3D Vp, Vs, And Vp/Vs microseismic tomography imaging on “MA” geothermal field: fluid saturation condition analysis

Abstract

“MA” Geothermal Field lies on andesitic volcano highlands in the center of Sunda Arc and has transition system between vapor and liquid-dominated reservoir. The reservoir characteristic has been changed over time caused by fluid injection and production. The objective of the research is to interpret the fluid saturation characteristic inside the reservoir based on 3D Vp, Vs, and Vp/Vs tomography image. Microearthquake (MEQ) activities in geothermal field may relate to changes in stress and fractures’ occurrence. P-and S-wave arrival time from 878 MEQ events are used for tomographic inversion. The initial hypocenters were determined using Geiger Adaptive Damping. To ensure the quality of data, we selected the events according to azimuthal gap and the recorded stations. The P-and S-wave arrival times are updated using waveform cross correlation. VELEST is used to determine 1D seismic velocities as input for SIMULPS12 tomographic inversion with hypocenter adjustment. The results show there are two clusters of MEQ events around “GB” and “WD” mountain. The distribution of MEQ around the fluid injection and production area may be related to weak zones and also reveal a structure with N-NE orientation. According to seismic velocity model, the production area is dominated with vapor (low Vp/Vs ratio value). Vapor dominated area has experienced pressure decrease caused by ongoing exploitation. There is also area that has high fracture density compared to other areas. Area that has high water saturation is situated around GB mountain (high Vp/Vs ratio value).