Heterogeneous Hydrolysis of a Thermally Degrading Tracer

Abstract

A major obstacle in commercializing geothermal resources is predicting the potential for cooling of production wells, particularly in the presence of cold water reinjection. Thermally degrading tracers such as phenyl acetate may enable advanced warning of “premature thermal breakthrough” if compounds with appropriate and known temperature-dependent reaction kinetics can be identified. This work investigates the potential for accelerated reaction rates in the presence of mineral surfaces (i.e., a heterogeneous reaction path). First, a homogeneous aqueous solution buffered with citric acid and sodium phosphate dibasic was spiked with phenyl acetate and the hydrolysis reaction was measured by spectrophotometry for temperatures ranging from 20 to 50 °C. We then performed heterogeneous experiments with silica beads in a packed-bed reactor. After submerging the silica beads for several weeks, the measured reaction kinetics suggested that the activation energy for the heterogeneous conditions agree well with the homogeneous case, but the pre-exponential factor increased by several orders of magnitude. Interestingly, the accelerated reaction rates were observed only after fluid-mineral contact had been established for several days. We suggest a rate-limited surface maturation processes to describe this time-dependency.