Natural Analogue Studies Research Articles

A natural analogue for CO 2 mineral sequestration in Miocene basalt in the Kuanhsi-Chutung area, Northwestern Taiwan

In general, CO 2 sequestration by carbonation is estimated by laboratory experimentation and geochemical simulation. In this study, however, estimation is based on a natural analogue study of the Miocene basalt in the Kuanhsi-Chutung area, Northwestern Taiwan. This region has great potential in terms of geological and geochemical environments for CO 2 sequestration. Outcropping Miocene basalt in the study area shows extensive serpentinization and carbonation. The carbon stable isotopes of carbonates lie on the depleted side of the Lohmann meteoric calcite line, which demonstrates that the carbonates most probably precipitate directly from meteoric fluid, and water–rock interaction is less involved in the carbonation process. Oxygen stable isotope examinations also show much depleted ratios, representative of product formation under low temperatures (∼50–90 °C). This translates to a depth of 1–2 km, which is a practical depth for a CO 2 sequestration reservoir. According to petrographic observation and electron microprobe analysis, the diopside grains in the basalt are resistant to serpentinization and carbonation; therefore, the fluid causing alteration is likely enriched with calcium and there must be additional sources of calcium for carbon mineralization. These derived geochemical properties of the fluid support the late Miocene sandstone and enclosed basalts as having high potential for being a CO 2 sequestration reservoir. Moreover, the existing geochemical environments allow for mineralogical assemblages of ultramafic xenoliths, indicating that forsterite, orthopyroxene and feldspar minerals are readily replaced by carbonates. Based on the mineral transformation in xenoliths, the capacity of CO 2 mineral sequestration of the Miocene basalt is semi-quantitatively estimated at 94.15 kg CO 2 chemically trapped per 1 m 3 basalt. With this value, total CO 2 sequestration capacity can be evaluated by a geophysical survey of the amount of viable Miocene basalt at the potential sites. Such a survey is required in the near future.

Preliminary study on calcium aluminosilicate glass as a potential host matrix for radioactive 90Sr—An approach based on natural analogue study

Given the environmental-, safety- and security risks associated with sealed radioactive sources it is important to identify suitable host matrices for 90Sr that is used for various peaceful applications. As SrO promotes phase separation within borosilicate melt, aluminosilicate bulk compositions belonging to anorthite–wollastonite–gehlenite stability field are studied in this work. Tests for their homogeneity, microstructural characteristics and resistance to phase separation narrowed the choice down to the composition CAS11 (CaO = 35 wt%, Al 2O 3 = 20 wt%, SiO 2 = 45 wt%). We find that up to 30 wt% SrO can be loaded in this glass without phase separation (into Ca, Sr-rich and Sr-poor, Si-rich domains). Leaching behaviour of the glasses differs depending on the content and distribution of Sr. In general, the elemental leach rates determined from conventional PCT experimental procedure yield values better than 10 −7 g cm −2 day −1 for both CAS11 base glass as well as SrO doped glass. It was noted that leach rates calculated on the basis of Ca 2+ and Sr 2+ were of the same order and bit higher compared to those calculated on the basis of Si 4+ and Al 3+. During accelerated leaching tests, zeolite and zeolite + epidote were found to have developed on CAS11 base glass and SrO doped glasses respectively. The Sr bulk diffusion coefficients is found to vary from ∼10 −15 to 10 −13 cm 2/s at temperature intervals as high as 725–850 °C. Based on the experimental observations, it is suggested that CAS11 glass can be used as host matrix of 90Sr for various applications of radioactive Sr-pencils.

Hyperalkaline groundwaters and tectonism in the Philippines: Significance to natural carbon capture and sequestration

Abstract The potential of using ophiolites for CCS has been pointed out before, but no case study has been conducted for the Philippines. Here, the potential for CCS in both ophiolites in general and ophiolites in the Philippines in particular is examined and discussed. Specific drawbacks of using ophiolites as a CO2 sink are presented for the first time, using information from natural analogue studies in the Philippines and Jordan.

