Tracking Fluid Flow Between IDDP-2 and the Current Production Reservoir in the Reykjanes Geothermal System in SW-Iceland, Using Drilling Fluid as Tracer

Abstract

The IDDP-2 well is a 4.6 km deep well in southwestern Iceland, that was drilled into supercritical conditions in the roots of the Reykjanes geothermal system as part of the Icelandic IDDP-project. It was partly supported by the DEEPEGS Horizon 2020 project, which aims at demonstrating the feasibility of deep enhanced geothermal systems (EGS) as a competitive energy alternative for commercial use. Evaluating the connection between the IDDP-2 well and the currently producing 280-300°C reservoir in the Reykjanes geothermal system would conventionally be done via a tracer test. This is however challenging because the temperature of the deep reservoir that the IDDP-2 well is drilled into, is predicted to be close to 500°C, exceeding the temperature tolerance of most tracer molecules. However, during a routine geochemical production monitoring of the Reykjanes production wells, an abrupt change in the chemical composition in a nearby production well, RN-12, was observed during drilling of the IDDP-2 well; with decreasing concentrations of elements such as Na, Cl, and Ca and an increase in the atmospheric gases N2 and Ar. This change in concentrations is probably due to injection of an air-saturated low-salinity fluid into the saline Reykjanes reservoir during drilling. We propose to use this change in concentration as a substitute for conventional tracers. Using an extended version of an 1D analytical tracer analysis we deduce the fluid flow between the two wells. Also, we predict the temperature evolution in RN-12 due to injection into the IDDP-2 well, assuming an injection rate of 100 kg/s with 50°C water. Two predictions were made: a pessimistic prediction for a wide flow channel that resulted in a 26°C cooling over 40 years and an optimistic prediction with a narrow flow channel that resulted in a less than 1°C cooling over 40 years.