[1] Geological repositories subject to the injection of large amounts of anthropogenic carbon dioxide will undergo chemical and mechanical instabilities for which there are currently little experimental data. This study reports on experiments where low and high P co 2 (8 MPa) aqueous fluids were injected into natural rock samples. The experiments were performed in flow-through triaxial cells, where the vertical and confining stresses, temperature, and pressure and composition of the fluid were separately controlled and monitored. The axial vertical strains of two limestones and one sandstone were continuously measured during separate experiments for several months, with a strain rate resolution of 10 À11 s À1. Fluids exiting the triaxial cells were continuously collected and their compositions analyzed. The high P co 2 fluids induced an increase in strain rates of the limestones by up to a factor of 5, compared to the low P co 2 fluids. Injection of high P co 2 fluids into the sandstone resulted in deformation rates one order of magnitude smaller than the limestones. The creep accelerating effect of high P co 2 fluids with respect to the limestones was mainly due to the acidification of the injected fluids, resulting in a significant increase in solubility and reaction kinetics of calcite. Compared to the limestones, the much weaker response of the sandstone was due to the much lower solubility and reactivity of quartz in high P co 2 fluids. In general, all samples showed a positive correlation between fluid flow rate and strain rate. X-ray tomography results revealed significant increases in porosity at the inlet portion of each core; the porosity increases were dependent on the original lithological structure and composition. The overall deformation of the samples is interpreted in terms of simultaneous dissolution reactions in pore spaces and intergranular pressure solution creep.

Résumé -Simulation numérique de l'effet du stockage souterrain de dioxyde de carbone sur la déformation des calcaires par dissolution sous contrainte : résultats préliminaires -Lors de l'injection de dioxyde de carbone dans un réservoir déplété ou dans un aquifère, la dissolution du CO 2 dans l'eau de formation produit une acidification. Ce phénomène accélère les réactions de dissolutionprécipitation avec la matrice rocheuse, et par conséquent, peut modifier notablement les propriétés mécaniques et hydrauliques des roches. De tels effets sont particulièrement importants dans les calcaires pour lesquels la solubilité et la réactivité des minéraux dépendent directement du pH, qui est lié à la pression partielle de CO 2 . Le mécanisme de déformation par dissolution-précipitation sous contrainte est contrôlé par un couplage entre des processus de dissolution et de précipitation des minéraux et une déformation macroscopique de la matrice. Ce mécanisme implique une dissolution aux joints de grains où la contrainte normale est élevée, une diffusion de la matière dissoute dans le fluide intergranulaire, et une précipitation de matière dans les pores où la pression est plus faible. Cela induit une compaction de la roche et une diminution de porosité contrôlées à la fois par l'indentation des grains et par la précipitation dans les pores. La percolation de fluides riches en CO 2 tend à accélérer la compaction et peut ainsi modifier les propriétés mécaniques du réservoir à long terme. Dans cet article, nous avons cherché à quantifier ce processus à l'aide d'un modèle numérique 2D qui couple les processus de dissolution et de précipitation à l'échelle des grains avec des transferts de matière à une échelle plus importante (quelques décimètres). Nous montrons que des pressions élevées de CO 2 (jusqu'à 30 MPa) accélèrent la vitesse de compaction des roches calcaires d'un facteur ∼50 à ∼75 et diminuent aussi leur viscosité. Abstract

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One of the major challenges associated with CO2 geological storage is the performance of the confining system over long timescales. In particular, the occurrence of CO2 leakage through existing wells could not only defeat the purpose of storage but also badly affect human health or the environment. Indeed, cement degradation and casing corrosion in injection, production or abandoned wells can create preferential channels over time, allowing migration of CO2 from the reservoir to shallower formations (e.g. aquifers), and/or to the surface. In this paper, a risk-based approach is proposed for well integrity and confinement performance management. The approach, based on Performance and Risk Management methodology (P&RTM), serves as a decision support tool. The major steps are (i) identifying the system and the sources of degradation through characterization and system analysis; (ii) quantifying their criticity through modeling, in terms of probability and severity, and (iii) establishing a risk mitigation plan. This methodology is based on experience in material aging and risk assessment of complex systems, like nuclear structures, where probabilistic simulations are performed. It accounts for all stakes involved in well integrity management and enables the full integration of uncertainties as part of risk estimation. The methodology presented here greatly improves common approaches based on "Features, Events, and Processes" as it quantifies risk levels. It provides useful and reliable tools to support decisions for well integrity management strategies or emergency plans. To that purpose, mitigation actions such as characterization/inspection, remediation (workover), design improvement or monitoring are valued based on a cost/benefits ratio. Moreover, updating risk assessment with incoming data allows for an evolving vision of risk levels to optimize interventions in time. This approach is successfully applied, leading to recommendations for safer and more efficient design, maintenance, and monitoring strategies.

