Calibrating Micro-Mechanical Parameters for 3D Parallel Bond Modeling of Ultra-Deep Carbonate Rocks: Effects of Confinement on Crack Propagation

Investigating rock mechanical properties at great depths is crucial for ultra-deep petroleum exploration, yet laboratory experiments under true in-situ conditions are challenging due to limited core availability and experimental constraints. The discrete element method (DEM) with the parallel bond model (PBM) offers an economic and repeatable alternative, but the selection of PBM microparameters strongly controls the macroscopic response of numerical specimens. To reduce trial-and-error efforts under complex micro-macro coupling, we propose an automatic calibration method that combines an adaptive proportional feedback control (APFC) with an inner–outer dual-loop (IODL) consistency strategy. The method is applied to Longwangmiao Formation limestone (LWm F.m.) using conventional triaxial compression and Brazilian disc tests, and the fracture mechanisms are interpreted with scanning electron microscopy (SEM). Results show that: as confining pressure increases from 10 to 120 MPa, the triaxial compressive strength increases from 192 to 678 MPa and crack propagation evolves from X-type shear macro-fractures to distributed fragmentation. Moreover, after peak stress, local tensile cracks develop on shear fracture surfaces and are nearly perpendicular to the loading direction, which is attributed to local stress concentration. The proposed approach provides an efficient tool for DEM microparameter calibration and clarifies confinement-controlled crack mechanisms relevant to wellbore stability and stimulation design in ultra-deep carbonate reservoirs. • Develops an automatic micro-parameter calibration framework for three-dimensional (3D) parallel bond modeling in the discrete element method (DEM). • Calibrates model microparameters against triaxial and Brazilian tests of ultra-deep Longwangmiao Formation limestone and validates crack patterns with scanning electron microscopy (SEM). • Reveals that increasing confining pressure (10–120 MPa) shifts crack propagation from X-type shear localization to distributed fragmentation. • Explains post-peak local tensile cracking on shear fractures using DEM crack-type statistics and SEM evidence.