  The effective stress law for the permeability of a limestone is studied
experimentally by performing constant head permeability tests in a triaxial
cell with different conditions of confining pressure and pore pressure. Test
results have shown that a pore pressure increase and a confining pressure
decrease both result in an increase of the permeability, and that the effect of
the pore pressure change on the variation of the permeability is more important
than the effect of a change of the confining pressure. A power law is proposed
for the variation of the permeability with the effective stress. The
permeability effective stress coefficient increases linearly with the
differential pressure and is greater than one as soon the differential pressure
exceeds few bars. The test results are well reproduced using the proposed
permeability-effective stress law. A conceptual pore-shell model based on a
detailed observation of the microstructure of the studied limestone is
proposed. This model is able to explain the experimental observations on the
effect of the total stress and of the pore pressure on the permeability of the
limestone. Effective stress coefficients for the stress-dependent permeability
which are greater than one are obtained. It is shown that the controlling
factor is the ratio of the different bulk moduli of the various constituents of
the rock. This ratio is studied experimentally by performing microhardness
tests.
