Natural State Model of the Nesjavellir Geothermal Field, Iceland

The Nesjavellir geothermal system in southern Iceland is very complex from both a thermal and hydrologic point of view. There are large pressure and temperature gradients in the wellfield and zones with drastically different pressure potentials. Thus, natural fluid flow is substantial in the system and flow patterns are complex. We have developed a twedimensional natural state model for the Nesjavellir system that matches reasonably well the observed pressure and temperature distributions. The match with field da t a has allowed determination of the energy recharge to the system and the permeability distribution. Fluids recharge the system at rate of 0.02 kg/s/m with an enthalpy of 1460 kJ/kg. The permeability in the main reservoir is estimated to be in the range of 1.5 to 2.0 md, which agrees well with injection test results from individual wells. Permeabilities in shallower reservoirs are about an order of magnitude higher. Most of the main reservoir is under twephase conditions, as are shallow aquifers in the southern part of the field. The model results also suggest tha t the low temperatures in the shallow part of the northern region of the field may be due to the young age of the system; Le., the system is gradually heating up. If this is the case the estimated age of the system near the wellfield is on the order of a few thousand years.