METHOD OF DETECTING GAS IN A FORMATION USING CAPTURE CROSS-SECTION FROM A PULSED NEUTRON DEVICE
Elemental analysis of an earth formation is performed using measurements from a gamma ray logging tool. From the elemental analysis, an estimate of the mineralogy of the formation is made. A prediction of the capture cross-section of the formation is made using the mineralogical analysis. The difference between the predicted capture cross-section and a measured capture cross-section is an indication of gas in the formation.
1 . A method of determining a presence of gas in an earth formation, the method comprising:
determining the presence of gas in an earth formation using a difference between an estimated capture cross-section of the earth formation and a predicted capture cross-section of the earth formation, wherein the estimated capture cross-section is estimated by a processor.
2 . The method of claim 1 , further comprising:
irradiating the earth formation using a source of radiation within a borehole;
measuring radiation from the earth formation responsive to the irradiation; and
using the measured radiation to estimate the estimated capture cross-section of the earth formation.
3 . The method of claim 2 wherein irradiating the earth formation further comprises using a pulsed neutron source, and measuring the radiation further comprises measuring gamma rays resulting from the irradiation.
4 . The method of claim 1 further comprising determining the predicted capture cross-section using a composition selected from: (i) an elemental composition, and (ii) a mineralogical composition.
5 . The method of claim 4 further comprising determining the composition using an elemental analysis of spectra of the measurements of the radiation.
6 . The method of claim 1 wherein estimating the capture cross-section of the earth formation further comprises performing summation of counts of the radiation over a time window substantially unaffected by a fluid in a borehole.
7 . The method of claim 1 further comprising correcting the predicted cross-section for a trace element.
8 . The method of claim 1 further comprising identifying the presence of gas by a crossover of a log of the estimated capture cross-section and a log of the predicted capture cross-section.
9 . The method of claim 2 further comprising conveying the source of radiation into the borehole on a conveyance device selected from: (i) a wireline, and (ii) a bottomhole assembly on a drilling tubular.
10 . An apparatus configured to determine a presence of gas in an earth formation, the apparatus comprising:
a source configured to be conveyed in a borehole and irradiate the earth formation;
a detector configured to measure radiation resulting from the irradiation of the earth formation; and
at least one processor configured to:
(i) use the measured gamma rays to estimate a capture cross-section of the earth formation; and
(ii) use a difference between the estimated capture cross-section and a predicted capture cross-section of the earth formation based on an estimated composition of the earth formation as an indication of the presence of gas.
11 . The apparatus of claim 10 , wherein the source further comprises a pulsed neutron source, and the radiation that the receiver is configured to measure further comprises gamma rays.
12 . The apparatus of claim 10 wherein the at least one processor is further configured to determine the predicted capture cross-section using a composition selected from: (i) an elemental composition, and (ii) a mineralogical composition.
13 . The apparatus of claim 12 wherein the at least one processor is further configured to determine the composition using an elemental analysis of spectra of the measured radiation.
14 . The apparatus of claim 10 wherein the at least one processor is further configured to estimate the capture cross-section of the earth formation by performing a summation of counts of the radiation over a time window substantially unaffected by a fluid in the borehole.
15 . The apparatus of claim 13 wherein the at least one processor is further configured to correct the predicted capture cross-section for a trace element.
16 . The apparatus of claim 10 wherein the at least one processor is further configured to identify the presence of gas by a crossover of a log of the estimated capture cross-section and a log of the predicted capture cross-section.
17 . The apparatus of claim 10 further comprising a conveyance device configured to convey the logging tool into the borehole, the conveyance device being selected from: (i) a wireline, and (ii) a bottomhole assembly on a drilling tubular.
18 . A computer-readable medium accessible to at least one processor, the computer-readable medium including instructions which, when executed, cause the at least one processor to:
estimate a capture cross-section of a formation using radiation measured by a detector responsive to irradiation of the formation by a source of irradiation in a borehole; and
determine the presence of a gas using a difference between the estimated capture cross-section and a predicted capture cross-section of the earth formation and an estimated composition of the earth formation.
19 . The medium of claim 18 further comprising at least one of: (i) a ROM, (ii) an EPROM, (iii) an EEPROM, (iv) a flash memory, and (v) an optical disk.