| Authors |
G.O. FRIDLEIFSSON, A. ALBERTSSON, A. STEFANSSON, G. THOROLFSSON, K.G. Msefin, K.V. MATTÃASDOTTIR, K. SIGURDSSON, Ó. SIGURDSSON, Þ. GISLASON, W.A. ELDERS, R.A. ZIERENBERG, E. BALI, E.A. GUÃNASON, F. ÓSKARSSON, T. B. WEISENBERGER |
| Abstract |
The DEEPEGS demonstration well IDDP-2, in the Reykjanes high-temperature geothermal field in SW Iceland, achieved several scientific- and engineering firsts. It is the deepest and hottest drill hole so far sited in an active mid-ocean spreading center. The reservoir fluid in the Reykjanes system is modified seawater. IDDP-2 penetrated an active supercritical hydrothermal environment at depths analogous to those postulated as the high temperature reaction zones feeding black smoker systems. IDDP-2 was drilled to a total depth of 4,659 m and cased with a production casing to almost 3 km depth. The well was angled towards the main up-flow zone of the Reykjanes geothermal system, so its vertical depth is about 4.5 km. Based on alteration mineral assemblages, joint inversion of geophysical data, and the rate of heating measurements, the bottom hole temperature appears to be 550-600°C. The DEEPEGS project received funding from the European Union’s Horizon 2020 Research and Innovation Programme with the principal aim of demonstrating the feasibility of Enhanced Geothermal Systems (EGS) to deliver renewable energy in Europe. The DEEPEGS aims to test stimulation technologies for EGS in deep wells in different geological settings, a high enthalpy system at Reykjanes with temperatures up to 550°C, and in two deep hydrothermal reservoirs in southern France with temperatures up to 220°C. For local administrative reasons only one French demonstration site could be drilled, located in Vendenheim, NE-France. A major problem encountered during drilling the IDDP-2 well in 2016-2017 was total loss of circulation below 2.5 km depth that continued to the final depth. After cementing the production casing at 2.941 m depth, total circulation losses continued despite 12 attempts to plug the loss zones with cement. At 3.2 km depth the operator abandoned the plugging attempts and drilled blind to the total depth. The deep feed zones indicate that the Reykjanes conventional reservoir needs be enlarged by at least 1 km downwards, from about 3 km down to ~4 km depth, which is a major achievement by the deep drilling. Some drill cutting samples were retrieved from the interval immediately above and below the production casing but drilling the remainder of the well proceeded without recovering any drill cuttings. The only rock samples recovered from the well came from 13 spot coring attempts. These cores are characteristic of a basaltic sheeted dyke complex, with hydrothermal alteration mineral assemblages ranging from greenschist to amphibolite facies, enabling investigation of water-rock interaction in the active roots of an analogue to submarine hydrothermal systems. Petrological and fluid inclusion studies imply that the current bottom hole temperature is likely to be close to 600°C. Earthquake activity monitored with a local seismic network during drilling of the deep well detected abundant small earthquakes (ML ≤ 2) within the depth range of 3-5 km. A zone at 3-5 km depth below the producing geothermal field, generally aseismic prior to drilling, became seismically active during the drilling. The total loss of circulation throughout the drilling and subsequent 1.5 year of re-injection tests demonstrate that an EGS system would be created with further reinjection into the 400-600°C hot environment. During the stimulation and injection test the production casing was damaged between 2,307-2,380 m depth, presumably due to cyclic heating-up and cooling of the casing, and the lowest part of a 4.5 km long stimulation pipe suffered from some corrosion. Attempts to inject chemical tracers were not as successful as anticipated, but regular chemical monitoring of neighboring wells showed clear dilution of the geothermal brine from the freshwater used during drilling and subsequent injection and stimulation tests. A flow testing experiment was planned to begin in April but was delayed until late summer 2019. Its purpose is to find out if the deep fluid can be used directly use to produce electricity, with or without chemical mitigation, and/or for other geothermal uses in the Reykjanes geothermal resource park. The other option is to use the well for deep re-injection to mine heat from the root zone of the Reykjanes geothermal reservoir and result in an EGS system, to improve the performance of the overlying geothermal reservoir. This DEEPEGS demonstration well is in an operational environment at precommercial scale, operating at TRL 7 level as all the infrastructure needed is already installed at the Reykjanes power plant. The resulting business model may bring the DEEPEGS concept to TRL level 8-9, depending on the result from the flow test and pilot study. |