Adsorption and mineral trapping dominate CO2 storage in coal systems

High-CO2 coal seams occur in many sedimentary basins world-wide and provide natural analogues of the processes likely to occur as a result of CO2 injection and storage in coal systems. CO2 is stored in coal predominantly as adsorbed molecules on micropore surfaces (adsorption trapping) that allow higher densities and greater volumes of CO2 at shallower depths than in sandstone and carbonate reservoirs. In the longer term CO2 will dissolve in formation water and react with minerals in the host formation (solubility/ionic trapping) and may be precipitated as carbonate minerals (mineral trapping). A recent study suggests that solubility trapping is the predominant CO2 sink in natural gas fields (Gilfillan et al., 2009), whereas our natural analogue studies in the Bowen and Gunnedah Basins, eastern Australia, indicate that CO2 has been stored in coal and sandstone formations since the Mesozoic through a combination of adsorption and mineral carbonation reactions. These differences reflect the different mineralogies and hydrogeology of the CO2-rich natural gas fields studied by Gilfillan et al. (2009) and coal systems. Our work shows that natural analogue studies of sites that have stored CO2 over geological time scales are essential to determine the mechanisms of CO2 storage and the level of characterisation required for storage site equivalents. (C) 2011 Published by Elsevier Ltd.

Do the sedimentary strata have power to neutralize leaking CO2 ?: A natural analogue study on past CO2 invasion and carbonate precipitation in the Cretaceous Izumi Group, SW Japan

Dawsonite, which frequently plays an important role in geochemical modelling of CO2 storage complex, is a rare mineral in the nature. This study reports mode of occurrences of natural dawsonite recently re-discovered in a classical locality of Japan (the Izumi Group, SW Japan), in comparison with our geochemical modelling of CO2 storage in sedimentary strata. The simulation has shown that dawsonite is only stable in the early stage of the evolution of CO2 storage complex even though an extensive precipitation of this mineral in the CO2-affected part within. Natural dawsonite forms mineral veins in a compact mudstone layer, and is also found to be a product of early stage of carbonate veining. The observation is concordant with the simulation result suggesting a temporal limit on the stability of dawsonite in the evolution of storage fluid with time. It is also probable that the precipitation occurred associated with the formation of fractures in mudstone to self-seal them effectively. The relation can be a support to assure the storage security, showing how a cap rock mudstone respond to pressurized CO2 which could be expected as “pressure build up” during the injection phase, and how a cap rock mudstone confine CO2 through self-sealing by mineral precipitation. The presence of self-sealed veins composed of carbonate(s) only stable in the early stage of the CO2 plume evolution may imply that some of the geochemical trapping mechanisms could be operative in the early stage of the CO2 trapping, a scenario a little bit different from that shown by the famous figure (i.e., Fig. 5.9) in the IPCC Special Report.

Fully coupled modeling of long term cement well seal stability in the presence of CO2

Abstract Evidence from experimental investigations, natural analogue studies and demonstration projects support the assertion that significant quantities of CO 2 will not leak from a properly selected and managed CO 2 storage complex. However, site operators must nevertheless justify the expectation of secure CO 2 containment to regulators and other stakeholders. Secure containment will depend upon well seal stability inter alia . This paper presents fully-coupled geochemical models of well seal stability, focusing on cement degradation. The models have been developed with the ambition of reducing the simulated chemical system to the simplest possible configuration that is able to reproduce key features from experimental and field observations. Simplifying the models in this way makes them suitable for inclusion in larger scale system-level models of the storage system, where the use of highly-detailed chemical models can be impractical. Two sets of models were constructed in order to simulate: (1) a cement carbonation experiment (9 days in duration); and (2) field observations of cement degradation from the ‘SACROC’ site (30 years of reaction time). Although some model input parameters are uncertain, the experimental system was successfully simulated and the model was subsequently ‘up-scaled’ and applied to the SACROC core data. Over longer time-scales (10 2 –10 3 years), the effects of parameter and thermodynamic data uncertainties on model output are more significant than at shorter time-scales. Although further work is required to bound the effects of these uncertainties, this paper illustrates the practicality of constructing simplified fully-coupled models to explore the significance of uncertainties by carrying out sensitivity calculations.

