Reservoir Monitoring of the Onuma Geothermal Field Using Fluid Flow Tomography

Fluid Flow Tomography (FFT) method is an improved mise-à-la-masse method, which is used as a reservoir monitoring method. This method is a 4-Dimensional (4-D) electrical method in order to visualize the transient subsurface fluid flow by measuring charged electric potentials due to subsurface resistivity and streaming potentials caused by subsurface fluid flow. This method is utilized to monitor the fluid flow during hydraulic fracturing of Hot Dry Rock project and periodical overhaul of geothermal power plant. This method is also used to monitor the flow of heavy crude oil in oil sand formation during thermal Enhanced Oil Recovery (EOR). The observed data of FFT method can be separated into charged electric potentials derived from subsurface resistivity structure and streaming potentials due to a permeable fluid flow. In this paper we describe the algorithm of three-dimensional resistivity inversion and three-dimensional probability tomography of self-potential data. A case study of FFT method was carried out in the Onuma geothermal area. As the results of 3-D analysis of resistivity data, distinguished low resistivity anomalies were detected and resistivity discontinuities where marked with resistivity changes were detected. As the results of 3-D analysis of self-potential data, the main production zone and reinjection zone were delineated from Charge Occurrence Probability (COP) distribution. We believe that the FFT method is a useful tool to monitor the dynamic change of fluid flow behavior in a reservoir during production and reinjection operations.