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Abstract

A method of determining the porosity of earth formations surrounding a wellbore despite shielding of that formation by well casing, cement or mud cake. The method includes producing fast neutrons in a wellbore and detecting a first epithermal neutron population resulting from passage of neutrons from the source through a designated first region and a second epithermal neutron population resulting from passage of neutrons through a designated second region. The relationship of the logarithms of a first and a second epithermal neutron population resulting from passage of neutrons through a plurality of known porosity homogenous mediums is determined and plotted as a first curve with each point representing a unique value of porosity. A second curve of the logarithms of the first and second epithermal neutron populations in the actual wellbore is plotted. Individual points on the second curve are then corrected to the first curve utilizing known attenuating factors to determine the porosity of the earth formation.

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First Claim