The large-scale gas injection test (Lasgit) was a full-scale demonstration experiment based on the Swedish KBS-3 repository concept, conducted at 420 m depth at the & Auml;sp & ouml; Hard Rock Laboratory. Six gas injection tests were conducted: four in a canister filter towards the bottom of the deposition hole (GT1/2/4/6) and two in a filter towards the top of the canister (GT3/5). The main conclusions were: (1) The movement of gas occurred at a pressure close to the local total stress; (2) Peak gas pressure was linked to the hydraulic permeability of the buffer and the ease at which gas could exit the deposition hole. Therefore, the maturity (hydraulic conductivity) of the buffer was a secondary control on gas entry and movement; (3) Gas was transported through a limited number of dilatant pathways. The pathways were small in relation to the total volume of the buffer, and temporally variable; (4) Over the timescale of the project, pathways sealed. Repeat gas injection tests showed partial re-activation of pathways formed in previous tests, which may have exploited the same weakness in the system on repeat testing; (5) (Gas movement did not weaken the KBS-3 engineered barrier system, with gas release being controlled.

Gas generation is an inevitable consequence of radioactive waste storage and disposal. The rate at which gas diffuses through host rocks and overlying strata is an important consideration in safety assessment. To examine the impact of material variability, British Geological Survey developed a methodology to manufacture 'synthetic rock' samples from mixtures of clay, sand, and silt. Diffusion experiments were conducted on these, and natural samples of Boom Clay and Eigenbilzen Sands. Samples were tested under an isotropic stress equivalent to 400 m burial, assessing anisotropy by measuring intrinsic permeability and diffusion normal and perpendicular to bedding. Boom Clay exhibited permeability and gas diffusion anisotropy ratios of 4.5 and 1.5, respectively. A semi-log relationship between permeability and diffusivity was observed. No significant correlation to mineralogy changes was found, indicating fabric and pore morphology are more important. This was supported by a correlation between porosity and diffusivity. X-ray computed tomography analysis suggested diffusivity was inversely proportional to bioturbation features and that stress promoted micro-crack closure which may also help to explain previous data dispersion. The study confirmed the usefulness of synthetic samples in exploring complex processes, generated data relevant to natural clay systems and showed diffusivity was not highly sensitive to large changes in permeability.

The final experiment (full canister test, FCT) of the large-scale gas injection test (LASGIT) aimed to explore the impact of gas volume on gas transport behaviour. Unlike previous tests, the FCT involved pressurising a full-scale KBS-3 canister up to ∼7100 kPa when pressure was held, allowing excess water to drain and to establish gas entry. Once drained, pressure gradually decreased by 220 kPa, indicating gas moving into the fully saturated clay buffer. Observations showed different pore pressure and total stress behaviour compared with earlier tests, but no major changes were seen in the buffer’s response, suggesting gas migration was unaffected by gas volume. The slow pressure decay shows gas travelled through a limited number of narrow pathways, which were not formed through tensile fracturing. A gas leak early in the FCT led to depressurisation and later pressurisation of the canister, causing a 50 μm expansion in its radius. This mechanical loading on the buffer, greater than the bentonite’s drainage capacity, caused pore pressure and radial stress changes. The expansion of the canister by pressurisation or thermal effects should therefore be considered in performance assessments.

Fluid-rock reactions are of great interest in many engineered geological storage and disposal systems where the long term integrity of the system is key, and where fluid seepage through a permeable rock may lead to reaction and convective transport of material through the formation. If an unsaturated fluid displaces formation fluid in equilibrium with a reactive porous medium, a reaction front develops, across which the invading fluid becomes saturated with the soluble matrix material. Depending on the composition of the invading fluid, it may initially be less dense than the formation fluid, but following reaction it may become denser than the formation fluid. If the invading fluid displaces the formation fluid downwards through the porous layer, the reaction front may then be stabilized by buoyancy, but a Rayleigh-Taylor type instability can develop at the interface between the reacted fluid and the original formation fluid ahead of the reaction front. We present a series of new analog experiments of this process by injecting aqueous sugar solutions into a porous layer containing saturated salt solution, salt powder, and glass ballotini. As an analog of a reaction front, a dissolution front develops as the aqueous sugar solution dissolves the salt powder and becomes denser than the saturated salt solution. The buoyancy instability then leads to a growing finger interface. If the buoyancy speed of the fluid, uB, is smaller than uF−uR, where the speed of the advancing fluid-fluid front is uF and the speed of the dissolution front is uR then the instability grows as if the system were unconfined. However, if uB>uF−uR, then the supply of fluid at the dissolution front limits the growth of the instability. We present an idealised model for the speed of the non-linear buoyancy-driven fingers, and we consider the implications of our results for the long term integrity of a number of geological storage systems. Published by the American Physical Society 2025

