Reactive transport modelling of cement-groundwater-rock interaction at the Grimsel Test Site

An in situ experiment at the Grimsel Test Site (Switzerland) to study water-cement-rock interaction in fractured granite was modelled. It consisted of a hardened cement source in a borehole intersecting a water conducting fracture. Grimsel groundwater was injected into this borehole. Two other boreholes at about 0.56 m and 1.12 m from the emplacement borehole were used to monitor the evolution of water composition for 5 years. The modelling approach was based on a 1D radial model for the emplacement borehole and a small volume of rock (fault gouge) around it, and a 2D model for the rest of the domain. The results of the 1D model were used as input for the 2D model. Both models showed dissolution of the fault gouge minerals. Results from the 1D model showed dissolution of portlandite in the cement with an increase in porosity. The 2D model showed a reduction in porosity in the fault gouge due to mineral precipitation. Near the emplacement borehole ettringite precipitated. At the centre of the plume there was precipitation of C-A-S-H and hydrotalcite. At the edge of the hyperalkaline plume calcite, hydrotalcite and illite precipitated.Peer ReviewedPostprint (author's final draft

Reactive transport modelling of a high-pH infiltration test in concrete

A laboratory-scale tracer test was carried out to characterize the transport properties of concrete from the Radioactive Waste Disposal Facility at El Cabril (Spain). A hyperalkaline solution (K-Ca-OH, pH = 13.2) was injected into a concrete sample under a high entry pressure in order to perform the experiment within a reasonable time span, obtaining a decrease of permeability by a factor of 1000. The concentrations of the tracers, major elements (Ca2+, SO42-, K+ and Na+) and pH were measured at the outlet of the concrete sample. A reactive transport model was built based on a double porosity conceptual model, which considers diffusion between a mobile zone, where water can flow, and an immobile zone without any advective transport. The numerical model assumed that all reactions took place in the immobile zone. The cement paste consists of C-S-H gel, portlandite, ettringite, calcite and gypsum, together with residual alite and belite. Two different models were compared, one with portlandite in equilibrium (high initial surface area) and another one with portlandite reaction controlled by kinetics (low initial surface area). Overall the results show dissolution of alite, belite, gypsum, quartz, C-S-H gel and ettringite and precipitation of portlandite and calcite. Permeability could have decreased due to mineral precipitation.Peer ReviewedPostprint (author's final draft

Fluid Flow Simulations of a Large-Scale Borehole Leakage Experiment

Borehole leakage is a common and complex issue. Understanding the fluid flow characteristics of a cemented area inside a borehole is crucial to monitor and quantify the wellbore integrity as well as to find solutions to minimise existing leakages. In order to improve our understanding of the flow behaviour of cemented boreholes, we investigated experimental data of a large-scale borehole leakage tests by means of numerical modelling using three different conceptual models. The experiment was performed with an autoclave system consisting of two vessels bridged by a cement-filled casing. After a partial bleed-off at the well-head, a sustained casing pressure was observed due to fluid flow through the cement–steel composite. The aim of our simulations is to investigate and quantify the permeability of the cement–steel composite. From our model results, we conclude that the flow occurred along a preferential flow path at the cement–steel interface. Thus, the inner part of the cement core was impermeable during the duration of the experiment. The preferential flow path can be described as a highly permeable and highly porous area with an aperture of about 5μm and a permeability of 3⋅10$^{−12}$m$^{2}$ (3 Darcy). It follows that the fluid flow characteristics of a cemented area inside a borehole cannot be described using one permeability value for the entire cement–steel composite. Furthermore, it can be concluded that the quality of the cement and the filling process regarding the cement–steel interface is crucial to minimize possible well leakages

Transport of water, vapour, heat and solutes in concrete for storing radioactive waste

