Reykjanes Geothermal Field Research Articles

Prospects for Power Generation of the Doublet Supercritical Geothermal System in Reykjanes Geothermal Field, Iceland

Supercritical geothermal resources are in the preliminary exploration stage as a new type of clean energy and there are no practical utilization projects. The IDDP-2 well at Reykjanes geothermal field in Iceland encountered supercritical geothermal conditions in 2017, with a maximum temperature of 535 °C. The system is still in the field experiment stage and no exploitation work has been carried out. Hence, a hypothetical doublet geothermal system was simulated based on IDDP-2 to study the power generation potential and favorable operating conditions for future development of supercritical geothermal resources. A multiphase flow model is established to predict the fluid and heat flow characteristics. Furthermore, sensitivity and economic analyses were performed to evaluate the expected commercial and environmental benefits of the supercritical geothermal system. The results show that the system’s evolution could be briefly divided into three stages according to the temperature variation. The power generation ranges between 5.4 MW~16.5 MW, and the levelized cost of electricity (LCOE) is 0.02 $/kWh. In addition, the system can reduce CO2 emissions, which are 1.2~7.75 Mt less than that of fossil fuel plants with the same installed capacity. The results prove the great development potential and commercial competitiveness of the supercritical geothermal system.

Interplay of large-scale tectonic deformation and local fluid injection investigated through seismicity patterns at the Reykjanes Geothermal Field, Iceland

SUMMARY Occurrence of seismicity sequences as consequence of fluid injection or extraction has long been studied and documented. Causal relations between injection parameters, such as injection pressure, injection rates, total injected volumes and injectivity, with seismicity derived parameters, such as seismicity rate, cumulative seismic moment, distance of seismicity (RT-plot), b-values, etc. have been derived. In addition, reservoir engineering parameters such as permeability/porosity relations and flow types play a role together with geology knowledge on fault and fracture properties, influenced by the stress field on different scales. In this paper, we study observed seismicity related to water injection at the Reykjanes Geothermal Field, Iceland. The region near the injection well did not experience seismicity before the start of injection. However, we observed continued seismic activity during the 3 months of injection in 2015, resulting in a cloud of about 700 events ranging in magnitude from Mw 0.7 to 3.3. We re-located these events using a modified double-difference algorithm and determined focal mechanism of event subsets. Characteristic for the site is that the events are bound to about 4 km distance to the injection point, and moreover known faults seem to act as barrier to fluids and seismicity. Several repeating sequences of seismicity, defined as bursts of seismicity have hypocenter migration velocities larger than 4 km d–1 and their dominant direction of propagation is away from the injection point towards larger depths. The seismic events within the bursts lack larger magnitude events, have elevated b-values (∼1.5) and consist of many multiplets. Except from the coinciding onset of seismicity with the start of fluid injection, no correlation between injection rates and volumes could be identified, neither could hydraulic diffusivity models explain observed seismicity patterns. Comparison of our results with investigations on background seismicity from 1995 to 2019 and from a seismic swarm in 1972 revealed similar focal mechanism patterns and burst-like seismicity patterns. We finally present a conceptual model where we propose that the observed seismicity patterns represent a stress release mechanism in the area close to the injection well, controlled by an interplay of local pore pressure and stress field changes with continued extensional stress build up at the Reykjanes Ridge.

Hydro-mechanical simulation and analysis of induced seismicity for a hydraulic stimulation test at the Reykjanes geothermal field, Iceland

The combination of seismic analysis with advanced physics-based simulation provides an opportunity to further understand injection-induced fault reactivation, including the hydro-mechanical interplay between different faults and the rock where they reside. Here, this is investigated based on data from hydraulic stimulation of a well at the Reykjanes geothermal field. Central is the development of an interdisciplinary framework for integration of different data types towards a 3D, hydro-mechanical and faulted geothermal reservoir simulation model. This work shows how seismic interpretations can improve simulation models and, reciprocally, how fully coupled physics-based modeling can add to seismic interpretations in analysis of fault reactivation.

