temperature and pressure in the svartsengi ... - Orkustofnun

777 CAUSES OF TEMPERATURE CHANGESExploitation of the Svartsengl reservoir has caused thewithdrawal of about 40 million tonnes of geothermal fluid(Thorhallsson, personal communication, 1983), This hascaused about 13 bar pressure drawdown In the reservoir . Adecrease In pressure causes increased boIling in the top ofthe reservoir. The recharge of cold water can increase aspressure decreases In the reservoir. Episodic cold inflowfrom the overlying ground water can take place In associationwith earthquakes. The decrease In the temperaturecould also be caused by a decrease In the lateral flow ofhot fluid. This chapter will deal with some of thepossible cooling processes in the Svartsengl reservoir.7.1 BoilingThe salini ty of the geothermal fluid of the Svartsengireservoir is roughly 2/3 that of sea water. The saturationtemperature of this salt solution is higher than thesaturation temperature of pure water by a maximum of 2°C ata depth of about 2000 m (Michaelides, 1981; Grant, 1982)Little or no correction is thus needed when using thesteam tables.Exploitation has caused a pressure drawdown which makes itpossible for the reservoir fluid t o boil at greater depththan it could initially. In other words. the drop inpressure caused by the exploitation has increased theboiling In the upper part of the reservoir. During boiling.thermal energy is used in changing the state of the fluidfrom liquid to vapour and this causes cooling in thereservoir. However, as the level of boiling is migratingdownwards, hotter and hotter water comes into boilingconditions. This boiling sustalns the steam cap locatedaround wells SG-2. SG-3 and SG-l O. The effect of theboiling Is therefore to increase the temperature of thesteam cap. The temperature of the steam z one was about