Cementitious materials are used as barriers in radioactive waste storage. Hence, transport processes in this kind of material are important. The objective of this thesis is to improve the understanding of transport of water (liquid and gas), heat and solutes in concrete. This is applied in the concrete from the low-and intermediate- level radioactive waste facility at El Cabril (Southern Spain). To do so, the following methodology was used. First, evaporation tests in concrete columns have been simulated in order to obtain thermo-hydraulic parameters. The conceptual model considers unsaturated liquid flow and transport of vapour and energy. The retention curve was estimated from relative humidity and gravimetric water content at the end of the test. Relative permeability, thermal conductivity and tortuosity factor for vapour diffusion were obtained by calibrating the numerical model. Results show that the vapour diffusion is the dominant water transport process above an evaporation front and liquid advection is dominant below it. Second, numerical models simulating the processes that take place inside the concrete cells have been carried out. Temperature and relative humidity measured by sensors in the cells and thermo-hydraulic parameters from laboratory tests have been used. Results show that temperature oscillations outside the cell create a temperature difference between the two sides of an air gap between the concrete containers and the wall of the cell. Water rises from the phreatic level into the wall of the cell through capillary rise. Water evaporates at the hot side (wall of the cell in summer and containers in winter) and diffuses as vapour from the hot to the cold side. Condensation is produced at the cold side. Consequently, water runs off to the drain. In order to avoid this phenomenon, various scenarios have been studied. Third, a laboratory-scale tracer test in concrete has been carried out using a high entry pressure. The conceptual model considers matrix diffusion between a mobile pore domain, were water can flow, and an immobile zone with only diffusion. Three geometries have been compared, considering the immobile zone as slabs, spheres or tubes. Porosity of the mobile zone and characteristic time were estimated by calibrating the model results to the measured breakthrough curves of deuterium and bromide. The calculated values show that the characteristic time depends on the geometry, and a similar porosity of the mobile zone was estimated for all geometries. Bromide behaviour could not be reproduced even when linear retardation was applied. Finally, reactive transport models of concrete has been applied in order to study the changes in mineralogy produced during the performance of the tracer test. All minerals are considered in the immobile zone. The cement paste consists of alite, belite, gypsum, calcite, C-S-H gel, portlandite and ettringite. The aggregates are composed of quartz. Overall model results show mineral dissolution of alite, belite, gypsum and quartz and precipitation of C-S-H gel, portlandite, ettringite and calcite. The model shows that the porosity in the immobile zone increases due to mineral dissolution. However, the conceptual used model cannot reproduce the changes in permeability.Els materials fets amb ciment s'utilitzen en l'emmagatzematge de residus radioactius. Per tant, els processos de transport en aquest tipus de materials són importants. L'objectiu d'aquesta tesi és millorar la comprensió del transport d'aigua (en fase líquida i gasosa), calor i soluts en el formigó. Concretament l'utilitzat en l'emmagatzematge de residus radioactius de mitja i baixa activitat del centre d'emmagatzematge El Cabril (Sud d'Espanya). Per tal de dur a terme aquest objectiu s'ha seguit la metodologia que es detalla a continuació. En primer lloc, s'han simulat assaigs d'evaporació en formigó per tal d'obtenir-ne els paràmetres termo-hidràulics. El model conceptual considera flux en medi no saturat, transport de vapor i d'energia. S'ha estimat la corba de retenció a partir de les dades d'humitat relativa i contingut gravimètric del final de l'assaig. Mitjançant la calibració del model numèric, s'ha obtingut la permeabilitat relativa, la conductivitat tèrmica i el factor de tortuositat de la difusió de vapor. Els resultats del model numèric mostren que el procés de difusió de vapor es dominant per sobre d'un front d'evaporació i l'advecció de líquid ho és per sota d'aquest front. En segon lloc, s'¿han fet models numèrics per simular els processos que tenen lloc en les estructures d'emmagatzematge de residus. S'han utilitzat dades de temperatura, humitat relativa mesurades per sensors en les cel·les d'emmagatzematge, i també paràmetres termo-hidràulics obtinguts a partir d'assaigs de laboratori. Els resultats mostren que les oscil·lacions de temperatura de l'exterior creen una diferencia de temperatura dins la cel·la d'emmagatzematge. Concretament, entre els dos costats d'un espai d'aire que hi ha entre la paret de la cel·la i els contenidors. L'aigua ascendeix des del nivell freàtic a la paret de la cel·la a través d'ascens capil·lar. Es produeix evaporació al costat calent (paret de la cel·la a l'estiu i contenidor a l'hivern), i es produeix condensació al costat fred. Com a conseqüència, es recull aigua al desguàs. S'han estudiat diferents casos per evitar aquest fenomen. En tercer lloc, s'ha realitzat un assaig de traçadors en formigó, a escala de laboratori, on s'han utilitzat elevades pressions d'infiltració. El model conceptual considera difusió en la matriu, entre una zona mòbil on l'aigua pot fluir i una zona immòbil on només hi ha difusió. S'han comparat tres geometries, considerant la zona immòbil com a blocs, esferes o tubs. La porositat de la zona mòbil i el temps característic s'han estimat calibrant el model numèric amb les corbes d'arribada de deuteri i bromur. Els resultats mostren que el temps característic depèn de la geometria utilitzada, i s'obté una porositat de la zona mòbil similar per a totes les geometries. El comportament del bromur no es pot reproduir encara que s'apliqui un retard lineal. Finalment, s'han utilitzat models de transport reactiu en formigó per estudiar els canvis en la mineralogia produïts durant l'assaig de traçadors. Tots els minerals estan en la zona immòbil. El ciment està format per alita, bel·lita, guix, calcita, gel C-S-H, portlandita i ettringita. Els agregats estan formats per quars. Els resultats mostren dissolució d'alita, bel·lita, guix i quars, i precipitació de gel C-S-H, portlandita, ettringita i calcita. El model mostra que la porositat de la zona immòbil augmenta degut a la dissolució de minerals. No obstant, el model conceptual no pot reproduir els canvis en la permeabilitat amb el model conceptual utilitzat.Postprint (published version

