Study On Three Detectors Scattering Gamma Logging In Cased Well

Three detectors scattering gamma logging can reduce the influence of statistical fluctuation and improve the measurement accuracy of low-porosity formation.It can be used to evaluate cement density and casing thickness in cased wells.In addition,it can also carry out cement quality inspection and casing corrosion evaluation.Therefore,it is necessary to study three detectors scattering gamma logging in cased wells.In this paper,the transverse geometric factors of three detectors in different environments and energy ranges are studied by Monte Carlo numerical simulation method,and the radial detection depth of three detectors under different conditions is obtained.Monte Carlo numerical simulation method is used to analyze the influence of borehole and formation factors such casing thickness,cement density and formation density on the scattering gamma counting rate of different spacing detectors.The forward models of different detectors are established by correlation function method,and the forward models are inverted by optimization method.The results show that the logging response law of backscatter detectors in different wellbore and formation environments is opposite to that of medium and far detectors;the detection depth of backscatter detector is the shallowest,mainly affected by casing,the detection depth of middle detector is far,affected by casing and cement,and the detection depth of far detector is the largest,mainly affected by formation and cement.When the formation density is less than2.7g/cm~3and cement sheath is well cemented,the three detectors inversion can obtain the accurate casing thickness and cement density;for the actual wells with formation density data,the inversion results are accurate,and the inversion results without formation density data are quite erroneous.