Time Lapse (4-D) Seismic Velocity Tomography Using Micro-Seismic Data in Geothermal Field

Abstract

Micro-seismic monitoring has become one of the primary methods for behavior of reservoir monitoring in geothermal field. Monitoring is conducted every time. In this study, we tried to observe the velocity changes as physical properties of reservoir within two different times. Those times are first time before the water injection phase in 2007 and second time after injection phase done in 2007-2008. We used these two different time periods of microearthquake data catalog to invert for seismic velocity cube structure by using tomographic method. Injection phase was initiated in second year. In this study, there are velocity changes from low velocity to high velocity after injection phase on injector wells area. It is caused by increasing bulk modulus of reservoir rocks that are initially dry or gas filled to be water saturated. It can be use to interpret the direction and delineation of fluid injected in reservoir. 4-D monitoring can be done by using the micro-seismic annually or any events such as injection phase, hydraulic fracture or etc. This study still continues to be developed to determine a few correction and tolerance must be treated due to imbalance in the number of data and tomography inversion is not produce the absolute value of velocity.