HEAT-LOSS TO THE SURROUNDING ROCK FORMATION FROM A GEOTHERMAL WELLBORE

The impact that heat loss to the surrounding formation may have on the fluid temperature and pressure in geothermal wells is investigated an d quantified. An extensive parametric study is conducted for three dif ferent rock temperature profiles at flow-rates of 50, 100 and 150 t/h over the range of 1-1000 days elapsed flow time. The flow-rate is iden tified as a key parameter determining the wellhead conditions, with th e elapsed time and the geotemperature gradient playing a secondary rol e. Scale analysis has been applied to derive a proportionality involvi ng the Peclet number, the total length of the well, the average temper ature difference between the flowing fluid and the surrounding formati on over the entire interval, and a decaying time function. The results of the scale analysis have been validated through numerical calculati ons over an extended range of single-phase and two-phase flow conditio ns. With the present model, some data from the Ogachi HDR experiments have been analyzed. Assuming appropriate temperature profiles and flow -rates reflecting the specific reservoir and rock conditions at Ogachi , we have obtained downhole temperature profiles and wellhead temperat ure histories very close to those observed during the experiments. For comparison, with no heat loss conditions assumed, calculated wellhead temperatures diverge by approximately 30 degrees C from the measured ones. (C) 1997 CNR.