Stress-controlled fracture closure and anisotropic flow in carbonate reservoirs: implications for CO₂ storage

Abstract: Natural fractures exert a first-order control on permeability, sealing capacity, and stress sensitivity of carbonate reservoirs, yet quantitative links between measured fracture-surface roughness and evolving transport properties remain poorly constrained across scales. We present a workflow that combines high-resolution optical surface profilometry with numerical closure and flow modelling to evaluate fracture hydraulic behaviour under effective stress, with implications for CO₂ storage efficiency and containment in fractured carbonates. A 4 × 4 cm fractured carbonate sample is split along the fracture plane to expose complementary surfaces, which are imaged using 3D optical microscopy in the metrology laboratory. Surface height grids are processed in Vision64 and exported for analysis. We computed roughness statistics and constructed aperture fields by digitally pairing the two surfaces, thereby enabling progressive mechanical closure to be simulated as either a prescribed displacement (closure) or field-stress-controlled loading.

Using closure-dependent aperture maps, we quantified transport anisotropy by solving pressure-driven flow through the fracture for orthogonal directions. Conductivity/permeability proxies are calculated using both cubic-law scaling and a spatially variable conductivity formulation (k ∝ b³) solved on the aperture grid. In parallel, capillary entry pressure is estimated from aperture distributions to evaluate stress-dependent sealing. Results show a nonlinear reduction in connected aperture with increasing closure, producing rapid declines in fracture conductivity and increases in capillary entry pressures as contact patches expand and percolating pathways collapse. Directional differences in flow and sealing metrics reveal pronounced anisotropy inherited from the surface topography, with dominant flow aligned with the most persistent connected channels.

Finally, stress-path sweeps (injection/depletion and shear ramp scenarios) demonstrate how effective normal stress and shear-related dilation can produce contrasting permeability–capillary responses, highlighting the potential for hysteresis and path dependence during CO₂ injection and pressure cycling. This integrated approach provides a quantitative bridge between laboratory-scale roughness measurements and stress-sensitive fracture transport, supporting improved parameterisation of fractured carbonate reservoirs in CO₂ storage models and risk assessment for leakage versus immobilisation.

Keywords: capillary entry pressure, fracture roughness, CO₂ storage integrity, anisotropy.