Article collectif : Contribution O. Bildstein (CEA Cadarache)
The modelling approach focuses on the phenomenology which has to be investigated to understand the long term evolution of CO2 storage. It is applied here in the framework of the
ANR “Géocarbone-Intégrité” project which is concerned with the confining properties of the caprock of depleted oil reservoirs or deep saline aquifers when CO2-SC is injected. After injection and migration into the reservoir, the CO2-SC bubble reaches the top of the structure and gets in physical contact with the caprock. At this point, different scenarios can be considered which will affect in a different manner the performance and safety of the industrial process of CO2 sequestration (see Pironon et al., this issue).
In the case of a homogeneous structure, the CO2-SC might not enter into the bulk of the caprock because the pressure build-up does reach the entry pressure. In this case, the CO2 can only migrate through the caprock in a dissolved form by diffusion in aqueous phase. The caprock would therefore undergo a diffusional alteration process. Existing single phase reaction-transport models such as Hytec (Van der Lee, 2005), Crunch (Steefel, 2001), and
PhreeqC (Parkhurst), and Alliances (e.g. AlliChess + Cast3m; Mugler, 2004; Montarnal et al.
2007) can address this problem and no further development would be required.
In the case where the pressure exceeds the entry pressure, the supercritical phase will migrate through the caprock by diffusion of the phase and/or capillary. In this scenario, full reactive multiphase transport codes are required to tackle the problem, e.g. Toughreact (Xu et al., ),
Coores (who?). The codes have to address the dissolution of CO2 in the aqueous phase and the interactions between dissolved CO2 and the minerals in the caprock. Note that some experimental studies has evidenced direct interactions between CO2-SC and minerals
(Regnault, Kohler): these phenomena might also need to be accounted for in the models if one wants to show if the effect is significant on the performance assessment.
A last scenario is also envisaged where, in the presence of an initial or reactivated fracture network (by mechanical, or geochemical processes, or both), the CO2-SC intrudes into the fracture and then migrate through the network. In this case, we would have to deal with reactive multiphase flow into fractures. An in-depth knowledge of multiphase flow and capillary effects in fractures would therefore be required (references: Xu, Glassley, other?).
In order to tackle the different scenarios rising from the safety assessment, a number of the previously mentioned codes are used at the CEA (e.g. Hytec, Crunch, Toughreact) as well as a computation platform which is implementing a series of codes. The Alliances platform was originally developed by the CEA, ANDRA and EDF to perform calculations of the long term evolution of radioactive waste disposal (Montarnal et al. 2006). Besides featuring the implementation of different existing (stand-alone) codes (components), the platform main advantage is to provide a unified set of data, multi-domain computation, and coupling between some components (e.g. flow, mass and heat transport, geochemistry). The physical capabilities of interest here include: hydraulic and mass transport in saturated and unsaturated media (Richards), advection/diffusion, mineral dissolution/precipitation, sorption factor (Kd,

Langmuir, …), full geochemistry, reactive transport, heat transport, (thermo-)mechanical hydraulics,…The most important codes implemented on the platform include: Cast3M (CEA),
Porflow (ACRI), Traces (IMFS), Mt3d (USGS) for flow and transport; Chess (CIG) and
Phreeqc (USGS) for the chemistry; and Cast3M (CEA) for the mechanical aspects and Aster
(EDF) for the THM aspects.
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The calculations are performed at the CEA on systems at two different scales: the scale of the experimental setup to determine the parameters of the reactivity of the caprock with CO2-SC, and the scale of the geological setting for the performance and safety assessment. To this day, the calculations focused on the geochemical reactivity in single phase systems, according to the experiments that are performed in the laboratory (Pironon et al., this issue). Tools such a
Hytec, Crunch, and Alliances are used for this purpose. When data on the reactive multiphase flow in caprocks become available, calculations will be performed first with the Toughreact code, and later with Alliances when this capability (under development) will be validated.
Two key developments which already exist in Alliances are investigated at the moment: multi-domain computation and uncertainty/sensitivity analysis. The first one enables to use different phenomenology in different part of the storage (e.g. multiphase flow in the reservoir, single phase transport in the caprock). The second one is a powerful tool to determine the sensitivity of results with regard to uncertainty concerning the parameters and database used for performance and safety assessment. A distribution function can be applied to key parameters and with different sampling methods can be used to prepare the set of input parameters. Conveniently, parallel runs can be performed for each set of data on different computers. The results can be interpreted in a sensitivity analysis using correlation coefficients between input and output parameters.
Future developments mainly concern the reactive multiphase flow in fractures reactivated by geochemical/geomechanical processes. This topic requires further scientific investigation for phenomenological knowledge as well as complex HMC coupling in codes.
Bengaouer et al.
Glassley
Parkhurst,
J. Pironon, G Hubert, J. Delay, A. Vinsot, O. Bildstein, M. Jullien, P. Chiquet, D. Broseta, V.
Lagneau (2007). Are “caprocks” safe seals for CO
2
?, this issue.
Regnault, Kohler
Steefel, 2001
Van der Lee, 2005
Xu et al.,
Ph. Montarnal, C. Mügler, J. Colin, M. Descotes, A. Dimier, E. Jacquot, “Presentation and use of a reactive transport code in porous media”, Physics and Chemistry of the Earth, 32
(2007), 507-517.

Ph. Montarnal, A. Bengaouer, C. Chavant, L. Loth, « ALLIANCES : Simulation Platform for
Nuclear Waste Disposal “,CMWR’06, Copenhagen, Denmark, 2006.
C. Mügler, Ph. Montarnal, A. Dimier, L. Trotignon, “Reactive transport modelling on the
ALLIANCES sofware platform”, CMWR’04, USA, 2004.