Hazardous radioactive waste must be removed from the biosphere, and geological storage is the universally favoured option for accomplishing this. In many cases, the repository designs include a clay buffer as part of the engineered barrier system (EBS) that surrounds the individual waste canisters and seals the disposal galleries. The emplacement of the EBS around the waste will generate small void spaces that must be closed to ensure that high permeability pathways do not develop. Many of the groundwaters at proposed disposal sites are saline, and this porewater chemistry, combined with the presence of technological voids may present a technical challenge to repository designers. In this study, a suite of experiments was conducted to examine the impact of fluid salinity in combination with the presence of a void space on the swelling behaviour of barrier bentonites. Both sodium and calcium bentonites were studied and the sample lengths were varied to provide an understanding of the role of axial strain on the homogenisation and swelling pressure development in the bentonite. After 100 days of testing the clay had swelled into the void space and differential swelling pressures (difference between the maximum and minimum swelling pressure recorded at a given time) had reduced substantially, highlighting the ability of the bentonite to expand into a void. However, clay that swelled into the void never generated significant swelling pressures during the testing period and differential pressures were still significant at the end of the tests. The calcium bentonite demonstrated much smaller peak swelling pressures than the sodium bentonite under the same salinity conditions, and in all tests, the generation of significant swelling pressure in the clay in the low-density end of the sample did not occur during the testing period. These findings suggest that the suppression of clay swelling at higher salinities is likely to strongly impact the void-filling process.

Understanding gas percolation through a clay layer or a shale formation is of great importance for the development of a geologic repository for nuclear waste disposal, a subsurface system for gas storage, and an engineering approach for hydrocarbon extraction from unconventional reservoirs. Gas injection experiments have revealed complex dynamic behaviours of gas percolation through water saturated compacted bentonite, characterized by a high breakthrough pressure, rapid breakthrough, a pressure/stress decay after the breakthrough, a relatively high migration rate, high-frequency periodic/nonperiodic variations in flow rate, stepwise rate reductions during relaxation, and low gas saturation over the whole process, all indicating channelling nature of the processes. Using linear stability analyses, we show that this channelling can autonomously emerge from the instability of the deformable interface between the injected gas and the compacted bentonite matrix driven by local stress concentration, pore dilation, and hydrologic gradient. Channel patterns formed would possess a fractal geometry. We further show that, once a percolating channel is established, the gas injected would percolate through the channel in a chain of gas bubbles, also due to the interface instability, resulting in periodic/chaotic variations in gas flow rate. Our work provides a unified explanation for key features observed for gas percolation in low-permeability deformable media. The work also suggests a possibility of designing an engineered barrier system for a nuclear waste repository that can have controllable gas release while limit water transport.

Understanding the impact of repository gas, generated from degradation of waste and its interaction with the host rock, is essential when assessing the performance and safety function of long-term disposal systems for radioactive waste. Numerical models based on conventional multi-phase flow theory have historically been applied to predict the outcome and impact of gas flow on different repository components. However, they remain unable to describe the full complexity of the physical processes observed in water-saturated experiments (e.g., creation of dilatant pathways) and thus, the development of novel representations for their description is required when assessing fully saturated clay-based systems. This was the primary focus of Task A within the international cooperative project DECOVALEX-2019 (D-2019) and refinement of these approaches is the primary focus of this study (Task B in the current phase of DECOVALEX-2023). This paper summarises development of enhanced numerical representations of key processes and compares the performance of each model against high-quality laboratory test data. Experimental data reveals that gas percolation in water-saturated compacted bentonite is characterised by four key features: (i) a quiescence phase, followed by (ii) the gas breakthrough, which leads to a (iii) peak value, which is then followed by (iv) a negative decay. Three models based on the multiphase flow theory have been developed. These models can provide good initial values and reasonable responses for gas breakthrough (although some of them still predict a too-smooth response). Peak gas pressure values are in general reasonably well captured, although maximum radial stress differences are observed at 48 mm from the base of the sample. Here, numerical peak values of 12.8 MPa are predicted, whereas experimental values are about 11 MPa. These models are also capable of providing a reasonable representation of the negative pressure decay following peak pressure. However, other key specific features (such as the timing of gas breakthrough) still require a better representation. The model simulations and their comparison with experimental data show that these models need to be further improved with respect to model parameter calibration, the numerical representation of spatial heterogeneities in material properties and flow localisation, and the upscaling of the related physical processes and parameters. To further understand gas flow localisation, a new conceptual model has been developed, which shows that discrete channels can possibly be induced through the instability of gas-bentonite interface during gas injection, thus providing a new perspective for modeling gas percolation in low-permeability deformable media.

