Chapter 7 - Deterministic inversion of galvanic resistivity, induction resistivity, propagation resistivity, and dielectric dispersion logs

Abstract

Borehole-based subsurface electromagnetic (EM) logs, namely galvanic resistivity (laterolog), induction resistivity, propagation resistivity, and dielectric dispersion logs, are commonly used for water saturation estimation in hydrocarbon-bearing formations. EM logs exhibit frequency dependence primarily due to the interfacial polarization effects arising from clay grain surfaces, conductive minerals, and charge blockage in pore throats. To limit operational expenses in near-wellbore subsurface characterization, operators commonly employ only one of the four EM logging tools operating at a specific frequency. Also, it is not a common practice to measure permittivity along with resistivity when using the laterolog and induction logging tools. Deployment of all the four EM tools in a single well followed by a joint interpretation of the multifrequency electrical conductivity and relative permittivity acquired over a broadband frequency range for water saturation estimation is also not a common practice. The proposed joint deterministic interpretation method uses the mechanistic PPIP-SCAIP model or PS model (explained in earlier chapters) that accounts for the interfacial polarization effects arising from clay and conductive mineral grains; thereby generating physically consistent water saturation estimates. The joint inversion of relative permittivity and electrical conductivity logs derived from a combination of the four aforementioned EM logs is challenging because of the limited data points corresponding to the discrete log-acquisition frequencies, which ranges from four to nine frequencies depending on the available EM log combinations. Added to that, in most cases, relative permittivity logs are not derived from galvanic resistivity and induction logs; consequently, the proposed joint interpretation will most likely lack permittivity data at laterolog and induction log frequencies.