Monitoring of Reinjected Fluid Using Repeat Resistivity at Ohaaki

Abstract

Repeated resistivity measurements made around BR41 reinjection well at the northwestern boundary of the Ohaaki geothennal field indicate large resistivity changes (up to reduction) during five years of injection. These changes have a extent of 200 to 300 m from the well. They are caused by hot (150?C) saline water (1200 chloride) replacing cool groundwater in a rhyolite aquifer at about 300 m depth. The pattern of resistivity changes suggests radial flow, but with a preferred flow direction to the northeast., along the boundary of the field, rather thantowards the production area of deep pressure drawdown.