The Larderello geothermal field: a review

Abstract This paper reviews published geological, geophysical, petrological and geochemical data, together with reservoir engineering parameters, of the Larderello geothermal reservoir of southern Tuscany, Italy. Some theories are proposed on the deep origin, evolution in time, and natural state of this geothermal field prior to its exploitation. The interaction between the vapor-dominated reservoir and surrounding aquifers is discussed. Analysis of hydrogeochemical data suggests that meteoric waters from shallow external aquifers contribute little to the production of steam. The vapor-dominated system and the cold external aquifers are separated by areas of very low permeability where physical conditions change greatly over short distances. The presence and significance of liquid phases in the peripheral areas of the vapor-dominated reservoir are interpreted as being due to steam condensation. The latter is a consequence of: (a) low permeability conditions in the deep buried parts of the carbonate reservoir formations in the north-northwestern sector of the field, and (b) by the mixing of ascending superheated steam from depth with cold CaSO4 meteoric waters circulating in unconfined shallow aquifers within the outcrops of the carbonate reservoir formations in the south-southeastern parts of the field. The presence of steam below these liquid-dominated sectors of the reservoir seems possible. The limited natural recharge of the system, the constant steam production rate for geothermal power generation (3000 t h−1, since 1951), the persistence of natural thermal features, and the fact that reinjection has had little effect on the characteristics of the fluid discharged, seem to indicate a steady state for the Larderello field. The main contribution to production seems to be from superheated steam ascending from depth and the in situ evaporation of water stored in the small pores of the rocks present in Larderello, in a system that is much larger than the one known at present.

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