Swelling-induced fracture closure in Opalinus Clay: An inert particle tracking approach

This study examines permeability reduction in fractured Opalinus Clay material due to swelling-induced self-sealing mechanisms under controlled triaxial state conditions. An artificial continuous fracture, oriented parallel to natural fractures, was introduced to simulate an extreme scenario. Inert particle tracking was used to quantify material displacement during swelling, allowing derivation of swelling factors, porosity, and density changes. These parameters were applied to calculate spatially resolved permeability, including a swelling-induced micro-fractured damage zone modeled using a fractal scaling approach. Micro-fracture apertures ranged from pore size at the swollen region boundary up to ∼300 nm at the fracture surface. Application to 3D X-ray Computed Tomography derived fracture geometries predicted permeability reduction from ∼1 × 10−11 m2 (open) to ∼5 × 10−17 m2 (swollen), consistent with experimental results. Achieving this reduction required a swollen volume approximately seven times greater than the initial fracture volume. With continued swelling, micro-fracture apertures may shrink to ∼80 nm, further reducing bulk permeability to ∼1 × 10−18 m2, only slightly higher than that of intact material.