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The Hellisheidi geothermal power plant, located in Iceland, is a combined heat and power double-flash geothermal plant with an installed capacity of 303.3 MW of electricity and 133 MW of hot water. This study aimed to elucidate the environmental impacts of the electricity and heat production from this double-flash geothermal power plant. In this vein, firstly, the most updated inventory of the plant was generated, and secondly, a life-cycle assessment approach based on the exergy allocation factor was carried out instead of applying the traditionally used allocations in terms of mass and energy. The functional unit was defined as the production of 1 kWh of electricity and 1 kWh of hot water for district heating. The life-cycle stages included the (i) construction, (ii) operation (including abatement operations and maintenance), and (iii) well closure of the geothermal plant. All of the life-cycle stages from construction to dismantling were considered. Finally, the results on the partitioning of the environmental impact to electricity and heat with exergy allocations showed that most of the impact should be charged to electricity, as expected. Furthermore, the distribution of the environmental impacts among the life-cycle stages determined that the construction stage was the most impactful for the electricity and heat production. This result was attributable to the large consumption of steel that was demanded during the construction of the geothermal power plant (geothermal wells, equipment, and buildings). Impacts due to the abatement stage demonstrated that this stage satisfactorily reduced the total impact attributed to the three life-cycle stages of the geothermal power plant. [ABSTRACT FROM AUTHOR]

Understanding injection‐induced microseismicity in geothermal systems can provide insight into reservoir connectedness. However, fault and reservoir complexity are difficult to represent in simple analytical models, which makes it difficult to discern clear relationships from incidental associations. Here, we have used data‐driven models to study how fluid injection and microseismicity are related in the Rotokawa (New Zealand) and Hellisheiði (Iceland) geothermal fields. We tested two classes of model: (a) lagged linear regression of seismicity rate as a function of well injection rates; and (b) systematic extraction of injection time series features that are then evaluated for associations with the seismicity. These models allowed us to determine which wells had the greatest correlation with microseismicity and to explain this association in a reservoir context. Finally, exploring different data types and transformations, we were unable to establish a link between rapid changes in injection rate and seismicity spikes, as suggested by some theoretical models. Plain Language Summary: To better understand the small earthquakes triggered in geothermal fields, we used two kinds of data‐driven models: time series feature engineering (a machine learning method) and the non‐negative linear regression method. We applied these techniques to microearthquakes recorded in the Rotokawa geothermal field, New Zealand, and the Húsmúli reinjection area in the Hellisheiði field, Iceland. We searched for associations and patterns hidden within the noisy injection and seismicity data. These patterns are further complicated by the complex injection schedules that involve several wells that switch off and on over time. We found a stronger association between injection and long‐term microseismicity rate as opposed to short‐term fluctuations driven by noise. We showed that particular injection wells are strongly associated with the microseismicity, which we interpret in terms of reservoir connectedness and stress changes in the rock. We found that injection rate and volume are the most useful data for understanding microseismicity, whereas there was no evidence that rapid injection rate changes triggered seismicity at these two locations. Key Points: Relative associations of geothermal wells with seismicity rate are evaluated using regression analysis and time series feature engineeringReservoir operational changes such as injection transfers between wells can affect the association between wells and microseismicityThere is a stronger correlation between injection and long‐term seismicity rate than short‐term seismicity rate at Rotokawa and Hellisheiði site [ABSTRACT FROM AUTHOR]

To improve the utilization rate of hot dry rock resources, it is necessary to recover the energy of geothermal tailwater and improve the net output work. An improved ammonia-water power cycle is proposed based on the Kalina cycle. Taking the geothermal parameters of the Husavik Power Plant in Iceland as the prototype (the water temperature of the geothermal well is 122 °C, and the tailwater temperature is 80 °C), the numerical simulation of the modified Kalina cycle is carried out by using Engineering Equation Solver software. The result shows that the net power output of the modified Kalina cycle increases by 10.5% compared to that of the Kalina cycle. In addition, the parametric study shows that the optimal ammonia concentration of the basic solution in the general area is 0.9. The lower the cooling water temperature is, the lower the turbine exhaust pressure and the higher the net power output. When the ammonia concentrations of the basic solution are 0.6, 0.7, 0.8 and 0.9, the optimum pressures are 29 bar, 36 bar, 41 bar and 45 bar, respectively. The results of this study will contribute to the utilization of geothermal energy in hot dry rock. [ABSTRACT FROM AUTHOR]

