Neutron-Excited Gamma-Ray Spectrometry for Well Logging

The location and monitoring of oil reserves in cased boreholes is becoming an increasingly important activity in the petroleum industry. Measurement of the macroscopic thermal-neutron capture cross-section of the formation surrounding the bore-hole has been quite successful in differentiating between oil and saline formation water in known lithologies. However, in many instances, the formation water salinity is low, variable, or unknown, and/or the lithology is complex. In such cases, much more information than the macroscopic cross-section is needed. Accordingly, a gamma-ray spectroscopy logging system has been developed which uses inelastic and capture gamma-radiation induced in a formation by 14 MeV neutrons from a pulsed neutron generator. This logging technique provides an accurate elemental determination of the formation and its fluid by spectroscopic analysis of the gamma-ray data. The spectrometry logging tool consists of a pulsed D-T accelerator, a NaI/PM tube detector, a multi-channel analyzer, and a digital telemetry system. Surface equipment includes an on-line computer to accumulate and unfold the spectra, to provide control of the downhole systems, and to provide the analog and magnetic tape outputs. The elemental information obtained by analysis of the gamma-ray spectra can be used in a variety of well logging applications. For example, the carbon-oxygen ratio (C/O) is used to obtain oil saturation. The observed silicon, iron and calcium concentrations are used to determine the makeup of the formation rock. Similarly, the ratio of hydrogen concentration to silicon and calcium concentration determines the formation porosity; chlorine and hydrogen outputs determine the water salinity.