Influence of dissolved ions on determination of oxygen isotope composition of aqueous solutions using the CO₂‐H₂O equilibration method

Abstract

The isotope salt effect and the kinetics of isotope exchange must be taken into account when the oxygen isotope composition of a saline aqueous solution is determined using the CO(2)-H(2)O equilibration method. Our experimental data and the proposed prediction method provide essential guidelines for the accurate δ(18)O analysis of saline aqueous solutions.

How long is enough: CO2‐H2O equilibration for δ18O analysis in saline formation waters?

“…Some researchers have suggested that different ions have different effects on the exchange rate of oxygen isotopes between CO 2 and H 2 O. [12,13] For example, a sequence of oxygen isotope exchange time has been suggested as: MgSO 4 > CaCl 2 = NaSO 4 > NaCl > MgCl 2 > KCl > H 2 O. [13] This study has indicated that the total dissolved salts (TDS) is the main factor controlling the equilibration time in petroleum formation waters.…”

“…[12,13] For example, a sequence of oxygen isotope exchange time has been suggested as: MgSO 4 > CaCl 2 = NaSO 4 > NaCl > MgCl 2 > KCl > H 2 O. [13] This study has indicated that the total dissolved salts (TDS) is the main factor controlling the equilibration time in petroleum formation waters. Although some ions (e.g., SO 4 2and K + ) are present at ten times higher concentrations in samples C and D than in samples A and B, the equilibrium times required for samples A and B are still double or triple of those for samples C and D, as shown in Tables 1 and 2.…”

“…Such ion‐enriched waters collected from petroleum exploration and production wells can contain a total dissolved salt content higher than 300 g/L, which will compromise the precision and accuracy of the δ 18 O analyses. It has been shown that the equilibration kinetics is a function of the salinity or ion strength of the water . The formation waters of petroleum reservoirs with high total dissolved salts (TDS) commonly have a relatively slow exchange rate between CO 2 and H 2 O, and a period of 24 h may not be enough for the O‐isotopes to reach equilibrium between CO 2 and the formation brines, resulting in erroneous and unreliable measurements.…”

“…It has been shown that the equilibration kinetics is a function of the salinity or ion strength of the water. [10][11][12][13] The formation waters of petroleum reservoirs with high total dissolved salts (TDS) commonly have a relatively slow exchange rate between CO 2 and H 2 O, and a period of 24 h may not be enough for the O-isotopes to reach equilibrium between CO 2 and the formation brines, resulting in erroneous and unreliable measurements. There has been no published study regarding how long the equilibration between CO 2 and formation brines with a specific salinity will take.…”

Enzymatically catalyzed CO2‐H2O equilibration for oxygen isotope analyses of aqueous samples

“…The oxygen isotope exchange between CO 2 and H 2 O proceeds until oxygen isotope equilibrium is attained, which usually takes ~24 h or longer in a closed reaction vessel at 25°C. This isotopic equilibration time can be reduced significantly by shaking the reaction vessel that contains the aqueous sample (e.g …”