Fluids emanating from geothermal areas contain trace quantities of methane and other simple hydrocarbons. These hydrocarbons are thought to derive from thermal cracking of organic matter dissolved in circulating meteoric or seawater or found in pre-existing organic-rich sedimentary rocks, but an abiotic origin has also been proposed. We measured the relative abundances of four CH 4 isotopologues (12CH 4 , 13CH 4 , 12CH 3 D, and 13CH 3 D) in hydrothermal gases discharged by steam vents and geothermal wells from Iceland and Nisyros island (Greece) in order to investigate the origin of methane. Measured methane samples yielded consistently low Δ13CH 3 D values (13CH 3 D abundance relative to stochastic) of 0.82–1.77‰, which correspond to high apparent temperatures of isotopologue equilibrium ( T Δ 13 CH 3 D = 278–490 °C). Hydrothermal well fluids from the Krafla and Námafjall geothermal fields in Iceland yielded the lowest Δ13CH 3 D values, and thus the highest Δ13CH 3 D-based temperatures averaging 438 - 45 + 55 °C. Those samples also show the most pronounced departures in δD CH4 and δ13C CH4 values expected for isotopic equilibrium with respect to δD H2O and δ13C CO2. In contrast, CH 4 samples from natural steam vents in other Iceland locations and in Nisyros have slightly higher Δ13CH 3 D values (with T Δ 13 CH 3 D = 351 - 35 + 42 °C) and have δD CH4 and δ13C CH4 values that are consistent with those expected for isotopic equilibrium with both H 2 O and CO 2. The short fluid residence times (1–50 years) in systems that are exploited for geothermal energy, such as Krafla and Námafjall, combined with the proximity of a hot magma chamber, favor the preservation of kinetic signals. The initial disequilibrium δD CH4 and δ13C CH4 values are consistent with a thermogenic origin from immature organics dissolved in hydrothermally heated groundwater, but an abiotic origin cannot be excluded. The high apparent Δ13CH 3 D-based temperatures at Krafla and Námafjall could therefore represent nonequilibrium signals associated with either pyrolysis or abiotic generation of CH 4 in a superheated vapor or supercritical water phase (>374 °C), considered to exist in the roots of the system above the magmatic heat source. Isotopologue equilibration calculations demonstrate that under such conditions (e.g. > 400 °C) kinetic signals would be erased in days to months, implying rapid migration and quenching of CH 4 into the overlying subcritical (<300 °C) hydrothermal reservoir fluids. In systems with longer fluid residence times such as Nisyros, equilibrium isotopologue distributions at temperatures of ~ 350 °C are consistent with long fluid residence times on the order of > 100 years. Our calculations further reveal that CO 2 –CH 4 isotopic equilibration requires unreasonably long fluid residence times, suggesting that any apparent 13C equilibrium may be coincidental. [ABSTRACT FROM AUTHOR]

The unexpected intersection of rhyolitic magma and retrieval of quenched glass particles at the Iceland Deep Drilling Project-1 geothermal well in 2009 at Krafla, Iceland, provide unprecedented opportunities to characterize the genesis, storage, and behavior of subsurface silicic magma. In this study, we analyzed the complete time series of glass particles retrieved after magma was intersected, in terms of distribution, chemistry, and vesicle textures.Detailed analysis of the particles revealed them to represent bimodal rhyolitic magma compositions and textures. Early-retrieved clear vesicular glass has higher SiO2, crystal, and vesicle contents than later-retrieved dense brown glass. The vesicle size and distribution of the brown glass also reveal several vesicle populations. The glass particles vary in δD from -120‰ to -80‰ and have dissolved water contents spanning 1.3-2 wt%, although the majority of glass particles exhibit a narrower range. Vesicular textures indicate that volatile overpressure release predominantly occurred prior to late-stage magma ascent, and we infer that vesiculation occurred in response to drilling-induced decompression. The textures and chemistry of the rhyolitic glasses are consistent with variable partial melting of host felsite. The drilling recovery sequence indicates that the clear magma (lower degree partial melt) overlays the brown magma (higher degree partial melt). The isotopes and water species support high temperature hydration of these partial melts by a mixed meteoric and magmatic composition fluid. The textural evidence for partial melting and lack of crystallization imply that magma production is ongoing, and the growing magma body thus has a high potential for geothermal energy extraction.In summary, transfer of heat and fluids into felsite triggered variable degrees of felsite partial melting and produced a hydrated rhyolite magma with chemical and textural heterogeneities that were then enhanced by drilling perturbations. Such partial melting could occur extensively in the crust above magma chambers, where complex intrusive systems can form and supply the heat and fluids required to re-melt the host rock. Our findings emphasize the need for higher resolution geophysical monitoring of restless calderas both for hazard assessment and geothermal prospecting. We also provide insight into how shallow silicic magma reacts to drilling, which could be key to future exploration of the use of magma bodies in geothermal energy. [ABSTRACT FROM AUTHOR]

