Unlocking the Effect of Pore Types on Dielectric Permittivity in Carbonates

Dielectric permittivity measurements provide vital interpretation answers for evaluating subsurface reservoirs, including water-filled porosity, salinity, and rock texture parameters. Dielectric permittivity is sensitive to pore-scale interfacial polarization, and it provides information on the fluids in rock pores and their connectivity. Carbonate rocks are complex in nature and display a variety of pore types. This study aims to assess the effect of carbonate pore types on dielectric permittivity response and provide a reliable correlation method. A total of 17 carbonate core samples were selected for this study. The core samples were cleaned and dried, after which their petrophysical properties, including porosity, and permeability were measured. Thin sections were prepared for all the cores, for petrographic description. Digital image analysis (DIA) was used to analyze the thin section images and to capture the different pore types and their attributes. The samples were then saturated with 30 kppm NaCl solution, and dielectric permittivity measurements were conducted on the samples at multi frequencies. The samples exhibit wide ranges of porosity (8.6-45%) and permeability (0.3-1500 mD). The thin section photographs revealed three dominant pore types in the examined samples: moldic, interparticle, and intercrystalline pores. Normalized dielectric permittivity measurements at 10 MHz show that rocks with dominant moldic pores exhibit the highest dielectric permittivity values. Interparticle pores display the lowest normalized dielectric permittivity, while intercrystalline pores fall in between. To understand the effect of pore geometry on permittivity, DIA parameters such as perimeter over area (PoA), dominant pore size (DOMSize), and aspect-ratio (AR) were assessed. Moldic pores generally have the lowest PoA and AR, and the highest DOMSize. Intercrystalline pores show the opposite, while interparticle pores fall in between. Low PoA and high DOMSize indicate a simple pore system, while high PoA and low DOMSize indicate a complex pore network. The platy shape and simple pore system of moldic pores could explain their high dielectric permittivity. Conversely, the low dielectric permittivity of interparticle pores could be attributed to their complex pore network. These observations allow for developing a new workflow to predict carbonate pore types from permittivity dielectric measurements. This study provides insights into the relationship between the carbonates pore types and dielectric permittivity response. For the first time, a relationship database is built between rock dielectric permittivity and DIA parameters. The results and observations allowed for the development of a workflow towards predicting carbonate pore types from dielectric permittivity measurements, a crucial step for advanced and accurate formation evaluation.