Experience of the application of a database of generic Features, Events and Processes (FEPs) targeted at geological storage of CO2

Abstract This paper reviews the application of a freely accessible on-line database of generic Features, Events and Processes (FEPs), designed to support the analysis of geological CO2 storage systems during performance assessments. The Generic CO2 FEP Database was established by Quintessa in 2004 through international collaboration under the auspices of the Weyburn project. Subsequently, development of the database has continued and its use has become widespread, with over 1000 people having registered to access the database. Most commonly, the database has been used as an audit tool to help build confidence that a systems analysis covers all relevant FEPs and to document transparently those FEPs that are not being considered. In other applications the generic FEP database has been screened to identify relevant FEPs that are then used directly to build conceptual models. As a generic resource, the Generic CO2 FEP Database covers the range of FEPs that might be relevant to assessments, from those associated with the storage formation and cap rock to potential impacts on humans and the environment. The range of applications to date demonstrates its use in support of different scales of assessment for different components of the system. Examples include total-systems models, assessments focusing solely on potential loss of containment from the storage formation and natural analogue studies of potential impacts. Over the past five years the use of the Generic CO2 FEP database has helped to build confidence in assessments relating to long-term geological storage. Additionally, the database represents a knowledge base relating to the potential performance and safety of storage systems. The experience gained from application of the database to date helps to inform the way in which it can be applied in future. The database continues to be developed, based on experience gained in its application. Recently references and links have been updated and a targeted review has revised descriptions and FEPs relating to the marine environment. Further targeted reviews and updates are planned. For the database to continue to structure the latest knowledge and understanding relating to geological storage, on-going feedback from its user base is sought.

A natural analogue study of CO2 -cement interaction: Carbonation of calcium silicate hydrate-bearing rocks from Northern Ireland

Natural analogues of Portland cement can provide insights into carbonation processes operating over timescales of interest to performance assessments of CO2 storage. Two such sites in Northern Ireland (Scawt Hill and Carneal Plug) have been investigated, where carbonation reactions of naturally-occurring cement minerals can be observed. Here, partially hydrated larnite-rich nodules have been reacting with atmospheric CO2 or dissolved bicarbonate ions at low temperatures over the last 10–20 thousand years. This has produced rims of carbonate minerals around hydrated low-temperature calcium silicate (CSH) minerals enclosing residual cores of primary high-temperature metamorphic calcium silicates and calcium aluminates (dominated by larnite, and often accompanied by wollastonite, spurrite, paraspurrite, brownmillerite, bredigite, andradite-grossular) with carbonation proceeding progressively from the outer margins of the nodules. Calcite and scawtite are the dominant secondary calcium carbonate minerals, but vaterite and aragonite have also formed. The carbonation produces dense, low-porosity carbonate rims. This is associated with a reduction in volume, accompanied by shrinkage and microfracturing of the underlying residual poorly crystalline CSH gel and its silica-rich alteration product, which has created significant secondary porosity in the altered material. Although some secondary calcium carbonate reaction products may partially mineralise the fractures, they do not seal the fractures completely, allowing further ingress of CO2/bicarbonate. That said, the ingress rate has not been sufficient to completely carbonate the nodules, even after several thousand years. Uncertainties remain in terms of quantifying carbonation reaction rates and the CO2/ HCO3-flux, and further work is needed to understand these potentially very useful analogues.

SHRIMP in situ isotopic analyses of REE, Pb and U in micro-minerals bearing fission products in the Oklo and Bangombé natural reactors: A review of a natural analogue study for the migration of fission products

In the Precambrian, parts of the Oklo, Okélobondo and Bangombé uranium deposits of the Republic of Gabon, central Africa, functioned as natural fission reactors. Many elements in the Oklo and Bangombé uranium deposits show variations in isotopic composition caused by a combination of nuclear fission, neutron capture and radioactive decay. Isotopic studies provide useful information to understand the behavior of radionuclides in geological media. In our recent work, in situ REE, Pb and U isotopic analyses of individual tiny minerals in and around reactor zones have been performed using a SHRIMP (Sensitive High Resolution Ion Microprobe). The isotopic results of the SHRIMP analyses on micro-minerals found in and around the Oklo and Bangombé natural reactors are reviewed in this paper. The data suggest the selective uptake behavior of (1) Ra into illite, and (2) Pu into apatite, (3) the formation process of secondary minerals bearing fissiogenic REE and depleted U, (4) evidence of nuggets (ɛ-particles) bearing fissiogenic platinum group elements (PGE), and (5) from the U–Pb systematics of highly altered zircons, the redistribution of U and Pb.

