MR31B-05
Porosity and permeability evolution of clay faults: in situ experiments
Wednesday, 16 December 2015: 09:00
302 (Moscone South)
Pierre Henry1, Yves Guglielmi2, Simon Seguy1, Melody Lefevre1, Irfan Ghani1, Guillaume Gent1, Raymi Castilla3, Claude Gout3, Pierre Dick4 and Christophe Nussbaum5, (1)Aix-Marseille Université, CNRS, IRD,, CEREGE UM34, Aix en Provence, France, (2)Aix Marseille University, Marseille Cedex 03, France, (3)TOTAL - Centre Scientifique et Technique Jean Feger, Pau, France, (4)IRSN Institut de Radioprotection et de Sûreté Nucléaire, Fontenay-aux-Roses cedex, France, (5)Swisstopo, Wabern, Switzerland
Abstract:
Fault models associating low permeability cores with high permeability damage zones are widely accepted, however, constitutive laws relating permeability with fault structure, stress, and strain remain poorly constrained. We here present preliminary results of hydromechanical experiments performed at the 10 m scale in fault zones in Toarcian and Aalenian black shale formations. Intact formations have a very low permeability (10-19 to 10-22 m2). One case (in IRSN’s Tournemire Underground Research Laboratory) displays a porosity increase in and around the fault core and abundant veins and calcite cemented small faults in the damage zone. The other case (Mont Terri Swisstopo Underground Research Laboratory) displays a porosity decrease in the fault core zone and few veins. However, under the present stress state, the static permeability of the fractured zones at both locations is higher than that of the intact formation by up to 3 orders of magnitude. During borehole pressurization tests three regimes of permeability variations are observed. (1) Fracture permeability first increases progressively as a function of fluid pressure (2) When a threshold is reached, permeability further increases by 100 or more, but strain as well as permeability variations remain in most part reversible. (3) When a steady pressure is maintained in the injection borehole (from 20 minutes to several days) flow rate tends to decrease with time. These results show that high transient permeability may locally occur in a fault zone under conditions when most of the deformation is reversible, opening the possibility of transient fluid migration decoupled from slip along faults that are not favorably oriented. However, during one test, more than 1 mm of irreversible slip occurred along one of the main interfaces, associated with a sudden increase in flow rate (from 11 to more than 40 l/min). This suggests that when slip occurs, it could result in permeability variations that may remain difficult to predict.
