Castelnuovo area, in Larderello geothermal field, was exploited intensively over a period of more than 35 years, with productive wells that are generally 300-500 m deep. As a consequence of this exploitation the formation pressure in the upper part of the reservoir decreased below 4 ata. A few deeper wells, to $800 m depth, have shown shut-in pressures in the 6 - 10 ata range during these last few years.

In 1974 a deep well, Sperimentale 2, was drilled in this area in order to test steam production possibilities from deep horizons of the reservoir where sufficiently high pressures were still expected to be found. Having reached a depth of 1266 m (Fig. 1) the hole was cased from the surface to 829 m in order to isolate the shallower formations. A circulation loss occurred at 834 m, which represented the only fracture identified during drilling in the open-hole section of the well. No other eventual fractures were noted at greater depths as drilling continued without a fluid return.

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