f Time lapse reservoir monitoring: geophysical applications from the geothermal industry

High enthalpy geothermal fields exploited for electricity production are routinely monitored using geophysics, including both the long term reservoir evolution and short term injection and interference tests. We present examples of induced micro-seismicity data, gravity and repeat levelling (ground subsidence) programs from some of the worlds most significant fields, including The Geysers in the US and Darajat in Indonesia. The evolution of reservoir mass balance is monitored through repeated, high precision levelling and gravity surveys, enhanced by simultaneous monitoring of ground water level changes in shallow boreholes. The mass balance tracks the ability of natural fluid recharge to keep up with net loss from production and only partial, condensed fluid re-injection. Accurate MEQ locations, obtained through iterative 3D tomographic modelling of Vp and Vs velocities and subsequent location updating, track production and injection fluid paths. Fluid characteristics can be monitored through spatial and temporal changes in the Vp/Vs ratio, and moment tensor analysis gives and indication of changing fracture systems. The rates of fluid movement associated with commercial geothermal production are of the same order of magnitude as those expected from large oil fields, suggesting the monitoring techniques are transferable. The fluid dynamics are somewhat different, naturally, but analogies such as steam and water flood monitoring are directly comparable.