Time-lapse resistivity imaging: CSEM-data 3-D double-difference inversion and application to the Reykjanes geothermal field

SUMMARY Time-lapse resistivity tomography bring valuable information on the physical changes occurring inside a geological reservoir. In this study, resistivity monitoring from controlled source electromagnetics (CSEM) data is investigated through synthetic and real data. We present three different schemes currently used to perform time-lapse inversions and compare these three methods: parallel, sequential and double difference. We demonstrate on synthetic tests that double difference scheme is the best way to perform time-lapse inversion when the survey parameters are fixed between the different time-lapse acquisitions. We show that double difference inversion allows to remove the imprint of correlated noise distortions, static shifts and most of the non-linearity of the inversion process including numerical noise and acquisition footprint. It also appears that this approach is robust against the baseline resistivity model quality, and even a rough starting resistivity model built from borehole logs or basic geological knowledge can be sufficient to map the time-lapse changes at their right positions. We perform these comparisons with real land time-lapse CSEM data acquired one year apart over the Reykjanes geothermal field.

Casing-to-casing resistance study performed at Reykjanes geothermal field in Iceland to estimate fracture connectivity

Successful drilling of geothermal wells into fractured hot rock through which geothermal fluid flows is critical to ensure efficient operation of geothermal power plants. This study focuses on investigating a casing-to-casing resistance method for estimating fracture connectivity between wells at Reykjanes geothermal reservoir in Iceland to better understand the fluid-flow in the reservoir. Conductive well casings were successfully used as electrodes to transfer the electric current deeper into the fractured reservoir in an electrical resistance survey. The low resistance measured between wells implies that the wells are connected to the main production zone at Reykjanes and that the current is travelling through fractures filled with conductive geothermal brine from one well casing to another rather than going through a less conductive rock. The measured resistance is compared to an analytical study showing the promising potential the casing-to-casing resistance method has for gaining valuable information about the fracture connectivity in geothermal reservoirs.

Detection of Gold Nanoparticles in Hydrothermal Fluids

The transport and deposition of gold from colloidal suspensions in hydrothermal fluids has been a persistent theme in ore deposits research. Studies of active geothermal systems show that a complete model of gold transport must include both dissolved and particulate forms. However, samples of the hydrothermal fluids are commonly spiked with aqua regia after collection in order to put any solids back into solution, thus preventing a quantitative assessment of the particle load. Although attempts have been made to filter the solids, gold nanoparticles (Au NPs) will mostly pass the 0.2-µm filters that are in common use, and a simple technique for analyzing suspended particles in the liquids has been lacking. In this study, we demonstrate how time-resolved acquisition of mass 197 in a conventional inductively coupled plasma-mass spectrometer (ICP-MS) can be used to detect and measure Au NPs in the filtered liquids, with an example of well-characterized fluids from the Reykjanes geothermal field on Iceland. The technique allows for precise monitoring of the solution as it is introduced into the plasma with the capability of identifying individual particles carried in suspension. Results show that Au particles passing the 0.2-µm filters are abundant in the studied samples, and measurements of the individual particles can be used to determine their size. The experiment highlights the potential of emerging ICP-MS techniques, including very fast data acquisition and multielement analysis of single particles in timeof-flight mode, for characterization of NPs in hydrothermal fluids.

Monitoring geothermal reservoir developments with the Controlled-Source Electro-Magnetic method — A calibration study on the Reykjanes geothermal field

