Jon F. Harrington

Gas migration in clay barriers

Controlled flow-rate gas injection experiments have been performed on pre-compacted samples of KBS-3 specification Mx80 1 buffer bentonite using helium as a safe replacement for hydrogen. By simultaneously applying a confining pressure and backpressure, specimens were isotropically-consolidated and fully water-saturated under predetermined effective stress conditions, before injecting gas using a syringe pump. Ingoing and outgoing gas fluxes were monitored. All tests exhibited a conspicuous threshold pressure for breakthrough, fractionally larger than the sum of the swelling pressure and the backpressure All tests showed a post-peak negative transient leading to steady-state gas flow. Using a stepped history of flow rate, the flow law was shown to be nonlinear. With the injection pump stationary (i.e. zero applied flow rate), gas pressure declined with time to a finite value. When gas flow was re-established, the threshold value for gas breakthrough was found to be significantly lower than in virgin clay. There is strong evidence to suggest that the capillary threshold for gas entry is of such a magnitude that normal two-phase flow is impossible. Gas entry and breakthrough are therefore accompanied by the development of pathways which propagate through the clay from gas source to sink. In the absence of these pressure-induced pathways, initially water-saturated bentonite is impermeable to gas.

Gas transport properties of clays and mudrocks

Abstract Controlled flow rate gas injection experiments have been performed on clay and mudrock samples using helium as a permeant. By simultaneously applying a confining stress and back-pressure, specimens were isotropically consolidated and fully water saturated under predetermined effective stress conditions, before injecting gas at a very slow rate using a syringe pump. Ingoing and outgoing gas fluxes were monitored. All tests exhibit a conspicuous threshold pressure for gas breakthrough. All tests showed a post-peak negative transient leading to steady-state gas flow. On the basis of a stepped history of flow rate, the flow law was shown to be nonlinear (i.e. non-Darcian). With the injection pump stationary (i.e. zero flow rate), gas pressure declined with time to a finite value. No gas flow was ever detected at excess gas pressures less than this lower threshold. When gas flow was re-established, the threshold for gas breakthrough was found to be significantly lower than in the virgin clay. There is strong evidence to suggest that the capillarity restrictions on gas penetration of the intergranular pores of saturated clays and mudrocks are of such a magnitude that normal two-phase flow is impossible. Gas therefore does not occupy, or flow through, the intergranular porosity of the clay matrix. In the absence of pressure-induced cracks, water-saturated clays and mudrocks are totally impermeable to gas. The measured gas permeability of a clay-rich medium is a dependent variable rather than a material property, as it depends on the number of pressure-induced pathways in the plane normal to the flow, together with the width and aperture distributions of these pathways. The experiments suggest that the flow pathways open under high gas pressure conditions and partially close if gas pressure falls, thus providing a possible explanation of the nonlinearity of the flow law. Reliance on conventional two-phase flow theory is inadvisable when attempting to quantify gas transport in initially water-saturated clay-rich materials.

Gas flow in Callovo-Oxfordian claystone (COx): results from laboratory and field-scale measurements

Abstract To understand the fate and impact of gas produced within a repository for radioactive waste, a series of laboratory and field scale experiments have been performed on the Callovo-Oxfordian claystone (COx), the proposed host rock for the French repository. Results show the movement of gas is through a localized network of pathways, whose properties vary temporarily and spatially within the claystone. Significant evidence exists from detailed laboratory studies for the movement of gas along highly unstable pathways, whose aperture and geometry vary as a function of local stress, gas and porewater pressures. The coupling of these parameters results in the development of significant time-dependent effects, impacting on all aspects of COx behaviour, from gas breakthrough time, to the control of deformation processes. Variations in gas entry, breakthrough and steady-state pressures are indicative of microstructural heterogeneity which exerts an important control on the movement of gas. The localization of gas flow is also evident in preliminary results from the large scale gas injection test (PGZ) where gas flow is initially focussed within the excavation damaged zone (EDZ), which acts as a preferential pathway for gas. Numerical models based on conventional two-phase flow theory are unable to adequately describe the detailed observations from laboratory tests.

