Pressure Testing Experiment at the Swiss Mont Terri Rock Laboratory

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By James Craig

15 November 2016

 The Swiss are making an important commitment to long-term CO2 storage through international collaboration and research. This is notably evident at the Mont Terri Rock Laboratory situated in the Jura Mountains about two hours drive north of Lausanne. This facility has been deliberately constructed within an impermeable shale called the Opalinus Clay which is a potential substrate for high-level radio-active waste storage. The Mont Terri facility also offers ideal conditions for testing cap rock sealant properties in CO2 storage sites.

The Opalinus Clay has very low permeabilities in the order of nano darcy levels. It is also dissected by a fault at the Mont Terri site which provides a natural setting for measuring changes in permeability, pressure and associated microseismicity under controlled conditions. The results of an in-situ fault reactivation experiment were presented in the Geomechanics sesstion on the first day of the GHGT-13 conference in Lausanne. Three boreholes were drilled into the faulted Opalinus Clay. Four sections of the faulted formation were selected representing different sections of the structure. Each section was sealed by borehole packers allowing pressurised water to be injected into the fault. Three other boreholes were used to detect any associated microseismicity. Pressure was increased in a series of incremental steps until dilation occurred and a simultaneous drop in pressure followed by a very minor microseismic event. The scale of this in-situ experiment does need to be placed in context. The shear on the fault was on the scale of micromillmeters to ~0.4mm and the radius of the induced seismic event occurred over a radius of 1.2m. The entire experiment from initial pressurisation to dilation, slip and movement took place over a span of 150 seconds. The value of this type of experiment is that it demonstrates how increases in fluid pressure can change fault properties for short periods. It also shows how properties such as permeability can vary across a fault.

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