Controlled flow-rate gas injection experiments have been performed on
pre-compacted samples of KBS-3 specification M×801 buffer
bentonite using helium as a safe replacement for hydrogen. By simultaneously
applying a confining pressure and backpressure, specimens were
isotropically-consolidated and fully water-saturated under pre-determined
effective stress conditions, before injecting gas using a syringe pump.
Ingoing and outgoing gas fluxes were monitored. All tests exhibited a
conspicuous threshold pressure for breakthrough, somewhat larger than the
sum of the swelling pressure and the backpressure. All tests showed a
post-peak negative transient leading to steady-state gas flow. Using a
stepped history of flow rate, the flow law was shown to be nonlinear. With
the injection pump stationary (i.e. zero applied flow rate), gas pressure
declined with time to a finite value. When gas flow was reestablished, the
threshold value for gas breakthrough was found to be significantly lower
than in virgin clay. There is strong evidence to suggest that the capillary
pressure for the penetration of interparticle pore space of buffer bentonite
is of such a magnitude that normal two-phase flow is impossible. Gas entry
and breakthrough is therefore accompanied by the development of microcracks
which propagate through the clay from gas source to sink. The experiments
suggest that these pathways open under high gas pressure conditions and
partially close if gas pressure falls, providing a possible explanation of
the nonlinearity of the flow law.