Navajo Sandstone-Brine-CO2 Interactions: Implications for Geological Carbon Sequestration

The injection of CO2 into deep saline aquifers is presently an attractive option for greenhouse gas mitigation. The Navajo sandstone or its geological equivalent Nugget sandstone is selected as the target storage for large volume injection tests. To test the feasibility of this strategy, a total of three hydrothermal experiments involving Navajo sandstone dissolution in acidic brine at 200oC and 250-300 bars were conducted. The first experiment (batch) was sandstone interaction with CO2 impregnated brine; the second experiment (batch) was sandstone dissolution in CO2-free acidic brine, served as a control; the third was a flow-through experiment. The solution chemistry data indicated that the dissolved SiO2 increased gradually and pH increased slowly with reaction progress. Surprisingly, there was only a slight decrease for dissolved CO2 concentration during the first experiment. Analysis of sandstones (SEM and XRD) following the three experiments showed that typical dissolution features (e.g., terraces, kinks, steps) were evident on the surface of the K-feldspar. The secondary minerals were mainly clay minerals. Mineral saturation indices were calculated with speciation-solubility modeling. The solution was generally undersaturated with respect to indigenous minerals (quartz and K-feldspars), and supersaturated with respect to clay and carbonate minerals (if with CO2).