Reactive Transport Modelling of the Long-Term Interaction between Carbon Steel and MX-80 Bentonite at 25 °C

The geological disposal in deep bedrock repositories is the preferred option for the management of high-level radioactive waste (HLW). In some of these concepts, carbon steel is considered as a potential canister material and bentonites are planned as backfill material to protect metallic waste containers. Therefore, a 1D radial reactive transport model has been developed in order to better understand the processes occurring during the long-term iron-bentonite interaction. The numerical model accounts for diffusion, aqueous complexation reactions, mineral dissolution/precipitation and cation exchange at a constant temperature of 25 °C under anoxic conditions. Our results suggest that Fe is sorbed at the montmorillonite surface via cation exchange in the short-term, and it is consumed by formation of the secondary phases in the long-term. The numerical model predicts precipitation of nontronite, magnetite and greenalite as corrosion products. Calcite precipitates due to cation exchange in the short-term and due to montmorillonite dissolution in the long-term. Results further reveal a significant increase in pH in the long-term, while dissolution/precipitation reactions result in limited variations of the porosity. A sensitivity analysis has also been performed to test the effect of selected parameters, such as corrosion rate, diffusion coefficient and composition of the bentonite porewater, on the corrosion processes. Overall, outcomes suggest that the predicted main corrosion products in the long-term are Fe-silicate minerals, such phases thus should deserve further attention as a chemical barrier in the diffusion of radionuclides to the repository far field

Fractured core experiments to study water-rock-cement interaction under CO2 storage conditions

