Self-sealing experiments and gas injection tests in a backfilled microtunnel of the Mont Terri URL

This paper describes a large-scale experiment on gas transport and hydromechanical processes around underground structures as part of a long-term geoscientific research programme at the Mont Terri Underground Rock Laboratory in the Jura Mountains of Switzerland. A horizontal microtunnel with a diameter of 1 m and a length of 13 m was drilled in an overconsolidated claystone formation. After installing monitoring instruments in the open tunnel, the end of the tunnel was backfilled with sand (test section) and a large hydraulic packer was emplaced in the seal section. The packer was inflated and subsequently the test interval was saturated with a synthetic pore-water. Following saturation an extended programme of hydraulic testing was performed over a two year period. A series of gas injection tests was then performed over a period of approximately 1.5 years. Following this first series of gas injections, a long post-gas hydraulic test has been initiated. The paper presents data and interpretation of the gas injections and subsequent hydraulic testing. The ability of the excavation damage zone to transport gas at pressures below fracturing is demonstrated. The post-gas hydraulic performance is considered and related to the self-sealing of the damage zone observed during saturation and hydraulic testing.