Mechanical Versus Mineralogical Brittleness Indices Across Various Shale Plays

Abstract

Abstract In hydraulic fracturing, it is natural to think that regions of brittle rock are prime candidates for effective fracture network creation or may be relatively easy to fracture. Consequently, various forms of brittleness indices have been introduced, in part, to decide where to place perforation clusters in multi-stage horizontal completions. These brittleness indices are often calculated using correlations between elastic parameters and mineralogical data, primarily because mineralogical information is readily obtained from wireline logs, cores, or cuttings. This presents two unresolved issues. First, while it is likely that a relationship between brittle failure and stiffness qualitatively exists, the efficacy of leveraging these indices in a quantitative manner remains uncertain. Second, there are limited examples comparing direct rock core measurements to indices calculated from mineralogical data, leaving room for ambiguity in what the magnitude of brittleness indices mean to hydraulic fracture initiation and propagation. This study aims to clarify the definition of brittleness by comparing laboratory-derived rock mechanical properties, mineralogical and mechanical brittleness indices. Shales evaluated include the Floyd, Haynesville, Eagle Ford, and Barnett. Rock properties measured in this study are Young's modulus, Brinell hardness, tensile strength, and compressive strength. The mineralogical content of the specimens was determined by the x-ray diffraction (XRD) technique. A mineral-defined brittleness index is computed from the XRD data, while the mechanical brittleness indices are computed from tensile and compressive strength testing of core from these shale formations. Results show little agreement among the mechanically based indices, but hardness measurement trends coincide with elasticity-based mechanical indices and mineralogical indices. Additional data is needed verify the possible relationship between tensile strength and mineralogy. Strong correlation exists between mineralogical-based brittleness and Brinell hardness in the Eagle Ford and Haynesville shales. The Brinell hardness index could be applied in lieu of brittleness with the added benefit of estimating proppant embedment.