June 20, 2019
Journal Article

Chemical Trapping of CO2 by Clay Minerals at Reservoir Conditions: Two Mechanisms Observed by In Situ High Pressure and Temperature Experiments


The reaction of clay minerals in sedimentary rocks with supercritical CO2-rich fluids is poorly understood, but has potential implications for enhanced oil and gas production and geological C-sequestration. Experiments performed in situ at temperature and pressure relevant to reservoir conditions (T = 323 K and Pfluid = 90 bar) show that trioctahedral clay minerals can react with supercritical CO2 to produce carbonate phases by both ion exchange/precipitation reactions and dissolution/reprecipitation on a timescale of hours under certain conditions. The dissolution/reprecipitation reactions were observed in a synthetic, high surface area/high edge site surface area smectite (laponite) exchanged with Ca2+, Cs+, and tetramethyl ammonium (TMA+), and the ion exchange/precipitation mechanism was observed for a Pb-exchanged natural smectite (hectorite). Novel X-ray diffraction and NMR and infrared spectroscopic tools provide in situ observation of the reactions in real time supported by a suite of ex situ analyses. For the laponites, IR data show that HCO3- ion forms at water contents as small as ~5 H2O molecules/exchangeable cation. When the exchangeable cation is Ca2+, the IR data show the formation of carbonate as well, and the NMR results show formation of amorphous calcium carbonate at low water contents in addition to HCO3-. Laponites equilibrated at 100% R.H. at atmospheric conditions and then exposed to scCO2 generate additional, more mobile HCO3- ion and exhibit evidence of clay dissolution leading to a poorly crystalline or amorphous hydrous magnesium carbonate/bicarbonate that forms from Mg2+ released by dissolution of the octahedral sheet. The 100% R.H. sample with exchangeable Ca2+ also forms calcite, vaterite and aragonite precipitates. Comparison of these results with those already published in the literature suggest that a high edge site surface area is crucial to this process occurring on a short timescale. In the Pb-exchanged hectorite exposed to scCO2, once a critical humidity threshold is reached, cerussite (PbCO3) rapidly forms concurrent with exchange of interlayer Pb2+ with H3O+ formed by reaction of CO2 with water on the clay surface. Given that such a reaction is not observed on a similar timescale with more common Ca2+ or Na+ in hectorite and other smectites, the low solubility of cerussite appears to be the thermodynamic driving force for this process, meaning that such reactions are expected to occur more slowly in most natural samples. However, the results suggest that both types of CO2-capturing processes could occur on short and/or geological timescales during CO2 flooding of oil and gas reservoirs, in C-sequestration reservoirs, and as the result of fracking using CO2-based fluids.

Revised: February 10, 2020 | Published: June 20, 2019


Bowers G.M., J.S. Loring, H.T. Schaef, S.S. Cunniff, E.D. Walter, S.D. Burton, and R.K. Larsen, et al. 2019. Chemical Trapping of CO2 by Clay Minerals at Reservoir Conditions: Two Mechanisms Observed by In Situ High Pressure and Temperature Experiments. ACS Earth and Space Chemistry 3, no. 6:1034-1046. PNNL-SA-140918. doi:10.1021/acsearthspacechem.9b00038