Contribution à :
ARE »CAP ROCKS« SAFE SEALS FOR CO2?
Intro-Conclusion-Synthèse: (J. Pironon et G. Hubert)
L'étanchéité des roches de couverture: l'expérience des pétroliers (M. Lescanne)
La caractérisation in situ d'une roche de couverture du point de vue chimique et
mécanique: la démarche ANDRA (J. Delay, A. Vinsot)
Geochemical and geomechanical reactivity: an experimental approach for caprock
integrity (O. Bildstein, M. Jullien)
The experimental studies aim at gaining knowledge of the processes and determining the
controlling parameters of these processes in order to assess the long term sequestration of CO2.
The experiments mainly focus on the confinement properties of the caprock (mostly
argillaceous) and on the mechanisms by which CO2 might leak and escape from the depleted
gas/oil reservoir or the saline aquifer. To investigate the geochemical reactivity, two main types
of experiments were designed to explore the two key scenarios of the performance and safety
assessment: batch systems to look at the geochemical reactivity of CO2 (in dissolved and
potentially in supercritical form) with the minerals in the caprock, and percolation systems to
look at reactive flow of supercritical CO2 (CO2-SC) through chemically or mechanically
activated fractures.
The first step of this experimental approach is to finely characterise the minerals in the caprock
of interest in order to determine the geochemical, geomechanical and petrophysical parameters
of the rock and to quantify the confinement capabilities at the initial state, i.e. before alteration.
The second step is to perform the same characterisation after alteration to detect any evolution
of geochemical, geomechanical, flow and transport parameters in order to assess the impact on
the caprock integrity. For the mineralogy, FTIR, X-ray diffraction (XRD) and DTA are the
basic techniques used to perform this task. Scanning electron microscopy (SEM) coupled to an
energy-dispersion spectrometer (EDS) and TEM are used to look at more precise texture or
nature of minerals. In the framework of a French National Research Agency program, the
ANR-Géocarbone “caprock integrity” project, a limited zone restricted to the Paris Basin was
explored and the callovo-oxfordian geological layers were identified as potential shale and marl
caprocks for reservoirs adapted to CO2 sequestration. This choice was also related to the
opportunity to study samples coming from an exploited oil reservoir in Saint Martin-deBossenay (SMB, South-East of Paris) and from the underground research laboratory at Bure
(URLB, East of Paris). The first site is to become the first French pilot site for the injection of
CO2, while the second was set up for the study of deep geological disposal of radioactive
wastes (operated by the French waste management agency, Andra). This geological layer
shows a remarkable homogeneity over long distances in terms of mineral composition: mainly
carbonates (calcite), clays (interstratified illite/smectite and illite), and quartz.
The operating mode for the batch experiments is to start reactivity experiments with an initial
water composition as close as possible to the formation brine or a composition at equilibrium
with the mineral assemblage. This water composition is then modified to match the expected
conditions after CO2 injection: equilibration with CO2 gas or CO2-SC. For the batch experiments
performed at the CEA in Cadarache with the SMB samples, the rock samples are crushed and
reduced to powder (< 500 m), to maximize the reactive specific surface, and then placed into a
titanium autoclave where fluids are maintained at constant temperature and pressure. The initial
water was synthesised from the composition given by Azaroual et al. (1997). Four sets of
experiments were systematically carried out: SMB with brine, SMB with brine acidified with
CO2(g), SMB with dry CO2-SC, and SMB with brine and CO2-SC. Two temperatures were chosen
for the experiments and maintained at 150 bars during 30/90 days: 80°C which is the
temperature at depth of the caprock at SMB, and 150°C which allows for an activation of slow
reactions in order to extrapolate results for the long term assessment of the caprock reactivity.
For each tests, three replicates of brine were analysed before and after reaction by inducted
coupled Plasma – Atomic Emission Spectrometer (ICP-AES). Preliminary results show that the
reactivity of minerals with dry CO2-SC is not significant (this result has to be confirmed, see
Regnault et al., 2005). At the opposite, significant changes were found when brine was present,
resulting in a destabilisation of clay minerals and precipitation of calcium sulfates and
carbonates. These results are supported by geochemical modelling which will be further
analysed and included in larger scale modelling approaches to assess the impact on
confinement properties of the caprock.
The same kind of experiments were carried out with pure homoionic Ca-Montmorillonite and
with Bure claystone at INPL-Nancy. In this case, the crushed samples are placed into small
golden capsule cells along with the brine and dry ice. The conditions of experiment are similar
to those described above. Different solutions were used: brine which was previously
equilibrated during 1 week with the powder, the same brine acidified with CO2(g), with CO2-SC.
A series of experiments are performed at a pressure of 150 bars and a temperature of 80°C and
150°C during 2 and 6 months.
For the second type of experiments on original device was designed at the CEA in Cadarache to
look at the percolation of CO2-SC through fractured samples (Figure 1). It is composed of a
triaxial confinement cell with a series of injection pumps for CO2. The mechanical integrity of
the fractured sample is assured by the counter-pressure exerted by a confining fluid on the
protective tube containing the sample. At the moment, this device is in a testing phase. In
parallel, geomechanical characterisation are also performed in order to determine the
deformation properties of the caprock (elastic properties, fracturing strength, …). This type of
characterization are planned for pristine caprock plugs and for plugs that underwent
degradation due to the interaction with CO2-SC and brine to evaluate the damage caused by
geochemical alteration.
hc
Figure 1. Triaxial percolation cell for reactive CO2 –SC flow-through experiments at the CEA Cadarache.
The oven (left pictures) containing the experimental cell (in the middle) allows for thorough temperature
control. CO2 gas is injected with a series of pumps (at the left and right of the oven) which maintain the
pressure inside the cell. CO2 becomes supercritical due to temperature increase in the heating coil (hc). It
then reacts with the fractured sample inside the cell (right picture).
Les approches expérimentales au laboratoire intégrité capillaire (D. Broseta, P. Chiquet)
La modélisation numérique hydrogéochimique du comportement d'une roche de
couverture, extrapolations dans le temps (V. Lagneau)
References:
Azaroual M, Fouillac C., Matray J.M., 1997. Solubility of silica polymorphs in electrolyte
solutions, II. Activity of aqueous silica and solid silica polymorphs in deep solutions from the
sedimentary Paris Basin, Chemical Geology, 140, 3-4, pp. 67-179.
Regnault O., Lagneau V., Catalette H., Schneider H., 2005. Etude expérimentale de la réactivité
du CO2 supercritique vis-à-vis de phases minérales pures. Implications pour la séquestration
géologique du CO2. C. R. Geoscience 337, pp. 1331-1339.