Abstract: Shale gas development in commercial scale benefits from the key technology— staged fracturing of horizontal wells, and the evaluation of shale fracability is also the key for the technology. Shale fracture toughness is an important factor for the evaluation of fracability. Based on the microstructure of Model I fracture, combining the fracture mechanics with the fractal theory, this paper established a new calculation method of shale fracture toughness. The surface energy of shale can be calculated by means of crystal splitting, and the calculation results can be obtained easily via rock density and acoustic time. This paper presents a comparison between the fractal calculation results and the experimental test results and traditional predicting results. The comparison shows that the mean error of fractal calculation is 3.63% and the mean error of traditional predicting is ?10.53%, validating the accuracy of the new method. Referring to the horizontal well logging data of horizontal wells, this method can be used to establish the wellbore model-I fracture toughness index continuity. Considering the relationship between the fracability level and the probability of stimulated reservoir volume, the class III and class II fracability level is preferred. The fractal calculation method of model I fracture toughness of shale is important in the perspective of quantitative evaluation of shale gas reservoir fracability.

Key words: shale rock, model I crack, fracture toughness, fractal, fracture toughness index