Highlights • Supercritical fluids form around magmatic intrusions from conductive heating of surrounding subcritical geothermal fluids. • Supercritical fluid formation leads to a quartz-dominated contact aureole. • Supercritical fluids have similar volatile contents as surrounding subcritical geothermal fluids. • Condensation of liquid from ascending supercritical fluids above the intrusion generates acid thermal fluids. Abstract High-enthalpy supercritical geothermal fluids were obtained from the IDDP-1 well at Krafla, Iceland, which had a discharge temperature of ∼440 °C and specific enthalpy of ∼3200 kJ kg−1. Utilization of such fluids may multiply power production from geothermal well fields. However, the origin of supercritical fluids in the roots of volcanic geothermal systems is poorly understood. Here, we propose that an important mechanism of such supercritical fluid formation is conductive heating of surrounding subcritical geothermal fluids near a shallow intrusion. Predictions from hydrologic and chemical models of supercritical fluid formation and ascent are compared with measured fluid compositions from the Krafla geothermal system. Supercritical fluids formed by close to isobaric heating of liquid geothermal groundwater display similar volatile concentrations (C, S, B, Cl, F) as the initial fluid. In contrast, the low concentrations of non-volatile elements (Si, Na, K, Ca, Mg, Al, Fe) in the supercritical fluid result from intensive, quartz-dominated mineral deposition near the magmatic intrusion during boiling of liquid to dryness. Liquid condensed out of ascending supercritical fluid has a low pH (∼2) due to the dissociation of volatile components like HCl. Such 'acid' geothermal fluids have been encountered in wells in the Krafla system. However, the chemical signatures of supercritical fluid ascent are likely to be overprinted by mixing of the acid fluids with cooler subcritical fluids, fluid-rock interaction, depressurization boiling and phase segregation. [ABSTRACT FROM AUTHOR]

• Modeling of injection-induced fault reactivation at Reykjanes, Iceland. • 3D faulted reservoir model based on integration of different data types. • Simulation of fully coupled hydro-mechanical processes and fault reactivation. • Combined induced seismicity analysis and physics-based simulation. • Improved understanding of interactions between faults in hydraulic stimulation. 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. [Display omitted] [ABSTRACT FROM AUTHOR]

• Soft stimulation treatment procedures were successfully applied at field scale in a densely populated area. • Hydraulic performance of well RV-43 was increased by hydraulic stimulation. • Moment magnitudes of induced seismic events were below 0. Reykjavik is almost entirely heated by geothermal energy. Yet, recent growth of the city significantly increased the heat demand. Past experiences in Iceland's capital region showed that hydraulic stimulation of existing geothermal wells is suited to improve hydraulic performance and energy supply. However, fluid injection may also trigger felt or even damaging earthquakes, which are of concern in populated areas and pose a significant risk to stimulation operations. Consequently, soft stimulation concepts have been developed to increase geothermal well performance while minimizing environmental effects such as induced seismicity. In a demonstration project of hydraulic soft stimulation in October 2019, more than 20.000 m³ of water were injected into well RV-43 in Reykjavik in multiple stages and with different injection schemes. The hydraulic performance of the well was improved without inducing felt seismicity. An a priori seismic risk assessment was conducted and for the first time the risk was continuously updated by an adaptive traffic light system supported by a sophisticated realtime microseismic monitoring. Our results confirm that it is possible to improve the performance of geothermal wells in Reykjavik and worldwide with acceptable technical, economic, and environmental risks. Here we provide an overview of the entire stimulation project including site description, stimulation design, zonal isolation, logging, seismic risk assessment and mitigation measures, realtime seismic, hydraulic and chemical monitoring, and stimulation results and challenges. [ABSTRACT FROM AUTHOR]

