Experimental study of the stress dependence of the absolute and relative permeabilities of some tight gas sandstones

Connected porosity and dry gas permeability at 5 MPa hydrostatic stress have been measured for thirteen tight gas sandstone samples originating from a Rotliegend sandstone reservoir in Germany. The samples have then been preserved into hermetic chambers at fixed relative humidity (RH) values, until mass (and water saturation level) stabilisation. This allows a first characterisation of pore size distribution showing that most of the pore network is constituted with pores with entrance radii greater than 50 nm. The relative gas permeability was then measured for different water saturation and hydrostatic stress (up to 60 MPa). The analysis of the results has allowed us to identify the limiting conditions (water saturation and hydrostatic stress) for the apparition of a hydraulic cut-off, also called the “permeability jail”. Dry gas permeability at low confining pressure (5 MPa) has appeared to be a good indicator of the sensitivity of a sample to both hydraulic stress and liquid saturation. This gives an efficient tool to predict the global behaviour of the gas reservoir despite its heterogeneity with a single gas permeability measure.