Suzanne J. T. Hangx

Thermal loads associated with Carbon Capture and Storage (CCS) operations in depleted hydrocarbon reservoirs are known to affect the performance of cement seals along CO2 injection wells, yet wellbore cement behaviour under realistic downhole temperature and pressure conditions remains poorly constrained. This study presents confined thermal cycling and porewater freeze–thaw experiments on API Class G HSR Portland cement to evaluate its volumetric response, mechanical integrity, and permeability evolution under conditions representative of CO2 injection operations. Imposing temperature variations from 80 °C to –20 °C at constant confining pressures of up to 20 MPa produced reversible contraction–expansion behaviour, superimposed by progressive permanent compaction, particularly at higher confinement. Under near drained conditions, apparent volumetric thermal expansion coefficients averaged (5±3)·10-5 m3 m-3 K-1 and reached up to 25·10-5 m3 m-3 K-1, markedly exceeding values commonly adopted in wellbore integrity models. Subsequent triaxial testing shows thermally cycled samples exhibit a 5–30% reduction in compressive strength and a transition from discrete shear failure to more pervasive microcracking, likely linked to freeze–thaw induced damage. Thermal cycling treatment had limited impact on cement matrix permeability, which remained within industry guidelines for effective zonal isolation. Our results show that downhole-representative thermal expansion behaviour must be incorporated into predictive modelling of CO₂ injection well integrity, as commonly assumed values for the cement thermal expansion coefficient derived from atmospheric laboratory testing lead to underestimation of thermally induced stresses and debonding potential.

Salt caverns are formed by solution mining and may be used for energy storage purposes after the production phase. During the operational lifetime, and in particular during abandonment, the spatial and temporal distribution of stress changes and deformation around caverns leads to convergence of salt around the caverns. In turn, this may lead to surface subsidence and potentially affect cavern integrity. Deformation around salt caverns is governed by different creep mechanisms, encompassing transient and steady-state creep stages. Steady-state creep is governed by a combination of dislocation and diffusion creep mechanisms. Deformation due to dislocation creep is dominant at relatively high stresses, whereas grain size-sensitive diffusion creep, particularly pressure solution, contribute significantly at low stresses. When modelling rock salt behaviour, these mechanisms need to be properly accounted for. Recently, studies have suggested that a threshold differential stress may exist below which pressure solution does not take place, which needs to be accounted for. In addition, dynamic recrystallisation may take place through grain boundary migration, driven by differences in energy stored in neighbouring grains due to dislocation creep strain. The process of grain boundary migration reduces the (work hardening) energy in the system as old grains are consumed by new ones, causing weakening. Furthermore, incorporation of transient creep is generally based on the description given in the Munson and Dawson model.In this study, we aim to simulate different cavern operation phases, such as leaching, production, and abandonment, to analyse the effect of transient creep, pressure solution creep and its threshold stress on the stress and deformation evolution around the cavern. The coupled effects of these complex creep characteristics on cavern behaviour have not yet been studied in detail. Such, more extensive coupling, are needed to better align laboratory- and field-based observations of salt mechanical behaviour, and apply it to large-scale numerical models. The commercial mechanical simulator FLAC (Fast Lagrangian analysis of continua) has been used to develop a 2D model for a single cavern system, which can be used to examine cavern convergence, subsidence and cavern integrity. An empirical model is used to define the threshold strain limit for dynamic recrystallisation by grain boundary migration, analogous to the Munson-Dawson strain limits for transient creep.The results show a significant effect of pressure solution creep on stress and deformation behaviour around the cavern. In the production phase, the transient creep does not show any significant effect on cavern behaviour; however, it could be important under varying loading conditions. The extent and magnitude of convergence and subsidence are dependent on the rate of pressure solution creep and its threshold stress. A preliminary analysis of the onset of the dynamic recrystallisation around the cavern suggests that DRX may be active in the lower regions of the cavern.

Understanding how granular salt compacts is essential for estimating the timescales required to seal backfilled galleries and shafts in radioactive waste repositories hosted in rock salt. It is well established that the presence of brine, even as thin liquid films on grain surfaces, strongly accelerates compaction by enabling fluid-assisted grain boundary diffusion, or pressure solution. For this reason, the addition of liquid brine to salt backfill in radioactive waste repositories hosted in rock salt has been considered as a means to accelerate compaction and reduce sealing timescales. However, the presence of liquid brine also enhances corrosion of the waste canister and promotes gas generation, which has therefore been used as an argument against the intentional addition of brine.Previous studies on salt-moisture interaction show that adsorbed fluid films retain liquid-like properties down to relative humidities of at least 40%, suggesting that pressure solution can operate well below full saturation. However, compaction behavior at relative humidities below the deliquescence point of salt (about 75% RH) remains poorly constrained. We present compaction experiments on fine-grained sodium chloride conducted under controlled relative humidities between 53% and 73%. Under these conditions, pressure solution is the dominant compaction mechanism, although compaction rates are 2-4 orders of magnitude lower than when pore spaces are fully saturated with brine. Nevertheless, our results demonstrate that humid air alone significantly accelerates the compaction of granular salt compared to dry conditions, with pressure-solution creep rates increasing systematically with relative humidity. These findings suggest that, in repository backfill, moisture from the surrounding host rock or from ventilation systems may be sufficient to induce pressure-solution-controlled compaction, even in the absence of intentionally added liquid brine or brine inflow from the host rock. Interaction between granular salt backfill and humid air therefore plays a key role in backfill evolution, with important implications for compaction rates, sealing timescales and the long-term containment of radioactive waste.