In wellbores used for CO2 injection in Geological Carbon Storage (GCS), Portland cement is placed between the steel casing and the surrounding rock to prevent gas or fluid leakages. During and after CO2 injection, the resulting CO2-rich acid water may deteriorate the cement and favour undesired leaking. The objective of this study is to understand the processes that take place in the contact between Portland cement and sedimentary rocks under GCS conditions. In this study, artificially fractured cores made of a half cement cylinder and a half sedimentary rock cylinder (limestone, sandstone and marl) were reacted with synthetic brines at 25ºC and 10-3.4 bar CO2 and 60ºC and 130 bar CO2 (atmospheric and supercritical CO2 conditions, respectively) by means of percolation experiments. Variation in the aqueous chemistry (concentrations of Ca, SO4, Mg, K, Na, Si, and Al) was monitored over time. At the end of the experiments, the fractured cores were examined by SEM-EDS, XRD, and XCMT to evaluate the changes in the mineralogical content and structure. Results showed a pH increase in all injected solutions: up to 7-11 in the atmospheric experiments and up to 6 under supercritical CO2 conditions. Ca and Si output concentrations were also higher than the initial ones, whereas a sulphate deficit was observed. Dissolved calcium and silicon and removal of sulphate were higher under supercritical CO2 conditions. Overall, under all conditions, the release of Ca and Si was attributed to dissolution of portlandite and C-S-H from the cement, and the sulphate depletion was caused by gypsum precipitation. The SEM-EDS analyses showed a significant alteration of the cement and rock surfaces along the fracture walls. On the rock side, dissolution of calcite and precipitation of aragonite and gypsum were detected. On the altered cement wall, precipitation of aragonite and gypsum were identified. Under atmospheric conditions, an extensive layer of a hydrated Mg phase was formed. Under supercritical CO2 conditions, an increase in cement porosity and fracture aperture was visible. The observed chemical and mineralogical changes allow us to compare the hydrogeochemical response of artificially fractured cores by changing temperature and PCO2. Under CO2 supercritical conditions, the main chemical processes were dissolution of calcite and gypsum precipitation. Cement degradation occurred through dissolution of portlandite and C-S-H. Under atmospheric conditions, similar alteration processes occurred although at a slower rate, along with the formation of a Mg hydrated layer on the cement and rock fracture walls. These experiments are providing evidence of the reactivity of cement and sedimentary rock fractures to be considered in the management of GCS systems. Moreover, 2D reactive transport simulations of the variations in the experimental aqueous chemistry and core mineralogy will be used to quantify the kinetics of the acid brine-cement-rock interaction and to evaluate the integrity of the cemented annulus in GCS wellbores. Acknowledgements This study was financed by projects CGL2017-82331-R (Spanish Ministry of Economy and Competitiveness), with contribution of FEDER founds, and 2017SGR 1733 (Catalan Government)

Reactive transport modelling of a high-pH infiltration test in concrete

A laboratory-scale tracer test was carried out to characterize the transport properties of concrete from the Radioactive Waste Disposal Facility at El Cabril (Spain). A hyperalkaline solution (K-Ca-OH, pH = 13.2) was injected into a concrete sample under a high entry pressure in order to perform the experiment within a reasonable time span, obtaining a decrease of permeability by a factor of 1000. The concentrations of the tracers, major elements (Ca2+, SO4 2−, K+ and Na+) and pH were measured at the outlet of the concrete sample. A reactive transport model was built based on a double porosity conceptual model, which considers diffusion between a mobile zone, where water can flow, and an immobile zone without any advective transport. The numerical model assumed that all reactions took place in the immobile zone. The cement paste consists of C-S-H gel, portlandite, ettringite, calcite and gypsum, together with residual alite and belite. Two different models were compared, one with portlandite in equilibrium (high initial surface area) and another one with portlandite reaction controlled by kinetics (low initial surface area). Overall the results show dissolution of alite, belite, gypsum, quartz, C-S-H gel and ettringite and precipitation of portlandite and calcite. Permeability could have decreased due to mineral precipitation. © 2017 Elsevier LtdThe authors would like to thank Jordi Illa and Salvador Galí (Universitat de Barcelona) for their help in the X-Ray diffraction analysis. We acknowledge financial support of the Spanish Ministry of Economy and Competitivity through the project HEART (CGL2010-18450), a Research Grant from the Technical University of Catalonia (UPC) and ENRESA (Spanish Nuclear Waste Management Company).Peer reviewe

Reactive transport modelling of cement-groundwater-rock interaction at the Grimsel Test Site

