The geological sequestration of CO₂ is considered to have great potential. However, the heterogeneity that characterizes most geological reservoirs significantly affects the migration, trapping, and leakage of CO₂. The present study involved a pore-scale investigation of the effects of heterogeneity in pore size and wettability using stratified sand packs. A stratified glass bead (BZ06+BZ02) pack and a quartz+dolomite pack were introduced in flow experiments at gaseous CO₂ (gCO₂) and supercritical CO₂ (scCO₂) conditions. Different injection rates, namely, 0.1, 0.3, and 0.6 mL/min, were applied to the drainage while a constant rate of 0.1 mL/min was maintained for the imbibition. High resolution CT imaging and subsequent processing of the images were used to analyze the effects of heterogeneity in pore size and wettability. The results indicate that heterogeneity in pore size and wettability alters the intrinsic percolation abilities of the individual layers of the stratified structure and weaken the linear correlation between CO₂ volume and slice-averaged porosity. It was also observed that water films sometimes exist in the layers with smaller pore spaces after drainage and it can trigger snap off events.

Conclusions:

In this study, the effects of heterogeneity in pore size and wettability was investigated at pore scale using a micro-CT machine. Two stratified sand packs, namely, BZ06+BZ02 and quartz+dolomite packs, were used to perform displacement experiments. In the experiments, CO₂ was injected at different flow rates in drainage under gaseous and supercritical conditions, and water was re-injected at a constant flow rate during imbibition. Though the pore sizes of the utilized packs are larger than those of real natural rock cores, the similar pore structure gives us a better understanding of the nature of pore-scale CO₂ displacement in saline aquifers. The main findings of this study are summarized as follows.

For stratified sand packs with different pore sizes, a change in the injection rate or CO₂ phase mainly affects CO₂ distribution in the layer with smaller pores during drainage. With increasing injection rates or displacement of the gCO₂, CO₂ tends to form channels and exhibits a low displacement efficiency. In stratified sand packs with a different wettability, an increase in the injection rate or gCO₂ displacement gives rise to a more unstable displacement in both the weak and strong water-wet layers.

Stratified structures with variable pore sizes or wettability change the intrinsic percolation ability of the constituent layers, particularly those with smaller pore spaces or stronger water-wet characteristics. Stratified structures also weaken the linear correlation between CO₂ volume and slice-averaged porosity observed in homogenous cores. Moreover, CO₂ saturation sharply changes in the transition region between two layers during drainage. A considerable amount of water films remains in the small pore space layers of stratified structures after drainage, and this could trigger the occurrence of snap off events even during drainage. In addition, during the imbibition process, capillary trapping is more likely to occur in the layers with small pore spaces or strong water-wet characteristics, or the regions between two layers.

The results have been published on Greenhouse Gas Science Technology 7:972–987 (2017).