Characterization of Meso-Mechanical Properties and Fracture Mechanism of Dolomite Based on Combined Nanoindentation-SEM Technique

The mesomechanical properties of dolostone are critical for reservoir stimulation. Focusing on the dolostone from the Shunbei Oil and Gas Field, this study employed nanoindentation combined with SEM, EDS, and XRD to investigate its micromechanical behavior. The samples are predominantly composed of dolomite, with minor amounts of calcite and silicates, exhibiting heterogeneity in both mineral phases and pore structures. Nanoindentation results indicate that the elastic moduli are concentrated in the range of 90–120 GPa, with hardness values of 3–5 GPa and maximum indentation depths of 1.0–1.4 μm, reflecting high brittleness. Dense regions with a modulus of 149.09 GPa exhibit few cracks, whereas low-modulus regions at 109.2 GPa develop radial cracks. The fracture toughness ranges from 3.6 to 10.3 MPa·m0.5, and microdefects significantly degrade this toughness. The elastic modulus shows a moderate positive correlation with hardness; meanwhile, fracture toughness correlates positively with the elastic modulus and weakly with hardness, reflecting the synergistic control exerted by dense crystalline domains and defects. Furthermore, the elastic modulus varies nonlinearly with indentation depth, and fracture toughness exhibits a negative power-law correlation with depth, confirming the coupling effect between depth dependence and heterogeneity. This study establishes quantitative correlations among micromechanical heterogeneity, mineral phases, and pores. It provides a mesomechanical basis for fracturing optimization and wellbore stability in ultra-deep carbonate reservoirs, thereby expanding the application of nanoindentation techniques.