T Tracer testing is the most definitive method for learning about thehydrothermal properties of a geothermal reservoir. Tracers are usually inert.Tracer chemicals released into the environment are usually through injection wells,and are distributed throughout the reservoir by natural flow patterns. In thefollowing days or months, samples were collected from other wells throughout thegeothermal area, and tested for the presence of tracer. The concentration of tracerand travel through the reservoir at all times is monitored from various observationpoints which allow the researcher to model the hydrothermal characteristics of thereservoir. The tracer is used in geothermal reservoir engineering to determine theconnectivity between injection and production wells, and to prevent early coolingin the reservoir. Tracers that used in this field are 1.5 NDSA and 1.6 NDSA whichits use are only suitable for a water-dominated reservoir that has a temperatureresistance of up to 350 C. Tracer first appeared from the AN injection well to theproduction well on the first day more precisely at the 18th hour and from the AMinjection well to the production well on the second day. From this tracer sample,the data is processed in the TRINV software, which will then get the mass parametermultiplied by the tracer, this parameter which will be processed in Excel to find therecovery factor value. The result of the smallest recovery factor value from AMinjection well is 4.3% and the biggest recovery factor is 6.9%, while from ANinjection well the smallest recovery factor is 8.7% and the biggest recovery factoris 29%. From the recovery factor, we get the average cross-sectional area valuethat later can be used to predict the cooling of wells in the next 30 years. From thecooling prediction of wells for the next 30 years with a fixed rate at 361 kg/s andwith different temperatures it does not seem to have such a large impact on eachwell except the AC production well, the AC production well experiences the mostsignificant temperature change among other production wells, AC production wellhave decrease in temperature to 170 C with final temperature around 2430C in 30years by injection fluid rate of 361 kg/s at temperature 800C. From the result of thecooling effect on Yanara geothermal field it is suggested to always maintain theinjection fluid flow rate below 361 kg/s so there is no big decrease in productionfluid temperature.