Exploring the Effect of Fractures and Microstructure on Brittleness Index in the Barnett Shale

Summary The effectiveness of fracture stimulation techniques depends on the microstructural features which control the rock strength. We analyze brittleness index, fractures, and microstructure of the Barnett Shale for a better understanding of the correlation between mechanical properties, mineralogy, and pore geometry. The complexity of multiple minerals, pore geometries, and pore inclusions are modeled using the self-consistent approximation (SCA) model, with consideration of statistical distributions of pores and cracks in shales. The method is applied to core samples and well log data from the Barnett Shale to invert the aspect ratio (ratio of short axis to long axis) of pores, and to estimate crack density, and the proportion of stiff pores and cracks in the Barnett Shale. The inverted crack density gives an average estimate of the pore space geometry. Results show that the aspect ratio for the Barnett Shale varies between 0.01 and 1 and has a dominant value of 0.1. Analysis on the core data indicates that both quartz and carbonate minerals contribute to increased crack density. Comparison reveals good correlations between the brittleness indices defined in terms of (λ+2μ)/λ and Poisson’s ratio. While the brittleness index defined by Young’s modulus is not consistent with the definition in terms of (λ+2μ)/λ and Poisson’s ratio, Young’s modulus is a good indicator for the variation of crack density. Results of pore-type inversion show that the variation of pore types coincides with inverted crack density.