Experimental percolation of supercritical CO2 through a caprock

Abstract

Abstract Leakage of CO 2 into the atmosphere is the most crucial concern for geological storage of anthropogenic CO 2 . Leakage routes could develop through existing wells and pipelines, but also by natural migration of CO 2 rich pore-fluid through the caprock and in fault zones. Therefore, a thorough geological characterization of the prospective formation identifying seal capacity, integrity and possible migration pathways must be performed prior to injection of CO 2 . A caprock is a low permeable confining layer trapping CO 2 stored in a reservoir rock. Although the caprock acts as a seal, its lower boundary will be in contact with CO 2 saturated pore water or even pure CO 2 . Chemical interaction between the pore fluid and the caprock may change its material properties. The aim of this study is to increase our understanding of the interaction between CO 2 and caprock, focusing on the microstructural properties of the rock. A flow through cell is used to flood a shale core (40 mm long and 38 mm diameter) with supercritical CO 2 at a temperature of 35 ∘ C and at pressures above 7.5 MPa. A pressure gradient is applied across the sample to obtain a breakthrough of CO 2 in the core. During flooding both axial and radial strain of the core are measured together with the acoustic velocities in the axial direction. The measurements are performed both for the brine saturated core and at various stages during flooding with CO 2 . The experimental results suggest flow of CO 2 along defined pathways within the shale. These pathways are likely to be controlled by the increasing pore pressure at the bottom of the sample which allows reopening of the cracks in the lower part of the sample, allowing CO 2 to enter the shale. Flow of CO 2 into the cracks in the lower part of the sample is followed by percolation of CO 2 in the upper part of the sample where the effective pressure is higher and crack reopening is less likely to occur.