Geological disposal of radioactive waste is being planned by many countries. Bentonite clay is often included in facility design, providing a barrier to radionuclide migration. Gas, generated by the waste or corrosion of waste canisters, may disrupt the properties of the bentonite. Robust prediction of this interaction is, therefore, necessary to demonstrate safe facility evolution. In some cases, gas may deform the clay, resulting in localised flow; however, the nature of this deformation has been widely debated. Accurate numerical representation of this behaviour has been limited by a shortage of information on the degree/distribution of deformation. Using experimental data from gas injection tests in bentonite, we show that first order fluctuations in the stress field can provide this information. We show that hundreds of microdeformation events can be detected, with similar characteristics to established fracturing phenomena, including earthquakes and acoustic emissions. We also demonstrate that stress field disruption (i) is spatially localised and (ii) has characteristics consistent with gas pathway ‘opening’ and ‘closure’ as gas enters and exits the clay, respectively. This new methodology offers fundamental insight and a new opportunity to parameterise and constrain gas advection models in clays and shales, substantially improving our capacity for safe facility design.

Understanding the gas movement phenomenon within the deep geological repository is essential for assessing the disposal system's long-term stability. The primary gas transport mechanism through the bentonite is dilatancy-controlled flow, which differs from gas flow in general porous media. This flow is characterized by gas movement through microcracks created under relatively high gas pressure conditions, and the intrinsic permeability, air-entry pressure, and mechanical strength of the medium change due to the generation and propagation of these microcracks. Therefore, dilatancy-controlled flow cannot be simulated using the classical two-phase flow modeling technique. This study constructed the H2MD (two-phase hydraulic-mechanical- damage) numerical model by combining a damage model to simulate material degradation and the resulting change in intrinsic permeability with a classical two-phase flow model. In addition, the numerical model was tested against a 1D laboratory gas injection test investing gas flow mechanisms in the buffer, and a sensitivity analysis was performed on tensile strength, a key factor in the damage model for gas movement phenomenon. In the validation study, the proposed model successfully simulated the key features observed in the test: rapid stress and pressure increase trends, changes in intrinsic permeability due to damage, and the resulting flow rate. In addition, the effect of heterogeneity on the strength characteristics of each material and interfaces between materials was analyzed through field-scale test simulations, and the applicability of the model to upscaling analysis was examined. The study of heterogeneity effects confirmed that incorporating the strength characteristics of interfaces accurately simulates the gas flow path observed in actual tests. However, the model overestimated the gas flow before the gas breakthrough and underestimated the evolution of the damaged area within the buffer. Therefore, additional research on relative permeability and mechanical constitutive models is needed to improve the reliability of the current model.

Abstract Nuclear energy currently contributes about 10% of the world’s electricity supply but, at present, there are only interim storage facilities for the accumulated 2.6 x 108 kg of spent nuclear fuel. Geological storage is the universally favoured option for permanent removal of this highly radioactive waste from the biosphere with the first purpose-designed and constructed permanent disposal site, the Onkalo facility in Finland, expected to become operational in the next few years. Envisioned and planned geological disposal facilities for high-level radioactive waste will commonly include a bentonite clay buffer called the Engineered Barrier System (EBS); this flow barrier both surrounds the waste cannisters and backfills the disposal galleries. However, questions are being raised about the long-term performance of bentonite for waste containment due to specific issues associated with inhomogeneous fluid-rock interaction occurring within the material. We apply a novel combination of μ-MRI methods and 2H labels to monitor flow in barrier materials in unprecedented detail and resolution to reveal micro-scale fluid-flow variations caused by nano- and microheterogeneities not possible via other techniques. We demonstrate the fluid penetration front becomes fragmented, and that fragmentation is governed by heterogeneities in the clay’s pore structure; both must be considered in host rock characterisation and long-term EBS design, manufacture, and performance.