Surface geophysical monitoring techniques are important tools for geothermal reservoir management as they provide unique information on the reservoir development away from boreholes. For magmatic environments, electromagnetic (EM) methods are attractive monitoring tools as they allow to characterize the reservoir and hence potentially monitor changes related to fluid injection/production. Indeed, the electrical resistivity of reservoir rocks is highly dependent on the volume, chemistry and phase of the in-situ geothermal brine (e.g. liquid, vapor, supercritical).Passive EM techniques (e.g. magnetotellurics or MT) are traditionally used for geothermal exploration and a few recent studies have demonstrated its potential for monitoring reservoir development. One of the main challenges is though the presence of cultural noise and/or variability of the Earth magnetic field that can obfuscate the EM signals of interest. We have investigated the benefits and drawbacks of active EM surveying (Controlled-Source EM or CSEM) to tackle this challenge, first with a synthetic study and subsequently with an actual time-lapse survey acquired in 2016 and 2017 over the Reykjanes geothermal field in Iceland before (baseline) and after (monitor) the thermal stimulation of the supercritical RN-15/IDDP-2 geothermal well.The synthetic study showed that for geothermal fields having a resistivity structure similar to the Reykjanes field (i.e. a conductive caprock overlying a more resistive higher temperature reservoir), CSEM and MT measurements can both detect resistivity changes within the deep resistive reservoir, provided that measurement errors are small. Variations in many survey parameters (e.g. errors in receiver position/orientation, differences in recording devices, variations of near surface conditions, external noise) can create significant time-lapse CSEM measurement errors. Our actual time-lapse survey showed that when similar CSEM equipment is used during the baseline and monitor surveys and systematically d-GPS positioned, the remaining key parameter controlling the survey repeatability is the level of external noise. Since the influence of external noise on CSEM data can be artificially reduced (e.g. by increasing the transmitter dipolar moment), it offers the possibility to adapt the survey design to increase the chance of detecting the time-lapse signals of interest. On the contrary, little control is possible on the MT signal to noise ratio and hence repeatability.The time-lapse EM survey acquired over the Reykjanes geothermal reservoir showed indeed that a high CSEM survey repeatability can be achieved with electric field measurements (within a few percent) but that time-lapse MT survey is a challenging task because of the high level of cultural noise in this industrialized environment. To assess the quality of our CSEM dataset, we inverted the data and confronted the resulting resistivity model with the resistivity logged in the RN-15/IDDP-2 well. We obtained a good match up to 2-3km depth, i.e. enough to image the caprock and the liquid-dominated reservoir but not deep enough to image the reservoir in supercritical conditions. To obtain such an image, we had to jointly invert legacy MT data with our CSEM data. On the monitoring aspects, the analysis of changes in electric fields did not allow to identify any CSEM signal related to the thermal stimulation of the RN-15/IDDP-2 well. One possible explanation is the weakness of the time-lapse CSEM signal compared the achieved CSEM survey repeatability as a result of a limited resistivity change over a limited volume within the reservoir. Future reservoir developments in the supercritical reservoir (e.g. hydraulic stimulation, long-term fluid circulation) will most likely generate stronger resistivity variations over a larger volume than during the thermal stimulation of the well. This calibration study provides the basic information for deciding when and how an EM monitor survey must be performed for the monitoring of the Reykjanes geothermal reservoir but also for the definition of the monitoring strategy of similar high-enthalpy geothermal reservoirs.

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

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 <300 °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 (ML ≤ 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.

Tectonic Control of the Reykjanes Geothermal Field in the Oblique Rift of SW Iceland: From Regional to Reservoir Scales

This paper presents a multidisciplinary structural analysis of the Reykjanes Peninsula where Holocene deformation of a young oblique rift controls the geothermal processes in presence of a transform segment. The new structural map from aerial images and outcrops is correlated with selected surface and subsurface data and shows a complex pattern: NNE extensional rift structures, N-S dextral and ENE sinistral oblique-slip Riedel shears of the transform zone, and WNW and NW dextral oblique-slip faults. Shear fractures are more common, and along with the NNE fractures, they compartmentalise the crustal blocks at any scale. The fractures are within two ENE Riedel shear zones, indicating a minimum 7.5 km wide transform zone. The greatly deformed Southern Riedel Shear Zone is bounded to the north and the south by the 1972 and the 2013 earthquake swarms. This shear zone contains the geothermal field in a highly fractured block to the west of a major NW structure. Some of the deformations are: a) clockwise rotation of rift structures by the 1972 earthquake zone, inducing local compression; b) magma injection into extensional and oblique-slip shear fractures; c) reactivation of rift structures by transform zone earthquakes; d) tectonic control of reservoir boundaries by WNW and ENE shear fractures, and the distribution of surface alteration, fumaroles, CO2 flux, reservoir fluid flow and the overall shape of pressure drawdown by N-S, ENE, WNW/NW and NNE fractures. Results demonstrate the role of seismo-tectonic boundaries beyond which fault types and density change, with implications for permeability.

