Resolution of through tubing fluid flow and behind casing fluid flow in multiple completion wells

A method for detecting the direction, linear flow rate and volume flow rate of undesired behind casing water migration in an upper producing zone of a multiple completion well operating on gas lift, wherein the upper completion zone has a production tubing string of known diameter passing therethrough from a lower completion zone which contains water cut fluid flowing at a substantially known volume flow rate in an upward direction, substantially without interrupting production from either of the completion zones, comprising the steps of:

• (a) locating a well tool sized and adapted for passage through production tubing and having a source of fast neutrons at least some of which having sufficient energy to cause the nuclear reaction O.sup.16 (n,p) N.sup.16 and at least two gamma ray detectors longitudinally spaced from said source and each other, in an upper producing zone of the well at a location adjacent either above or below producing perforations to be investigated;
• (b) irradiating the borehole environs with fast neutrons from said source;
• (c) detecting at said detector locations in at least two separate energy regions of the gamma ray energy spectrum, gamma rays substantially caused by the decay of the unstable isotope N.sup.16 comprising the molecular structure of any undesired behind casing water migration and any water cut flowing in the production tubing from the lower producing zone, and generating at least four separate count signals representative thereof;
• (d) combining said at least four separate count signals according to a first predetermined relationship to derive an indication of the linear flow rate of any undesired water flow behind the casing in the upper producing zone;
• and
• (e) combining the known volume flow rate and linear flow rate of the water cut in the production tubing from the lower producing zone and said at least four count signals according to a second predetermined relationship to derive an indication of the volume flow rate of the undesired water flow behind the casing in the upper producing zone.