Temperature sensitivity of reactive ester tracers for measuring CO2 residual trapping capacity

• Reactive chemical tracers are used to estimate formation residual CO2 saturation, Sc.
• The supercritical CO2/water partition coefficient is sensitive to temperature.
• These changes must be accounted for in simulations of tracer behavior to obtain Sc.

An understanding of the fundamental properties of chemical tracers (e.g. partitioning and sorption) is critical for assessing their subsurface behavior and to accurately model and interpret field tracer data. Carbon dioxide–water (CO2/H2O) partition coefficients have been previously determined for the reactive ester tracers, propylene glycol diacetate, triacetin and tripropionin, suggesting their suitability for assessing the residual trapping capacity of a geological formation for CO2 in a single well chemical tracer test. Understanding the temperature dependent behavior of chemical tracers significantly enhances their utility in field trials leading to a more accurate estimate of reservoir properties. However, very little information is known on the relationship between temperature and the partitioning process for these chemical tracers. In this paper, the temperature dependence of the partitioning process has been determined for temperatures ranging from 60 to 120 °C. The variations in the partition coefficient with temperature were shown to correlate reasonably well with the van't Hoff equation for all tracers. The enthalpy associated with the partitioning process was determined by a regression fit to this equation and was shown to be the smallest for propylene glycol diacetate and the largest for tripropionin. It was found that the estimated enthalpy is approximately linearly correlated with the number of carbon atoms in the organic ester. Furthermore, we have shown that the temperature dependence of the tracer partition coefficient causes significant variation in the estimate of residual saturation.