Salt caverns are formed in the subsurface during solution mining of salt. After the end of salt production, caverns need to be safely abandoned or may be repurposed for storage of energy carriers such as hydrogen. Salt caverns locally disturb subsurface stresses, leading to creep of the surrounding rock salt. Creep can cause cavern convergence at depth and may result in surface subsidence, with consequences for infrastructure and public safety. Accurate forecasting of cavern stability during abandonment or assessment of suitability for storage requires a deep understanding of the grain scale deformation mechanisms and processes controlling rock salt strength and creep rate. For rock salt, important deformation mechanisms are dislocation creep and pressure solution creep. Laboratory experiments have shown that dynamic recrystallization (DRX) associated with dislocation creep can be activated and contribute to mechanical weakening. However, the weakening effect of DRX is not included in engineering constitutive laws used in salt cavern numerical models. These laws are commonly based on low-strain laboratory experiments, where the influence of DRX is limited, and microstructural data are relatively rarely reported. The aim of this study is to experimentally investigate the dominant DRX process in deforming natural rock salt and its effect on the mechanical behaviour.Lab experiments have been carried out on natural wet salt samples from the Zechstein formation. We conducted constant strain rate experiments using a triaxial compression apparatus. Experiments were performed at a confining pressure of 20 MPa and a temperature of 125 °C, using constant displacement rates corresponding to strain rates of approximately 5 × 10⁻⁵ s⁻¹ and 5 × 10⁻⁷ s⁻¹, up to 30–40% axial strain. After the experiment, all samples were studied using optical microscopy. Electron backscatter diffraction (EBSD) analysis was performed on the starting material and on two deformed samples, one from each strain-rate condition.For all samples, we observed an initial transient creep stage followed by a quasi-steady state stage. The transition to quasi-steady occurred at a strain of about 10% for samples deformed at a strain rate of ~5 × 10-7 s-1. For samples deformed at the faster strain rate of ~5 × 10-5 s-1, continuous hardening occurred up to axial strains of 30%, with a gradually decreasing hardening rate approaching steady state. Light optical and EBSD microstructural analysis revealed grains with a dense substructure including subgrain walls, euhedral shape grains with low to no substructure, and grains with irregular shaped grain boundaries including bulges. We infer that the dominant deformation mechanism in the tested natural samples was dislocation creep, providing sufficient local differences in dislocation density to activate DRX dominated by grain boundary migration processes. DRX led to rheological weakening and quasi-steady deformation. We are working on robust understanding of the parameters controlling DRX as this is essential to evaluate the zones prone to weakening by DRX around salt caverns.

Fluid extraction from sandstone reservoirs leads to reservoir compaction, potentially inducing surface subsidence and seismicity, as observed in the Groningen Gas Field, Netherlands. Such compaction is partly elastic, but can additionally be caused by instantaneous plastic and rate/time‐dependent processes, such as subcritical crack growth, meaning that compaction may continue even if production is stopped. Despite the need to evaluate the impact of post‐abandonment reservoir behavior (10–100 years), few mechanism‐based, rate/time‐dependent compaction laws exist. Compaction due to grain breakage, either via critical or subcritical crack growth, is driven by tensile stresses acting on surface and volume flaws. We performed high‐resolution 3D linear elastic finite element method simulations on simplified grain assemblies to investigate the effect of stress–strain boundary conditions, porosity and mineralogical variations on grain‐scale stress fields. Our simulations showed tensile stress concentrations at grain contact edges and on pore walls, which increased in magnitude with increasing aggregate porosity and local porosity variation. The fraction of surface area with tensile stresses sufficient to extend flaws with a size up to showed a clear correlation with compactive yield envelopes for the Groningen reservoir sandstone. This suggests that compactive failure is related to the probability of pre‐existing surface flaws, falling in a pore surface region where the Griffith criterion is satisfied. A preliminary, time‐independent failure probability model, using the observed tensile stress distribution, qualitatively predicts a non‐linear increase in grain cracking during deviatoric loading, and suggests a new route to predict sandstone compaction through brittle grain failure.