Mineral sequestration of carbon dioxide in basalt: A pre-injection overview of the CarbFix project

In this paper we describe the thermodynamic and kinetic basis for mineral storage of carbon dioxide in basaltic rock, and how this storage can be optimized. Mineral storage is facilitated by the dissolution of CO 2 into the aqueous phase. The amount of water required for this dissolution decreases with decreased temperature, decreased salinity, and increased pressure. Experimental and field evidence suggest that the factor limiting the rate of mineral fixation of carbon in silicate rocks is the release rate of divalent cations from silicate minerals and glasses. Ultramafic rocks and basalts, in glassy state, are the most promising rock types for the mineral sequestration of CO 2 because of their relatively fast dissolution rate, high concentration of divalent cations, and abundance at the Earth's surface. Admixture of flue gases, such as SO 2 and HF, will enhance the dissolution rates of silicate minerals and glasses. Elevated temperature increases dissolution rates but porosity of reactive rock formations decreases rapidly with increasing temperature. Reduced conditions enhance mineral carbonation as reduced iron can precipitate in carbonate minerals. Elevated CO 2 partial pressure increases the relative amount of carbonate minerals over other secondary minerals formed. The feasibility to fix CO 2 by carbonation in basaltic rocks will be tested in the CarbFix project by: (1) injection of CO 2 charged waters into basaltic rocks in SW Iceland, (2) laboratory experiments, (3) studies of natural analogues, and (4) geochemical modelling.

Natural analogue study on geological disposal of high level nuclear waste

Natural analogue study on geological disposal of high level nuclear waste

Natural analogue studies of waste disposal

Natural analogue studies of waste disposal

Role of paleohydrogeological studies from the perspective of the natural analogue study of the Tono uranium deposit

Role of paleohydrogeological studies from the perspective of the natural analogue study of the Tono uranium deposit

The impact of CO2 on shallow groundwater chemistry: observations at a natural analog site and implications for carbon sequestration

In a natural analog study of risks associated with carbon sequestration, impacts of CO2 on shallow groundwater quality have been measured in a sandstone aquifer in New Mexico, USA. Despite relatively high levels of dissolved CO2, originating from depth and producing geysering at one well, pH depression and consequent trace element mobility are relatively minor effects due to the buffering capacity of the aquifer. However, local contamination due to influx of brackish waters in a subset of wells is significant. Geochemical modeling of major ion concentrations suggests that high alkalinity and carbonate mineral dissolution buffers pH changes due to CO2 influx. Analysis of trends in dissolved trace elements, chloride, and CO2 reveal no evidence of in situ trace element mobilization. There is clear evidence, however, that As, U, and Pb are locally co-transported into the aquifer with CO2-rich brackish water. This study illustrates the role that local geochemical conditions will play in determining the effectiveness of monitoring strategies for CO2 leakage. For example, if buffering is significant, pH monitoring may not effectively detect CO2 leakage. This study also highlights potential complications that CO2 carrier fluids, such as brackish waters, pose in monitoring impacts of geologic sequestration.

