Neutron log measurement of moisture in unsaturated basalt: progress and problems

The research described here examines whether neutron logs can be used in unsaturated basalt to quantitatively estimate moisture content, and, in tandem with other wireline logs, to determine the relative contribution to neutron log response by minerals containing chemically bound H. Our results show that it should be possible to quantitatively correlate neutron log response to the amount of both bound and unbound H regardless of saturated or unsaturated conditions. Such a correlation is possible only if bulk density, saturated porosity and neutron log response are already known in saturated conditions. It is not yet known, however, what the exact form of this correlation should be. To evaluate the candidate correlation equations, we compare measured permeability in unsaturated basalts with the calculated H content derived from these correlations. As the result of this evaluation, we identify intervals of clay-free impermeable, unsaturated basalt with apparently high H content. The neutron log response in these intervals may be caused by localized variations of the neutron slowing-down length, L(s), due to low concentration of H in the formation and borehole environment. When L(s) is greater than the source-detector spacing of a neutron logging tool, the neutron log response can invert, resulting in potentially faulty interpretation of neutron logs. These measurement uncertainties in unsaturated basalts might be overcome by using a neutron logging tool with both a higher-flux neutron source and increased source-detector spacing, or a neutron tool with several detectors placed at different spacing from the source.