The Iceland Deep Drilling Project 4.5 km deep well, IDDP-2, in the seawater-recharged Reykjanes geothermal field in SW Iceland has successfully reached its supercritical target

Abstract. The Iceland Deep Drilling Project research well RN-15/IDDP-2 at Reykjanes, Iceland, reached its target of supercritical conditions at a depth of 4.5 km in January 2017. After only 6 days of heating, the measured bottom hole temperature was 426 °C, and the fluid pressure was 34 MPa. The southern tip of the Reykjanes peninsula is the landward extension of the Mid-Atlantic Ridge in Iceland. Reykjanes is unique among Icelandic geothermal systems in that it is recharged by seawater, which has a critical point of 406 °C at 29.8 MPa. The geologic setting and fluid characteristics at Reykjanes provide a geochemical analog that allows us to investigate the roots of a mid-ocean ridge submarine black smoker hydrothermal system. Drilling began with deepening an existing 2.5 km deep vertical production well (RN-15) to 3 km depth, followed by inclined drilling directed towards the main upflow zone of the system, for a total slant depth of 4659 m ( ∼ 4.5 km vertical depth). Total circulation losses of drilling fluid were encountered below 2.5 km, which could not be cured using lost circulation blocking materials or multiple cement jobs. Accordingly, drilling continued to the total depth without return of drill cuttings. Thirteen spot coring attempts were made below 3 km depth. Rocks in the cores are basalts and dolerites with alteration ranging from upper greenschist facies to amphibolite facies, suggesting that formation temperatures at depth exceed 450 °C. High-permeability circulation-fluid loss zones (feed points or feed zones) were detected at multiple depth levels below 3 km depth to bottom. The largest circulation losses (most permeable zones) occurred between the bottom of the casing and 3.4 km depth. Permeable zones encountered below 3.4 km accepted less than 5 % of the injected water. Currently, the project is attempting soft stimulation to increase deep permeability. While it is too early to speculate on the energy potential of this well and its economics, the IDDP-2 is a milestone in the development of geothermal resources and the study of hydrothermal systems. It is the first well that successfully encountered supercritical hydrothermal conditions, with potential high-power output, and in which on-going hydrothermal metamorphism at amphibolite facies conditions can be observed. The next step will be to carry out flow testing and fluid sampling to determine the chemical and thermodynamic properties of the formation fluids.

Lithogeochemical approaches in geothermal system characterization: An application to the Reykjanes geothermal field, Iceland

This article presents the results of a comprehensive lithogeochemical study that was conducted using downhole rock samples from the Reykjanes geothermal system in Iceland. Magmatic fractionation trends established for least-altered rocks were used to correct mass change estimates for pseudo-enrichments/-depletions due to fluid-rock interaction. Net mass changes of altered rocks in the Reykjanes geothermal system range from −26 to +22g/100gp, and are dominated by changes in SiO2>CaO>MgO>Na2O,K2O, S>C. Trace elements undergoing the greatest net mass changes are Ba>Zn>Cu, Ni>Sr>Rb>As>Cd. The lithogeochemical data are interpreted with petrographic data and data from parallel studies of mineral chemistry (detailed in separate manuscripts). Trends in the distribution of reactive-mobile elements correspond closely to the large-scale hydrology of the Reykjanes geothermal system, as determined by measured isotherms and hydrothermal mineral zonation. A hypothesis of deep inflow to the system from the southeast is supported by lithogeochemical and mineralogical evidence and downhole temperature measurements. Whole-rock mass changes of specific components counter their documented enrichments and depletions in the Reykjanes geothermal fluid relative to the initial seawater input. Volume estimates for the Reykjanes geothermal system are made based on the redistribution of select reactive-mobile elements.

