Influence of geomechanical loading configuration on modeling stress-dependent flow and transport in 3D fractured rocks

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Boundary conditions (BCs) in geomechanical modeling are typically prescribed as pure stress, pure displacement, or mixed formulations. However, their effects on flow and transport in three-dimensional fractured rock systems have not been systematically investigated, creating uncertainty in stress-dependent hydraulic response. This paper systematically examines the influence of different mechanical BCs on deformation, flow and transport in fracture networks. Four BC types are examined: (i) pure stress (BC Type 1), with all six boundaries stress-loaded; (ii) single roller (BC Type 2), with one roller-constrained boundary; (iii) triple roller (BC Type 3), with three roller-constrained boundaries; and (iv) buffer boundary (BC Type 4), where stress is applied through buffer plates of finite stiffness. Multiple realizations of discrete fracture networks are generated, each assuming a constant fracture size and subjected to varying stress ratios. Additionally, different fracture sizes are explored to assess size-dependent effects. Results show that for systems with small fractures, BCs have negligible influence on flow and transport. In contrast, for systems with large fractures, BCs strongly affect mechanical deformation, hydraulic behavior, and transport responses. The deformation decreases progressively as more roller constraints are introduced. These findings demonstrate that mechanical BC assumptions can significantly influence coupled stress-flow-transport processes in fractured rocks, particularly in networks with large fractures. The implications extend directly to engineered subsurface systems like CO 2 sequestration, enhanced geothermal systems, deep geologic repositories, and hydrocarbon recovery, where BC choices govern predictions of injectivity, storage integrity, resource extraction, and long-term hydraulic performance.

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