| 摘要 |
A system for determining formation gas saturation in a cased wellbore which includes a neutron logging tool having a source, a first neutron detector offset towards the wellbore primarily sensitive to the wellbore and a second detector spaced further from the source and offset towards the formation, so that the second detector is sensitive primarily to liquid-filled porosity in the formation. The detectors are encased in a neutron moderator. The system includes a method of determining gas saturation in the formation by measuring count rates at the detectors, generating a wellbore gas saturation from the first detector counting rate, generating a ratio of counting rates of the second and first detectors to generate a value of liquid-filled porosity in the formation, and calculating a gas saturation in the formation by combining the liquid-filled porosity with a measurement of total porosity. The total porosity measurement can be obtained from core samples or other well logs run before the casing in set in the wellbore.
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