Petro-elastic model of the multiple pore-crack structure of carbonate rocks based on digital cores

Underground carbonate deposits are widespread worldwide and have considerable hydrocarbon potential. They are generally characterized by a complex microscopic structure that affects the properties of the macroscopic fluid flow and the relevant petrophysical behavior. In recent years, advances in digital technology have helped reveal the microstructures (i.e., pore connections, cracks, pore size and radius, etc.) of rocks in the subsurface. In this work, drill cores (cylinder) are taken from a deep carbonate deposit in the Sichuan Basin in western China to perform computed tomography (CT) scans, thin sections and mineral analysis. The characteristics of lithology and pore structure are investigated. Ultrasonic experiments with different fluid types and pressures are conducted to determine rock samples’ wave velocities, attenuation and crack porosity. The experimental data show that the rocks have low porosity/permeability and a complex pore/crack system, leading to significant pressure, crack and fluid type effects on the velocities, dispersion and attenuation. We develop a model of multiple pore-crack structures for carbonates by considering the complex structure and fluid properties. Digital cores are reconstructed based on CT scans, image processing and threshold segmentation. The aspect ratios of pores and cracks are extracted with their volume fractions to simulate the rock skeleton with the differential effective medium theory. The Biot–Rayleigh wave propagation equations are applied to simulate the effects of different pore and fluid types on the velocity and attenuation of P-waves. The agreement between the modeling results and the ultrasonic and log data confirms that the model can validly reproduce the wave responses.