Quantifying Water Saturation in a Source Rock Gas Play Using Multi-Frequency Dielectric Measurements

Abstract

One of the challenges in exploring tight source rock plays is accurately estimating water saturation using downhole measurements. The conventional approach of using resistivity along with a saturation equation has many uncertainties, especially in the exploration setting when the treasure chest of core measurements is still lacking. Even with core data available, non-standard core handling practices often lead to misleading interpretations and incorrect calibrations. Multi-frequency dielectric dispersion measurements provide a great way to overcome the challenges faced by the conventional resistivity based methods. These tools measure formation dielectric permittivity and conductivity at multiple frequencies. The large contrast between the permittivity of water and permittivity of rock, hydrocarbon and kerogen enables the tool to measure water volume. The dispersion observed in permittivity and conductivity with frequency is a function of the water textural parameters and salinity. Thus, this measurement does not need external textural and salinity parameters as inputs, but rather, computes these as outputs which can be used in the conventional resistivity based approach. Multi-frequency dielectric measurements were carried out on the Diyab source rock gas play in Abu Dhabi. Comparisons to the resistivity based method led to an initial uncertainty in identifying zones of low water saturation in the source rock play, however, a core analysis program was designed to address these uncertainties. Core NMR, core dielectric permittivity, retort, total organic carbon (TOC) were among the several measurements that were carried out to characterize the formation and compute the fluid volumes in the rock. Despite the uncertainties associated with heterogeneity seen in the source rock gas play, a better match was observed between the dielectric measured water volumes and core results as compared to the resistivity based water volumes. This was due to high textural variation in the rock which was confirmed by dielectric dispersion measurements. Relative permittivity of the rock matrix was recognized as a key parameter influencing dielectric dispersion results. Core measurements were used to calibrate the calcite matrix permittivity for the Diyab formation. Limitations were recognized (and addressed) with certain core measurement sample preparation techniques which could lead to confusing results in the absence of other measurements. Core NMR was recognized as an essential measurement to quantify water volume in this study. This is the first time that a source rock gas play has been analyzed in Abu Dhabi with such an extensive core and log program, and it is imperative that best practice methods be established to improve and add value to future interpretations. Dielectric dispersion measurement has proved to be immensely valuable in quantifying the water saturation in the source rock gas play in Abu Dhabi, overcoming several challenges we face with conventional resistivity based methods.