Structural and Elastic Properties of Carbonate Rocks With Different Pore Types Based on Digital and Theoretical Rock Physics

Abstract

Carbonates are characterized by a complex system of pores, caves, vugs and fractures that significantly influence fluid flow and the physical behaviors of rocks. Six rock samples are taken from a carbonate reservoir in China's Sichuan Basin to perform computed tomography (CT), X-ray diffraction and thin section analyses. The samples are classified into fractured, fractured-vuggy and pore-cavity types based on their microstructural properties. Ultrasonic and low frequency tests are performed with different pressures and fluids to measure the frequency dependence of the elastic properties. The relationships between the pore types and the elastic properties are investigated, showing that there is no direct correlation between velocity and porosity for these tight carbonates. Furthermore, the elastic properties of rocks with different structure types are quite different, suggesting that the pore structure dominates the elastic velocities. The CT data are used to reconstruct digital rocks to analyze the complex pore structure. We apply a finite difference (FD) method to estimate the elastic velocities. However, the FD simulations give higher values than the ultrasonic measurements. The discrepancy is due to the limited accuracy of the CT scans, which does not capture the micro-pore structures of rocks. We consider the microscopic pores and cracks and develop a reformulated rock physics model by incorporating the theories of differential equivalent medium and squirt flow based on the simulated elastic moduli. The model can effectively interpret the experimental multi-frequency data and describe the wave response of the carbonates with different pore types. This work contributes to characterize the multiscale pore structure and understand the structural and acoustic properties of carbonate rocks. It bridges multi-frequency data and provides relevant insights and methods by integrating digital and theoretical rock physics.