Quantitative evaluation of rock brittleness based on the energy dissipation principle, an application to type II mode crack

Rock brittleness is the key parameter in the evaluation of “sweet spot” in the shale reservoir which is characterized with low porosity and low permeability, and numerous methods are presented to qualitatively reveal the essence of rock brittleness. However, the quantitative evaluation of this property is still a tough question with continued attention. This paper proposes a new quantitative evaluation method based on the energy dissipation principle in fracture mechanics, which interprets the degree of rock brittle fracturing as energy efficiency contributing to the new generated surface energy of fractures. The fractal geometry and fractal dimension are introduced to describe fracture surface with intricate geometry. The experimentally constructed correlation between volume density of rock fracture and fractal dimension is incorporated into energy dissipation and fractal geometry of fracture. Finally, the energy-balance equation, which corresponds to the rock brittleness index with volume density of rock fracture, is established. The brittleness indexes of different rock are calculated and compared, and the influence of the confining pressure on the brittleness is also studied. Sichuan Longmaxi shale formation of X well in southwest China is analyzed, which verified the reliability of this quantitative evaluation method in pinpointing the fracturing candidate in hydraulic fracturing engineering.