Gas migration in pre-compacted bentonite under elevated pore-water pressure conditions

Pre-compacted bentonite has long been proposed as a primary component of an engineered barrier system for the safe geological disposal of radioactive waste. Selection of properties such as the clay composition, compaction-state and clay-to-sand ratio varies in different disposal concepts. However, a sound understanding of the gas transport properties of the barrier material is often considered a necessary part of safety case development for a geological disposal facility. In this study, results are presented from two gas injection experiments conducted in Mx80 bentonite, under elevated pore-water pressure conditions. Test observations indicate that the conditions necessary for gas to enter this material are remarkably consistent, irrespective of the applied water pressure. As expected, an association is noted between the total stress experienced by the clay and the gas pressure at the moment of entry. Gas migration is interpreted as occurring by the formation and propagation of dilatant pathways within the bentonite. Local pore-pressure and stress measurements indicate that significant reworking of the clay can occur, resulting in meta-stable episodes of ‘pressure-cycling’, as gas seeks a stable escape pathway. These findings demonstrate the potential for ‘phases’ of pathway development and propagation within the buffer, resulting in successive migration episodes over the repository lifetime. Experiments also show the potential for gas entry into the buffer to occur as a result of declining pore-water pressure conditions. As such, the influence of significant deviations from hydrostatic conditions (for example, resulting from glacial loading) should not be neglected when considering gas interaction with the buffer over long timescales

Gas transport properties through intact and fractured Callovo-Oxfordian mudstones

A series of controlled water and gas experiments was undertaken on samples of Callovo-Oxfordian (COx) mudstone using a synthetic fluid and helium gas. Data from this study demonstrate that the advective movement of gas through COx is accompanied by dilation of the original fabric (i.e. the formation of pressure-induced microfissures) at gas pressures significantly below that of the minimum principal stress. Flow occurs through a local network of unstable pathways, the properties of which vary temporally and spatially within the mudstone. The coupling of variables results in the development of significant time-dependent effects affecting many aspects of COx behaviour, from the gas breakthrough time to the control of deformation processes. Variations in gas entry, breakthrough and steady-state pressures may result from the arbitrary nature of the flow pathways and/or microstructural heterogeneity. Under these conditions, the data suggest that gas flow is along pressure-induced preferential pathways, where permeability is a dependent variable related to the number, width and aperture distributions of these features. This has important implications for modelling gas migration through low permeability, clay-rich materials

Bentonite permeability at elevated temperature

Repository designs frequently favour geological disposal of radioactive waste with a backfill material occupying void space around the waste. The backfill material must tolerate the high temperatures produced by decaying radioactive waste to prevent its failure or degradation, leading to increased hydraulic conductivity and reduced sealing performance. The results of four experiments investigating the effect of temperature on the permeability of a bentonite backfill are presented. Bentonite is a clay commonly proposed as the backfill in repository designs because of its high swelling capacity and very low permeability. The experiments were conducted in two sets of purpose-built, temperature controlled apparatus, designed to simulate isotropic pressure and constant volume conditions within the testing range of 4–6 MPa average effective stress. The response of bentonite during thermal loading at temperatures up to 200 °C was investigated, extending the previously considered temperature range. The results provide details of bentonite’s intrinsic permeability, total stress, swelling pressure and porewater pressure during thermal cycles. We find that bentonite’s hydraulic properties are sensitive to thermal loading and the type of imposed boundary condition. However, the permeability change is not large and can mostly be accounted for by water viscosity changes. Thus, under 150 °C, temperature has a minimal impact on bentonite’s hydraulic permeabilit

Gas network development in a precompacted bentonite experiment: evidence of generation and evolution

In a deep geological disposal facility for radioactive waste, precompacted bentonite is proposed as a sealing material for the isolation of boreholes, disposal galleries and deposition holes. The advective movement of repository gas in bentonite has been linked to the development of new porosity and propagation of dilatant pathways. For the first time we present a detailed analysis of stress field data during the generation and evolution of a gas network. A new experimental dataset, from a highly instrumented test, clearly shows the strong coupling between stress, gas pressure and flow in bentonite. Multiple discrete propagation events are observed, demonstrating spatial variability and time-dependency as permeability within the clay develops. Analysis of the stress data before, during and after gas entry indicates a heterogeneous stress field initially develops, resulting from the development of these pathways. The flow network is dynamic and continues to spatially evolve after gas entry, such that permeability under these conditions must be time-dependent in nature. Perturbation of the stress field is significant before all major gas outflow events, presumably resulting from the requirement to propagate an effective gas network before outflow is possible. In contrast, no major flow perturbations are detected which did not correlate with fluctuations in the stress field. The controls on the distribution and geometry of the resulting flow network are unclear, as well as its long-term evolution and stability. These will be beneficial in the assessment of gas pressure evolution as part of safety case development

Stress field disruption allows gas-driven microdeformation in bentonite to be quantified

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

An experimental study of the potential for fault reactivation during changes in gas and pore-water pressure

