DE2650345

Undesirable water which may be present behind the casing of a prodn. well can be detected by lowering into the well a fast neutron source which produces the nuclear reaction O16 (n,p)N16. Two detectors at a certain distance from each other and from the source measure the gamma radiation, generated by the decay of the unstable isotope N16. Their output signals produce the information about the rate of flow of oxygen nuclei and of water. This furnishes more accurate results than acoustic methods which do not detect micro annuli between the casing and the cement ring. A clear discrimination between water flow inside and outside the casing is possible.