Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
In this paper, we describe an outcrop to characterize the effect of fracture spacing and type on larger scale effective elasticity, which is measured for the first time in-situ with a Schmidt hammer. The outcrop is dominated by lime mudstones and belongs to the deformation zone of the St Clement fault, in southern France. Our results suggest that small spacing of faults, open fractures and styolites leads to lesser effective Young's modulus, whereas small sealed fracture spacing leads to greater effective Young's modulus. These relationships are compatible with theoretical models of effective elasticity. Using Amadei and Savage (1993) approach, we define a non-linear model that relates Schmidt hammer rebound to spacing by fracture type. A hemisphere with a radius of 40 to similar to 200 cm is the rheological volume characterized by the Schmidt hammer. Results of model inversion demonstrate that variations of Schmidt hammer rebound over the outcrop can be used to estimate fracture type and stiffness. Stiffness of sealed fractures is 2-3 orders of magnitude greater than the stiffness of faults, stylolites and open fractures. This result is consistent with an increase of the rate of interseismic stress build-up of major faults with sealing of fractures in their damage zone.
ISSN: 0191-8141

hal-00669954
https://hal.archives-ouvertes.fr/hal-00669954
DOI : 10.1016/j.jsg.2011.09.009

Éditeur(s) : HAL CCSD Elsevier
Résumé : Deficient sealing in the well cement plugs and annulus due to mechanical fracturing is an important risk of CO2 leakage from the reservoir to others permeable layers and the surface. Such situation was reproduced at laboratory scale in order to determine the hydro-chemical control on the fracture permeability. Specifically, we investigated the effect of CO2 rich-brine flowing through fractured Portland cements at T = 60 °C and P = 10 MPa and variable flow rates. We showed that carbonation process is dominant and induces permeability decrease and leakage mitigation for extremely low flow rate whereas for high flow rate injection the permeability remains constant due to the precipitation of a low density secondary Si-rich phase which maintains the initial fracture aperture. At intermediate flow rate the hydraulic aperture can increase due to the densification of the material triggered by the net precipitation of low porosity calcite. These results emphasize that more complex behaviors than those considered from batch experiments may take place in the vicinity of flowing fractures. Specifically, the redissolution of the neoformed calcite as well as the development of amorphous phases, both controlled by the CO2-rich brine renewing rate in the fracture may prevent the healing fracture.
ISSN: 1750-5836

hal-00857555
https://hal.archives-ouvertes.fr/hal-00857555
DOI : 10.1016/j.ijggc.2013.04.002

Éditeur(s) : HAL CCSD Elsevier
Résumé : We investigate experimentally the alteration of fractured class-G cement flowed by CO2-rich brine. The experiment mimics a mechanically damaged rough-walled fractured cement annulus at temperature 60 °C and pressure 10 MPa. The experiment consists of flowing a reservoir-equilibrated brine mixed with CO2 (partial pressure of 2.3 MPa) through the fracture of average aperture 14 μm at constant flow rate (100 μL min−1). This flow rate corresponds to pressure gradient representative of an average in situ hydrodynamic condition. Results indicate an intense alteration of the cement with a large removal of mass at the scale of the sample. However, the fracture alteration patterns are triggered by the initial heterogeneity of the fracture aperture; the aperture of the low aperture zones tends to decrease due to calcite precipitation whereas preferential paths develop in the zones of higher aperture associated. Nevertheless, the expected large permeability increase triggered by the mass removal is mitigated by the precipitation of a low density Si-rich amorphous material. The alteration rate will decrease with time because of the increasing distance of diffusion between the fracture where the reactants are actively renewed by advection and the portlandite and C-S-H dissolution fronts. The different zones of reaction can be adequately modeled by a simple 1D diffusion-reaction model using published kinetics coefficients and extrapolation to larger times than the experiment time can be drawn. Altogether, and in addition to the previous studies of the alteration of fractured well cement annulus, this study shows that the leakage potential is strongly controlled by the initial distribution of the aperture along the fracture: low aperture zones will tend to self-heal while localized flow in connected high aperture paths will be perennial.
ISSN: 1750-5836

hal-01346006
https://hal.archives-ouvertes.fr/hal-01346006
DOI : 10.1016/j.ijggc.2016.01.032