Renewable energy provides a low-carbon alternative to power generation in the UK. However, the resultant supply varies on daily, weekly and seasonal cycles, such that for green energies to be fully exploited new grid-scale energy storage systems must be implemented. Two pilot facilities in Germany and the United States have demonstrated the potential of the Earth as a battery to store compressed air, using off-peak surplus energy. Natural accumulations of salt (halite deposits) in the UK represent a large and untapped natural storage reservoir for compressed air with the ability to provide instantaneous green energy to meet peak demand. To realise the potential of this emerging technology, a detailed knowledge of the relationship between mechanics, chemistry and geological properties is required to optimise cavern design, storage potential and economic feasibility. The variable stresses imposed on the rock matrix by gas storage, combined with the cyclic nature of cavern pressurisation are barriers to deployment that need to be addressed to enable large-scale adoption of schemes. Well-designed field experiments are a lynchpin for advancing research in this area, especially when supported by state-of-the-art characterisation and modelling techniques. The research facility at STFC’s Boulby Underground Laboratory presents the ideal location to tackle these fundamental issues to optimise “Battery Earth”.

Understanding gas migration in compacted clay materials, e.g., bentonite and claystone, is important for the design and performance assessment of an engineered barrier system of a radioactive waste repository system, as well as many practical applications. Existing field and laboratory data on gas migration processes in low-permeability clay materials demonstrate the complexity of flow and transport processes, including various types of instabilities, caused by nonlinear dynamics of coupled processes of liquid–gas exchange, dilation, fracturing, fracture healing, etc., which cannot be described by classical models of fluid dynamics in porous media. We here show that the complexity of gas migration processes can be explained using a phenomenological concept of nonlinear dynamics and deterministic chaos theory. To do so, we analyzed gas pressure and gas influx (i.e., input) and outflux (i.e., output), recorded during the gas injection experiment in the compact Mx80-D bentonite sample, and calculated a set of the diagnostic parameters of nonlinear dynamics and chaos, such a global embedding dimension, a correlation dimension, an information dimension, and a spectrum of Lyapunov exponents, as well as plotted 2D and 3D pseudo-phase-space strange attractors, based on the univariate influx and outflux time series data. These results indicate the presence of phenomena of low-dimensional deterministic chaotic behavior of gas migration in bentonite. In particular, during the onset of gas influx in the bentonite core, before the breakthrough, the development of gas flow pathways is characterized by the process of chaotic gas diffusion. After the breakthrough, with inlet-to-outlet movement of gas, the prevailing process is chaotic advection. During the final phase of the experiment, with no influx to the sample, the relaxation pattern of gas outflux is resumed back to a process of chaotic diffusion. The types of data analysis and a proposed phenomenological model can be used to establish the basic principles of experimental data-gathering, modeling predictions, and a research design.

In a repository for radioactive waste hosted in a clay formation, hydrogen and other gases may be generated due to the corrosion of metallic materials under anoxic conditions, the radioactive decay of waste and the radiolysis of water. If the gas production rate exceeds the gas diffusion rate within the pores of the clay, a discrete gas phase will form and accumulate until its pressure becomes large enough to exceed the entry pressure of the surrounding material, at which point dilatant, advective flow of gas is expected to occur. The purpose of Task An under DECOVALEX-2019 is to better represent the processes governing the advective movement of gas in both low-permeability argillaceous repository host rocks and clay-based engineered barriers within numerical codes. In this paper special attention is given to the mechanisms controlling gas entry, flow and pathway sealing and their impact on the performance of the engineered clay barrier. Previous work suggests gas flow is accompanied by the creation of dilatant pathways whose properties change temporally and spatially within the medium. Thus, four new types of approaches have been developed: (i) standard two-phase flow models (continuous techniques) incorporating a range of different mechanical deformation behaviours, (ii) enhanced two-phase flow models in which fractures are embedded within a plastic material (continuous techniques) or incorporated into the model using a rigid-body-spring network (discrete approaches), (iii) a single-phase model incorporating a creep damage function in which only gas flow is considered, and (iv) a conceptual approach used to examine the chaotic nature of gas flow. The outputs from these different approaches are compared. This is an essential step as the choice of modelling approach strongly impacts the representation and prediction of gas flow in a future repository. In addition, experience gained through this task is of direct relevance to other clay-based engineering issues where immiscible gas flow is a consideration including hydrocarbon migration, carbon capture and storage, shale gas and landfill design. This paper summarises the outcomes of work in Task A conducted between May 2016 and May 2019 and provides a brief overview of the experimental data and a synthesis of the work of the participating modelling teams.

