Estimation of crack porosity in deep carbonate reservoirs based on attenuation rock physics templates

Carbonate reservoirs in the S area of the Tarim Basin (China), with low porosity, complex fracture systems and dissolved pores, are ultra-deep hydrocarbon resources. Especially, cracks play an important role in reservoir connectivity and storage space. To study the effect of cracks on reservoir, we have performed measurements on limestone samples under different confining pressures, and used the self-consistent approximation model and Biot-Rayleigh theory to simulate. Based on the dependence of seismic Q on the cracks, a multiscale attenuation template (RPT) is built, based on P-wave attenuation, impedance and phase velocity ratio. Then, calibration of the RPT is performed at ultrasonic and seismic frequencies to estimate the total and crack porosities of reservoirs. The results indicate that the distribution of the microcracks and pores were obvious different in the reservoirs. The total porosity is low (mostly < 5%), which is consistent with the geological structure and well-log data. The crack porosity is mainly between 0.06% and 0.12%, its behavior shows a good agreement with the actual oil production reports.