A new approach to the interpretation of nuclear borehole logs

Abstract

Abstract When interpreting nuclear borehole logs it is usually assumed for simplicity that the tool signal and a formation property, e.g. its density, are uniquely related. However, while the tool signal will usually depend primarily on one formation property, it will also be affected by other formation properties. To take this multi-parameter dependence of all the nuclear tools into account, a new approach to nuclear log interpretation has been developed. In this approach four nuclear tools are considered: the gamma density tool, the neutron porosity tool, the pulsed neutron tool and the natural gamma tool. The responses of these four tools depend on formation density (ρ), porosity (φ), neutron absorption (σ a ), and geological type (including shale content). For each of the four tools a physical model has been developed. The signals measured with the four tools and the four models represent together four equations with four unknowns: ρ, φ, σ a and geological type. These four equations may be solved by iteration and a consistent set of formation properties obtained. The interpretation approach has been implemented on a personal computer. Examples of results of such interpretations are presented.