Abstract - Gas Threshold Pressure Test performed at the Mt Terri Rock Laboratory: Experimental Data and Data Analysis – Migration of gases in argillaceous formations increasingly attracts the attention of various geoscientific disciplines for purposes like CO2 sequestration, gas storage in geological formations and disposal of hazardous wastes. Quantitative assessment of gas transport in such ultra-low permeability formations requires information on gas transport properties of the argillaceous formation. A series of hydro- and gas tests in boreholes was performed at the Mont Terri Rock Laboratory in Switzerland to investigate gas transport in the Opalinus Clay, an indurated clay formation of Jurassic age. This paper presents the results of a comprehensive test campaign in the shaly facies of the Opalinus Clay, comprising a “classical” hydrotest with water injections followed by pressure recovery sequences and an extended gas threshold pressure test. A detailed interpretation of both the hydraulic and gas test sequences was conducted using numerical simulations for the data analysis (software TOUGH2/iTOUGH2). In contrast to former test analyses, the sequential interpretation of the hydro- and gas test sequences provided a consistent set of single-phase hydraulic parameters and two-phase flow parameters. The use of independent information from laboratory testing (porosity, capillary pressure curves) constrained distinctly the inverse problem of parameter fitting. Discrimination between different parametric models of the relative permeability was impaired by inherent limitations of the field data due to non-ideal test conditions. Nevertheless, a classical two-phase flow type capillary pressure - relative permeability relationship based on the Van Genuchten - Mualem approach was found to be valid. A supplementary design study was conducted to optimise the test procedures to better distinguish between relative permeability models. It is shown, that long term (> 1 year) injection tests are more adequate to constrain the models. This finding is important for the design of future gas threshold pressure tests in indurated clays.