Identifying the pore structure and permeability anisotropy of coral reef limestone based on CT image analysis

Abstract

Coral reef limestone is a biogenic limestone with a hyperdeveloped pore structure. The complex structural features of coral reef limestone can directly affect its mechanical and seepage characteristics, which are not yet well-understood. The pore structure information of two coral reef limestone specimens, including one with coarse pores (CR-C) and one with fine pores (CR-F), was obtained by combining CT scanning technology, 3D visualization reconstruction, and pore identification algorithms. The pore structure characteristics were interpreted considering the porosity’s spatial variability, fractal dimension, and tensorial anisotropy. A seepage tensor derived from the geometry component of Hagen-Poiseuille’s law was proposed to assess the seepage anisotropy, which was then verified through seepage simulations along different directions. It was found that the spatial distribution of the porosity of CR-C was less uniform than that of CR-F. Both testing specimens had distinct pore structures with obvious directionality that varied with sampling size. Seepage simulations on a representative elementary volume (REV) of CR-C revealed that the permeability along the identified dominant seepage direction was indeed the largest. Some empirical correlations between the permeability and the deviation angle from the dominant seepage direction were established. This study highlights the importance of considering structural anisotropy when interpreting the properties of coral reef limestone.