Migration and storage characteristics of supercritical CO2 in anisotropic sandstones with clay interlayers based on X-CT experiments

Abstract

To date, many laboratory-based core scale (i.e. flooding) studies have focused on relatively homogeneous rock samples (e.g. Berea sandstone) to study the migration characteristics of supercritical carbon dioxide (SC-CO2) and evaluate these rocks for their potential geological CO2 storage. Clay interlayers with low porosity and permeability developed in sandstones are often encountered in target reservoirs and can have an obvious influence on the distribution of injected SC-CO2 and the overall storage capacity of the reservoir. To understand the impact of clay interlayers (which act as thin layers of low porosity/permeability rock with the surrounding rock being relatively homogeneous with much higher porosity/permeability), samples containing these interlayers were cored in two different orientations and both samples were subjected to core flooding experiments. A high-resolution X-ray computed tomography (X-CT) scanner was used to monitor the behavior of the fluid inside the samples during the experiment. Using X-CT iamges and differential pressure data, the migration and distribution characteristics of SC-CO2 inside the samples during the drainage process (i.e. the process where SC-CO2 displaces brine) are analyzed. The results show that the drainage process for both samples occurs in three steps: channel creation, channel extension and channel stabilization. However, we find that the orientation of the clay interlayer has a significant effect on the SC-CO2 migration characteristics and storage capacity and that the SC-CO2 generally avoids the clay interlayer during drainage. When the injection direction is parallel to the clay interlayer, it acts like a shunt to separate the SC-CO2 flow and any fingering phenomenon is separated on either side of the clay interlayer with little crossflow. When the injection direction is nearly perpendicular to the clay interlayer, it acts as a barrier which prevents SC-CO2 moving ahead and affects the pressure propagation. The final distribution of SC-CO2 resembles sweep behavior before the clay interlayer; however, fingering phenomenon occurs after the clay interlayer. The displacement efficiency and storage capacity decreased by 43.14% and 40.74% by changing the orientation of a less than one-millimeter thick clay interlayer vertical to the flooding direction.