Geochemical impact of re-injecting geothermal waste waters: example, Larderello, Italy

Abstract

Abstract The impaccts of re-injection of geothermal waste waters on the physico-chemical characteristics of the fluids of the Larderello vapour-dominated geothermal field in Tuscany (Italy) were monitored using deuterium and oxygen-18 as ‘natural’ tracers. The observed variations were due mainly to mixing between the re-injected and deep components. Large isotopic fractionations occur at depth during the evaporation of the re-injected water in the reservoir 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 from the monitored wells. Maps showing the isotopic distribution of the steam and the distribution of the gas content have been generated for the whole Larderello geothermal field. Evaluation of data collected before and after re-injection reveals a good distinction between natural inflow induced by exploitation, and artificial recharge with waste fluid. Gases other than N 2 in the wells affected by injection are substantially diluted since condensates are formed by evaporation. Isotopic variations of H 2 , CH 4 , and CO 2 have been used to describe the disequilibrium conditions among gas components. This disequilibrium is induced by the settling of a liquid plume at the producing level in the reservoir.