Natural gas behavior in shallow geologic layers as natural analogues of discharge of CO2

Abstract In order to obtain public agreement on carbon dioxide (CO2) geological sto rage, it is required to assess the impact of leaked CO2 to our environment and dispel the anxieties as well as to d emonstrate technically the long-term stability of the geological CO2 storage system. As an essential issue to the assessment of the impact w e should understand CO2 behavior in sh allow geologic layers. From this point of view the authors carried out an inv estigation on natural gas behavior in shallow geologic layers in the vicinity of a natural gas reservoir as one of natural analogues of the behavior of leaked CO2 arriving from the deep storage layer. In this report the authors introduce the scheme of the investigation and the currently obtained results of the natural analogue study. The scheme of the investigation aims proposing the procedure to predict the movement of CO2 in shallow geologic layers, that is to say, geological surveying, numerical modeling, computing and estimating the impact of the CO2 to our living environment. The surveyed area is located in the Mobara gas field near Tokyo, Japan. It is said that the natural gas is dissolved in the brine in the turbidite sandstone layers of Pliocene-Pleistocene at the depth more than 200 meters below sea level. The land area is utilized as rice field and we can observe the spotted natural gas gushing when the rice field is filled with water. In this area the authors performed the exploratory investigation, borehole logging, electrical prospecting, underground radio detection, isotope analysis as well as natural gas flux measurement. They also studied the gas movement in the shallow layers by means of numerical analysis. The results of these efforts are briefly described as follows: The natural gas is not biologically originated near the surface but comes from rather deeper zone. The i mpermeable clayey layer traps the natural gas in gaseous phase and the geological structure such as faults and formations of the sedimentary layers restricts the footprint of the discharge of natural gas at the surface.

Hydrogeochemical Modeling for Natural Analogue Study of CO2 Leakage due to Matsushiro Earthquake Swarm

Abstract For natural analogue study of CO2 leakage, the Matsushiro field in central Japan was selected to investigate hydrogeochemical behavior due to CO2 leakage from a fault zone during the period of the Matsushiro Earthquake Swarm between 1965 and 1967. One of the main objectives of the modeling study is to understand effects of hydrogeological characteristics of shallow aquifers and seal mechanism due to mineral precipitation in a fault zone. Reactive geochemical transport simulations were carried out using TOUGHREACT. Based on the historical data including discharge rate, chemical composition of water and seismic activity, we developed a numerical model to reproduce the geochemical behavior during and after the swarm earthquakes. A two-dimensional model (2000 m×1800 m) for the fault zone using the Multiple Interacting Continua (MINC) method was set up to evaluate geochemical effects during degassing and mixing of brine upwelling along the fault zone. The simulation results can capture the observed trend at the Matsushiro field. Supercritical and gaseous CO2 occur in the deep and the shallow fracture zones, respectively. During the earthquake swarm, CO2 saturated brine moves up along the fracture zone and CO2 degassing occurs in the shallow zone. Subsequently, water pH increases and a small amount of calcite precipitates. Lateral groundwater flow in shallow aquifers, which transports the CO2 away from the fault zone, makes CO2 surface discharge small. At the same time, calcite dissolves after the swarm due to lateral fresh water recharge. The result is consistent with the fact that no calcite observed in the core from the 80 m borehole drilled during the course of this study, indicating that CO2 discharge is likely affected by lateral shallow groundwater flow. Measurements of surface CO2 flux and soil gas δ 13 C indicated that little seepage of CO2 occurs in the area of the fan deposits at present, 40 years after the earthquake swarm. Using the same model, long term leakage simulations (if supercritical CO2 leaked along fault zone from the storage reservoir) were carried out to test whether CO2 surface leakage occurs or not. Two simulations were performed, with CO2 upflow rates of 50,000 and 100,000 kg/year in an area of 25 m×25 m. CO2 surface leakage does not occur in the first case, while CO2 leakage occurs in the second case. CO2 surface leakage is reduced because CO2 dissolves in groundwater flowing laterally in shallow aquifer. These simulations indicate that it is important to understand hydrogeological characteristics of aquifers underlain by reservoir as well as cap rocks.

