COMPETITIVE INFLUENCE OF H2S ON CO2 MEASUREMENTS IN GROUNDWATER BY MULTIPLE VOLUMETRIC EXPANSION

Geological carbon capture and sequestration (CCS) has been proposed as a way to lessen global climate change due to fossil fuel combustion, but the potential migration of CO₂ and mobilized pollutants to groundwater associated with these sites is a concern. The CarboQC carbonation meter is a method of CO₂ analysis in water that is based on the ideal gas law by measuring equilibrium temperature and pressure resulting from the multiple expansion of a sealed sample volume. This method is effective using carbonated beverages and most natural waters because the solubility of potentially interfering O₂ and N₂ gases is over 50 times lower in water than it is for CO₂. However, hydrogen sulfide (H₂S), which is even more soluble than CO₂ in water, is occasionally encountered in very high concentrations in groundwater samples, especially those associated with high-S petroleum deposits or natural gas production. In this study, we examined the effects of H₂S associated with water samples from a natural spring in Virginia and saline produced waters from enhanced oil recovery (EOR) wells in Texas on the measurement of CO₂ using the CarboQC meter. Both copper sulfate and zinc acetate were tested as ways of eliminating the H₂S fraction via precipitation of metal sulfides and determining the CO₂ concentration by difference. Copper sulfide was rapidly precipitated in EOR well samples containing 500-800 mg/L H₂S, but the reaction simultaneously lowered the pH to ca. 1.5. As a result, detectable CO₂ levels were actually increased due to the net acidification of the sample. The potential for this analytical approach to be effective in such waters is considered and discussed relative to recently observed effects on the mutual interaction between the two gases and their respective solubilities in water.