Experimental Investigation on Damage Behavior of Unconventional Reservoir Rocks Based on Key Stress Thresholds

Key stress thresholds—crack initiation stress (Ci), crack damage stress (Cd), and peak strength (Cp)—delineate the progressive stages of crack development, yet their utility for characterizing rock damage in unconventional reservoirs remains insufficiently explored. This study investigates rock damage behavior using triaxial compression failure tests on three tight sandstones and seven shales. Ci, Cd, and Cp are determined via the lateral strain response method, volumetric strain reversal, and peak deviatoric stress, respectively. The normalized ratios Ci/Cp and Cd/Cp are introduced as descriptors of crack initiation and damage acceleration timing. Three damage modes emerge: Group 1 (high Ci/Cp > 0.65, Cd/Cp > 0.9) is brittle, with prolonged elasticity and sharp post-peak failure; Group 3 (low Ci/Cp < 0.5, Cd/Cp < 0.75) is ductile-dominated, with early cracking and gradual stress decline; Group 2 represents a mixed mode. Strain energy analysis corroborates the classification—brittle samples retain Ue/U > 0.75 at peak stress, while ductile samples develop Ud/U of 0.4–0.5. Two conventional brittleness indices can identify highly brittle rocks but fail to distinguish mixed from ductile modes. In contrast, Ci/Cp and Cd/Cp reliably capture damage evolution across the full failure process, offering a physically grounded framework for hydraulic fracturing target selection in unconventional reservoirs.