Evolution of gas migration properties in a field-scale bentonite engineered barrier system

The large-scale gas injection test (Lasgit) was a full-scale demonstration experiment based on the Swedish KBS-3 repository concept, conducted at 420 m depth at the Äspö Hard Rock Laboratory. Six gas injection tests were conducted: four in a canister filter towards the bottom of the deposition hole (GT1/2/4/6) and two in a filter towards the top of the canister (GT3/5). The main conclusions were: (1) The movement of gas occurred at a pressure close to the local total stress; (2) Peak gas pressure was linked to the hydraulic permeability of the buffer and the ease at which gas could exit the deposition hole. Therefore, the maturity (hydraulic conductivity) of the buffer was a secondary control on gas entry and movement; (3) Gas was transported through a limited number of dilatant pathways. The pathways were small in relation to the total volume of the buffer, and temporally variable; (4) Over the timescale of the project, pathways sealed. Repeat gas injection tests showed partial re-activation of pathways formed in previous tests, which may have exploited the same weakness in the system on repeat testing; (5) (Gas movement did not weaken the KBS-3 engineered barrier system, with gas release being controlled.