Understanding Geothermal Reservoirs: Iddp-2, Iceland

Abstract

IDDP-2 well was drilled as a deepening of the existing RN-15 well in the Reykjanes geothermal field in 2016-2017. Well reached to 4659 m (MD) depth and is at the time the deepest and hottest well drilled. The target of the well was to reach geothermal resources at supercritical temperatures (above 406 °C and 298 bar for sea water), i.e. depths below 3000 m. Reykjanes geothermal field is located at the tip of the Reykjanes peninsula, in the southwestern part of Iceland. The geothermal system situates at the place where the Mid-Atlantic Ridge rises from the oceanic floor and is centered in a prominent rift-zone graben and bounded by NE-SW trending zone of normal faults and NW-SE transform faults. There is a lot of subsurface data available for the actively exploited, upper part of the geothermal field. However, there was limited data available for the lower part of the geothermal reservoir, below 3000 m. There was only MT data available for the area to get predrill temperature estimation. Temperature was estimated based on available resistivity from magnetotelluric inversion, density from gravity inversion, and from Bayesian rock-physics inversion. While-drilling and post-drill research was based on what limited data was available for the use. Geological section was built based on drilling parameters from drilling reports. Separated geological units in most cases indicate changes in rock parametres, except from depths from 3000 to 3200 m, that is based on descriptions from cuttings (by ISOR). Well log data was analysed and the best data was gathered from image log data analysis. Available core intervals were described and thin section descriptions are underway for better understanding of processes occurred in the reservoir. Based on rock parametres, geological description was divided into several units. These units were verified by zonation based on image log data analysis. There is almost a perfect match between the results from these two datasets. Additionally, thin section analysis was used to verify metamorphic facies that also coincide very well with the main changes in the subsurface. Units obtained from geological section and well descriptions coincide with temperature predictions from geophysical study. Based on all the results, reservoir has been divided into several sections that characterize changes in rocks due the temperature changes and related processes in the reservoir. Most important changes occur in three different depths. (i) At ca 3400 m, reservoir reaches 400 °C temperature. This depth also marks the change in rock properties, as well as change from convective system to conductive system. (ii) Interval from 3700 (3750) to 3900 m indicates the main change in rock parametres and is well documented in geological section and on image log. It is also marked by change from greenschists facies to amphibolite facies. (iii) Starting at ca 4500 m depth, rocks reach 500 °C and there are first indications for the start of the transition from brittle to ductile rock. However, the well did not reach the interval where rock would be fully ductile. This research was partly funded by the EC Horizon 2020 project DEEPGS.

AAPG Datapages/Search and Discovery Article #90345 © 2018 AAPG European Region, Geothermal Cross Over Technology Workshop, Part II, Utrecht, The Netherlands, April 17-18, 2018