Abstract

Differentiating brittle zones from ductile zones in low permeability shale formations is imperative for efficient hydraulic fracturing stimulation. The brittleness index (BI) is used to describe the rock resistance to hydraulic fracture initiation and propagation and measures the ease at which complex fracture networks can be created. In this study, we constructed brittleness templates through the correlation of fundamental rock properties and geomechanical characterization. We then employed the templates to distinguish the brittle, ductile, and brittle-ductile transition zones in the Longmaxi shale gas reservoir, Sichuan Basin of southern China. The approach works in two steps. First, we suggest a new expression for the mineralogical BI by their respective weights based on the analysis of correlation coefficients between mechanical testing and XRD results. Second, we correlate TOC, porosity, pore fluid, natural fractures, and improved BI model with multiple elastic properties to define the brittle, ductile, and transitional zones in the Longmaxi shale gas reservoir of China. Compared with the traditional mineralogy-based BI definition, the improved BI model differentiates the brittle and ductile zones and provides a better sense of the most suitable fracturing regions. Our results show that the brittleness templates, which combine fundamental rock properties, improved BI model, and geomechanical characterization led to identifying favorable zones for hydraulic fracturing and enhanced shale characterization. The proposed brittleness templates’ effectiveness was verified using data from horizontal wells, offset wells, shale gas wells from different origins, laboratory core testing, and seismic inversion of BI across the studied wells.