Quantifying CO2 capillary heterogeneity trapping through experiments, data analysis, and simulation

Abstract

CO2 geologic storage is an integral part of the climate change solutions in that it allows society to transition toward carbon neutral energy while still relying on fossil fuels. Both the primary and secondary trapping mechanisms immobilize the injected CO2 in the reservoir and keep it securely trapped. This work focuses on one such secondary trapping mechanism, residual trapping, which is caused by capillary forces induced by natural geological heterogeneities both at the pore scale and the mesoscale (millimeter scale). To differentiate between residual trapping at the two different scales, we call the former pore-scale residual trapping and the latter capillary heterogeneity trapping. The mechanisms and the various factors affecting pore-scale residual trapping have been extensively studied previously. However, only a handful of studies have examined the role mesoscale heterogeneity plays in capillary heterogeneity trapping. This work mainly focuses on predicting, identifying, and quantifying the CO2 capillary heterogeneity trapping potential of geological formations. We first use coreflooding experiments to investigate which parameter best represents the degree of mesoscale heterogeneity in the context of residual trapping in order to predict the amount of CO2 capillary heterogeneity trapping with an empirical relationship. Then, we use statistical analysis of the data to examine the effects of mesoscale heterogeneity on capillary dominated flow behaviors and to identify and visualize CO2 capillary heterogeneity trapping behavior in sandstone core samples. Finally, we use an improved percolation model to simulate and quantify the effects of mesoscale heterogeneity on residual trapping and the extent of such effects as compared to the pore-scale trapping mechanism. The data analysis results have confirmed that post-imbibition CO2 capillary heterogeneity trapping indeed occurs upstream of capillary barriers. Furthermore, it can also occur within the capillary barriers themselves. Both experiments and simulations have shown that CO2 capillary heterogeneity trapping increases with the degree of mesoscale heterogeneity. Macroscopic percolation simulation results have shown that the capillary heterogeneity trapping contribution can be quite large for heterogeneous rocks with good mesoscale capillary barriers. Experimental results show that the variance in the drainage CO2 saturation fields created under capillary dominated flow conditions can best capture the degree of mesoscale heterogeneity in terms of predicting CO2 capillary heterogeneity trapping