Gas migration in clay barriers

Controlled flow-rate gas injection experiments have been performed on pre-c ompacted samples of KBS-3 specification Mx80(1)buffer bentonite using heliu m as a safe replacement for hydrogen. By simultaneously applying a confinin g pressure and backpressure, specimens were isotropically-consolidated and fully water-saturated under predetermined effective stress conditions, befo re injecting gas using a syringe pump. Ingoing and outgoing gas fluxes were monitored. All tests exhibited a conspicuous threshold pressure for breakt hrough, fractionally larger than the sum of the swelling pressure and the b ackpressure. All tests showed a post-peak negative transient leading to ste ady-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 applie d flow rate), gas pressure declined with time to a finite value. When gas h ow was re-established, the threshold value for gas breakthrough was found t o be significantly lower than in virgin clay. There is strong evidence to s uggest that the capillary threshold for gas entry is of such a magnitude th at normal two-phase flow is impossible. Gas entry and breakthrough are ther efore accompanied by the development of pathways which propagate through th e clay from gas source to sink. In the absence of these pressure-induced pa thways, initially water-saturated bentonite is impermeable to gas. (C) 1999 Published by Elsevier Science B.V. All rights reserved.