Stable isotope study of reinjection processes in the Larderello geothermal field

Abstract

The effects of reinjected waters on the physico-chemical characteristics of the fluids of the Larderello vapor-dominated geothermal field were monitored using deuterium and oxygen-18 as “natural” tracers. During and after reinjection, strong short-term variations of the isotopic composition of the steam were observed near the reinjection area. These variations were due mainly to mixing between the reinjected and ‘deep’ components. Large isotopic fractionations occur at depth during the evaporation of the reinjected water, and may affect the evaluation of the “recovered” fluid using a simple mixing model. Stable isotope and gas/steam ratios are closely correlated in the fluid collected in the monitored wells, in agreement with the hypothesis that an effective mixing process occurs at depth, accompanied by the formation of a liquid plume subjected to different degrees of evaporation. The non-linear correlation between isotope and gas/ steam ratios has been interpreted with a model where the deep areal chemical differences in the original fluids are modified by reinjection.