MINERAL REACTIVITY OF THE LIMESTONE ROCK SAMPLES FOLLOWING THE INJECTION OF CO2-RICH SOLUTIONS: RESULTS AND OBSERVATIONS

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CHARACTERIZATION OF PRE- AND POST- EXPERIMENTAL SAMPLES

A complete description of the mineral composition of the sample as well as the architecture of its porous network before and after experiments were performed. The workflow for the description of the rock and the extracted information are detailed in figure 41. A petrographic observation of the rock in thin section and fresh fractured rock (“Fresh” means that rock samples have not undergone any experimental treatment), coupled with X-ray tomographic imaging performed on cylindrical samples of 10 mm x 10 mm was used to describe the microstructure of the rock. Quantitative data were also obtained through petrophysical measurements (porosity, permeability) and mineralogical analysis. The whole core-plug was analyzed by X-Ray microtomography before and after experiments.

Protocol of measurement and data acquisition.

The rock sample are the same than for the permeability analysis. This analysis is performed in a second stage because the filling of the sample with mercury clogs the porosity of the rock which can’t be re-used after measurement.
The measurements proceed in two phases: (1) a low pressure phase with pressure from 0.01 to 0.150 MPa and (2) a high pressure phase with pressure from 0.150 to 220 MPa. To perform analysis measurement, the sample is loaded into a penetrometer (Fig. 43). The penetrometer is sealed and placed in a low pressure chamber. A pressure of 50 μm of mercury is applied in order to remove air before the penetrometer’s cup and capillary stem are automatically backfilled with mercury. Aspressure on the filled penetrometer increases, mercury intrudes into the sample’s pores, beginning with those pores of largest diameter. This requires that mercury moves from the capillary stem into the cup. As the cannula is surrounded by a metal plating, it is possible to measure the decreasing of capacitance of the condenser formed by the metal plating and the mercury of the cannula. These data determine the volume of mercury injected in the porous network of the sample every time the pressure increases. From these data, it is possible to plot cumulative pore volume versus entry pore radius (Fig. 44a), thereby producing a pore-size distribution histogram (Fig. 44b).

Sampling protocol of the pre- and post-petrophysical analyses.

Six cylindrical samples cored in the direction perpendicular to the horizontal sedimentary bedding of the rock (Kv permeability) and one cored vertically (Kh permeability) at different part of the Lavoux block were used for the native petrophysical properties analyses. After each experiments, 5 samples with the same dimensions were drilled at different regions of the core-plug especially at the vicinity of the edge (Fig. 45). 30 samples have thus be analyzed after experiments performing Purcell tests (porosity) and the standard method of nitrogen (permeability). It should be noticed that no evidence of the existence of high permeability zones in the small rock sample has to be emphasized to validate the data and to realize a comparative analysis between the experiments.

Scanning Electron Microscopy (SEM)

The SEM is a microscope that uses electrons instead of light to form an image. It uses a focused beam of high-energy electrons to generate a variety of signals at the surface of solid specimens. The signals deriving from electron-sample interactions are mainly due to secondary electrons (SE), backscattered electron (BSE) and X photons:
Secondary electrons are very sensitive to the topography of the sample. Imaging with secondary electrons consequently provides information about morphology and surface topography. The study of the initial and aged samples gives information about the morphology of dissolution features and the location of precipitated phases.
Back Scattered Electron result from elastic scattering of the primary beam with the nucleus of the atom at the sample surface. The likelihood of backscattering increases with the atomic number (Z) of the material. High-Z materials give a stronger signal (brightness) than low-Z materials, thereby giving image contrast from element mass differences. This technic gives an image of the chemical composition of the sample.
X photons: The ionic bombardment of the sample surface by the primary electrons of high energy also leads to X photon emissions. The wavelength of the X photons emissions depends on the targeted atoms. The X photon analysis gives the chemical composition of the sample.

Concept and data acquirement procedure.

