Analysis and Comparison of Natural Shear and Induced Tensile Fractures for Caprock Leakage Assessment

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Abstract
Fluid flow in low-permeability caprocks is largely controlled by fractures. However, these fractures are often oversimplified as planar structures. Furthermore, existing studies provide limited insights into the comparisons between naturally occurring shear fractures and artificially induced tensile fractures. As such, it is yet to be quantified how flow models induced fractures adequately capture flow in natural subsurface fractures, which are difficult to obtain. Here, we examine three natural shear fractures and two induced tensile fractures from the same low-permeability lithology. Using high-resolution synchrotron X-ray computed micro-tomography, we extracted three-dimensional fracture void geometries to analyse aperture distributions, surface roughness, and spatial correlation patterns. We then compared measured fracture transmissivities against theoretical predictions from parallel-plate models (cubic law) and direct numerical simulations (DNS) of flow to evaluate consistency and uncertainties. The results reveal that despite important differences in heterogeneity, induced and natural fractures can generally yield similar flow in caprocks. Our comparisons further indicate that the choice of flow estimation method can introduce more uncertainty than the fracture opening mode. These findings highlight the importance of anchoring modelling approaches when predicting permeability in fractured caprock. They also demonstrate the usability of laboratory-induced tensile fractures to study subsurface flow.