Title: Prediction of Injection-Induced Cooling using Tracer Test Data in the Hellisheiði Geothermal Field, SW-Iceland Comparison of Numerical and Analytical Modelling Approaches

Abstract

The Hellisheiði geothermal system is a fracture-controlled geothermal system located in the south part of the Hengill volcanic system in SW-Iceland. The bedrock mainly consists of a sequence of basaltic lavas, hyaloclastites and intrusions. Abundant extensional and transform faults reflect its location near a triple junction. The objective of this study is to construct a numerical reservoir model of the Hellisheiði geothermal system with a particular focus on reinjection-induced cooling in the Húsmúli subfield. A numerical model is calibrated using natural state temperature profiles, as well as recent tracer data collected in 2019, and is used to predict magnitude and timescale of cooling due to long term reinjection. Using the dual porosity approach, the model is able to achieve a fairly good match to field data for three monitoring wells (HE-31, HE-48, HE-48). The tracer flow in the numerical model constructed underscores the role of the NE oriented fault structures as the main conduits for tracer migration, and the role of transform structures as flow barriers. The model predict an annual temperature decrease of less than 1°C for wells HE-31 and HE-48. On the other hand, although very little tracer appeared in well HE-33 suggesting little risk of injection-induced cooling, the model suggests it may be affected by cooling due to cold water recharge from neighbouring formations. The cooling predictions are compared with those of a simple one-dimensional model (TRINV software). Despite its simple internal structure, TRINV provides greater tracer simulation accuracy. This study supports the importance of reinjection that not only provides pressure support but also helps to counteract colder marginal recharge from surrounding formations.