The petrophysical properties of geothermal reservoir rocks have been investigated at
in-situ conditions in the laboratory. For this purpose, cylindrical samples from a
reservoir in volcanic rocks (Iceland) and samples of Fontainebleau sandstone, a
typical porous reservoir rock, were subjected to elevated temperatures, pressures,
and pore-pressures, using a pore-fluid with well-defined salinity. Electrical
reisitivity and seismic velocities were measured, with increasing temperature (Tmax:
250°C) at constant pressure and pore-pressure, similar to reservoir conditions. After
several temperature cycles, pore-pressure was slowly reduced at a constant
temperature of 150°C by pumping pore-fluid out of the sample, until the liquid –steam
phase transition was reached. 

When the phase transition was reached, no further change in pore-pressure was
observable, despite the continuous release of pore-fluid from the sample. The loss in
fluid volume was compensated by steam, until the phase transition was completed for
the entire porespace. Only when pore-fluid was pumped out beyond this point the
pore-pressure continued to drop.

During this process, electrical resistivity showed a gradual increase with increasing
steam content in the pore-fluid, while the seismic signals (p-waves only) showed a
slight decrease in velocity. Increasing pore-pressure again and that way approaching
the phase transition from the steam field did not result in the initial resistivities
of the fully-saturated rock.

These preliminary results suggest that the liquid-steam transition in geothermal
reservoirs should be detectable by geophysical methods.

Campus SIAF, SP del Monte Volterrano
Localita' Il Cipresso
Volterra, Italy