Carbon Capture and Storage, as a solution to mitigate the increase in greenhouse gases emissions in the atmosphere, is still bringing intensive worldwide R&D activities. In particular, significant acceleration of in situ CCS experiments supports technical developments as well as acceptability of this technology. Among the major risks identified to this technology, wells are often considered to be the weakest spots with respect to CO2 confinement in the geological reservoir. Therefore, long-term well integrity performance assessment is one of the critical steps that must be addressed before large scale CCS technology deployment is accepted as a safe solution to reduce CO2 emissions. A risk-based methodology associated with well integrity is proposed within CO2 geological storage. The main objectives of this approach are to identify and quantify risks associated with CO2 leakages along wells over time (from tens to thousands of years), to evaluate risks and to propose relevant actions to reduce unacceptable risks. The methodological framework emphasized the use of the risk concept as a relevant criterion to (i) evaluate the overall performance of well confinement with respect to different stakes, (ii) include different levels of uncertainty associated to the studied system, and (iii) provide a reliable decision making support. For the quantification of risk, a coupled CO2 flow model (gas flow and degradation processes) was used to identify possible leakage pathways along the wellbore and quantify possible CO2 leakage towards sensitive targets (surface, fresh water, any aquifers…) for different scenarios. This approach offers an operational response to some of the challenges inherent to well integrity management over well lifecycle. This paper focuses on the application of the methodology to a synthetic case based on an existing well. The practical outcomes and the added values will be presented:an objective and structured process,scenarios identification and quantification of CO2 migration along the wellbore for each scenario,risk mapping,and operational action plans for risk treatment of well integrity.

One of the major technological issues for CO 2 injection (for EOR, CCS, etc.) is the long-term behavior of cement-based materials used to ensure the overall sealing performance of the storage wells. When water is present, the CO 2 after injection can react chemically with the cement (i.e. carbonation). How do the CO 2 -enriched formation fluids changes the cement's chemistry and properties? Could the sealing efficiency of the wells be affected by these changes?The objectives of our experimental program are to assess the kinetics and phenomenology of the changes that occur in different class-G Portland cements exposed to CO 2 -enriched aqueous fluids at 8 MPa and two different temperatures.The experimental program presented in this paper consists of: a first carbonation test (Test # 1) using neat G cement, at a temperature of 90°C (194° F) and a pressure (supercritical CO 2 above water) of 8 MPa (1160 psi); a second carbonation test (Test # 2) using G cement with silica flour (to prevent strength retrogression), at a temperature of 140°C (284°F) and at the same CO 2 pressure of 8Mpa. Finally, coupled chemo-mechanical tests (dynamic tests) are underway on similar class-G cement and similar CO 2 -rich water.All the samples were prepared according to ISO/API specifications. The experimental set-up simulates downhole "static" conditions: the samples were immersed in water in a cell thermally regulated and pressurized by CO 2 . Cement samples were exposed to CO 2 -saturated water for various lengths of time (from one week to 3 months) and were characterized using advanced methods for chemical and mineralogical analysis (X-ray tomography, SEM, XRD, TGA-TDA…) and mechanical testing.The main preliminary results show a reactive front (characterized by carbonation) progressing from the fluid-sample interface towards the sample centre. The carbonation front moves faster during Test 2 (at higher temperature) than during Test 1 (at lower temperature). SEM images of Test 2 also show a thin layer of dissolved carbonate at the sample's surface. The carbonated cement areas exhibit increased density and greater compressive strength.The results of coupled chemo-mechanical tests with injection of CO 2 -enriched water in samples under deviatoric stress show that the CO 2 flow rate in the cement rapidly decreases, finally resulting in carbonation clogging of the cement sample. These results seem consistent with reported field observations.

Summary One of the major challenges associated with the geological storage of carbon dioxide (CO2) is the performance of the confining system over long time scales. In particular, the occurrence of CO2 leakage through existing wells could not only defeat the purpose of storage, but also badly affect human health or the environment. Indeed, cement degradation and casing corrosion in injection, production, or abandoned wells can create preferential channels over time, allowing the migration of CO2 from the reservoir to shallower formations (e.g. aquifers), and/or to the surface. In this paper, a risk-based approach is proposed for well-integrity and confinement-performance management. The approach, based on Performance and Risk Management methodology (P&R™), serves as a decision-support tool. The major steps in this methodology are identifying the system and sources of degradation through characterization and system analysis; quantifying their criticality through modeling, in terms of probability and severity; and establishing a risk-mitigation plan. This methodology is based on experience in material aging and risk assessment of complex systems, such as nuclear structures where probabilistic simulations are performed. It accounts for all stakes involved in well-integrity management and enables the full integration of uncertainties as part of risk estimation. The methodology presented here greatly improves common approaches based on "features, events, and processes" because it quantifies risk levels. It provides useful and reliable tools to support decisions for well-integrity-management strategies or emergency plans. To that purpose, mitigation actions such as characterization/inspection, remediation (workover), design improvement, or monitoring are valued on the basis of a cost/benefit ratio. Moreover, updating the risk assessment with incoming data allows for an evolving vision of risk levels to optimize interventions over time. This approach has been applied successfully, leading to recommendations for safer and more-efficient design, maintenance, and monitoring strategies.