Large scale gas injection test (Lasgit) performed at the Äspö Hard Rock Laboratory: summary report 2007

The deposition hole was closed on the 1st February 2005 signifying the start of the hydration phase. Groundwater inflow through a number of conductive discrete fractures resulted in elevated porewater pressures leading to the formation of conductive channels (piping), the extrusion of bentonite from the hole and the discharge of groundwater to the gallery floor. This problem was addressed by drilling two pressure-relief holes in the surrounding rock mass. Artificial hydration began on the 18th May 2005 after 106 days of testing. Initial attempts to raise porewater pressure in the artificial hydration arrays often resulted in the formation of preferential pathways. These pressure dependent features were not focused in one location but occurred at multiple sites at different times in the test history. These pathways appear to be relatively short lived, closing when water pressure is reduced. It was determined that both pressure relief holes should remain open until the bentonite had generated sufficient swelling pressure to withstand the high water pressure in the system when these holes are closed. Packers were installed into the pressure relief holes on 23rd March 2006 and sections in them closed off over the period to 5th July 2006. There was no repeat of the formation of piping through discrete channels so, on 20th November 2006, pressures to the artificial hydration filters on the canister were increased to 2350 kPa

The impact of boundary conditions on CO2 capacity estimation in aquifers

The boundary conditions of an aquifer determine the extent to which fluids (including formation water and CO2) and pressure can be transferred into adjacent geological formations, either laterally or vertically. Aquifer boundaries can be faults, lithological boundaries, formation pinch-outs, salt walls, or outcrop. In many cases compliance with regulations preventing CO2 storage influencing areas outside artificial boundaries defined by non-geological criteria (international boundaries; license limits) may be necessary. A bounded aquifer is not necessarily a closed aquifer. The identification of an aquifer’s boundary conditions determines how CO2 storage capacity is estimated in the earliest screening and characterization stages. There are different static capacity estimation methods in use for closed systems and open systems. The method used has a significant impact on the final capacity estimate. The recent EU Directive (2009/31/EC) stated that where more than one storage site within a single “hydraulic unit” (bounded aquifer volume) is being considered, the characterization process should account for potential pressure interactions. The pressure interplay of multiple sites (or even the pressure footprint of just one site) is heavily influenced by boundary conditions

Large scale gas injection test (Lasgit) performed at the Äspö Hard Rock Laboratory. Summary report 2008

This report describes the set-up, operation and observations from the first 1385 days (3.8 years) of the large scale gas injection test (Lasgit) experiment conducted at the Äspö Hard Rock Laboratory. During this time the bentonite buffer has been artificially hydrated and has given new insight into the evolution of the buffer. After 2 years (849 days) of artificial hydration a canister filter was identified to perform a series of hydraulic and gas tests, a period that lasted 268 days. The results from the gas test showed that the full-scale bentonite buffer behaved in a similar way to previous laboratory experiments. This confirms the up-scaling of laboratory observations with the addition of considerable information on the stress responses throughout the deposition hole. During the gas testing stage, the buffer was continued to artificially hydrate. Hydraulic results, from controlled and uncontrolled events, show that the buffer continues to mature and has yet to reach full maturation. Lasgit has yielded high quality data relating to the hydration of the bentonite and the evolution in hydrogeological properties adjacent to the deposition hole. The initial hydraulic and gas injection tests confirm the correct working of all control and data acquisition systems. Lasgit has been in successful operation for in excess of 1385 days

Final report of FORGE WP4.1.1: the stress-path permeameter experiment conducted on Callovo-Oxfordian Claystone

