Time-Lapse Seismic Tomography Analysis For Reservoir Characterization Within A Liquid-Dominated Geothermal System In Sumatra, Indonesia

Microearthquake (MEQ) is a typical geothermal geophysics surveillance method at exploitation phase. The objective of this monitoring technique primarily to investigate velocity changes within the reservoir for certain period of time due to production and reinjection activities. The MEQ observation in this particular geothermal field was carried out continuously using permanent local monitoring network. During three years period of monitoring, there were recorded over 22,000 P wave and S wave observations from earthquake which having gap angle less than 180 degrees. All of the P and S wave onset are hand-picked where the shallow portion of the initial 1D velocity structure derived from sonic log reading from several wells. From the beginning of the observation, there were additional reinjection well pads at the southern part due to additional power generations at the field. To monitor the changes of velocity in the vicinity of production and reinjection wells, travel time seismic tomographic inversions were performed to generate the three-dimensional Vp and Vp⁄Vs structure of the reservoir at each time period. The ray tracing was calculated using pseudo-bending technique whereas the tomographic parametrization, including grid geometry and damping determination, was tested using the so-called checkerboard resolution test. Based on the tomographic inversion results, observed an extensive high Vp⁄Vs anomaly from elevation 1 km at the southern part of the field which attributed to the fluids addition from the reinjection wells. This process may increases Vp by increasing the bulk modulus of rocks while at the same time decreases Vs. The velocity model at the production part were not well resolved due to the MEQ event mostly located in the vicinity of reinjection well pads. However, subtle low Vp⁄Vs anomaly at the same depth still could be detected at the northern part of the field. To enhance the Vp⁄Vs anomaly, we also construct the relative Vp⁄Vs map (∆Vp⁄Vs) between each period. The high ∆Vp⁄Vs from each epoch are consistent with the operational and reservoir dynamics of the field. Continuous MEQ surveillance and time-lapse monitoring of Vp, Vs and Vp⁄Vs have a critical contribution in geothermal exploitation phase due to its ability to provide information of reservoir properties characterization.