The removal of potentially harmful radioactive waste from the anthroposphere will require disposal in geological repositories, the designs of which often favour the inclusion of a clay backfill or engineered barrier around the waste. Bentonite is often proposed as this engineered barrier and understanding its long-term performance and behaviour is vital in establishing the safety case for its usage. There are many different compositions of bentonite that exist and much research has focussed on the properties and behaviour of both sodium (Na) and calcium (Ca) bentonites. This study focusses on the results of a swelling test on Bulgarian Ca bentonite that showed an unusual gel formation at the expanding front, unobserved in previous tests of this type using the sodium bentonite MX80. The Bulgarian Ca bentonite was able to swell to completely fill an internal void space over the duration of the test, with a thin gel layer present on one end of the sample. The properties of the gel, along with the rest of the bulk sample, have been investigated using ESEM, EXDA and XRD analyses and the formation mechanism has been attributed to the migration of nanoparticulate smectite through a more silica-rich matrix of the bentonite substrate. The migration of smectite clay out of the bulk of the sample has important implications for bentonite erosion where this engineered barrier interacts with flowing groundwater in repository host rocks.

Bentonites are commonly proposed for use in the geological disposal of high heat generating radioactive wastes. Repository designs include bentonite as a buffer to occupy the void space around the waste canisters because of the favourable properties it can exhibit that enhance the isolation and containment functionality of the repository. Many repository concepts introduce small voids within the bentonite because the buffer is incorporated as individual bricks stacked around the waste canisters. These voids must be closed to prevent the persistence of high permeability pathways for fluids. As bentonite hydrates, it expands and can exert a considerable swelling pressure on the surrounding host rock. The bentonite also expands to fill the engineering cavities inherently present in the repository, but the non-uniform development of total stress and pore pressure could cause persistent material heterogeneities to occur. This is likely to be exacerbated by the thermal gradients existing between the hot waste and the temperature of the surrounding host rock in the early stages of repository postclosure. Whilst this is an area of ongoing research, the final extent of bentonite homogenisation within the repository and for how long property variations persist, is not well understood. In this study, four tests were conducted on pre-compacted, sodium-activated MX80 bentonite samples placed next to a water-filled engineering void, to examine the effect of elevated temperature on the development of swelling and swelling pressure as a function of sample size. The sample lengths were chosen to give small bentonite-to-void ratios, and to represent extremities of behaviour, such that an acceptable upper limit of void size might be established. The results demonstrated that even under extreme bentonite-to-void ratios, the bentonite was able to swell and completely fill the void space, exerting a small but measurable swelling pressure. Under the conditions of this study, the results have shown larger end-of-test swelling pressures and higher final dry densities along their entire length than shown in equivalent tests conducted at ambient temperature. In addition, the elevated temperature tests showed a rapid initial increase and then decrease in swelling pressure at the start of testing, approaching an asymptote in swelling pressure more quickly, whilst uptaking less water than in the ambient temperature case. This implied that heating the bentonite reduced the test duration by about 60%, which is most likely explained by a reduction in the viscosity of the test permeant at higher temperatures.

The DECOVALEX Project is an on-going international research collaboration, established in 1992, to advance the understanding and modeling of coupled Thermal (T), Hydrological (H), Mechanical (M) and Chemical (C) processes in geological in geological systems. DECOVALEX was initially motivated by the recognition that prediction of these coupled effects is an essential part of the performance and safety assessment of geologic disposal systems for radioactive waste and spent nuclear fuel. Later it was realized that these processes also play a critical role in other subsurface engineering activities, such as subsurface CO2 storage, enhanced geothermal systems, and unconventional oil and gas production through hydraulic fracturing. Research teams from many countries (e.g., Canada, China, Czech Republic, Finland, France, Germany, Japan, Republic of Korea, Spain, Sweden, Switzerland, Taiwan, United Kingdom, and the United States) various institutions have participated in the DECOVALEX Project over the years, providing a wide range of perspectives and solutions to these complex problems. These institutions represent radioactive waste management organizations, national research institutes, regulatory agencies, universities, as well as industry and consulting groups. The overall aim of DECOVALEX-2019 was to increase the understanding of various thermo-hydro-mechanical-chemical processes of importance for radionuclide release and transport from a repository to the biosphere and how they can be described and modelled using mathematical models. The scientific and technical objectives are: to increase the basic understanding of T-H-M-C coupled processes in fractured rocks (crystalline, sedimentary, argillaceous) and buffer materials; to investigate the predictive capabilities of different codes to field experiments and to perform verification of codes; to exchange experimental data, and improve the understanding of the constitutive behavior of crystalline and argillaceous rock masses and buffer materials; and to perform THMC calculations in a performance/safety assessment context.

