Steam and Brine Zones Prediction Inside an Operated Geothermal Reservoir Based on Seismic Velocities Produced by Double Difference Tomography

Abstract

The knowledge about reservoir condition is one of the key factors for sustainable production in a geothermal field. Monitoring of geothermal reservoir by using micro-earthquake (MEQ) method has been implemented recently in several operated fields in Indonesia. Double difference seismic tomography is one of tomographic method that uses the absolute and the relative travel times which could be used to monitor geothermal reservoir by analyzing the distribution of produced Vp, Vs, and Vp/Vs ratio. This method had been implemented to a set of MEQ data that were recorded within a relatively short period of two months in 2005. Altogether, 17 seismometers were deployed in the field. Catalog data was created by re-picked waveform data in order to determined P and S wave arrival times. Micro-earthquake event were classified based on the difference between S and P arrival times (ts-tp) that were less than or equal to 3 seconds. In total 303 micro-earthquake events had been successfully identified and picked. This number is equivalent to 1798 P-wave and 1750 S-wave picked arrival times. Initial location was calculated using Geiger’s method and 1-D two layers system was used as initial velocity model. Hypocenter positions which were determined by Geiger’s method were relocated by using VELEST software in order to obtain a better velocity model and hypocenter positions. Furthermore, the double-difference tomographic method is implemented in order to obtain the best 3-D velocity model and hypocenter positions. Altogether, 197 events which have a gap value less than 180o between epicenter and station were used for input data of double difference tomography. Low Vp and Vs anomaly values from -20 % to -10 % of the background velocity model are concentrated in the northern part of research area. The existence of those anomalies and the appearance of low Vp/Vs ratio from 1.6 to 1.7 at elevation of +0.5 km to 1.0 km are interpreted as steam zone inside geothermal reservoir. The zone which has a high Vp anomaly values from 0 % to 10 % of the background velocity model, low Vs anomaly values from -15 % to 0 %, and high Vp/Vs ratio of 1.8 to 2.0 is interpreted as brine zone.