Tracking the Movement of External Reinjected Geothermal Fluid (Out-Field) Using Repeat Resistivity

Abstract

External (Out-Field) reinjection of hot saline water into cool groundwater aquifers accomplishes the environmental objective of avoiding surface disposal effects, while minimising the detrimental effects of premature feinjection returns to production wells, and enhancing sustainable resource use. Tracking the fate of this fluid can be difficult without an extensive and expensive network of monitor bores. Resistivity monitoring can be used to track the flow direction and lateral extent of the injected water, because of its contrasting conductivity. An example from Qhaaki, New Zealand, dlustrates the use of the repeat resistivity method for this purpose. Changes of up 80 56% were detected over 5 years of injection hto a non-thermal aquifer at about 3QOm depth. The slow rate of change implies that thermal and alteration effects are probably significant causes. The pattern of change implies quasi-radial flow to about 25Um radius, but with a prefened direction parallel to the boundary of the field rather than towards the centre of production. This result supports the inference that the injection and production aquifers are poorly connected, as deduced kom the absence of tracer returns. External reinjection, in this setting, appears to be a successful strategy.