An assessment method for shale fracability based on fractal theory and fracture toughness

•The complex crack micromorphology is studied by fracture mechanics and fractal theory.•The model-I/II fracture toughness values are calculated by using the fractal-based method.•The continuous profiles of the model-I/II fracture toughness of the shale wellbore are obtained.•An fast assessment method for shale fracability based on fractal theory and fracture toughness. Staged fracturing of a horizontal well may cause artificial fractures to intersect with weak structural planes, including a great number of natural fractures and stratifications in the shale, which will then form a complicated fracture network. Model-I and model-II are two common models of artificial fractures, and the fracture toughness of a different model is also different. This work starts with researching the micromorphology of model-I/II crack (fracture) and is on the basis of the fracture mechanics and the fractal theory. The fractal calculation method is applied to calculate the model-I/II fracture toughness, while the crystal splitting method is used to calculate the surface energy of the shale. The calculation results are obtained by using two parameters: density logging data and interval transit time. The comparison between the calculation results and the test results shows that the mean errors are 0.22% and −0.46%, respectively, which verifies the accuracy of the new method in calculating the fracture toughness. With reference to the shale horizontal well logging data, the integrated normalized fracture toughness index profile is calculated by using the fractal method. And, we have verified that it is accurate to use the fracability assessment method that is based on the fractal theory and the fracture toughness to guide the shale volume stimulation.