• Steel well casings used as electrodes in a resistance study. • Low resistance measured at Reykjanes due to high salinity of geothermal fluid. • Electric current travels between casings through brine-filled fractures. • Casings transfer electric current deeper into the ground than surface electrodes. 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. [ABSTRACT FROM AUTHOR]

• Corrosion study of casing material from IDDP-1. • Cracking and corrosion of low to medium carbon steels is preventing its long time use in IDDP-1 conditions. • Methane cracking shown for the first time in material from geothermal well. The Iceland Deep Drilling Project (IDDP) well IDDP-1 in the Krafla geothermal field in Iceland is the most powerful geothermal well on record, with a maximum wellhead temperature of 452 °C and 140 bar pressures. In the end, the well had to be abandoned and closed permanently due to several casing failures, including a number of collapses and tensile coupling failures of the innermost casing – the production casing. The history of this first IDDP well is discussed in this paper, revealing the conditions the carbon steel was exposed to. The metal samples degrading mechanism was investigated by mechanical testing and microstructure analysis. For the first time, material used in geothermal steam in situ above 400 °C for a long period was investigated and a new alarming cracking mechanism detected. This cracking mechanism due to formation of methane inside the carbon steel structure results in severe cracks and as a result the steel loses its strength and ductility. This study reveals the need for review of material selection for high-temperature geothermal wells. [ABSTRACT FROM AUTHOR]

• Predictable trace element enrichments and depletions in the Reykjanes system. • Boiling exerts a major influence on the enrichment of metals. • High concentrations of Au and Ag and Pb indicate accumulation in reservoir fluids. • Three quarters of the metal budget is deposited at depth or in the upflow zone. Mineral precipitation in the seawater-dominated Reykjanes geothermal system on the Mid-Atlantic Ridge, Iceland is caused by abrupt, artificially induced, pressure and temperature changes as deep high-temperature liquids are drawn from reservoir rocks up through the geothermal wells. Sulfide scales within these wells represent a complete profile of mineral precipitation through a seafloor hydrothermal system, from the deep reservoir to the low-temperature silica-rich surface discharge. Mineral scales have formed under a range of conditions from high pressures and temperatures at depth (>2 km) to boiling conditions in the upflow zone and at the surface. Consistent trace element enrichments, similar to those in black smoker chimneys, are documented: Cu, Zn, Cd, Co, Te, V, Ni, Mo, W, Sn, Fe and S are enriched at higher pressures and temperatures in the deepest scales, Zn and Cu, Bi, Pb, Ag, As, Sb, Ga, Hg, Tl, U, and Th are enriched at lower temperatures and pressures nearer to the surface. A number of elements (e.g., Co, Se, Cd, Zn, Cu, and Au) are deposited in both high- and low-pressure scales, but are hosted by distinctly different minerals. Other trace elements, such as Pb, Ag, and Ga, are strongly partitioned into low-temperature minerals, such as galena (Pb, Ag) and clays (Ga). Boiling and destabilization of metal-bearing aqueous complexes are the dominant control on the deposition of most metals (particularly Au). Other metals (e.g., Cu and Se) may also have been transported in the vapor phase. Very large enrichments of Au, Ag and Pb in the scales (e.g., 948 ppm Au, 23,200 ppm Ag, and 18.8 wt.% Pb) versus average concentrations in black smoker chimneys likely reflect that some elements are preferentially deposited in boiling systems. A mass accumulation of 5.7 t/yr of massive sulfide was calculated for one high-temperature production well, equating to metal fluxes of 1.7 t/yr Zn, 0.3 t/yr Cu, 23 kg/yr Pb, 4.1 kg/yr Ag, and 0.5 kg/yr Au. At least three quarters of the major and trace element load is precipitated within the well before reaching the surface. We suggest that a similar proportion of metals may be deposited below the seafloor in submarine hydrothermal systems where significant boiling has occurred. Mass accumulation estimations over the lifetime of the Reykjanes system may indicate significant enrichment of Zn, Pb, Au, and Ag relative to both modern and ancient mafic-dominated seafloor massive sulfide deposits, and highlights the potential for metal enrichment and accumulation in the deep parts of geothermal systems. [ABSTRACT FROM AUTHOR]