A three-dimensional classification for mathematical pore shape description in complex carbonate reservoir rocks

Porous carbonates display some of the most complex porosity networks in reservoir rocks. This requires a quantitative geometric description of the complex (micro)structure of the rocks. Modern computer tomography techniques permit acquiring detailed information concerning the porosity network in three dimensions. These datasets allow a more objective pore classification based on mathematical parameters. In this study, ratios of the longest, intermediate, shortest dimensions and compactness of the pore shapes, based on an approximating ellipsoid, are analysed to obtain a thorough and objective description of pore shapes. Using intrinsic properties of the latter, the classification can be used at every resolution scale. Five shape classes are defined: rod, blade, plate, cuboid and cube. An additional advantage of this classification is that the data provide information about the orientation of the pores. This allows assessing the anisotropy of the porosity parameter. Apart from having an objective pore-type classification, analysing the shape and orientation of the pores permits to study the relationship with other important petrophysical rock characteristics such as permeability, acoustic properties and rock mechanical behaviour.