Brittle Evaluation Based on Energy Evolution at Pre-Peak and Post-Peak Stage

Rock brittleness is one of the important mechanical properties of rock mass, which is so crucial for accurately evaluating hydraulic fracturing of oil and gas reservoir and rock bursting engineering. Existing methods of rock brittleness based on energy theory were summarized, and limitations of these indexes were analyzed in detail. In this study, energy evolution characteristics at pre-peak and post-peak stage are comprehensively considered. A new method to determine brittleness index of rocks based on complete stress-strain curves is established, which more reasonably describes the rock brittleness. To verify the rationality of the method, four sets of rock mechanics tests are collected to test the new index. Test results show that the new brittleness index gradually increases with the increase of confining pressure. Under low confining pressure, both coal and group 1 of shale exhibit strong brittleness, while under high confining pressure, the brittleness of red sandstone and group 2 of shale is obviously weakened, showing that the characteristics of brittle-ductile transition of rocks with increasing of the confining pressure. Then in actual slope engineering, the rationality of the new brittleness index is further validated by the tests of slate, which may be expected to offer some references for evaluating rock brittleness.