Micromechanical modeling of ultrasonic velocity for pore-structure and porosity characterization considering anisotropy in carbonate samples

Abstract

This work presents an approach to characterize the pore-structure and anisotropy in carbonate samples based on the Effective Medium Method (EMM). It considers a matrix with spheroidal inclusions which induce a transverse anisotropy. The compressional wave (VP), vertical (VSV) and horizontal (VSH) shear wave velocities are estimated taking into account parameters as characteristic length, frequency, angle of wave incidence, aspect ratio, mineralogy, and pore-filling fluid to predict pore shape in carbonates. Ranges of aspect ratios are shown to discriminate different pore types: intercrystalline, intergranular, moldic, and vuggy. The angle of wave incidence is a determinant parameter in the estimation of VP(0º, 45º, 90º), VSV(0º) and VSH(90º) to calculate dynamic anisotropic Young’s modulus (E33) and Poisson’s ratio (v31), as well as the Thomsen parameters, Epsilon, Gamma and Delta for quantification of the anisotropic pore-structure. The obtained results establish that the size, as well as the pore-structure, have a more significant impact on the elastic properties when the porosity takes values greater than 4% for the three frequencies, ultrasonic, sonic, and seismic. This investigation predicts the pore-structure and pore-size to improve characterization and elastic properties modeling of carbonate reservoirs. Validation of results includes porosity measurements and ultrasonic velocity data for different carbonate samples.