Estimasi Zona Saturasi Fluida dalam Reservoir Panas Bumi Berdasarkan Tomografi Seismik Atenuasi : Studi Kasus Lapangan Panas Bumi Wayang Windu
A good geothermal reservoir depends on the knowledge about reservoir geometry, temperature, saturations, state of saturations, pore pressure, porosity and permeability. The knowledge about fluid saturation zone inside reservoir is very important in order to determine the location of additional production wells and will be valuable information for reservoir management. The degree of fluid saturation associates with the vapor fraction, and it will provide potentially high energy conversion.
In this study, subsurface attenuation structure in Wayang Windu geothermal field is estimated by using micro-earthquake (MEQ) data induced by fluid injection activities for a period time of around 2 months within 2005 and is recorded by 17 seismometers. The data input for attenuation tomography is attenuation operator (t*), which was calculated by spectral fitting method. Both of calculated t* from 1,157 P- and 1,111 S-waves were used as input data, which are generated from 175 MEQ events. It has been used a time window length of 0.001 s to 0.0554 s to determine t* value and the average of corner frequency of 18.96 Hz. In order to invert Qp and Qs structures by using attenuation tomographic inversion, the knowledge about 3-D Vp and Vs structures is taken from previous study. After inversion process, it has been observed obvious low Qp and Qs values around reservoir zone that coincide well with the zone of low Vp/Vs ratio. These anomalies suggest associated with steam saturated zone. As conclusion, the results of attenuation tomographic inversion have given a new insight in determining reservoir characteristics, especially in estimating steam saturated zone inside geothermal reservoir.