秋田県奥奥八九郎温泉における炭酸塩シンター—アラレ石・方解石共生堆積物—

Carbonate hot spring is a natural chemical reaction field for understanding CO2 geological sequestration as a natural analogue. Natural analogue studies are particularly important to understand the kinetics of mineral precipitation which has potential difficulties in experimental investigations. Carbonate sinter is frequently formed in and around carbonate hot spring, which can be suitable to elucidate mechanisms of carbonate precipitation associated with flushing CO2. Oku-Okuhachikuro hot spring, located in Kosaka town, Akita Prefecture, NE Japan, is an artificial hot spring after drilling of exploration for the Kuroko-deposits, and it is still active where carbonate sinter has still been forming continuously for more than thirty years after drilling. The temperature of spring water is 44 °C and water pH is 6.2, with discharge rate of 0.08 m3/min. The average chemical compositions of sinter correspond about 80 wt% CaCO3, and 4 wt% Fe2O3, associated with minor (<1 wt% each) SiO2, MnO, MgO, Na2O and K2O. Carbonate sinter is mainly composed of aragonite with a small amount of calcite; an intimate occurrence of these two forms of CaCO3 is the most characteristic feature of this locality. However, mineral assemblage, texture and structure of carbonate sinter are different in relation to the distance from the blowout point. Near the blowout point, the sinter is well solidified and shows laminar structure having both of calcite and aragonite. Thickness of Ca-rich laminar ranges from 20 to 150 μm and Fe-rich one is from 10 to 80 μm. Calcite and aragonite assemblage is mainly observed in Ca-rich layer. The Fe-rich layer, however, is composed only of aragonite. The sinter along downstream becomes porous and is monomineralic having aragonite as CaCO3. The observed relations on the special distribution of aragonite/calcite and the possible stability relations of these phases through EPMA and TG-DTA analyses suggest an importance of minor elements (Fe, etc.) in the precipitation of metastable carbonates: this possible effect of the precipitation of metastable phases should be taken into account in the consideration of geochemical processes of CO2 mineral trapping.

Stratigraphy of the PB-1 Well, Nopal I Uranium Deposit, Sierra Peña Blanca, Chihuahua, Mexico

The Nopal I site in the Peña Blanca uranium district has a number of geologic and hydrologic similarities to the proposed high-level radioactive waste repository at Yucca Mountain, making it a useful analogue to evaluate process models for radionuclide transport. The PB-1 well was drilled in 2003 at the Nopal I uranium deposit as part of a DOE-sponsored natural analogue study to constrain processes affecting radionuclide transport. The well penetrates through the Tertiary volcanic section down to Cretaceous limestone and intersects the regional aquifer system. The well, drilled along the margin of the Nopal I ore body, was continuously cored to a depth of 250 m, thus providing an opportunity to document the local stratigraphy. Detailed observations of these units were afforded through petrographic description and rockproperty measurements of the core, together with geophysical logs of the well. The uppermost unit encountered in the PB-1 well is the Nopal Formation, a densely welded, crystal-rich, rhyolitic ashflow tuff. This cored section is highly altered and devitrified, with kaolinite, quartz, chlorite, and montmorillonite replacing feldspars and much of the groundmass. Breccia zones within the tuff contain fracture fillings of hematite, limonite, goethite, jarosite, and opal. A zone of intense clay alteration, encountered in the depth interval 17.45-22.30 m, was interpreted to represent the basal vitrophyre of this unit. Underlying the Nopal Formation is the Coloradas Formation, which consists of a welded lithic-rich rhyolitic ash-flow tuff. The cored section of this unit has undergone devitrification and oxidation, and has a similar alteration mineralogy to that observed in the Nopal tuff. A sharp contact between the Coloradas tuff and the underlying Pozos Formation was observed at a depth of 136.38 m. The Pozos Formation consists of poorly sorted conglomerate containing clasts of subangular to subrounded fragments of volcanic rocks, limestone, and chert. Three thin (2-6 m) intervals of intercalated pumiceous tuffs are present within this unit. The contact between the Pozos Formation and the underlying Cretaceous limestone basement was encountered at a depth of 244.40 m. The water table is located at a depth of ~223 m. Several zones with elevated radioactivity in the PB-1 core occur above the current water table. These zones may be associated with changes in redox conditions that could have resulted in the precipitation of uraninite from downward-flowing waters transporting U from the overlying Nopal deposit. All of the intersected units have low (typically submillidarcy) matrix permeability, thus fluid flow in this area is dominated by fracture flow. These stratigraphic and rock-property observations can be used to constrain flow and transport models for the Peña Blanca natural analogue.

