Field experiments of fault activation in shales
Yves Guglielmi1, Pierre Henry1, Claude Gout2 and Pierre Dick3
1
- CEREGE (UMR7330), Aix-Marseille University, CNRS-IRD, 13330 Marseille, France
2
– TOTAL S.A., Exploration Production Research and Development, Avenue Larribau – 64018
Pau Cedex – France
3
- IRSN/PRP-DGE/SRTG, Laboratoire d'Etude et de recherche sur les Transferts et les
Interactions dans les Sols, BP 17, 92262 Fontenay-aux-Roses Cedex
The key questions about fault reactivation in shales concern the potential enhanced fluid
leakage and seismicity through a previously low-permeable aseismic shale formation. Here we
selected a critically-stressed fault cross-cutting the underground research facility of
Tournemire (France) which is nested in Toarcian shales. Several inclined boreholes were
drilled across the 6m thick fault zone characterized by a fractured damage zone (FDZ) and a
central 2.4m thick complex core (FC) made of multiple shear bands with gouge intercalated to
highly fractured lenses. Fault core was pressurized using a straddle packer system defining a
2.4m long injection chamber normal to the fault surface. A three-dimensional displacement
sensor set in the chamber, and a distributed deformation flute set in a monitoring hole one
meter apart from the injection hole allowed continuous synchronous coupled monitoring of
fault movements, injection pressure and flow rate. An anticlockwise 0.2x10-3 m fault
movement associated to a 0.1 x10-3 m normal component initiates at the FC-FDZ boundary,
and then propagates firstly in the entire FC, secondly in the FDZ. Significant fluid leakage
occurs over decameter scale distances when shear slip affects the entire fault zone, leakage
being a factor-of-2-to-5 higher in the FDZ compared to the FC for the same magnitudes of slip
displacements. If the overall fault reactivation roughly is in good accordance with the regional
state of stresses, leakage occurring under shear movements characterized by very small
dilation rates and a large part of these movements being aseismic highlight a complex
influence of fault geology on its reactivation. Results are that shale faults slight reactivations,
for example conducted below the fracturing pressure, may display factor of 10-to-100
permeability enhancements without generating so much seismicity. Such results may be of
importance in evaluating fault seal integrity in oil and gas exploration, production or CO2
storage or underground excavations monitoring.