This report describes in detail the stress-path permeameter (SPP) apparatus and the test programme conducted on Callovo-Oxfordian (COx) Claystone from the Bure underground research laboratory (URL) in France. Funding for this study has been provided by the French radioactive waste management operator, Andra, the European Union (FORGE Project, Project number 230357) and the British Geological Survey through its well-founded laboratory programme and the Geosphere Containment project (part of the BGS core strategic programme). The results from the first test conducted using the SPP showed that COx has a very pronounced time-dependent component of deformation. This had implications for the following test conducted on COx and also has implications when comparing tests that have been deformed at a much faster rate. Test SPP_COx-1 was conducted with water as a test permeant at constant pore-pressure along a pre-defined stress-path. Volumetric deformation was observed during 16 steps along the stress-path, with considerable time-dependent deformation and anisotropy seen in radial strain. The 16th stage saw the sample fail through the formation of a fracture after the sample had experienced constant stress conditions for 5.5 days; this emphasises the observed time dependent deformation. The results from test SPP_COx-2 clearly showed that the sample dilated at the onset of gas propagation; dilatancy was observed in three radial and one axial direction. A component of this volumetric deformation was associated with changes in pore-pressure. However, pore-pressure variation cannot account for the full amount of strain recorded and a proportion of the strain observed was the result of gas migration by dilatant pathway formation. Prior to the sample attaining steady-state flow, outflow from the sample slowly reduced and the conductive features experienced self-sealing. Gas injection pressure was raised and back-pressure was carefully lowered; neither course of action re-initiated flow through the sample

CO 2 storage: setting a simple bound on potential leakage through the overburden in the North Sea Basin

So-called ‘gas chimneys’ are likely to provide the main geological risk for out-of-reservoir CO2 migration in thick post-rift overburden successions such as typify the central and northern North Sea. Here we postulate that, in the North Sea, such chimneys formed in the geological past, with a likely peak activity at the end of the ice-age, and are currently rather dormant. With this postulate we set a bound on possible bulk migration rates considering both advective and diffusive flow and based on a hypothetical CO2 storage site at 800 m depth. Calculated migration velocities into the overburden, by either advection or diffusion, are very low, at less than one metre per thousand years. Consequently flux rates are also very low, several orders of magnitude below the leakage thresholds that have been suggested as ensuring effective mitigation performance. Time-lapse seismic reflection data from the Sleipner storage site, which is located beneath some small chimney features, show no evidence of CO2 migration into the overburden. This cannot prove the postulate, because the time interval spanned by the seismic surveys is just a few years, but it is nevertheless consistent with it

Consolidation and rebound properties of Opalinus Clay : a long-term, fully drained test

A specimen of Opalinus clay from Mont Terri has been subjected to stress testing over a period of 532 days. Testing was undertaken by changing either (or both) of the axial and confining stresses in sharp steps followed by periods of between 4 and 82 days during which time the specimen was allowed to adjust to the new stress state. In this way, the drained consolidation, creep and rebound behaviour of an Opalinus clay specimen was examined. The test material was subjected to a maximum average effective stress of 38.3 MPa. Volumetric strain data for both volume change and porewater displacement measurements indicate a small inflection in the standard geotechnical plot of void ratio against the logarithm of average effective stress at a value between 20 and 22 MPa. The negative slope of the consolidation curve (α) based on volume change measurements exhibits a general trend of increasing magnitude as effective stress rises. Even though the data do not exhibit the sharp increase in α indicative of classic virgin consolidation behaviour, it would appear that plastic yielding is occurring at an average effective stress below 20 MPa. Analysis of net porewater flow measurements suggest original interstitial fluid was not expelled from the specimen until average effective stress exceeded 20 MPa. Given the data available, an estimate for the preconsolidation stress in the region of 20 to 25 MPa seems reasonable. As effective stress rises the duration of the strain transients lengthen. As the induration state of the mudrock increases, strain traces are characterised by less well-defined transients, indicative of time-dependent plastic yielding at high effective stresses. The volumetric strain data for both volume change and porewater displacement shows similar transient behaviour. These results give an average principal strain ratio of 0.252, suggesting the material is either mechanically anisotropic or behaving as a non-ideal elastic medium. Specific storage values derived from porewater displacement measurements show a general decreasing trend with increasing average effective stress and are in the range 1.5 to 12.5 × 10-6 m-1. Data from volume change measurements are less sensitive to changes in effective stress and are in the range 1.2 to 17.5 × 10-6 m-1. Elastic constants derived for undrained quantities are significantly higher than those for drained conditions by approximately one order of magnitude. Data suggests there is a transition in behaviour centred around an average effective stress of approximately 20 MPa. Analysis of creep curves can be broken down into three distinct responses. The Lemaitre model, as applied to Opalinus clay by Boidy (Boidy et al., 2002), was applied to the current test data. However the published model parameters failed to adequately fit the current data. Minor alteration of these parameters enabled modelling of the longer-term volumetric responses to be undertaken. The Lemaitre model did not predict the initial stage of creep very effectively. A much slower response time was seen in the current data, which was absent in the work by previous researchers. A power-law creep model was established. In general the fit was adequate for the volumetric strain observed, although these data exhibited some noise. In contrast, the fit of the axial strain data was not adequate and even the subdividing of the data into the individual creep stages failed to give an acceptable fit. A combination of power-law for the initial response and Lemaitre for the longer response may achieve a better prediction for this test stage. A numerical simulation was run using the 2-dimensional coupled flow and deformation code STAFAN. Two phases of the testing were modelled separately. During Phase 1, the model was used in an attempt to fit the creep data. A reasonable fit was made to the first step axial strain data, but the extrapolation to later stages showed a progressive deviation from the data. In addition, the model made poor predictions for the radial strain and porewater flow data in all steps. These observations indicate that both the assumptions of linear elasticity and isotropic deformation are probably invalid for this specimen. During the second phase of testing, the axial and confining stresses were raised synchronously in a series of seven 4 MPa steps. In view of the results of the Phase 1 modelling, it was decided to treat each step of Phase 2 as a separate test and to use the model to parameterise the changing state of the specimen. Young’s modulus was significantly lower than those derived from volume and porewater displacement measurements, which can be explained by the over prediction of radial strain due to the simple linear-elastic assumption in the STAFAN model. It has been shown that the linear elastic deformation model is not a good analogue for the behaviour of this specimen. There are clear indications of non-linear responses to stress changes in the data and it seems likely that some form of viscoelastic or viscoplastic model should be adopted. In addition, the axial and radial strain responses would seem to be anisotropic, bringing further complexity to the model that should be employed