Evidence for gas-induced pathways in clay using a nanoparticle injection technique

Abstract Corrosion, water radiolysis and microbial degradation will result in the generation of gas within repositories designed for the geological disposal of high-level radioactive waste. It is therefore crucial in the design of such facilities that the relevant mechanisms allowing gas migration through repository materials, both engineered barriers and clay-based candidate host rocks, are correctly identified. In Belgium, the Boom Clay represents a candidate host material for which the advective gas breakthrough characteristics and transport properties have been extensively tested and are well defined by numerous studies. The Boom Clay displays a significant capacity for self-sealing and both laboratory and field tests indicate that advective gas transport occurs not by visco-capillary flow, but instead through the formation of pressure-induced dilatant pathways. In this study, we present results from a gas injection test designed to demonstrate the presence of these features by injecting nanoparticulate tracers with helium gas into a sample of Boom Clay. The results provide conclusive evidence for the formation of transient, dilatant gas pathways within a candidate clay-based host rock. This technique provides a novel diagnostic tool for the identification of processes governing multi-phase flow, supporting robust long-term assessments of repository performance.

Observational evidence confirms modelling of the long-term integrity of CO2-reservoir caprocks

Storage of anthropogenic CO2 in geological formations relies on a caprock as the primary seal preventing buoyant super-critical CO2 escaping. Although natural CO2 reservoirs demonstrate that CO2 may be stored safely for millions of years, uncertainty remains in predicting how caprocks will react with CO2-bearing brines. This uncertainty poses a significant challenge to the risk assessment of geological carbon storage. Here we describe mineral reaction fronts in a CO2 reservoir-caprock system exposed to CO2 over a timescale comparable with that needed for geological carbon storage. The propagation of the reaction front is retarded by redox-sensitive mineral dissolution reactions and carbonate precipitation, which reduces its penetration into the caprock to ∼7 cm in ∼105 years. This distance is an order-of-magnitude smaller than previous predictions. The results attest to the significance of transport-limited reactions to the long-term integrity of sealing behaviour in caprocks exposed to CO2.

Experimental observations of mechanical dilation at the onset of gas flow in Callovo-Oxfordian claystone

Abstract Understanding the mechanisms controlling the advective movement of gas and its potential impact on a geological disposal facility (GDF) for radioactive waste is important to performance assessment. In a clay-based GDF, four primary phenomenological models can be defined to describe gas flow: (i) diffusion and/or solution within interstitial water; (ii) visco-capillary (or two-phase) flow in the original porosity of the fabric; (iii) flow along localized dilatant pathways (micro-fissuring); and (iv) gas fracturing of the rock. To investigate which mechanism(s) control the movement of gas, two independent experimental studies on Callovo-Oxfordian claystone (COx) have been undertaken at the British Geological Survey (BGS) and LAEGO–ENSG Nancy (LAEGO). The study conducted at BGS used a triaxial apparatus specifically designed to resolve very small volumetric (axial and radial) strains potentially associated with the onset of gas flow. The LAEGO study utilized a triaxial setup with axial and radial strains measured by strain gauges glued to the sample. Both studies were conducted on COx at in situ stresses representative of the Bure Underground Research Laboratory (URL), with flux and pressure of gas and water carefully monitored throughout long-duration experiments. A four-stage model has been postulated to explain the experimental results. Stage 1: gas enters at the gas entry pressure. Gas propagation is along dilatant pathways that exploit the pore network of the material. Around each pathway the fabric compresses, which may lead to localized movement of water away from the pathways. Stage 2: the dendritic flow path network has reached the mid-plane of the sample, resulting in acceleration of the observed radial strain. During this stage, outflow from the sample also develops. Stage 3: gas has reached the backpressure end of the sample with end-to-end movement of gas. Dilation continues, indicating that gas pathway numbers have increased. Stage 4: gas-fracturing occurs with a significant tensile fracture forming, resulting in failure of the sample. Both studies clearly showed that as gas started to move through the COx, the sample underwent mechanical dilation (i.e. an increase in sample volume). Under in situ conditions, the onset of dilation (micro-fissuring) is a necessary precursor for the advective movement of gas.

The role of stress history on the flow of fluids through fractures

Abstract Understanding flow along fractures and faults is of importance to the performance assessment (PA) of a geological disposal facility (GDF) for radioactive waste. Flow can occur along pre-existing fractures in the host-rock or along fractures created during the construction of the GDF within the excavation damage zone (EDZ). The complex fracture network will have a range of orientations and will exist within a complex stress regime. Critical stress theory suggests that fractures close to localized shear failure are critically stressed and therefore most conductive to fluid flow. Analysis of fault geometry and stress conditions at Sellafield has revealed that no features were found to be, or even close to being, classified as critically stressed, despite some being conductive. In order to understand the underlying reasons why non-critically stressed fractures were conductive a series of laboratory experiments were performed. A bespoke angled shear rig (ASR) was built in order to study the relationship between fluid flow (water and gas) through a fracture surface as a function of normal load. Fluid flow reduced with an increase in normal load, as expected. During unloading considerable hysteresis was seen in flow and shear stress. Fracture flow was only partially recovered for water injection, whereas gas flow increased remarkably during unloading. The ratio of shear stress to normal stress seems to control the fluid flow properties during the unloading stage of the experiment demonstrating its significance in fracture flow. The exhumation of the Sellafield area during the Palaeogene-Neogene resulted in considerable stress relaxation and in fractures becoming non-critically stressed. The hysteresis in shear stress during uplift has resulted in faults remaining, or becoming, conductive. The field and laboratory observations illustrate that understanding the stress-history of a fractured rock mass is essential, and a mere understanding of the current stress regime is insufficient to estimate the flow characteristics of present-day fractures.

