Unconventional shale poses many challenges for exploitation due to the complexity in mineralogy, the presence of kerogen, laminations, and the low permeability; thus in most cases, hydraulic fracturing is needed for economic recovery. One of the most important parameters for engineers is the tensile strength of the shale reservoir rocks, which is affected by the amount of ductile vs brittle minerals, the amount of kerogen, the textures of the rock, and the saturation states. In this study we investigate the effect on the tensile strength measured from Brazilian tests of the lamination and mineral assemblage, using computed tomographic (CT) imaging and X-ray diffraction (XRD).
A group of Mancos and Eagle Ford samples from commercial sources were prescreened (to exclude samples with noticeable fractures) and tested for the Brazilian tensile strength at different orientation relative to the bedding plane, in addition to CT scan and X ray diffraction; Scanning Electron microscopy (SEM) is performed on a selected number of samples. Bulk XRD analysis is performed on pulverized rock subsamples and also on polished rock samples; large scaled lamination analysis is carried out on CT images; SEM images are used to evaluate whether fine scale fractures and mineral orientation has any effect on the tensile strength measured at different relative positions.
Our results show that the amount of ductile clay minerals is the most important factor; overall the brittle-quartz-and-calcite-rich Eagle Ford samples have higher average tensile strength (∼700psi) whereas the ductile, clay-rich Mancos samples have a lower average tensile strength (∼400psi). Within each group, the trend of the tensile strength and the relative amount of brittle minerals also exists. The sharply defined laminations of Eagle Ford shales results in a larger degree of anisotropy of tensile strength than these in Mancos Shale, and in the Eagle Ford samples there are more shear failures along the weak lamination planes. The failure along the laminations appears related to sedimentary features and not to mineralogy.