Microstructural effects on mechanical properties of shaly sandstone

Abstract

Understanding the mechanical properties of shaly sandstone is of great importance in reservoir geomechanics. Because of the lack of core data, measurements based on acoustic wave velocities are often conducted. In such measurements, the mechanical properties such as uniaxial compressive strength (UCS) and Young's modulus (E) are related to the porosity of sandstone and shaly sandstone. However, in many instances, the porosity fails to directly correlate to mechanical properties especially when dispersed clays are present. Therefore, in this study, a set of uniaxial compressive tests on a number of different shaly sandstones was performed. The X-ray computed tomography was conducted on the representative samples from each type of sandstone to assess the porosity, rock composition, and texture. Independent compositional analysis was also carried out on all representative samples to link the samples' composition to their microstructure. Interestingly, it was found that the mechanical properties of these shaly sandstones are well-correlated to the combined clay volume and effective porosity in a multivariate analysis. It was concluded that the univariate analysis solely performed on porosity cannot always correlate well to the mechanical properties of sandstone containing clays. Therefore, a combination of clay volume and effective porosity is more accurately correlated to mechanical properties.