Gas migration experiments in bentonite: implications for numerical modelling

Abstract In the Swedish KBS-3 repository concept, there is potential for gas to be generated from corrosion of ferrous materials under anoxic conditions, combined with the radioactive decay of the waste and radiolysis of water. A full understanding of the probable behaviour of this gas phase within the engineered barrier system (EBS) is therefore required for performance assessment. We demonstrate key features from gas transport experiments on pre-compacted Mx80 bentonite, under laboratory and field conditions, and discuss their implications in terms of a conceptual model for gas migration behaviour. On both scales, major gas entry is seen to occur close to the sum of the porewater and swelling pressures of the bentonite. In addition, gas pressure at breakthrough is profoundly sensitive to the number and location of available sinks for gas escape. Observations of breakthrough can be explained by the creation of dilatational pathways, resulting in localized changes in the monitored porewater pressures and total stresses. These pathways are highly unstable, evolving spatially and temporally, and must consequently influence the gas permeability as their distribution/geometry develops. Such observations are poorly embodied by conventional concepts of two-phase flow, which do not fully represent the key processes involved. Although dilatancy based models provide a better description of these processes, the paucity of data limits further development and validation of these models at present.

The visualization of flow paths in experimental studies of clay-rich materials

Abstract One of the most challenging aspects of understanding the flow of gas and water during testing in clay-rich low-permeability materials is the difficulty in visualizing localized flow. Whilst understanding has been increased using X-ray Computed-tomography (CT) scanning, synchrotron X-ray imaging and Nuclear Magnetic Resonance (NMR) imaging, real-time testing is problematic under realistic in situ conditions confining pressures, which require steel pressure vessels. These methods tend not to have the nano-metre scale resolution necessary for clay mineral visualization, and are generally not compatible with the long duration necessary to investigate flow in such materials. Therefore other methods are necessary to visualize flow paths during post-mortem analysis of test samples. Several methodologies have been established at the British Geological Survey (BGS), in order to visualize flow paths both directly and indirectly. These include: (1) the injection of fluorescein-stained water or deuterium oxide; (2) the introduction of nanoparticles that are transported by carrier gas; (3) the use of radiologically tagged gas; and (4) the development of apparatus for the direct visualization of clay. These methodologies have greatly increased our understanding of the transport of water and gas through intact and fractured clay-rich materials. The body of evidence for gas transport through the formation of dilatant pathways is now considerable. This study presents observations using a new apparatus to directly visualize the flow of gas in a kaolinite paste. The results presented provide an insight into the flow of gas in clay-rich rocks. The flow of gas through dilatant pathways has been shown in a number of argillaceous materials (Angeli et al., 2009; Autio et al., 2006; Cuss et al., 2014; Harrington et al., 2012). These pathways are pressure induced and an increase in gas pressure leads to the dilation of pathways. Once the gas breakthrough occurs, pressure decreases and pathways begin to close. This new approach is providing a unique insight into the complex processes involved during the onset, development and closure of these dilatant gas pathways.

Microbiological influences on fracture surfaces of intact mudstone and the implications for geological disposal of radioactive waste

Abstract The significance of the potential impacts of microbial activity on the transport properties of host rocks for geological repositories is an area of active research. Most recent work has focused on granitic environments. This paper describes pilot studies investigating changes in transport properties that are produced by microbial activity in sedimentary rock environments in northern Japan. For the first time, these short experiments (39 days maximum) have shown that the denitrifying bacteria, Pseudomonas denitrificans, can survive and thrive when injected into flow-through column experiments containing fractured diatomaceous mudstone and synthetic groundwater under pressurized conditions. Although there were few significant changes in the fluid chemistry, changes in the permeability of the biotic column, which can be explained by the observed biofilm formation, were quantitatively monitored. These same methodologies could also be adapted to obtain information from cores originating from a variety of geological environments including oil reservoirs, aquifers and toxic waste disposal sites to provide an understanding of the impact of microbial activity on the transport of a range of solutes, such as groundwater contaminants and gases (e.g. injected carbon dioxide).