A new and economically attractive type of geothermal resource was recently discovered in the Krafla volcanic system, Iceland, consisting of supercritical water at 450 °C immediately above a 2-km deep ...magma body. Although utilizing such supercritical resources could multiply power production from geothermal wells, the abundance, location and size of similar resources are undefined. Here we present the first numerical simulations of supercritical geothermal resource formation, showing that they are an integral part of magma-driven geothermal systems. Potentially exploitable resources form in rocks with a brittle-ductile transition temperature higher than 450 °C, such as basalt. Water temperatures and enthalpies can exceed 400 °C and 3 MJ kg(-1), depending on host rock permeability. Conventional high-enthalpy resources result from mixing of ascending supercritical and cooler surrounding water. Our models reproduce the measured thermal conditions of the resource discovered at Krafla. Similar resources may be widespread below conventional high-enthalpy geothermal systems.
A series of hydrothermal experiments have been conducted to determine equilibrium D/H fractionations between brucite and water as a function of temperature (200–600 °C), pressure (2.1–800 MPa), and ...dissolved NaCl (0–5 molal or 0–22.6 wt%). Along with our previous study, a total of 39 data points show that pressure and dissolved NaCl both increase the D/H fractionation factor at a given temperature. Theoretical treatment allowed a first direct comparison of experimental and theoretical results of brucite-water D/H fractionation at a zero pressure limit. After correcting the pressure effects on brucite D/H partition function ratios (β-factor), the isotope effects of pressure and dissolved NaCl on the D/H β-factor of water were for the first time rigorously evaluated. There is a good linear relationship between the D/H β-factor of the aqueous NaCl solutions and their densities of at a given temperature. A good, pseudo-linear relationship observed between the density of aqueous NaCl solutions and the maximum intensity of the asymmetric OH vibrational frequency of water in turn suggests that this frequency is a fundamental property that determines the β-factor of water at elevated temperatures and pressures. Our study suggests that the density of aqueous solutions (ρaq-soln) could be used as a primary parameter to predict the effects of pressure and NaCl compositions on the β-factor of water under geologic conditions encountered in the crust and upper mantle: Δ103lnβwat/Δρaq-soln = −0.0111 (±0.0012), where Δ is a difference in the value of 103lnβwat orρaq-soln, and ρaq-soln in kg m−3.
To optimize the behaviour of an ATES (aquifer thermal energy storage), to estimate its efficiency and to identify the optimal well locations, the planned installation was simulated with a FE-FV ...(finite element-finite volume) simulator with realistic water properties, created on the basis of the CSMP++ (Complex Systems Modelling Platform) software library.
Simulation results show that storage efficiency increases with the distance between injection and production wells and decreases with increasing injection temperature. Results also support a storage design where the storage wells are placed near the walls. Groundwater flow does not affect storage efficiency significantly, proving that the double concrete walls act as a sufficient thermal insulator.
The full operation cycle of an ATES with an optimal well placement was simulated: 120 days of hot water injection (charging), 60 days of production (discharging). The predicted energy and exergy efficiency are 35.6% and 27.4%, respectively. The storage supplies between ∼300 kW and ∼120 kW of thermal energy the first 60 days of hot water production.
•Aquifer thermal energy storage was simulated using FE-FV (finite element-finite volume) method.•Water properties from water EOS (equation of state) and geological properties from a pilot site were used.•Results support a storage design where the storage wells are placed near the walls.•Groundwater flow does not affect storage efficiency significantly.
It has long been recognized that quartz precipitation from circulating hydrothermal fluids may reduce porosity and permeability near intrusions. However, the magnitude of permeability changes and ...potential feedbacks between flow, heat transfer, and quartz precipitation/dissolution remain largely unquantified. Here, we present numerical simulations of fluid convection around upper crustal intrusions which explicitly incorporate the feedback between quartz solubility and rock permeability. As groundwater is heated to ~350°C, silica dissolves from the host rock, increasing porosity and permeability. Further heating to supercritical conditions leads to intensive quartz precipitation and consequent permeability reduction. The initial host rock permeability and porosity are found to be main controls on the magnitude and timescales of permeability changes. While the permeability changes induced by quartz precipitation are moderate in host rocks with a primary porosity ≥ 0.05, quartz precipitation may reduce rock permeability by more than an order of magnitude in host rocks with a primary porosity of 0.025. Zones of quartz precipitation transiently change locations as the intrusion cools, thereby limiting the clogging effect, except for host rocks with low initial porosity. This permeability reduction occurs in timescales of hundreds of years in host rocks with initial high permeability and thousands of years in host rocks with intermediate permeability.