An in situ experiment at the Grimsel Test Site (Switzerland) to study water-cement-rock interaction in fractured granite was modelled. It consisted of a hardened cement source in a borehole intersecting a water conducting fracture. Grimsel groundwater was injected into this borehole. Two other boreholes at about 0.56 m and 1.12 m from the emplacement borehole were used to monitor the evolution of water composition for 5 years. The modelling approach was based on a 1D radial model for the emplacement borehole and a small volume of rock (fault gouge) around it, and a 2D model for the rest of the domain. The results of the 1D model were used as input for the 2D model. Both models showed dissolution of the fault gouge minerals. Results from the 1D model showed dissolution of portlandite in the cement with an increase in porosity. The 2D model showed a reduction in porosity in the fault gouge due to mineral precipitation. Near the emplacement borehole ettringite precipitated. At the centre of the plume there was precipitation of C-A-S-H and hydrotalcite. At the edge of the hyperalkaline plume calcite, hydrotalcite and illite precipitated.Peer Reviewe

Interaction between CO2-rich acidic water, hydrated Portland cement and sedimentary rocks: Column experiments and reactive transport modeling

Percolation experiments, using columns filled with alternating layers of hydrated Portland cement and crushed sedimentary rocks, were conducted at PCO2 = 10 bar and 60 °C. Limestone, sandstone and marl were representative of reservoir and cap rocks for a geologic CO2 storage site. The injected solution was at equilibrium with gypsum and equilibrated with the CO2. The main reactions were the dissolution of the calcite that constitutes the rocks and the hydrotalcite and portlandite of the Portland cement. The resulting porewaters were supersaturated with respect to aragonite and gypsum, leading to their precipitation. 2D reactive transport simulations successfully reproduced the experimental aqueous chemistry changes caused by the major dissolution of calcite, portlandite and hydrotalcite together with the precipitation of aragonite, dolomite (cement carbonation), gypsum and alunite. Porosity increased to different extents in both cement and rock. Cement degradation was noticeable in all the cases, but even more in the sandstone experiment.Thanks are due to Jordi Bellés (IDAEA-CSIC) and Maite Romero and Eva Prats (Scientific and Technical Services of the University of Barcelona) for their help in assisting in the laboratory, ICP-AES and SEM-EDX analyses, respectively. This study was financed by projects CGL2014-54831-C3-1-R and CGL2017-82331-R (Spanish Ministry of Economy and Competitiveness), with contribution from FEDER funds, and by projects CEX2018-000794-S (Spanish Ministry of Science and Innovation) and 2017SGR 1733 (Catalan Government). The manuscript has greatly benefited from the thorough comments of the Associated Editor, Dr. Karen H. Johannenson, and three anonymous reviewers.Peer reviewe

Reactive transport modelling of a high-pH infiltration test in concrete

A laboratory-scale tracer test was carried out to characterize the transport properties of concrete from the Radioactive Waste Disposal Facility at El Cabril (Spain). A hyperalkaline solution (K-Ca-OH, pH = 13.2) was injected into a concrete sample under a high entry pressure in order to perform the experiment within a reasonable time span, obtaining a decrease of permeability by a factor of 1000. The concentrations of the tracers, major elements (Ca2+, SO42-, K+ and Na+) and pH were measured at the outlet of the concrete sample. A reactive transport model was built based on a double porosity conceptual model, which considers diffusion between a mobile zone, where water can flow, and an immobile zone without any advective transport. The numerical model assumed that all reactions took place in the immobile zone. The cement paste consists of C-S-H gel, portlandite, ettringite, calcite and gypsum, together with residual alite and belite. Two different models were compared, one with portlandite in equilibrium (high initial surface area) and another one with portlandite reaction controlled by kinetics (low initial surface area). Overall the results show dissolution of alite, belite, gypsum, quartz, C-S-H gel and ettringite and precipitation of portlandite and calcite. Permeability could have decreased due to mineral precipitation.Peer Reviewe