Reservoir Assessment of the Ölfus-Bakki Low-Temperature Geothermal Area, SW Iceland

Abstract

The Ölfus-Bakki geothermal area in SW Iceland is located in the S and SE margins of the high-enthalpy systems of Hveragerði and Hengill, respectively. This geothermal field contains a productive region, denominated the Bakki field, which produces fluid at temperatures in the range of 100-130 °C. Since 1979, the geothermal resource has supplied thermal energy to the town of Þorlákshöfn, mainly for space heating and aquaculture. The central objective of this study is to provide a reservoir assessment supported by the integration of the available information for this location. The methodology employed included 1) the interpretation of well logging data for pressure and temperature, 2) the interpretation of geochemical data, 3) the mapping of baric, thermal and geochemical anomalies, 4) the preparation of a volumetric assessment of the reservoir and 5) the preparation of a lumped parameter model for the Bakki field. The combination of these methods allows an improved understanding of the natural state of the reservoir as well as its response to production under different scenarios. Data from nearly 24 wells was analyzed. Given the availability of data, the scope of the present assessment is constrained to the uppermost 1000 m of the reservoir. Analysis of the patterns of pressure and formation temperature allowed the identification of a convective system migrating southwards, very likely associated to the neighboring high-enthalpy geothermal system at Hveragerði. Chemical analyses permitted characterizing the fluids from the entire region into distinctive units. This characterization provided valuable clues to discern fluid provenance, as well as to identify possible recharge zones. Results from the volumetric assessment, based on a Monte Carlo simulation, indicated a mean reservoir capacity between 210-730 MWth for lifetimes between 30-100 years, with a 90% confidence interval of 98-1200 MWth. Lumped parameter modeling indicates that Ölfus-Bakki is an open system with unconfined aquifers. Current utilization is considered to be sustainable for the next 300 years given that production keeps growing at a similar rate compared to the past 20 years.