A rock brittleness evaluation method in interbedded reservoirs based on statistical damage constitutive

The significant heterogeneity of sandstone and mudstone interbedded reservoirs complicates the accurate evaluation of formation brittleness, thereby affecting the optimization of fracturing stage design. This paper first derives a statistical damage constitutive model to predict rock stress-strain curves under in-situ conditions, and the application of the model has been validated by experimental results. It then develops an energy-based brittleness evaluation procedure that accounts for the effects of lithology, physical properties and in-situ conditions on rock brittleness at a logging scale. The results indicate that highly brittle rocks can store significant elastic energy during loading and release it abruptly without requiring additional energy input. Rocks with a low Young's modulus can exhibit high brittleness, as the post-peak mechanical behavior of rock controls its overall brittleness. As confining pressure increases, the brittleness of rocks decreases, rocks with the same Young's modulus exhibits a 44.4% increase in brittleness when confining pressure decreases by 20 MPa. The newly proposed method effectively evaluates the reservoir energy-based brittleness index at the logging scale. This evaluation method is more sensitive to strongly heterogeneous formations and is more objective considering the cumulative impact of internal and external conditions without biased energy weighting.