Proppant packs are essential for maintaining fracture conductivity in enhanced geothermal systems (EGS). However, under the EGS high closure stresses and elevated temperatures, the long-term hydraulic performance is strongly affected by particle crushing, resulting in permeability reduction. This study presents a three-dimensional discrete element modeling investigation of proppant pack settlement, particle breakage, and permeability evolution under stress conditions representative of the Utah FORGE geothermal site. Simulations are performed using Particle Flow Code in three dimensions (PFC3D), in which particle fracture is captured through a Fragment Replacement Method (FRM). Breakage is triggered when a stress derived from the maximum contact force exceeds a size-dependent characteristic strength, calibrated using single-particle diametral compression test data. Loading is applied incrementally under oedometric conditions, with breakage events followed by mechanical re-equilibration to mitigate numerical artifacts. Pack-scale void ratio evolution is quantified from volumetric changes associated with particle rearrangement and fragmentation, while permeability evolution is estimated using the Kozeny–Carman relationship. Results show that allowing particle breakage leads to a more compliant macroscopic response, progressive broadening of the particle size distribution, and a sustained reduction in permeability driven by the generation of fines and reorganization of the contact network. Breakage is found to be distributed throughout the pack and governed primarily by internal force-chain evolution rather than boundary effects. These findings highlight the mechanical response to proppant crushing and provide a quantitative framework for evaluating trade-offs between particle size and strength when selecting proppants to maintain fracture conductivity in EGS and unconventional energy reservoirs.

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