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Gas transport properties of clays and mudrocks

Abstract Controlled flow rate gas injection experiments have been performed on clay and mudrock samples using helium as a permeant. By simultaneously applying a confining stress and back-pressure, specimens were isotropically consolidated and fully water saturated under predetermined effective stress conditions, before injecting gas at a very slow rate using a syringe pump. Ingoing and outgoing gas fluxes were monitored. All tests exhibit a conspicuous threshold pressure for gas breakthrough. All tests showed a post-peak negative transient leading to steady-state gas flow. On the basis of a stepped history of flow rate, the flow law was shown to be nonlinear (i.e. non-Darcian). With the injection pump stationary (i.e. zero flow rate), gas pressure declined with time to a finite value. No gas flow was ever detected at excess gas pressures less than this lower threshold. When gas flow was re-established, the threshold for gas breakthrough was found to be significantly lower than in the virgin clay. There is strong evidence to suggest that the capillarity restrictions on gas penetration of the intergranular pores of saturated clays and mudrocks are of such a magnitude that normal two-phase flow is impossible. Gas therefore does not occupy, or flow through, the intergranular porosity of the clay matrix. In the absence of pressure-induced cracks, water-saturated clays and mudrocks are totally impermeable to gas. The measured gas permeability of a clay-rich medium is a dependent variable rather than a material property, as it depends on the number of pressure-induced pathways in the plane normal to the flow, together with the width and aperture distributions of these pathways. The experiments suggest that the flow pathways open under high gas pressure conditions and partially close if gas pressure falls, thus providing a possible explanation of the nonlinearity of the flow law. Reliance on conventional two-phase flow theory is inadvisable when attempting to quantify gas transport in initially water-saturated clay-rich materials.