Experimental Measurements and Multi-Scale Modeling of the Relative Gas Permeability of a Caprock

Abstract

An experimental characterisation of a low permeability caprock is first reported. Two different methods are compared to measure the porosity: water imbibition and weighting versus gas (Argon) pycnometry. Water measured porosities appear to systematically underestimate gas measured porosities. A capillary pressure curve is derived from successive equilibrium of samples at imposed relative humidities. The relative gas permeability is thoroughly measured on the same samples. Gas porosity and effective gas permeability measurements have been carried out under different confining pressures. All measured relative gas permeabilities lie on a master curve and present a low critical water saturation beyond which the effective gas permeability nearly vanishes. In an attempt to model this last feature, the salient properties of the microstructure are extracted from electron microscopy images to propose a micro-macro model of the relative gas permeability. The multi-scale modeling is performed within the framework of random media or so-called continuum micromechanics, including localised flow effects within interfaces. The self-consistent homogenisation scheme allows retrieval of a critical water saturation, whose value is governed by the shape of the elementary particles of the material and the localisation of water within the pore space.