Water-rock interaction in the Bakki low-temperature geothermal field, SW-Iceland

Abstract

Water-rock interaction is a fundamental process in natural geothermal systems, both in high-temperature and low-temperature ones. The Bakki geothermal field which is utilized for a district heating system, is a low-temperature geothermal system with the maximum measured temperatures varying from 120 to 137°C in the wells. A relict high temperature alteration is evidenced by the occurrence of the mineral epidote. Reservoir rocks are alternatively basalt lavas and basaltic hyaloclastites. A silica-enthalpy mixing model indicates that the reservoir temperature may exceed 150°C. Use of CHILLER, a program for computing water-rock reactions, for mixing model suggests that the change in chemical composition in well HJ-01 revealed by chemical monitoring may be interpreted by dilution of the geothermal water with fresh local groundwater in a ratio of about 0.5-0.6. Based on the thermal fluid chemistry as well as the secondary minerals occurring, a modelling study was performed by using CHILLER. Due to the complicated alteration history of the area, two water-rock interaction models, one at present temperatures and one at high temperature, were applied. Most of the important alteration minerals such as calcite, laumontite, stilbite, clinochlore, epidote-ord ...