ＣＯ２地中貯留のナチュラルアナログ―松代での地化学調査―

Environmental assessments and safety control during and after CO2 injection are essential for CO2 geological storage, and we are required to evaluate long-term environmental changes and safety. However, long-term changes are difficult to detect directly because the leakage of CO2 is expected to be small and the evaluation is, sometimes, requested to cover more than 1,000 years. To solve this problem, a natural-analogue study, which inquires into environmental changes at present through a comparison with past geological phenomena, is one possible approach. When the Matsushiro earthquake swarm began in 1965, a large volume of subsurface water accompanied by CO2 gas was discharged along fracture zones. A natural-analogue study on the CO2 discharge during the earthquake swarm should be helpful to create a scenario of leakage and a guideline for the safety of CO2 geological storage. Surveys of the CO2 content in soil gas and CO2 flux emissions from the surface were carried out with carbon isotope ratio measurements to understand the current state at Matsushiro, and to make a conceptual model for environmental assessments and safety control. From geological and geophysical points of view, it is said that deep water gushing out from the surface caused the swarm of earthquakes. As this deep water is still gushing out, we planned to measure CO2 concentrations in soil gas and CO2 flux to examine present CO2 activities at Matsushiro. Because CO2 in the soil is also produced by activities of microbes, however, we decided to measure the isotope ratio of the carbon to distinguish CO2 in deep groundwater origin from that produced by microbes. We selected five survey lines and three survey areas based on previous geochemical measurements and fissure distribution during the earthquake swarm, and measured CO2 concentration in soil, CO2 flux, and isotope ratio. Although there were survey points on the thick fan deposit where CO2 concentration in the soil and CO2 flux were high, the isotope ratio indicated that the carbon is produced by the activity of microbes. On the other hand, the isotope ratio of the samples collected from the thin fan deposit area shows deep subsurface water as the origin. An investigation well was drilled into the basement. Subsurface water samples were collected near the bottom of the well in the igneous rock formation. Geochemical analyses and carbon isotope ratio measurements show higher concentrations of chloride and abiogenic CO2, indicating that groundwater of a deep origin with CO2 is still rising. We are now making a conceptual model of hydrogeological history at the next step. This natural analogue study of CO2 seepage could indicate the importance of understanding shallow hydrogeological characteristics in a CO2 storage field.

A New Natural Analogue Study of the Interaction of Low-Alkali Cement Leachates and the Bentonite Buffer of a Radioactive Waste Repository

AbstractBentonite plays a significant barrier role in many radioactive waste repository designs, where it has been chosen due to its favourable properties such as plasticity, swelling capacity, colloid filtration, low hydraulic conductivity and its stability in relevant geological environments. However, bentonite is unstable at high pH meaning that it could lose its favourable properties if interacted with hyperalkaline leachates from concrete construction materials (e.g. tunnel liners, grouts, etc.), seals and plugs and/or cementitious wastes in a repository. This fact has forced several national programmes to assess alternative construction and sealing materials such as low alkali cements. Recently, it has been assumed that the lower pH (typically pH 10-11) leachates of such cements will degrade bentonite to a much lesser degree than ‘standard’ OPC-based cement leachates (generally with an initial pH&gt;13).To date, few laboratory orin situURL (underground rock laboratory) data are available to support the use of low alkali cements in conjunction with bentonites, partly because of the very slow kinetics involved. Consequently, a new project has focussed on finding an appropriate natural analogue site to provide long-term supporting data which will avoid the kinetic constraints of laboratory and URL experiments. Early results have identified an initial, very promising site at Mangatarem in the Philippines, where a quarry excavating bentonite and zeolites is found in the sedimentary carapace of the Zambales ophiolite. In the immediate vicinity of the quarry, ophiolite-derived hyperalkaline groundwaters are present and further field work (including geophysics surveys and borehole drilling) are now being planned to assess regional bentonite/hyperalkaline groundwater interaction. This paper presents an overview of the current status of the project and assesses the relevance of the study to improving understanding of low-alkali cement leachate/bentonite interaction.