Electromagnetic dispersion and sensitivity characteristics of carbonate reservoirs

The need to develop a new measurement method for the identification of favorable hydrocarbon-producing zones in carbonate reservoirs is of great importance due to their complex pore structure and lithology. We have evaluated fundamental experimental data acquired from carbonate formation samples in a buried-hill reservoir to demonstrate the magnitude of the complex resistivity (CR) dispersion effect in oil-saturated carbonate rocks and the possibility of identifying hydrocarbon-bearing carbonate sections. Our experimental data set included CR measurements on four carbonate core samples with various degrees of oil saturation, and at each saturation level, CR measurements were acquired in a frequency range between 10 Hz and 100 kHz. The experimental data indicated that the intensity of the CR dispersion effect increased with oil saturation above a specific critical excitation frequency. The experimental spectra of the carbonate samples were fitted with the Debye model and the Cole-Cole model (CCM), which can be used to interpret the dispersion effect over a wide frequency range. Based on the experimental data and the inverted CCM parameters, a sensitivity study was carried out with the crosswell frequency- domain electromagnetic (EM) modeling. Multifrequency EM data were acquired directly by calculating the Maxwell equation with the CR. Our experimental data and forward modeling results indicated that measurement of induced polarization and EM coupling effects can be an effective means for identifying carbonate hydrocarbon reservoirs and for quantitatively evaluating the effect of formation saturation on favorable conditions.