Complex resistivity spectra and pore geometry for predictions of reservoir properties in carbonate rocks

Abstract

Measurements of complex resistivity spectra (CRS) are performed on core plugs from a wide variety of carbonates in a log sweep from 0.1 to 100,000Hz inside a pressurized chamber at varying effective pressures up to 20MPa. We quantify the CRS curves by extracting the slope of both real and imaginary part of complex resistivity in the 10,000–100,000Hz range. Pore geometries are quantified with thin-section digital image analysis (DIA) from optical light microscopy. The dataset includes 330 carbonate core plug samples from twelve different study areas and hence includes a highly diverse range of carbonate rock types. This should make our results applicable to most carbonate rocks.Pore geometry parameters derived from DIA, such as Dominant Pore Size (DOMsize) and Perimeter over Area (PoA), correlate well with petrophysical properties such as cementation factor and permeability. However, when modeling those properties, higher correlation coefficients are achieved with CRS than with DIA parameters. Using CRS and model constants tuned to the sub-datasets, cementation exponents are predicted with R2=0.91 and permeability with R2=0.84. The correlation coefficient of a universal equation for all 330 samples is still high for cementation factors with R2=0.80, but less for permeability with R2=0.48.The results show that CRS in carbonates are directly related to permeability and formation factors, and greatly improve reservoir property estimates. This study also highlights the usability of low-frequency CRS data as a measure of flow and storage properties in carbonate rocks. The transfer of this methodology to wireline applications would result in more accurate and continuous permeability and cementation factor predictions from well logs.