The southern boundary of Larderello geothermal field

Abstract

The southern area of the Larderello geothermal field presents some typical characteristics of the peripheral areas of vapor-dominated systems where the reservoir passes from confined to unconfined conditions. In the natural state, steam rising from depth in upflow zones migrates laterally towards the zones where the reservoir becomes unconfined, partly condensing and partly feeding natural manifestations. Two different zones are recognizable: (a) Monterotondo, where the vapor-dominated system and the cold external aquifers are separated by a low permeability “barrier” and the physical conditions change strongly over short distances. (b) Lagoni Rossi-Carboli, where the two systems are connected, at shallow depth, through thin, isolated, permeable channels and interfere with one another. The physical conditions there vary gradually with distance. The gas content in the fluid produced increases with the distance from the upflow zones, but at a lower rate in the vicinity of confinement boundaries than in zones in which the reservoir is confined throughout. The accumulation of gas is avoided by the venting to the atmosphere of the laterally migrating steam. Water recharge increased to a significant rate only when the reservoir pressure declined and water influx took the place of the original steam outflow through the shallow permeable formations.