X-Ray micro-tomography is a non-destructive and non-invasive technique used to explore the architecture of solid samples in 3-D with a spatial resolution that can reach less than one micrometer. In this work, tomography is used to access to the pore network and its evolution during the different experiments.
The principle of this technique is based on the measurement of the attenuation coefficients of a solid through which an X-ray photon beam passes. When a X-ray photon passes through a material, it interacts with the different components of the medium. The incident X-ray photons progressively disappear as they pass through the solid matrix. This phenomenon is called “attenuation”.
The level of attenuation depends on the characteristics of the radiation (frequency, wavelength and energy), but also on the thickness of the material as well as its density (and thus its porosity) and chemical composition. The absorbed beam are recorded with a photon detector X. This detector measures the X-Ray attenuation and quantify the linear attenuation coefficient m, defined from Lambert-Beer’s law: ⁄ = exp( −ℎ) (20).
where I0 is the incident X-ray intensity and I is the intensity remaining after the X-ray passed through a sample with thickness h.
The density variation is at the origin of the contrast detected in radioscopy of absorption of the X-rays. The obtained projections on the detector, supply 2-D radiographies in 16 bits. Image acquisition is repeated under different angles of rotation. The obtained sections are then reconstructed using the appropriate algorithms forming a threedimensional image of the sample. This technique reveals the characteristics of the internal structure of the sample: size, shape, spatial distribution of the elements relative to each other, heterogeneities and defects (pores, inclusions, mineral phases …).
The acquisition device (Fig. 47a) is made of: (1) a X-Ray generator (defined by its range of energy), (2) a rotation platform where the sample is located and (3) a data acquisition system composed of a X-ray detector with a CCD (Charge Coupled Device) camera (2-D-pixel arrays).

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Image processing protocol for surface curvature analysis

To approximate the distribution of surface curvature that define the geometry of the rock/void interface, the elaborated method of analysis firstly requires an average smoothing of the voxels composing the processed images by triangulation. An automatic meshing is generated using the “generate surface” module in the Avizo THERMOFISCHER software package. The creation of this triangular approximation of the non-planar surface patches is performed using a generalized marching cubes algorithm developed by Hege et al 1997 [179]. This algorithm is mainly built on the subdivision of a grid cell into a number of smaller sub-cells as depicted in simplified form in the following diagram.

Image Materiel and method.

The continuous in-situ pH monitoring was performed with commercial high pressure in-situ silver silver chloride electrode probes (ENDRESS + HAUSER CONDUCTA INC#, USA) at temperatures of 20°C and 60°C. The probes are inserted in the system using a 316SS tee (Fig. 53a). They are located in the by-pass at the outlets of the two autoclaves. The components (tubes and tees) of the by-pass positioned after the MIRAGES-2 injection autoclave are heated at 60°C thanks to a heater cable. As a consequence, the measurements are performed on the percolating fluid in the same pressure/temperature conditions than in the experimental autoclaves. The liquid junction metal is porous Teflon and the probe material is 316SS and glass (Fig. 53b). The probes have a pH range from 0 to 14. The working conditions of the electrodes are normally limited to 70 bar and 105°C. However the pressure header located at the top of the probes allows to work up to 150 bar, but this conditions of use reduced the lifetime of the probes.

Table of contents :

