1. n. [Formation Evaluation]

That part of the effect of gas on the neutron porosity measurement that is not explained by differences in hydrogen index. By using the concept of hydrogen index, the only significant contributor to the neutron porosity in a gas zone is the liquid-filled porosity, since the hydrogen indices of gas and matrix are close to zero. However, the resultant liquid-filled porosity is found to be too low. The error comes from treating the gas-filled porosity as matrix. If this matrix is excavated and replaced with gas, the correct response can be predicted.

See related terms: hydrogen index,  neutron porosity