Tritium tracer Injection test at Wayang Windu Geothermal Field, West Java, Indonesia

The Wayang Windu Geothermal Field (WWGF), with an installed capacity of 227 MWe (Units 1 & 2), has been in production for eighteen (18) years. Currently, there are twenty-nine (29) active production wells, located mainly in the northern and central areas of the field (Gambung-Wayang). About 100 kg/s of condensate and 60 kg/s of brine are disposed of (by gravity) to three (3) injection wells in the southern (Windu) area, with less than 5% recharge to the reservoir (NDSA tracer test). A tracer injection test was conducted in 2014 and 2016 to better understand the vertical connectivity between the deep brine and the steam cap reservoir in the northern Gambung area. The tracer used was tritium in water or tritiated water (HTO). Around 20 ml (equal to 20-23 Ci) of tritium was injected using capillary tubing to the depth of deep liquid feed zones in two wells, MBE-5 and WWS-1, located in the central area of the field. Tracer returns were monitored mostly in dry steam wells in the northern Gambung area. Interpretation of the tracer return profiles included 1) the tracer breakthrough time; 2) time of maximum concentration, which reflects the average fluid velocity; 3) the width of the tracer 'pulse', which reflects the flow path dispersion; and 4) tracer recovery as a function of time. Three typical tracers return profiles were observed (fast, intermediate, and slow); the shape of the tracer 'pulse' was consistent with a single flow-path. Monitoring wells WWQ-5 and WWT-2 showed the highest mass recovery and the fastest breakthrough times. Total mass breakthrough calculated from the tritium tracer (using TRINV calculation) was about 5%, which is similar to results from earlier liquid tracer tests (NDSA), albeit for a different area of the field (laterally and vertically). Despite the low percentage of mass breakthrough, the data may provide some insight for assessing possible future central-deep infield-injection strategy. The different tracer returns profiles observed in the monitoring wells may contribute to refining the structural interpretation and identifying preferred fluid pathways. Overall, the results of the tritium tracer test provide some insight about the heterogeneity of the fracture network that controls how the tracers are dispersed.