A New Brittleness Evaluation Index for Reservoir Rocks Based on Fuzzy Analytic Hierarchy Process and Energy Dissipation

Accurate and quantitative evaluation of rock brittleness is essential for optimizing hydraulic fracturing parameter design. Specific aspects of rock brittleness can be characterized based on mineral composition, strength parameter, stress- strain, and elasticity parameter. However, in characterizing the effect of high confining pressure on rock brittleness, these advantages have not been shown for various reasons. The essence of rock fracture resides in the accrual and dissipation of energy, concomitant with the transformation of three distinct energy modalities: elastic energy, dissipated energy, and post- peak release energy. In this study, we proposed a new brittle index for reservoir rocks considering the interactions of the energy evolution process in different stages. The associated B-ii values are computed through the analysis of energy evolution based on the fuzzy analytic hierarchy analysis process (FAHP) method. Tri-axial compression tests were conducted on granite, shale, and sandstone specimens under different confining pressures for the validation of the proposed brittleness evaluation (BE) index. The results indicate a decreasing trend in the brittleness index with increasing confining pressure. Comparative analysis with other existing brittleness evaluation indexes demonstrates that the new brittleness evaluation index offers a more precise evaluation of the brittleness of various rock types.