Method and apparatus for reducing matrix density effects on porosity measurements during epithermal neutron porosity well logging

摘要

The porosity of the subsurface earth formation surrounding a borehole is investigated by measuring the populations of epithermal neutrons at detector locations longitudinally spaced from a neutron source. During this measuring process, a detector is located at the matrix density neutral (MDN) distance from the neutron source at which the matrix density effects of the earth formation are significantly reduced. The porosity measurements are determined from neutron counts detected at both the MDN location and other detector locations to derive formation porosity measurements which have reduced lithology and matrix density effects. Because of these reduced effects, the derived formation porosity values do not need to be substantially corrected to account for lithology and matrix density effects. In addition, the formation lithology and matrix density may be obtained from measurements taken at the MDN location and other detector locations.