Final report of FORGE WP3.1.1: the large scale gas injection test (Lasgit) performed at the Äspö Hard Rock Laboratory

This report summarises the set-up, operation and observations from the first 2890 days (7.9 years) of the large scale gas injection test (Lasgit) experiment conducted at the Äspö Hard Rock Laboratory. During this time the bentonite buffer has been artificially hydrated and has given new insight into the evolution of the buffer. Three gas injection tests have been conducted during the duration of Lasgit. The first two tests were conducted in the lower array of injection filters at FL903. Both of these tests showed similar behaviour with a well-defined pressure peak; spontaneous negative transient; evidence of dynamic behaviour and unstable gas pathways; asymptote close to stress. The results were remarkably qualitatively similar to the laboratory test results. However, the high gas entry pressures seen in the laboratory were not seen in Lasgit as stress state is much lower due to non-complete hydration of the buffer and the expansion of the buffer to fill construction voids. The third gas test was conducted in an upper array filter (FU910). The response at the time of gas peak pressure was subtly dissimilar to that seen at FL903 with two peak pressures. Lasgit has confirmed the coupling between gas, stress and pore-water pressure for flow before and after major gas entry at the field scale. All observations suggest mechanisms of pathway propagation and dilatancy predominate. In all three gas tests the propagation was through localised features and the general movement direction was towards the bottom of the deposition hole in the direction of the prevailing stress gradient. The injection tests have shown that the interface between barriers is a key part of the system. Gas appears to have exited the deposition hole in Gas test 2, but failed to find a way out during Gas test 3; where gas continued to migrate along the canister/buffer interface. Throughout the history of Lasgit parts of the system have been artificially and naturally hydrated. Hydraulic results, from controlled and uncontrolled events, show that the buffer continues to mature and has yet to reach full maturation. Hydration of the clay is progressing well but sections of bentonite remain in suction and in hydraulic disequilibrium

Evidence of localised gas propagation pathways in a field-scale bentonite engineered barrier system: results from three gas injection tests in the large scale gas injection test (Lasgit)

Three gas injection tests have been conducted during a large scale gas injection test (Lasgit) performed at the Äspö Hard Rock Laboratory, Sweden. Lasgit is a full-scale experiment based on the Swedish KBS-3 repository concept, examining the processes controlling gas and water flow in highly water-saturated compact buffer bentonite. Three preliminary gas injection tests have been performed. The first two tests were conducted in the lower array of injection filters (FL903). Both of these tests showed similar behaviour that corresponded with laboratory observations. The third gas test was conducted in an upper array filter (FU910), which gave a subtly dissimilar response at major gas entry with an initial pressure drop followed by a secondary gas peak pressure. Lasgit has confirmed the coupling between gas, stress and pore-water pressure for flow before and after major gas entry at the field scale. All observations suggest mechanisms of pathway propagation and dilatancy predominate. In all three gas tests the propagation was through localised features that tended to exploit the interface between the copper canister and the bentonite buffer. Considerable evidence exists for the development of a highly-dynamic, tortuous network of pressure induced pathways which evolves both temporally and geospatially within the clay, opening and closing probably due to local changes in gas pressure and or effective stress

