Mechanical characteristics and factors controlling brittleness of organic-rich continental shales

Brittleness is an essential parameter in the evaluation of wellbore stability and hydraulic fracturing design of shale reservoirs. It is an inherent geomechanical property controlled by the mineral composition, sedimentary structure, and geological condition. However, traditional brittleness evaluation methods based on the elastic parameters and mineral composition analyses are limited by the anisotropic petrophysical behaviors of continental organic-rich shale. In this paper, we analyzed the contributing mechanical properties of individual mineral components and organic contents of shale using laboratory stress experiments. Based on the principle of energy conservation and post-peak deformation, we evaluated the brittleness and analyzed the main factors influencing shale brittleness. In addition, we established a new method of quantifying the representative shale Brittle-Ductile Index (BDI) via combined consideration of its organic matter content, mineral composition, and mechanics characteristics. The results identified quartz and feldspar as the main brittle minerals with a substantial contribution to shale brittleness. Conversely, the organic matter and carbonate minerals have little effects on the pre-peak elastic parameters but are very sensitive to the post-peak rupture parameters and significantly influence the brittle shale fractures. Specifically, the calculated BDI is high in areas of rich in organic matter or developments of shale lamination. This usually leads to brittle fracturing along the weak surfaces when the stress is mainly applied in the bedding directions. In contrast, the BDI is small in carbonate rich shales as mineral particles are heavily compacted which disperses the stress, resulting in scattered ductile fractures.