FREMGANGSMAATE OG APPARAT FOR MAALING AV UOENSKET VANNINNTRENGNING I ET BOREHULL

1505446 Measuring flow-rate of water in borehole casing TEXACO DEVELOPMENT CORP 15 Sept 1976 [3 Nov 1975(3)] 38143/76 Heading G1A The volume flow-rate of undesired water flow behind the casing 102 of a borehole e.g. along a cement lining channel 110, is measured by the use of neutron activation analysis apparatus comprising a fast neutron source 126 and longitudinally spaced detectors D 1 , D 2 , the source causing activation of oxygen nuclei to form unstable nitrogen N16, the gamma radiation decay from which is detected. The Specification provides an expression relating the ratio of the counts from the two detectors with the linear velocity v of the water flow. Using v, the volume flow V may be calculated from a further expression relating V to (among other known quantities) the count rate for a particular detector, the velocity v, the spacing S between the source and the particular detector, and the distance R from the particular detector to the centre of the water flow if R can be provided estimation or measurement. The value of R may be measured by two methods. In the first, pulse height analysis techniques are used to compare the counts in two gamma energy ranges, the ratio obtained being correlated with a calibration curve, Figs. 5 and 6 (not shown). In the second a specialized detector construction is used, Fig. 7. A central scintillator crystal 71 and an outer cylindrical crystal 72 (NaI or CsI-thallium activated) are respectively coupled to photo-multipliers and the ratio of the counts correlated with a calibration curve to find R, Fig. 8 (not shown). The activation analysis tool may be operated with a continuous source or preferably using a pulsed source. Different counting windows are used in each case, Figs. 4 and 5 (not shown). Typically, a Deuterium-tritium type accelerator is used to produce 14 MeV neutrons in bursts of 1 ms. The detectors are gated off for the initial period so that prompt gammas are not detected, then gated on for 6 ms. before the next accelerator pulse Fig. 11 (not shown). Synchronization pulses aid data handling as does the use of opposite polarity pulses from the individual detectors. A computer evaluates the value V at different heights sensed at 112. Measurements are taken with the logging tool fixed or slow moving. The tool is constructed in module form so that the source may be attached below the detectors for measurements in upward flowing water and vice versa, Figs. 9A-9C (not shown).