Quantitative Evaluation of Shale Brittleness Based on Brittle-Sensitive Index and Energy Evolution-Based Fuzzy Analytic Hierarchy Process

A quantitative method for evaluating brittleness is pivotal in optimizing hydraulic fracturing of shale. Shale reservoirs are characterized by diverse components and complex diagenetic environment, resulting in multiple factors influencing the brittleness feature, which makes the traditional brittleness index hardly applied widely. This study proposes a new quantitative brittleness model for shale based on the significance assignment of the brittle-sensitive index (BSI) at different stage during the loading. This study divides the complete stress–strain curve into three stages and acquires corresponding BSI based on the brittle failure evolution. Then, a fuzzy analytic hierarchy process (FAHP) is proposed to determine the contribution weight coefficients of BSI at each stage to the brittle behavior of shale, where a comparison matrix is constructed based on the evolution of energy accumulating and releasing during the loading to avoid the experience lacking and consistency issues. Experimental evaluations of two sets of reservoir shale show that the new model can effectively indicate the variation of brittleness with affecting factors, such as brittle–plastic mineral content, porosity, burial depth, and confining pressure, which demonstrate its accuracy and superiorities. Comparative analysis of the published models reveals that the new model has stronger sensitivity to the buried depth. The procedure of weight determination can effectively explain the prediction of brittle variation under confining pressure. Furthermore, the proposed model, combined with the controlled artificial shale, indicates that the brittleness decreases with the kerogen content. The results are of great importance in optimizing hydraulic fracturing of shale gas reservoirs.