PART 1 – THE CO2-DISSOLVED PROJECT
I. PRINCIPE
PART 2 – STATE OF THE ART
I. LES ROCHES CARBONATEES
I.1. Caractéristiques pétrographiques des roches carbonatées
I.2. Caractéristiques pétrophysiques des roches carbonatés
I.2.1. La porosité (l’espace poreux)
I.2.2. La perméabilité
I.2.3. La surface spécifique (interface fluide /roche) et la surface réactive
I.3. Nomenclature et classification des roches carbonatées
II. APPROCHE THEORIQUE DE LA REACTIVITE DES RESERVOIRS CARBONATES PAR FORÇAGE HYDROGEOCHIMIQUE ANTHROPIQUE
II.1. Système calco-carbonique et réactions de dissolution/ précipitation (échelles microscopiques)
II.1.1. Dissolution du CO2
II.1.2. Aspect thermodynamique des phénomènes de dissolution/précipitation
II.1.3. Aspect cinétique des phénomènes de dissolution/précipitation
II.1.3.1. Mécanisme de contrôle de la vitesse globale de réaction
II.1.3.2. Paramètres définissant la cinétique de dissolution/ précipitation
II.2. Transport réactif dans une roche calcaire poreuse (classification des modèles de dissolution (introduction Pe-Da)
II.2.1. Description du transport réactif multi-échelle
II.2.2. Adimensionnement du transport réactif et classification des déformations
PART 3 – EXPERIMENTAL APPROACH
I. INTRODUCTION
II. MATERIALS
II.1. Carbonate rock sample selected for experiments: the “Lavoux limestone”
II.2. Description of the injection well materials
II.2.1. Cement
II.2.2. Injection tube steel
III. PRE-DIMENSIONING MODELING OF THE MIRAGES-2 EXPERIMENTS 
III.1. Procedure of the numerical experiment used to define the injection conditions of the flowthrough experiments
III.2. Results
III.2.1. Impact of the flowrate
III.2.2. Impact of the amount of dissolved CO2 in the injected solution
III.3. Conclusion of the pre-dimensioning modeling
IV. THE EXPERIMENTAL BENCH: MIRAGES-2
IV.1. Sample design and preparation process
IV.2. Description of the MIRAGES-2 experiment
IV.2.1. Device for the CO2/solution mixture
IV.2.2. Device for the radial injection of the CO2 rich solution (MIRAGES-2)
IV.2.3. In-situ thermodynamic and chemical monitoring of the experiment
V. EXPERIMENTAL PROTOCOL
V.1. Protocol of injection
V.2. Determination of the initial fluid chemistry and preparation procedure
VI. DESCRIPTION OF THE EXPERIMENT
VII. CHARACTERIZATION OF PRE- AND POST- EXPERIMENTAL SAMPLES 
VII.1. Investigation of the petrophysical properties of the rock
VII.1.1. Permeability
VII.1.2. Porosimetry
VII.1.2.1. Concept
VII.1.2.2. Protocol of measurement and data acquisition
VII.1.3. Sampling protocol of the pre- and post-petrophysical analyses
VII.2. Investigation of the evolution of the structural properties of rock by imaging
VII.2.1. Thin section observation
VII.2.2. Scanning Electron Microscopy (SEM)
VII.2.3. X-Ray micro-tomography
VII.2.3.1. Concept and data acquirement procedure
VII.2.3.2. Image processing protocol
VII.2.3.3. Surface roughness analysis protocol
VII.2.3.3.1. Surface curvature concept
VII.2.3.3.2. Image processing protocol for surface curvature analysis
VII.2.3.4. Description of the analyzed samples
VII.2.4. In and ex-situ chemical analysis
VII.2.4.1. In-situ high pressure pH measurement
VII.2.4.1.1. Image Materiel and method
VII.2.4.1.2. pH probes calibration
VII.2.4.2. In-situ Raman spectroscopy under high pressure/high temperature hydrothermal conditions
VII.2.4.2.1. In-situ Raman: material and method
VII.2.4.2.2. Results of the in-situ Raman calibration at 20°C
VII.2.4.2.3. in-situ Raman calibration at 60°C
VII.2.4.3. Solution chemistry analysis by ICP-AES and ion chromatography (IC)
PART 4 – EXPERIMENTAL MODELLING OF THE INJECTION OF CO2 IN DISSOLVED FORM IN GEOLOGICAL RESERVOIR: RESULTS AND OBSERVATIONS
I. INITIAL CHARACTERIZATION OF THE ROCK SAMPLES
I.1. Petrographic analysis
I.2. Petrophysical analysis
I.3. Chemical analysis
II. MINERAL REACTIVITY OF THE LIMESTONE ROCK SAMPLES FOLLOWING THE INJECTION OF CO2-RICH SOLUTIONS: RESULTS AND OBSERVATIONS
II.1. Results of in-situ data acquisition
II.1.1. Pressure/Temperature and mass flow recording
II.1.2. pH measurement
II.1.3. Carbonate speciation
II.2. Chemical analysis of the aqueous solution
II.3. Sample alteration: Structural characterization
II.3.1. Macroscopic observation (post-experimental core-plug external observation and CT-scan images analysis)
II.3.2. Petrophysical parameters
II.3.2. Microscopic observation (SEM)
PART 5 – DISCUSSION: SPATIO-TEMPORAL EVOLUTION OF REACTIVE TRANSPORT PHENOMENA
I. pH VARIATION AND MASS BALANCE
II. DOMINANT MECHANISMS IN THE INITIATION OF DISSOLUTION PATTERNS: A DIMENSIONLESS NUMBER DISCUSSION
III. SPATIO-TEMPORAL EVOLUTION OF THE DISSOLUTION PROCESS
IV. EVIDENCE OF PRECIPITATION PHENOMENA
PART 6 – STRUCTURAL CONTROL OF A DISSOLUTION NETWORK IN A LIMESTONE RESERVOIR FORCED BY RADIAL INJECTION OF CO2 SATURATED SOLUTION
ABSTRACT
PAPER
CONCLUSION
NOMENCLATURE
REFERENCES BIBLIOGRAPHIQUES

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