High-frequency dielectric dispersion has been shown to be sensitive to the water volume, the water salinity, and the water tortuosity within the pore space. Dielectric dispersion data can be inverted to estimate water saturation in conditions where the interpretation of resistivity with traditional saturation equations is problematic, such as low and variable water salinities, complex texture, and unknown wettability. There is, however, a recognized limitation: salinity can be estimated only in the low to medium range. Above about 60 ppk, water salinity, which influences the estimated water volume, must be fixed in the dielectric interpretation.
The thermal neutron capture cross-section (sigma) measurement primarily responds to the volume and salinity of the water that is present in the region of the formation seen by the tool. It provides an excellent complement to dielectric measurements for several reasons. Sigma is highly sensitive to the salinity of the water phase, particularly at high salinities. Both sigma and dielectric measurements have a similar volume of investigation and so can be interpreted in a joint inversion.
We developed a joint inversion of dielectric dispersion and neutron sigma measurements that solves for the water volume, water salinity, and tortuosity of the water phase. The addition of neutron sigma to the dielectric dispersion measurements extends the interpretation to high-salinity conditions, and enables the capability of quantifying water salinity in conditions where this was previously not possible.
We illustrate the benefits of this joint inversion on some log examples. In a well drilled with water-based mud and fully invaded, the inversion provides a robust estimate of filtrate salinity even for highly saline filtrate and reduces the uncertainty on the estimated residual hydrocarbon. In the case of oil-based mud invasion, the method quantifies the salinity of the formation water even for moderately to highly saline water.