The DECOVALEX Project is an on-going international research collaboration, established in 1992, to advance the understanding and modeling of coupled Thermal (T), Hydrological (H), Mechanical (M) and Chemical (C) processes in geological in geological systems. This document is the final report of Task A which was proposed by the DECOVALEX Project and coordinated by the British Geological Survey (BGS), presenting the technical definitions of the problems studied, approaches applied, achievements made and outstanding issues for future research. The purpose of Task A under DECOVALEX-2019 is to better understand the processes governing the advective movement of gas in both low-permeability argillaceous repository host rocks and clay-based engineered barriers. Special attention is given to the mechanisms controlling gas entry, flow and pathway sealing and their impact on the performance of the engineered clay barrier. Previous work suggests gas flow may be accompanied by the creation of dilatant pathways whose properties change temporally and spatially within the medium. Thus, new numerical representations for the quantitative prediction of gas migration fluxes through argillaceous rock formations have been developed. These provide an invaluable tool with which to assess the impact of gas flow on repository layout and therefore design of any future facility. In addition, experience gained through this task is of direct relevance to other clay-based engineering issues where immiscible gas flow is a consideration including shale gas, hydrocarbon migration, carbon capture and storage and landfill design.

In a geological repository, the disposal of radioactive waste will result in the creation of engineering voids. Bentonite is commonly proposed as a sealing material as a result of its high swelling capacity. As the bentonite expands, the non-uniform development of porewater pressure and its coupling to total stress within the bentonite, may impair homogenisation. In this study we present results from five laboratory tests performed on sodium- and calcium-based bentonites to examine their swelling potential and capacity to homogenise over extreme bentonite-to-void ratios. Results demonstrate that even under these extreme ratios, the bentonite is able to swell and ultimately fill each void, creating a small swelling pressure. The swelling pressure development is spatially complex and time-consuming, and does not appear to be influenced by friction. Instead, it is characterised by plastic yielding of the clay with 70%–80% of the volume change associated with clay expansion adjacent to the void. This leads to heterogeneity illustrated by the presence of persistent differential stresses and the non-uniform distribution of moisture contents. Increases in the moisture content were measured but did not always correlate with the development of swelling pressure. This disequilibrium of the system is likely a reflection of the test durations and the slow evolution in the rates of change in swelling and porewater pressure beyond 130 days. Given the length of the experimental tests presented here, the time required to achieve full homogenisation of the clay is likely to be many years, if it occurs at all. Gravity segregation was also present in horizontal tests, further impairing clay homogenisation. However, as presented in this paper, it is possible to define functional relationships describing the bentonite swelling potential across engineering voids of differing size. This information will assist in establishing a safety case for bentonite usage in geological radioactive waste disposal.

Capture and subsurface storage of CO2 is widely viewed as being a necessary component of any strategy to minimise and control the continued increase in average global temperatures. Existing oil and gas reservoirs can be re-used for carbon storage, providing a substantial fraction of the vast amounts of subsurface storage space that will be required for the implementation of carbon storage at an industrial scale. Carbon capture and storage (CCS) in depleted reservoirs aims to ensure subsurface containment, both to satisfy safety considerations, and to provide con fidence that the containment will continue over the necessary timescales. Other technical issues that need to be addressed include the risk of unintended subsurface events, such as induced seismicity. Minimisation of these risks is key to building con fidence in CCS technology, both in relation to financing/liability, and the development and maintenance of public acceptance. These factors may be of particular importance with regard to CCS projects involving depleted hydrocarbon reservoirs, where the mechanical effects of production activities must also be considered. Given the importance of caprock behaviour in this context, several previously published geomechanical caprock studies of depleted hydrocarbon reservoirs are identified and reviewed, comprising experimental and numerical studies of fourteen CCS pilot sites in depleted hydrocarbon reservoirs, in seven countries (Algeria, Australia, Finland, France, Germany, Netherlands, Norway, UK). Particular emphasis is placed on the amount and types of data collected, the mathematical methods and codes used to conduct geo-mechanical analysis, and the relationship between geomechanical aspects and public perception. Sound geo-mechanical assessment, acting to help minimise operational and financial/liability risks, and the careful re-cognition of the impact of public perception are two key factors that can contribute to the development of a successful CCS project in a depleted hydrocarbon reservoir.