Gas migration in clay barriers

摘要

Controlled flow-rate gas injection experimentshave been performed on pre-compacted samples of KBS-3specification Mx801buffer bentonite using helium as a safereplacement for hydrogen. By simultaneously applying aconfining pressure and backpressure, specimens wereisotropically-consolidated and fully water-saturated underpredetermined effective stress conditions, before injecting gasusing a syringe pump. Ingoing and outgoing gas fluxes weremonitored. All tests exhibited a conspicuous threshold pressurefor breakthrough, fractionally larger than the sum of the swellingpressure and the backpressure. All tests showed a post-peaknegative transient leading to steady-state gas flow. Using astepped history of flow rate, the flow law was shown to benonlinear. With the infection pump stationary (i.e.zero appliedflow rate), gas pressure declined wit time to a finite value. Whengas flow was re-established, the threshold value for gas breadthrough was found to be significantly lower than in virgin clay.There is strong evidence to suggest that the capillary threshold forgas entry is of such a magnitude that normal two-phase flow isimpossible. Gas entry and breakthrough are thereforeaccompanied by the development of pathways which propagatethrough the clay from gas source to sink. In the absence of thesepressure-induced pathway, initially water-saturated bentonite is impermeable.