INFLUENCE OF H2S ON THE ANALYSIS OF CO2 USING THE VOLUMETRIC EXPANSION METHOD IN WELL WATERS ASSOCIATED WITH CO2-ENHANCED OIL RECOVERY

Carbon dioxide-enhanced oil recovery (EOR) is a method used to supplement conventional water flooding approaches and flush residual crude oil from subsurface rock formations towards production wells for recovery. CO₂ is injected along with water under pressure and it moves through the pore spaces of the rock, mixes with and lowers the viscosity of the oil, and helps displace oil from the rock formation. The concentration of CO₂ in production water is needed to determine CO₂ breakthrough rates at the production well, as well as identify potential paths of CO₂ leakage into groundwater reservoirs. The CarboQC carbonation meter is used to measure the concentration of CO₂ in water under field conditions by monitoring the temperature and pressure changes during the multiple expansion of a sealed volume of water and calculating the P_CO2 from the ideal gas law. This method is effective using carbonated beverages and most natural waters because the solubility of potentially interfering common gases such as O₂ and N₂ is more than 50 times lower in water than it is for CO₂. Hydrogen sulfide (H₂S), which is even more soluble than CO₂ in water, is sometimes encountered in very high concentrations in water associated with high-sulfur petroleum deposits or natural gas production, and this directly interferes with CO₂ analysis by volumetric expansion. In this study, we examined the effects of H₂S on CO₂ measurements in saline produced waters from EOR wells in Texas. Two methods were tested as ways to subtract H₂S from the dissolved gas fraction. Sulfide was oxidized with hydrogen peroxide, which produced elemental sulfur or sulfate as a reaction product depending on the pH of the water sample, but this generated gas pressure due to peroxide reaction with other organics. Copper sulfate was added to eliminate the dissolved sulfide via precipitation of highly insoluble copper sulfides. Copper sulfides were rapidly precipitated in EOR well samples, but the reaction simultaneously lowered the pH relative to the amount of sulfide present. This resulted in a variable increase in the measured CO₂ due to the conversion of bicarbonate ion to carbonic acid. The potential for this analytical approach to be effective in such waters is further discussed relative to dissolved inorganic carbon content in water and the potential interactions between CO₂ and H₂S.