NaCl is the most common solute in aqueous fluids in many geological settings but the thermodynamic properties of binary NaCl-H2O solutions have remained understudied at pressures above 0.5 GPa. We ...report the first high-pressure (>0.5 GPa) density data for NaCl-H2O fluids derived from acoustic velocity measurements in 1m and 3m NaCl solutions in diamond anvil cells to 673 K and 4.5 GPa using Brillouin scattering spectroscopy. An empirical equation of state (EoS) for NaCl-H2O fluids has been generated fitting the new density data together with literature data for water, and used for extrapolation of thermodynamic data to 1073 K and 4.8m NaCl at 0.5-4.5 GPa, spanning the conditions for fluids expelled by subducted slabs. The EoS yields densities for binary NaCl-H2O solutions with uncertainty lower than 0.3-0.5% below 673 K and lower than 2-4% in the extrapolation region. The EoS may allow extrapolations beyond 1073 K although the resulting uncertainties are difficult to assess. Densities for NaCl-H2O fluids computed from the EoS at 0.5 GPa generally agree within 1% with published data below 673 K, although differences of up to 5% are observed at 1073 K. The experimentally-based EoS is more reliable in the calculation of pure water fugacities and phase equilibria involving fluid phases than previous formulations, particularly in the high pressure range. The presence of dissolved NaCl substantially decreases the activity of water, indicating large non-ideality in NaCl aqueous fluids at subduction zone conditions. This behavior has important implications for the localization of dehydration reactions in the slab, as the dehydration boundaries of hydrous phases shift toward shallower depths in the presence of dissolved NaCl in the fluid.
The densities of aqueous solutions in the systems Na2SO4–H2O and Na2SO4–NaCl–H2O were determined experimentally at temperatures from (298.15 to 523.15) K and pressures up to 70 MPa, and over a wide ...range of salt concentrations (up to 5 mol·kg–1 of NaCl and up to 1.2 mol·kg–1 of Na2SO4). The measurements were conducted in vibrating-tube densimeters permitting density determinations with an accuracy varying from (2.8·10–5 to 1.0·10–4) g·cm–3 depending on temperature. The Pitzer equation for the apparent molar volume was parametrized on the basis of the newly obtained experimental data and data from literature. The resulting model for binary Na2SO4–H2O and ternary Na2SO4–NaCl–H2O solutions can be applied across the entire range of temperatures, pressures, and compositions covered by this study. Most importantly, it is the first model capable of calculating the mean apparent molar volume of the mixed solutes and, consequently, the density of solution in aqueous system Na2SO4–NaCl–H2O at elevated temperatures and pressures. The volumetric properties derived from the parametrized model for mixed electrolyte solutions (i.e., partial molar volume, limiting partial molar volume, and excess molar volume of components) provide an access to quantitative evaluation of the effect of pressure on the activity coefficients of electrolytes in multicomponent aqueous solutions. Calculation of the pressure dependence of a chemical equilibrium is thus much facilitated by the presented models, which can assist in modeling of aqueous systems at elevated temperatures and pressures.
The advances of computational approaches of enhanced sampling allow us to calculate the free energy of chemical reactions from constrained molecular dynamics 8. ...we can use this method to derivate ...the thermodynamic properties of metal complexation reactions. Molecular dynamics simulation also provides a new approach to investigate the structural and thermodynamic properties of minerals and melts; thus, now we are able to predict the trace element partitioning between minerals and melts. The prime example of such system-scale modeling is that of fluid flow around a cooling pluton, for which robust understanding emerged when the full properties of water, correct treatment of boiling, and temperature-dependent permeability models were introduced by Hayba and Ingebritsen 9. Besides epithermal and magmatic-hydrothermal deposits, simulation studies have focused on basin-hosted ore-forming systems and on submarine massive sulfide deposits along active spreading ridges and submarine arc volcanoes.
This study investigates the thermo-hydraulic implications of three geologic scenarios for characterizing the geothermal hydrology of Basse-Terre Island, Guadeloupe. Despite newly acquired ...magnetotelluric, petrophysical, and geologic data, flow patterns and heat sources have remained elusive. Our simulations were performed in 2D, on a cross section going from La Soufrière volcano in the south to the operating Bouillante geothermal field near the west coast. Simulation results are compared to geologic constraints such as the temperature profile measured at Bouillante and the timing of volcanic activity in the area, which may be indicative of new heat sources at depth. The simulations indicate that during lateral flow from La Soufrière, geothermal fluids would cool too much to explain the temperature at Bouillante. Two other scenarios were found to explain the current thermal structure of the Bouillante geothermal system: a young (ca. 5000 years) and more local magmatic intrusion at depth, or vertical corridors of enhanced permeability that tap hot and porous formations at a few km depth. Without further geologic evidence, neither of these two scenarios can be preferred. The second magma chamber scenario would indicate a more complex magmatic history of the island than previously established. The study shows that geologically constrained scenarios of regional geothermal hydrology can be meaningfully tested with current numerical simulation techniques, providing further insights for geothermal exploration.
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