CO 2 migration processes in argillaceous rocks: pressure-driven volume flow and diffusion

Abstract

The capillary sealing efficiency of fine-grained sedimentary rocks has been investigated by gas (CO 2, N 2) breakthroughexperiments and CO 2 diffusion experiments. Experiments were performed on initially fully water-saturated samples. Absolute (single phase) permeability coefficients ( k abs), determined by steady-state flow tests, ranged between 6 × 10 −22 and 5.5 × 10 −19 m 2. Maximum effective permeabilities to the gas phase ( k eff), measured after gas breakthrough, ranged from 1 × 10 −23 up to 1.1 × 10 −18 m 2. Capillary displacement pressures ( P d) ranged from 0.06 to 6.7 MPa. However, for individual claystone samples, gas breakthrough with subsequent pressure-driven volume flow (Darcy flow) was not observed even at much higher differential pressures (15 MPa). In these instances, molecular diffusion is the dominating transport process. It was shown that for nominal permeability coefficients below 10 −24 m 2, a distinction between pressure-driven volume flow and diffusion processes is no longer possible. Repeated diffusion experiments with CO 2 on the same sample plug yielded varying effective diffusion coefficients ranging from 10 −9 to 10 −11 m 2/s. This variability is taken as an indication of complex mineral matrix interactions affecting the molecular transport properties.