Though the energy transition aims to phase out fossil fuels while continuing to exploit the subsurface for other storage solutions (e.g. geological CO2 storage, temporary hydrogen storage), natural gas, as a low-carbon energy carrier, will continue to play a role in our energy mix for the foreseeable future. In general, human activities in the subsurface change the physical and chemical environment, which in turn can lead to surface subsidence and induced seismicity. These phenomena may continue even after activities have stopped, as observed for natural gas extraction from the giant Groningen Gas Field in the Netherlands. They are largely caused by deformation in the reservoir rock, driven by fluid pressure changes. However, in-situ strain measurements from the Groningen Gas Field demonstrate that the clay-rich over- and underburden formations of the reservoir are also affected by these fluid pressure changes, displaying slow compaction. To make accurate predictions of reservoir deformation and to allow reliable assessment of the associated surface subsidence and induced seismicity, a detailed understanding of the deformation processes controlling deformation in these clay-rich formations is needed. Understanding which processes caused deformation in past (hydrocarbon) operations will help in understanding what may happen now that we plan to store other fluids in the subsurface.We performed rock mechanical experiments at in-situ conditions on the Opalinus Claystone (Switzerland), an analogue to the Groningen over- and underburden claystones, to assess the grain-scale mechanisms responsible for deformation. We designed an innovative and comprehensive multi-step experimental procedure that provides new, coherent data on the time-dependent deformation of clay-rich rocks. The experiments were performed in a triaxial compression apparatus, applying systematic steps of constant stress while controlling the pore fluid pressure in the sample. These steps were either stepped up or down in differential stress during an experiment. At each differential stress we systematically analyzed the instantaneous and time-dependent deformation.We observed general compaction of the samples upon increasing stress, and time-dependent expansion of the sample when stepping down in stress. Our results demonstrate that deformation in clay-rich rocks is strongly affected by the fluid-transport properties of the rock. We infer that sorption of fluids to the clay-rich matrix plays an important role in the deformation of clay-rich rocks, along with frictional slip controlling grain rearrangement. However, matters are complicated by slow diffusion of pore fluid pressure, which leads to an additional time-dependent component. Overall, our results demonstrate that over half of the observed deformation is permanent, even at low differential stresses. A detailed understanding of the time-dependent deformation of clay-rich rocks is crucial for accurate predictions of the impact of human activities in the subsurface, as sorption of fluid to the clay material may also be important during CO2 and hydrogen storage.

Rocksalt formations are critical candidates for storing natural gas, hydrogen, compressed air energy, and radioactive waste. While pure, undisturbed rock salt deposits exhibit low porosity and impermeability when buried deeply, excavation leads to near-field microcracking and dilatancy in the salt, increasing porosity and permeability. Over time, the connectivity of brine- or water-vapor-filled microcrack networks in deformation-damaged salt is expected to decrease, partly due to dissolution-precipitation healing. In this study, we employ 4D (time-resolved 3D) microtomography to investigate the long-term evolution of dilated grain boundary and microcrack networks developed in deformation-damaged natural salt through brine-assisted processes. Our findings reveal substantial microstructural modification and healing occurring over periods ranging from days to a few months. Cracks and dilated grain boundaries undergo crystallographic faceting, necking, and migration, effectively "recrystallizing" the material and resulting in increased tortuosity and decreased connectivity of the crack network. Understanding the complex interplay between microcracking, healing, and permeability changes in deformation-damaged rock salt is of utmost importance for optimizing storage and disposal applications in geomechanics and physical chemistry. Our research contributes valuable insights to this field and informs the sustainable development and management of rock salt formations for diverse energy storage and waste management needs.

In the upper crust, rock deformation commonly occurs in the presence of aqueous fluids, which are known to alter the material's response to stress. These fluids facilitate dissolution and precipitation, driving changes in the mineral composition and texture. The interaction between dissolution-precipitation and deformation in the upper crust has been studied in various natural geological cases, through experimentation, and modeling. Recently, there has been increased emphasis on understanding the role of fluids in deformation at the nanoscale. This experimental study aims to comprehend the effects of deformation on dissolution and precipitation at the nanoscale, specifically at the boundary between individual mineral grains.Our research has focused on understanding quartz dissolution and precipitation at the grain boundary scale at hydrostatic pressures and temperature conditions representative of the upper 5 km of the crust. We report preliminary findings from our experiments on nano-milled quartz crystals, representing natural grain boundary geometry, in contact with different aqueous solutions in a closed system. The pH, salinity, and concentration of Si in solution were systematically altered to assess their impact on dissolution rates. By using 18O-doped solutions, coupled with nanoscale secondary ion mass spectrometry and atomic force microscopy, we can track the dissolved and re-precipitated material, and monitor the changes in the geometry of the nano-milled quartz surface. This constitutes the initial step in our effort to further explore and quantify the effects of differential stress at the grain boundary. Additionally, we present results from a pilot study designed to test how differential normal stress impacts asperity dissolution within quartz-quartz contacts. These experiments aim to improve our understanding of dissolution rates in relation to pressure solution, a significant deformation mechanism in sedimentary and fault rocks.This project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie initial training network (FluidNET) grant agreement no. 956125.