Gold enrichment in active geothermal systems by accumulating colloidal suspensions

The origins of high-grade hydrothermal ore deposits are debated, but active geothermal systems provide important clues to their formation. The highest concentrations of gold are found in geothermal systems with direct links to island arc magmatism. Yet, similar concentrations have also been found in the absence of any input from arc magmas, for example, in the Reykjanes geothermal field, Iceland. Here we analyse brine samples taken from deep wells at Reykjanes and find that gold concentrations in the reservoir zone have increased over the past seven years from an average of 3 ppb to 14 ppb. The metal concentrations greatly exceed the maximum solubility of gold in the reservoir under saturated conditions and are now nearly two orders of magnitude higher than in mid-ocean ridge black smoker fluids—the direct analogues of Reykjanes deep liquids. We suggest that ongoing extraction of brine, the resulting pressure drop, and increased boiling have caused gold to drop out of solution and become trapped in the reservoir as a colloidal suspension. This process may explain how the stock of metal in the reservoirs of fossil geothermal systems could have increased over time and thus become available for the formation of gold-rich ore deposits.

Application of diffuse gas flux measurements and soil gas analysis to geothermal exploration and environmental monitoring: Example from the Reykjanes geothermal field, SW Iceland

Soil gas geochemistry and diffuse CO2 flux from soil, together with gas concentrations in 7 production wells and 10 steam vents in the Reykjanes geothermal field (Iceland), were studied with the aim of: 1) studying the potential use of the CO2/He ratio in soil gas to constrain CO2 retention in calcite in the bedrock; and 2) quantifying the effect of 100 MWe power production from the Reykjanes geothermal reservoir on surface activity. The presence of biogenic CO2 in the soil gas prevented the use of the CO2/He ratio for the quantification of CO2 retention in calcite. The CO2/He ratio in about 20 soil gas samples was, however, low and consistent with CO2 retention. Our results suggest that detailed soil gas and soil temperature mapping may provide excellent information for defining drilling targets. The surface activity in the Reykjanes geothermal field increased noticeably after the commissioning of the 100 MWe Reykjanes Power Plant in 2006. When increased steam flow from steam vents is taken into account the total increase in surface activity in Reykjanes between 2004 and 2007 is of the order of 50%.

Testing of the recently developed tectonomagmatic discrimination diagrams from hydrothermally altered igneous rocks of 7 geothermal fields

Recently developed multidimensional tectonomagmatic discrimination diagrams based on log-ratio variables of chemical elements, discordant outlier-free databases, and probability-based boundaries have been shown to work better than the earlier diagrams. Hydrothermally altered drilled well rock cuttings obtained from different depths of geothermal fields were used to test these diagrams to compare the inferred tectonic setting with the expected one. In spite of the hydrothermal alteration effects, these diagrams provided the following expected tectonic settings: (1) an arc setting for Ahuachapan and Berlin geothermal fields, El Salvador; (2) a rift setting for Cerro Prieto geothermal field, Mexico, and Tendaho geothermal field, Afar region; (3) a MORB setting for Reykjanes geothermal field, Iceland; (4) a transitional arc and collision setting for Roman volcanic provenance, Italy; and (5) an arc setting for Tongonan geothermal field, Philippines. The present study indicates that the recently developed multielement discriminant function based diagrams may be successfully applied to infer the original tectonic setting of hydrothermally altered rock samples and thus confirms the robustness of these diagrams.

A plan for a 5 km-deep borehole at Reykjanes, Iceland, into the root zone of a black smoker on land

Abstract. A summary workshop report describing the progress made so far by the Iceland Deep Drilling Project (IDDP) is presented below. The report provides recommendations concerning technical aspects related to deep drilling, and invites international participation in both the engineering and the scientific activities of the next phase of the IDDP. No issues were identified at the workshop that should rule out attempting the drilling, sampling and testing of the proposed IDDP-2 well. Although technically challenging, the consensus of the workshop was that the drilling of such a hot deep well, and producing potentially hostile fluids, is possible but requires careful contingency planning. The future well will be explored for supercritical fluid and/or superheated steam beneath the current production zone of the Reykjanes geothermal field in SW Iceland. This deep borehole will provide the first opportunity worldwide to directly investigate the root zone of a magma-hydrothermal system which is likely to be similar to those beneath the black smokers on the world-encircling mid-ocean rift systems.