Investigation of the Balçova-Narlıdere geothermal field with the tracers

ABSTRACT Balçova-Narhdere geothermal field is used for heating of 630.000 m2 closed area for the welfare of thousands of people living in the region. This application is one of the largest geothermal energy based district heating projects in the world. Natural circulation of geothermal fluid and its effect on the re-injected reservoir are the main research topics of this study. In the study area, periodical water analyses at totally 17 wells and springs, oxygene-18, deuterium and tritium analyses at 14 localities, non-condensed gas measurements on 7 wells and gas analyses at 4 localities were performed. Results of water analyses yield that thermal waters are generally Na-HCOj-Cl type waters, they have a neutral character and they are slightly saline waters with total dissolved solid (TDS) contents less than 2000 ppm. The amount of non-condensed gases in the reservoir is less than 1000 ppm. Travertine deposits isn't observed around the hot springs. These characteristics indicate that geothermal water in the Balçova-Narhdere field is "heated groundwater" or "peripheral water mixed with deep chloride waters". It is determined that the presence of recharging ground waters from the S W of the field. In order to predict reservoir rock temperature, cation and silica geothermometers and silica-chloride mixing model were utilized. Results of cation geothermometers should be interpreted cautiously since waters have not been fully equilibrated with Na, K and Mg ions. On the basis of silica geothermometers and silica-chloride mixing model, reservoir in temperature was estimated to be 168-190°C. Isotope analyses indicate that O content in the waters is low and the field has an intermediate enthalpy (<180 °C). In order to determine flow direction and velocity of re-injected waters in the reservoir, tracer tests were performed. In Turkey, tracer tests in a geothermal field were first conducted in this study. As tracer, uranin was used and concentration measurements were carried out with flourometer which has a detection limit of 0.02 ppb. Tracer tests of B-9 and BD-2 wellstook 14 months to conduct and were repeated 5 times. Results of tests reveal that waters injected from the B-9 well flow through the shallow aquifer with a velocity of 2.3-12.7 m/h and they arrive at shallow production wells in the vicinity of re-injected waters in maximum 40 hours. In order to examine effect of re-injection on the production wells, well head temperatures were measured periodically. On the basis of one year observation, it was calculated that a total of 1.89 x 106 m3 water were produced in the field and 1.2 x 106 m3 of this amount were re injected. 58% of total re-injection was made only to the B-9 well. It was determined that waters pumped into the shallow aquifer caused 6-7°C cooling of shallow aquifer in annual base. This study is one of outstanding works where both tracer material and cooling were observed within the same wells. If re-injection into the B-9 well continues at the same amount, it was determined that shallow aquifer could not be sufficient for central heating in coming 3 or 4 years. As a result of re-injection from the BD-2 well, any cooling signs were detected in deep production wells. However, due to some problems encountered in protection pipes and cementing of this well, it was determined that waters uprise to the surface and polluted chemically shallow aquifer particularly in re-injection applications of high discharge.