Experimental Study of the influence of PVC casing thickness and well fluid density on prompt fission neutron uranium logging and its corrections

Abstract

Prompt fission neutron uranium logging is a direct method for measuring uranium content.This method is not influenced by uranium-radium disequilibrium, and provides an accurate and direct way to obtain the uranium content of the formation. However, in field logging, the results of logging are susceptible to the influence of lithology, well fluid and casing. Thus, it must be corrected. Based on the“epithermal /thermal neutron counting rate ratio”(i.e., NE/T ) theory of uranium quantification, a uranium block model is built to simulate field production boreholes. Meanwhile, the influences of PVC casing thickness, well fluid composition and well fluid density on the decay rate of epithermal neutrons and thermal neutrons and the error correction method are investigated. The experimental results show that NE/T increases linearly with increasing thickness of the PVC casing, and the correction equation is μ PVC = -0.0501 dPVC + 1, with a goodness of fit R 2 = 0.9931. The effect of different well fluid compositions and densities on the decay rate of double neutrons is kaolinite ≥ montmorillonite < chlorite < illite. μ_mud follows a quadratic decay law with increasing well fluid density and the fit R 2 < 0.95. Finally, theaverage corrected uranium quantification error is within 5% of the actual uranium content.