Evolution of rock salt fracture geometry and mechanical properties during fluid–rock interactions

Abstract

Rock salt caverns offer a promising subsurface engineering solution for CO₂ storage, waste disposal, compressed air energy storage, and hydrogen storage, owing to their substantial extent, extremely low permeability, and minimal porosity. The long-term containment of injected gases, fluids or disposed materials relies on the integrity of cavern seals. However, fractures in the damaged zone formed during the wellbore drilling phase, storage operations, or the abandonment phase can compromise system integrity by acting as preferential flow paths and mechanically weak zones. This poses a challenge to the safety of subsurface storage throughout the operational lifespan of a salt cavern. Understanding the evolution of these fractures in terms of their physical and mechanical properties is therefore essential. In this thesis, we adopt an integrated experimental numerical approach to investigate both the dissolution behaviour and mechanical recovery of rock salt fractures.

During the leaching period, the walls of a rock salt cavern are exposed to undersaturated brine. Fractures in contact with undersaturated brine are expected to undergo dissolution, with the dissolved ions being transported and potentially precipitating at a further location. This process alters the geometry of the fractures over time. We develop a flow visualization experimental setup at the decimetre scale, combined with quantitative measurement techniques, to investigate how initial fracture geometry, salt concentration, and flow rate influence the evolution of rock salt fracture channels. The results show that dissolution within fractures is heterogeneous, with increased dissolution near the inlet and along fracture corners due to advection-dominated flow, leading to progressive smoothing of fracture surfaces. Fractures with larger initial surface areas exhibit higher dissolution rates and greater total mass loss compared to those with smaller surface areas, Moreover, the dissolution patterns observed in this study differ from those typically reported at the microscale. To further interpret the experimental observations, a coupled reactive transport model is developed using COMSOL Multiphysics to simulate fluid velocity distribution and solute concentration.

Following the leaching process, salt caverns are employed for energy storage and extraction, during which the surrounding rock salt, particularly near the upper and lower cavern walls, is subjected to alternating dry and wet conditions. Given rock salt’s inherent healing capabilities (i.e., mechanical recovery), we investigate the shear strength recovery of rock salt fractures through direct shear box experiments. The evolution of fracture surface geometry is further examined using digital microscopy and microcomputed tomography (micro-CT). Results reveal a log-linear relationship between healing and time, with wet conditions exhibiting a recovery rate approximately seven times greater than that under dry conditions, consistent with time-dependent frictional healing laws observed in gouge. Healing in dry samples can be attributed to plastic flow. Additionally, dynamic behaviour is characterised by an increase in the residual friction coefficient over time. Healing in wet samples can be attributed to pressure solution, recrystallisation, and possibly surface energy minimisation processes. A difference in stick-slip behaviour in terms of frequency and stress drop is observed between dry and wet samples. This difference may be due to the applicability of a distance-dependent model of stick-slip behaviour in dry conditions and a velocity-dependent model in wet conditions.

In addition, the eject of initial fracture surface roughness on healing behaviour is examined by preparing samples with smooth and rough surfaces. Under dry conditions, strength recovery is limited and observed only in smooth fractures. In wet conditions, smooth fractures exhibit substantial healing, whereas rough fractures show only minor improvement. These findings demonstrate a clear inhibitory eject of surface roughness on strength recovery in fully saturated environments. This eject is attributed to the larger aperture in rough surface samples, which hinders crystal growth across the fracture and impedes bridging between opposing surfaces.

Finally, an existing elasto-plastic constitutive law is shown to also be capable to simulate damage and healing behaviour of rock salt fractures, based on a cohesive zone model framework, enabling the prediction of strength recovery over time. Specifically, fracture initiation is governed by a hyperbolic yield surface in the Qn-Ts stress plane. The shrinkage (damage stage) and expansion (healing stage) of the yield surface are governed by two history-dependent variables: an internal variable (Wcr) represents the dissipated fracturing (plastic) work, as established in existing models. The expansion (healing stage), in contrast, is governed by an external variable (Ht), which we propose here to represent the progressive recovery of the mechanical properties of rock salt fractures over time. The key features of the existing formulation are demonstrated through single-element tests under various sequences of mechanical loading. Simulations with and without healing show good agreement with laboratory results for both non-healed and healed samples, confirming the model’s ability to capture the essential characteristics of fracture healing in rock salt. Additional simulations over healing periods from years to decades are performed to assess long term recovery behaviour.

This thesis examines the evolution of the physical properties of rock salt fractures during dissolution, the mechanical recovery of fractures, and the associated microstructural changes of fracture surfaces, using both experimental and numerical approaches. The findings provide insights into the design and performance of salt caverns for energy storage and radioactive waste disposal.