A New Evaluation Method for Rock Fracability Based on a Ternary Index

Accurately evaluating fracability is crucial for improving shale gas fracturing efficiency. This study proposes a new mechanical deformation modulus to characterize rock fracture modes under coupled effects of stress conditions and mechanical parameters. Combined with tensile strength and fracture toughness, a ternary-index fracability evaluation method is established covering the full process of “fracture initiation–propagation–network formation”. Taking intervals Q1–Q9 of Gulong Shale as the research object, experiments were conducted to classify main intervals into four mechanical models: (1) “low tensile–low toughness–low modulus” (Q2), where fractures crack and grow easily but exhibit small apertures and weak fracture-forming capacity; (2) “low tensile–low toughness–medium modulus” (Q1, Q3, Q6), where fractures crack and grow easily, forming low-angle intersecting fracture networks; (3) “low tensile–low toughness–high modulus” (Q7, Q9), where fractures crack and grow easily, creating large-aperture, high-angle through-going fracture networks; and (4) “high tensile–low toughness–high modulus” (Q4, Q5, Q8), where fractures crack with difficulty but grow easily, developing high-angle through-going shear fractures. The evaluation results are consistent with the actual fracability characteristics of the Gulong Shale. Compared with conventional evaluation methods, the ternary index evaluation method can more clearly reveal the progressive evolution process of fractures from crack to propagation and then to fracture network formation, providing a reliable basis for fracture network prediction and fracturing optimization.