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We used IR and XRD, with supporting theoretical calculations, to investigate the swelling behavior of Na+-, NH4+-, and Cs+-montmorillonites (SWy-2) in supercritical fluid mixtures of H2O, CO2, and CH4. Building on our prior work with Na-clay that demonstrated that H2O facilitated CO2intercalation at relatively low RH, here we show that increasing CO2/CH4ratios promote H2O intercalation and swelling of the Na-clay at progressively lower RH. In contrast to the Na-clay, CO2intercalated and expanded the Cs-clay even in the absence of H2O, while increasing fluid CO2/CH4ratios inhibited H2O intercalation. The NH4-clay displayed intermediate behavior. By comparing changes in the HOH bending vibration of H2O intercalated in the Cs-, NH4-, and Na-clays, we posit that CO2facilitated expansion of the Na-clay by participating in outer-sphere solvation of Na+and by disrupting the H-bond network of intercalated H2O. In no case did the pure CH4fluid induce expansion. Our experimental data can benchmark modeling studies aimed at predicting clay expansion in humidified fluids with varying ratios of CO2and CH4in real reservoir systems with implications for enhanced hydrocarbon recovery and CO2storage in subsurface environments.

In 2013, the Pacific Northwest National Laboratory led a geologic carbon sequestration field demonstration where ∼1000 tonnes of CO2was injected into several deep Columbia River Basalt zones near Wallula, Washington. Rock core samples extracted from the injection zone two years after CO2injection revealed nascent carbonate mineralization that was qualitatively consistent with expectations from laboratory experiments and reactive transport modeling. Here, we report on a new detailed analysis of the 2012 pre-injection and 2015 post-injection hydrologic tests that capitalizes on the difference in fluid properties between scCO2and water to assess changes in near-field, wellbore, and reservoir conditions that are apparent approximately two years following the end of injection. This comparative hydrologic test analysis method provides a new way to quantify the amount of injected CO2that was mineralized in the field test. Modeling results indicate that approximately 60% of the injected CO2was sequestered via mineralization within two years, with the resulting carbonates occupying ∼4% of the available reservoir pore space. The method presented here provides a new monitoring tool to assess the fate of CO2injected into chemically reactive basalt formations but could also be adapted for long-term monitoring and verification within more traditional subsurface carbon storage reservoirs.