Subsidence and Gravity Changes Induced by Exploitation in the Travale-Radicondoli Geothermal Field (Tuscany, Italy)

Abstract

Fifteen precise leveling surveys since 1973 have revealed that Travale-Radicondoli geothermal area is affected by subsidence at a maximum rate of 2.5 cm/year in the first 10 year period and about 2 cm/year since 1983.

Since 1979 eight microgravity surveys have detected in this area very small gravity changes of the order of a few uGal/year. At times these are at the limits of the confidence level of the experimental data (15 uGal), or just below the limits of the noise effects introduced by seasonal meteo-climatic conditions.

Maximum gravity changes, corrected for the subsidence effect (Ag) so not exceed a total range of a few tens of uGal and show a constant trend in time, but different characteristic distributions from zone to zone.

The eastern zone, the most intensely exploited part of the field, has been producing since 1973. Since then subsidence values of more than 40cm have been observed, associated with the largest negative Ag anomalies, although not exceeding the 30 uGal recorded since 1979. These negative Ag values could be attributed to a mass deficit in the reservoir rocks of the order of just 3% of the mass of fluids delivered by the local wells during the 1979-91 period.

This hypothesis is consistent with an actual quasi-equilibrium dynamic state of the fluids in the field where the exploited mass is almost entirely reintegrated by fluid recharge.

The western zone has gradually been exploited over the last few years and has experienced only marginal effects of subsidence so far, mostly due to the exploitation of the eastern basin. In this zone a positive trend of the Ag values is evident, yet they only just exceed the confidence level of the observed data.