claim 11 comprising at least one of:
(i) a ROM, (ii) an EPROM, (iii) an EEPROM, (iv) a Flash Memory, and (v) an optical disk.