Carbon sequestration: Characterisation of limestone dissolution during CO2 injection into carbonate rocks

The Challenge
Experiments to model the injection of CO₂ into sample of limestone rocks were designed. A technique was required for sample characterisation, especially to visualise the porosity changes in the rock samples caused by the injection of CO₂.

Background
When a CO₂ saturated saline solution is injected into limestone rocks, the permeability and porosity of the rocks change due to dissolution mechanisms. Precipitation of carbonates can also occur at the injection front. These two effects can have a significant influence on the stability of a system:  rocks near the point of injection increase in porosity, whereas far from the injection point precipitates could slow down or stop progression of the injection front.

Synchrotron techniques
X-ray microtomography at beamline ID19 was used to visualise in 3D the pore structure within rock samples before and after injection of CO₂ saturated brine. From the 3D map of each sample, porosity could be calculated by subtracting pore space from the total volume and permeability was estimated using a cluster labelling algorithm.

Results
A relationship between porosity and permeability was found for the samples. These results can be used to give indications on the behaviour of large-scale systems.

Figure 1. X-ray microtomography of a limestone rock sample acquired after injection of a CO2 solution. The cross-section (left) is displayed in grey scale corresponding to the X-ray absorption; the cubic sub-volume displays segmented data (voids in black, calcite in white); the medial axis skeleton of the pore space, in red, shows the increase of the pore space connectivity and anisotropy. The water flows from the top to the bottom. (Image credit: L. Luquot, CNRS).

How did the synchrotron experiment help?
It was fundamental for the non-destructive high-resolution visualisation of the rock samples. The porosity of the sample was mapped in 3D using a voxel size of 5.06 μm.

Client
This work was supported by TOTAL S.A., ADEME (Agence De l'Environnement et de la Maîtrise de l'Energie) and ANR “Captage et Stockage du CO₂” through the project “Geocarbone-Carbonatation”.

Further information
[1] X-ray microtomography characterization of porosity, permeability and reactive surface changes during dissolution, P. Gouze, L. Luquot, Journal of Contaminant Hydrology 120–121, 45–55 (2011).
[2] Experimental determination of porosity and permeability changes induced by injection of CO₂ into carbonate rocks, L. Luquot, P. Gouze, Chemical Geology 265, 148–159 (2009).