Brittle logging evaluation of marine shale based on rock mechanical characteristics：A case study of Wu-2 member in Hongxing area, eastern Sichuan

The accurate prediction of mudstone/shale reservoir brittleness index is directly related to the fracturing design and effect of the formation of interest. Wu-2 member is currently one of the main exploration targets for Permian shale gas in Hongxing area of eastern Sichuan. The lithology changes rapidly and the heterogeneity is strong, which seriously restricts the accurate evaluation of brittleness attributes. In response to this geological reality, this study focuses on Wu-2 Member shale reservoir in Hongxing area, and such mechanical experiments as triaxial compression and Brazilian tensile tests are conducted on four lithological types: high quartzose shale, high calcareous shale, limestone, and high clay shale. The focus is to analyze the mineral composition, mechanical elastic parameter characteristics, stress-strain characteristics, and brittleness features of the rock core. The experimental results demonstrate that high-quartz- content shale samples exhibit brittle shear fracture, accompanied by bedding splitting failure. Based on rock mechanics experiments, a highest-precision elastic-parameters-based evaluation method for the brittleness index logging was optimized. Regionally, the absolute error between the predicted results and logging interpretation results for the 11 pilot wells in the region of interest is generally less than 1.5%, with relative error within ±2%. The results show that the three-dimensional brittleness index is in good agreement with the calculated brittleness index of a single well, which indicates that this evaluation method enables the effective prediction of brittleness space distribution of shale in Wu-2 member. This study plays significant role in guiding the fracturing design in the region of interest.