The Iceland Deep Drilling Project at Reykjanes: Drilling into the root zone of a black smoker analog

Abstract

The aim of the Iceland Deep Drilling Project is to drill into supercritical geothermal systems and examine their economic potential. The exploratory well IDDP-2 was drilled in the Reykjanes geothermal field in SW Iceland, on the landward extension of the Mid-Atlantic Ridge. The Reykjanes geothermal field produces from a b300°C reservoir at 1 to 2.5 km depth and is unusual because it is recharged by seawater. The well was cased to 3000 m depth, and then angled towards the main up-flow zone of the system, to a total slant depth of 4659 m (~4500 m vertical depth). Based on alteration mineral assemblages, joint inversion of wireline logging, and rate of heating measurements, the bottom hole temperature is estimated to be about 535°C. The major problem encountered during drilling was the total loss of circulation below 3 km depth and continuing to the final depth. Drilling continued without recovering drill cuttings, consequently spot coring provided the only deep rock samples from the well. These cores are characteristic of a basaltic sheeted dike complex, with hydrothermal alteration mineral assemblages that range from greenschist to amphibolite facies, hornblende hornfels, and pyroxene hornfels, allowing the opportunity to investigate water-rock interaction in the active roots of an analog of a submarine hydrothermal system. As they have not yet been sampled, the composition of the deep fluids at Reykjanes is unknown at present. Cold water is currently being injected with the aim of enhancing permeability at depth, before allowing the well to heat up prior to flow tests planned for early 2019. The well has at least two fluid feed zones, a dominant one at 3.4 km depth and a second smaller one at 4.5 km. Extensive geophysical surveys of the Reykjanes Peninsula completed recently allow correlation of geophysical signals with rocks properties and in-situ conditions in the subsurface. Earthquake activity monitored with a local seismic network during drilling the IDDP-2 drilling detected abundant small earthquakes (M L ≤ 2) within the depth range of 3-5 km. A zone at 3-5 km depth below the producing geothermal field that was generally aseismic prior to drilling, but became seismically active during the drilling. The drilling of the IDDP-2 has achieved number of scientific and engineering firsts. It is the deepest and hottest drill hole so far sited on an active mid-ocean spreading center. It penetrated an active supercritical hydrothermal environment at depths analogous to those postulated as the high temperature reaction zones feeding black smoker systems.

Geothermal energy and ore-forming potential of 600 °C mid-ocean-ridge hydrothermal fluids

“…Drill-core samples recovered from the sheeted dike complex of this active spreading center also provide a window into processes operating deep beneath the Mid-Atlantic Ridge. At ∼4500 m depth, the metabasalt and metagabbro show hydrothermal alteration to amphiboleplagioclase-clinopyroxene-orthopyroxene at temperatures from 700 °C to >900 °C, with median temperatures of ∼800 °C (Friðleifsson et al, 2020;Zierenberg et al, 2021). Paragenetically later alteration assemblages record continuing alteration down to 600 °C in the latest-stage, crosscutting veins (Zierenberg et al, 2021).…”

“…The Iceland Deep Drilling Project (IDDP) explores the economic potential of geothermal fluids in the deep roots of active geothermal systems (Friðleifsson et al, 2020). This includes evaluation of the feasibility of exploiting supercritical fluid for energy production and/ or extracting economically valuable elements directly from this fluid (e.g., Maimoni, 1982;McKibben and Elders, 1985).…”

“…The IDDP-2 borehole was drilled in 2016-2017 by deepening an earlier borehole, RN-15, in the Reykjanes geothermal system in southwest Iceland (Fig. 1) to reach a vertical depth of ∼4500 m (total slant depth of 4659 m; Friðleifsson et al, 2020). This is the deepest high-temperature geothermal borehole in the world.…”

Cation-Exchange Capacity Distribution within Hydrothermal Systems and Its Relation to the Alteration Mineralogy and Electrical Resistivity

“…Well RN-15 [48] was selected from the Reykjanes geothermal field. Well RN-15 was vertically drilled in 2004 to a depth of 2507 m. The well was deepened (RN-15/IDDP-2) in 2016 and 2017 as part of the Iceland Deep Drilling Project (IDDP) to a total measured depth of 4659 m and reached supercritical fluid conditions [49][50][51]. Resistivity data for RN-15 has been depth corrected [52].…”

Temperature log simulations in high-enthalpy boreholes

“…DTS, which is considered better suited for use at elevated temperatures, gives erroneous temperature readings under high temperatures (> 300 °C) due to the chemical and thermal degradation of the optical fiber (Reinsch et al 2013;Laarossi et al 2019). To keep the temperature of the measuring device below its maximum tolerance, cooling through continuous injection during logging is necessary for extreme high-temperature boreholes (Friðleifsson et al 2018).…”