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Title: Evaluation of Experimentally Measured and Model-Calculated pH for Rock-Brine-CO2 Systems under Geologic CO2 Sequestration Conditions

Reliable pH estimation is essential for understanding the geochemical reactions that occur in rock-brine-CO2 systems when CO2 is injected into deep geologic formations for long-term storage. Due to a lack of reliable experimental methods, most laboratory studies of formation reactivities conducted under geologic CO2 sequestration (GCS) conditions have relied on thermodynamic modeling to estimate pH; however, the accuracy of these model predictions is typically uncertain. In this study, we expanded the measurement range of a spectrophotometric method for pH determination, and we applied the method to measure the pH in batch-reactor experiments utilizing rock samples from five ongoing GCS demonstration projects. A combination of color-changing pH indicators, bromophenol blue and bromocresol green, was shown to enable measurements over the pH range of 2.5-5.2. In-situ pH measurements were compared with pH values calculated using geochemical models. The effect of different thermodynamic databases on the accuracy of model prediction was evaluated. For rocks comprised of carbonate, siltstone, and sandstone, model results generally agreed well with experimentally measured pH; however, for basalt, significant differences were observed. These discrepancies may be due to the models’ failure to fully account for certain proton consuming and producing reactions that occur between the basalt minerals and CO2-saturatedmore » brine solutions.« less
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Resource Type:
Journal Article
Resource Relation:
Journal Name: Chemical Geology, 359:116-124
Research Org:
Pacific Northwest National Laboratory (PNNL), Richland, WA (US)
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Country of Publication:
United States
CO2 sequestration, pH measurement, spectrophotometric