The injection of CO2 into a depleted reservoir will alter the pore pressure, which if sufficiently perturbed could result in fault reactivation. This paper presents an experimental study of fault reactivation potential in fully saturated kaolinite and Ball Clay fault gouges. Clear differences were observed in fault reactivation pressure when water was injected, with the addition of mica/illite in Ball Clay seen to reduce the pressure necessary for reactivation. Slip occurred once pore-pressure within the gouge was sufficient to overcome the normal stress acting on the fault. During gas injection localised dilatant pathways are formed with approximately only 15 % of the fault observing an elevated gas pressure. This localisation is insufficient to overcome normal stress and so reactivation is not initiated. Therefore faults are more likely to conduct gas than to reactivate. The Mohr approach of assessing fault reactivity potential gave mixed results. Hydro-mechanical coupling, saturation state, mineralogical composition and time-dependent features of the clay require inclusion in this approach otherwise experiments that are predicted to be stable result in fault reactivation

An experimental study of the flow of gas along synthetic faults of varying orientation to the stress field: implications for performance assessment of radioactive waste disposal

Critical stress theory states that fault transmissivity is strongly dependent upon orientation with respect to the stress tensor. This paper describes an experimental study aimed at verifying critical stress theory using a bespoke angled shear rig designed to examine the relationship between gas flows along a kaolinite-filled synthetic fault as a function of fault dip. A total of 22 gas injection experiments were conducted on faults oriented 0°, 15°, 30°, and 45° to horizontal; both with and without active shear. Gas flow was seen to be complex; repeat gas injection testing showed a consistent gas entry pressure but considerably different, nonrepeatable, gas peak or breakthrough pressure. Gas flow occurred along discrete, dilatant pathways. The physics governing the pressure at which these features formed was repeatable; however, permeability was dependent on the number, distribution, and geometry of the resultant pathways. The nonrepeatable gas response suggests that the number of pathways was dependent on very subtle variations in gouge properties. No fault orientations were seen to exhibit nonflow characteristics, although critical stress theory predicted that two of the investigated fault angles should be effective seals. However, a small variation in gas entry pressure was seen with fault angle as a result of varying normal and shear stress acting on the gouge material. Shear was seen to enhance gas movement by reducing gas entry pressure and increased permeability once gas became mobile. Therefore, in kaolinite gouge-filled faults, shear is not an effective self-sealing mechanism to gas flow

An experimental study of the influence of stress history on fault slip during injection of supercritical CO2

The injection of super-critical CO2 into a depleted reservoir will alter the pore pressure of the basin, which if sufficiently perturbed could result in fault slip. Therefore, knowledge of the acceptable pressure limits is required in order to maintain fault stability. A two-part laboratory study was conducted on fully saturated kaolinite fault gouge to investigate this issue. Previously, we showed that fault slip occurred once pore-pressure within the gouge was sufficient to overcome the normal stress acting on the fault. For kaolinite, this behaviour occurred at a pressure similar to the yield stress. The current study shows that following a slow-reduction in the maximum principal stress, as would be expected through changes in effective stress, the reactivation pressure shows a stress memory. Consequently, the pressure necessary to initiate fault slip is similar to that required at the maximum stress encountered. Therefore, fault slip is at least partially controlled by the previous maximum stress and not the current stress state. During the slow reduction in normal stress, the flow characteristics of the fault remain unchanged until pore-pressure exceeds shear stress and does not increase significantly until it exceeds normal stress. This results in fault slip, which slows the rate of flow increase as shear is an effective self-sealing mechanism. These observations lead to the conclusion that stress history is a vital parameter when considering fault stability

The role of the stress-path and importance of stress history on the flow of water along fractures and faults; an experimental study conducted on kaolinite gouge and Callovo-Oxfordian mudstone

The flow of water along discontinuities, such as fractures or faults, is of paramount importance in understanding the hydromechanical response of an underground geological disposal facility for radioactive waste. This paper reports four experiments conducted on kaolinite gouge on a 30° slip-plane and on realistic fractures created in Callovo-Oxfordian mudstone (COx) from France. Test histories were conducted that initially loaded the gouge material in step changes in vertical stress, followed by unloading of the sample in similar steps. This loading-unloading history showed considerable hysteresis in hydraulic flow, with only partial recovery of fracture transmissivity. This demonstrates the importance of stress history on fracture flow; consideration of just the current stress acting upon a fracture or fault may result in inaccuracies of predicted hydraulic flow. The stress dependency of fracture flow in both kaolinite and COx can be described by a power-law or cubic relationship, which is likely to be dependent on the fracture roughness, thickness of gouge material, saturation state, permeability of the host material, and clay mineralogy (i.e. swelling potential). The observed response of fracture transmissivity to normal stress in COx is a complex superposition of mechanical response of the fracture and the swelling of clay in the fracture surface. The stress-dependency of flow was also seen to be dependent on orientation with respect to bedding. A fracture perpendicular with bedding accommodates greater compression and results in a lower transmissivity. The orientation dependence is related to the anisotropic swelling characteristics of CO

Caprock integrity and public perception studies of carbon storage in depleted hydrocarbon reservoirs

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 confidence 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 confidence 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 geomechanical analysis, and the relationship between geomechanical aspects and public perception. Sound geomechanical assessment, acting to help minimise operational and financial/liability risks, and the careful recognition 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