Effects of old landfills on groundwater quality. Phase 2, investigation of the Thriplow landfill 1996–1997

Disused sand and gravel excavations overlying the major Chalk aquifer at Thriplow in Cambridgeshire have been filled with domestic waste in two phases. One area (Phase 1) was filled between 1957–77 with little compaction of the refuse and was left uncapped, while Phase 2 was deposited between 1981–87 and capped with clay. Aerial photography and surface resistivity surveys indicate that the site geometry is complex, with several phases of landfilling into excavations of differing depths. Drilling through the waste indicates that leachate production and waste stabilisation proceed at different rates in capped and uncapped landfills. Analysis of leachate obtained by centrifugation or squeezing appears to give more insight into the pollution potential than do leach tests with distilled water. The Biological Methane Potential (BMP) of the waste appears to be related to the quantity of decomposable material but the chemical oxygen demand (COD) values are distorted by the presence of reduced metals. Too few boreholes have been drilled to define the leachate source in terms of its spatial distribution and little is known of how its composition has changed with time. However, hydraulic conductivity measurements on the landfill caps suggest that it is sufficiently permeable for all rainfall to potentially infiltrate the waste. Boreholes outside the landfill penetrate the Upper and Lower Chalk, and identify the Melbourn Rock and underlying Plenus Marls at the junction of the two formations about 20 m below ground level (bgl). Surface resistivity surveys using the BGS RESCAN system, confirm aerial photographs of the extent of the landfill and also suggest that leachate has migrated beyond the base of the landfill. Evidence of leachate migration in pre-existing screened boreholes completed above and below the Plenus Marls suggests that leachate is flowing above the Plenus Marls. Hydraulic head measurements whilst drilling a borehole to the base of the lower Chalk approx. 70 m bgl revealed the potential for upward groundwater flow through the Plenus Marls. Thus, previously-drilled boreholes penetrating the Plenus Marls are expected to recharge upwards into the shallow aquifer above the Plenus Marls diluting any leachate in the upper aquifer and distorting the flow regime. Several of these boreholes have subsequently been modified to stem the flow across the Plenus Marls. One borehole down-gradient to the west of the site revealed a large thickness of drift composed of both sand and clay rich material. This suggests the existence of a buried channel, the hydrogeological significance of which has yet to be assessed. Groundwater chemistry appears to be influenced by three major factors. (a) the landfill leachate (b) the composition of shallow groundwater in the top 10 m of the Chalk, and (c) the composition of water from the Lower Chalk. Limited groundwater monitoring data appear to display a cyclic variation in chloride concentration. The origin for this is not clear but it may correlate with cyclic variations in groundwater levels when the water table rises into the waste. Cyclic flushing of the landfill may release leachate into the aquifer giving rise to pulses of chloride. Alternatively changes in chloride may arise by the changing direction of groundwater flow which as yet has not been assessed. A conceptual hydrogeological model in which flow is limited to above the Plenus Marls has been used to develop a more appropriate groundwater flow and solute transport model. However, the model lacks data on aquifer properties, on contaminant inputs concentrations, fluxes and spatial variations, and there is a paucity of monitoring data for calibration. Nonetheless preliminary transport modelling using an equivalent porous medium approach shows that an effective porosity of about 5% best fits the regional data. Since this is much less than the total porosity of about 40% for the Chalk, it would appear that only part of the Chalk is available for flow but that matrix diffusion could play an important role in leachate attenuation. Discrete fracture modelling using the FRACTRAN code has allowed some scoping to be made of the hydraulic properties of the aquifer by comparison with chloride hydrographs, but these again need to be better conditioned by in-situ measurement of fracture distributions and transmissivities. A number of additional activities are required to improve the understanding of flow and contaminant transport at the site. These include better spatial definition of the waste distribution, improved data on the hydraulic properties of the Chalk aquifer, and the use of automatic monitoring to record temporal changes in groundwater chemistry and groundwater levels