HR: 10:50h
AN: GC12A-03    [Abstracts]
TI: Increase of mechano-chemical deformation of reservoir rocks subject to fluids with elevated partial pressure of CO2
AU: * Le Guen, Y
EM: yleguen@ujf-grenoble.fr
AF: LGIT Grenoble University, Maison des Géosciences BP53, Grenoble Cedex9, 38041 France
AU: Renard, F
EM: FRenard@ujf-grenoble.fr
AF: LGIT Grenoble University, Maison des Géosciences BP53, Grenoble Cedex9, 38041 France
AU: Hellmann, R
EM: Hellmann@ujf-grenoble.fr
AF: LGIT Grenoble University, Maison des Géosciences BP53, Grenoble Cedex9, 38041 France
AU: Collombet, M
EM: Marielle@earth.leeds.ac.uk
AF: School of Earth Sciences, Volcano Seismology Group, Leeds, LS2 9JT United Kingdom
AU: Tisserand, D
EM: DTissera@ujf-grenoble.fr
AF: LGIT Grenoble University, Maison des Géosciences BP53, Grenoble Cedex9, 38041 France
AU: Gratier, J
EM: Gratier@obs.ujf-grenoble.fr
AF: LGIT Grenoble University, Maison des Géosciences BP53, Grenoble Cedex9, 38041 France
AU: Brosse, E
EM: Etienne.Brosse@ifp.fr
AF: IFP, 1 & 4, avenue de Bois-Préau, Rueil-Malmaison Cede, 92852 France
AB: At present, carbon dioxide (CO₂) sequestration in deep saline aquifers is viewed as one of the most viable solutions for mitigating against the increasing anthropogenic release of greenhouse gases to the atmosphere. Injection of CO₂ into such environments results in an acidification of in situ pore waters. As a consequence, the pore waters become more reactive, which can lead to an increased rate of rock deformation due to enhanced dissolution-precipitation processes, and may result in potential modifications of the mechanical and hydrological properties of the rock.
One of the mechanisms that couples matrix deformation to the presence of fluids is intergranular pressure solution creep (IPS). This process involves dissolution at intergranular grain contacts subject to elevated stress, and precipitation in pore spaces subject to lower stress. This leads to an overall reduction in porosity and permeability due to both grain indentation and precipitation in pore spaces. The IPS process is particularly significant in carbonate rocks given that their solubility and dissolution kinetics are strongly dependent on pH, which in turn is dependent on p_CO2.
In order to understand the effects of elevated p_CO2 fluids (up to 8~MPa) on the mechanical strength of rocks, flow-through experiments are being conducted in triaxial cells. The samples consist of natural limestone plugs (L~=~50 mm, Ø~=~25 mm) that are subject to a temperature and stresses representative of conditions at 800~m depth (σ₁~=~16~MPa, σ₃~=~12 MPa, T~=~40°C). The fluid flow is set to a flow velocity of ~~5~cm/day, similar to that in aquifers. The vertical strain and the fluid chemistry at the outlet are continuously monitored. With our experimental setup, creep rates as low as 10^-12 s^-1 can be measured.
Our results show that the initial injection of elevated p_CO2 fluids into a dry sample subject to stress causes a very high rate of vertical deformation. With continuous fluid percolation through the sample, the rate of compaction decreases with time in a smooth and monotonic fashion, as a consequence of strain hardening of the sample. The rate of compaction creep is greater by a factor of 16 with respect to compaction in the presence of pure water. Moreover, in some cases, steady deformation is interrupted by transient periods of accelerated rates of compaction creep (up to 100× faster). Measured [Ca] is up to 60× higher than the solubility value of calcite associated with atmospheric p_CO2. Measured concentrations of Ca in the exit solution show a positive correlation with the rate of deformation. Taken together, these results demonstrate that a reservoir would require a certain amount of time to re-equilibrate its internal stress gradients when CO₂-rich fluids are injected.
DE: 1859 Rocks: physical properties
DE: 3902 Creep and deformation
DE: 5114 Permeability and porosity
SC: Global Climate Change [GC]
MN: Fall Meeting 2005