Caliper-behind-casing from pulsed neutron apparatus

1. A method of logging a borehole in a formation, the borehole having casing installed therein to a casing shoe, the borehole having tubing installed in the casing and extending beyond the casing shoe, the method comprising:

filling the tubing and an annulus between the tubing and the casing with a liquid;

injecting a gas into the annulus between the tubing and the casing to a first point above the casing shoe of the casing in the borehole;

obtaining a first image response of a portion of the borehole with the annulus filled with the injected gas above the casing shoe by operating a logging tool in the tubing along the borehole to image the borehole in the formation behind the casing;

injecting the gas into the annulus between the tubing and the casing to a second point below the casing shoe of the casing in the borehole;

obtaining a second image response of the portion of the borehole with the annulus filled with the injected gas below the casing shoe by operating the logging tool in the borehole to image the borehole in the formation behind the casing;

comparing the second image response to the first image response; and detecting a void between the casing and the borehole in the formation behind the casing based on the comparison.

2. The method of claim 1 , wherein filling with the liquid comprises filing with brine as the liquid; and wherein injecting the gas comprise injecting gaseous nitrogen as the gas.

3. The method of claim 1 , comprising obtaining, before the first image response, an initial image response of the portion of the borehole with the tubing and the annulus filled with the liquid by operating the logging tool in the tubing along the borehole to image the borehole in the formation behind the casing.

4. The method of claim 3 , further comprising calibrating the first image response based on the initial image response.

5. The method of claim 1 , further comprising estimating a volume of the detected void between the casing and the borehole in the formation behind the casing.

6. The method of claim 5 , further comprising estimating an amount of resin to fill the estimated volume of the detected void.

7. The method of claim 6 , further comprising pumping the estimated amount of resin to fill the detected void.

8. The method of claim 1 , wherein obtaining the second image response of the portion of the borehole with the annulus filled with the injected gas below the casing shoe by operating the logging tool in the tubing along the borehole to image the borehole in the formation behind the casing comprises:

making a first pass of the portion of the borehole with the logging tool;

making at least one second pass of the portion of the borehole with the logging tool after a period of time; and

comparing the second image response of the first pass to that of the at least one second pass.

9. The method of claim 8 , further comprising determining, from the comparison, that an interface between the liquid and the injected gas has not shifted.

10. The method of claim 8 , further comprising verifying, from the comparison, the second image response.

11. The method of claim 1 , wherein obtaining the first and second image responses by operating the logging tool in the tubing along the borehole to image the borehole in the formation behind the casing comprises operating a pulse neutron logging tool as the logging tool.

12. The method of claim 11 , wherein operating the pulse neutron logging tool comprises:

counting first bursts as a function of depth at a first detector of the pulsed neutron logging tool being a first distance the neutron source; counting second bursts as a function of depth at a second detector of the pulsed neutron logging tool being a second distance from the neutron source, the second distance greater than the first distance; and

calculating a burst ratio of the first burst count relative to the second burst count as a function of depth.

13. The method of claim 12 , wherein comparing the second image response to the first image response comprises subtracting the calculated burst ratio of the first image response as a function of depth from the calculated burst ratio of the second image response as a function of depth; and extrapolating caliper of the borehole as a function of depth from the difference based at least on a porosity of the formation in which the borehole is disposed.

14. The method of claim 1 , wherein injecting the gas into the annulus between the tubing and the casing to the first point above the casing shoe of the casing in the borehole comprises monitoring pressure of the injected gas for a predetermined amount of time; and calculating a leak rate of the injected gas as a function of the monitored pressure relative to the predetermined amount of time.

15. The method of claim 1 , wherein injecting the gas into the annulus between the tubing and the casing to the second point below the casing shoe of the casing in the borehole comprises monitoring pressure of the injected gas for a predetermined amount of time; and calculating a leak rate of the injected gas as a function of the monitored pressure relative to the predetermined amount of time.

16. The method of claim 1 , wherein detecting the void in the formation behind the casing based on the comparison comprises basing the detection on a porosity of the formation.

17. A method of logging a borehole in a formation, the borehole having casing installed therein to a casing shoe, the borehole having tubing installed in the casing and extending beyond the casing shoe, the method comprising:

filling the tubing and an annulus between the tubing and the casing with a liquid;

obtaining a first image response of a portion of the borehole with the annulus filled with the liquid by operating a logging tool in the tubing along the borehole;

injecting a gas into the annulus between the tubing and the casing to a point below the casing shoe of the casing in the borehole by injecting the gas directly at an open hole portion of the borehole below the casing shoe, thereby allowing the gas to enter a potential void in the borehole behind the casing;

removing any of the injected gas from the annulus while the gas is allowed to remain in the potential void;

obtaining a second image response of the portion of the borehole while the annulus is filled with the fluid and the potential void is filled with the gas by operating the logging tool in the tubing along the borehole;

comparing the second image response to the first image response; and

detecting the potential void in the formation behind the casing based on the comparison.

18. The method of claim 14 , wherein calculating the leak rate comprises predicting a washout in the formation behind the casing based on the calculated leak rate; and wherein detecting the void comprises estimating a volume of the washout in the formation behind the casing based on the comparison of the second image response to the first image response.

19. A method of logging a borehole in a formation, the borehole having casing installed therein to a casing shoe, the borehole having tubing installed in the casing and extending beyond the casing shoe, the method comprising:

filling the tubing and an annulus between the tubing and the casing with a liquid;

injecting a gas into the annulus between the tubing and the casing to a first point above the casing shoe of the casing in the borehole;

obtaining a first image response of a portion of the borehole with the annulus filled with the injected gas above the casing shoe by operating a logging tool in the tubing along the borehole;

injecting the gas into the annulus between the tubing and the casing to a second point below the casing shoe of the casing in the borehole;

obtaining a second image response of the portion of the borehole with the annulus filled with the injected gas below the casing shoe by operating the logging tool in the tubing along the borehole;

comparing the second image response to the first image response;

detecting a void in the formation behind the casing based on the comparison;

estimating a volume of the detected void in the formation; and

estimating an amount of resin to fill the estimated volume of the detected void.

20. The method of claim 19 , further comprising pumping the estimated amount of resin to fill the detected void.

21. A method of logging a borehole in a formation, the borehole having casing installed therein to a casing shoe, the borehole having tubing installed in the casing and extending beyond the casing shoe, the method comprising:

filling the tubing and an annulus between the tubing and the casing with a liquid;

obtaining a first image response of a portion of the borehole with the annulus filled with the liquid by operating a logging tool in the tubing along the borehole;

injecting a gas into the annulus between the tubing and the casing to a point below the casing shoe of the casing in the borehole, thereby allowing the gas to enter a potential void in the borehole behind the casing;

extracting the injected gas from the annulus by displacing the gas in the annulus with the liquid supplied through the tubing to remove any of the injected gas from the annulus while the gas is allowed to remain in the potential void;

obtaining a second image response of the portion of the borehole while the annulus is filled with the fluid and the potential void is filled with the gas by operating the logging tool in the tubing along the borehole;

comparing the second image response to the first image response; and

detecting the potential void in the formation behind the casing based on the comparison.