Thermodynamic Properties of Aqueous Species Calculated Using the HKF Model: How Do Different Thermodynamic and Electrostatic Models for Solvent Water Affect Calculated Aqueous Properties?

Thermodynamic properties of aqueous species are essential for modeling of fluid-rock interaction processes. The Helgeson-Kirkham-Flowers (HKF) model is widely used for calculating standard state thermodynamic properties of ions and complexes over a wide range of temperatures and pressures. To do this, the HKF model requires thermodynamic and electrostatic models of water solvent. In this study, we investigate and quantify the impact of choosing different models for calculating water solvent volumetric and dielectric properties, on the properties of aqueous species calculated using the HKF model. We identify temperature and pressure conditions at which the choice of different models can have a considerable effect on the properties of aqueous species and on fluid mineral equilibrium calculations. The investigated temperature and pressure intervals are 25–1000°C and 1–5 kbar, representative of upper to middle crustal levels, and of interest for modeling ore-forming processes. The thermodynamic and electrostatic models for water solvent considered are: Haar, Gallagher and Kell (1984), Wagner and Pruß (2002), and Zhang and Duan (2005), to calculate water volumetric properties, and Johnson and Norton (1991), Fernandez and others (1997), and Sverjensky and others (2014), to calculate water dielectric properties. We observe only small discrepancies in the calculated standard partial molal properties of aqueous species resulting from using different water thermodynamic models. However, large differences in the properties of charged species can be observed at higher temperatures (above 500°C) as a result of using different electrostatic models. Depending on the aqueous speciation and the reactions that control the chemical composition, the observed differences can vary. The discrepancy between various electrostatic models is attributed to the scarcity of experimental data at high temperatures. These discrepancies restrict the reliability of the geochemical modeling of hydrothermal and ore formation processes, and the retrieval of thermodynamic parameters from experimental data at elevated temperatures and pressures.ISSN:1468-8115ISSN:1468-812

Advanced well model for superhot and saline geothermal reservoirs

We present a new well model aimed at simulating deep and superhot geothermal wells within reservoir-scale flow models. The model uses a classic multi-segment approach to solve the well hydrodynamics but also includes several important features significantly expanding its capabilities. Firstly, we use thermodynamic tables allowing us to accurately model fluids at all relevant pressures, temperatures and salinity conditions up to magmatic conditions. The well model can account for the transport of dissolved NaCl salt and its potential precipitation in the form of halite. Secondly, the model includes an air phase and incorporates the transient displacement of the air-water interface in the well. This allows us to simulate the starting of the well using the air pressurization technique. Lastly, the well model is coupled to an unstructured reservoir grid on which magma bodies and feed zones can be explicitly represented. This paper introduces the technical details of the well model and presents several applications showcasing what insights could be gained concerning the performance of deep geothermal wells. We conducted two sets of simulation: first, we simulated a deep resource resulting from strongly enhanced heat flux with a well and a single feedzone; we assessed the effects of the feedzone's permeability, temperature and salinity on well starting and initial performance. In a second set of simulations, we used a more realistic hydrothermal system, driven by a magmatic intrusion. From the results we illustrate examples of which factors control the ability of the well to self-start, if and how air pressurization can aid starting wells, how production from a supercritical resource created near a magmatic intrusion may evolve over up to 200 years, and how halite precipitation may rapidly clog the well in case of production from saline superhot resources.ISSN:0375-650

Simulations of the IDDP-2 well, Reykjanes, Iceland, and its behavior in different operation scenarios

We present a well-reservoir modeling study aimed at better understanding one of the hottest geothermal well ever drilled, the IDDP-2 well in Reykjanes. To obtain realistic models of the well and reservoir we follow three main steps. First, we simulate the evolution of the reservoir following the emplacement of a magmatic intrusion thousands of years ago to obtain the most likely natural state of the geothermal system. The simulations show that the reservoir permeability structure largely controls its thermal evolution. Model validation is done by refining the permeability structure and other secondary parameters until the simulation results match the currently measured reservoir temperatures along the well. An important constraint is the reservoir temperature of about 550 °C at 4500 m depth, consistent with previous estimates from geophysical inversions and fluid inclusions obtained in core samples from the deepest part of the well. Second, we constrain the location and permeability of the feed zones by simulating and matching the results from a cold-water injectivity test. Third, we simulate the extensive cold-water injection phase that occurred in the 2017–2018 period. The obtained reservoir state is used as an initial condition for the simulation of well operations. With H2O-NaCl as a proxy to the reservoir's fluid composition, our simulation shows that in the deepest part of the well (from 4200 to 4500 m), the fluid naturally present in the reservoir would be in the vapor + halite thermodynamic field implying that halite scaling upon production could rapidly clog the well. Three different scenarios were investigated: (1) a scenario that mimics the actual history of the well and simulates how flow evolves over 12 years following the cold-water injection phase, to better understand the current thermo-hydraulic state of the well and predict its behavior in the upcoming years; (2) a hypothetical scenario of how the well would have evolved without the preceding, long-term cold-water injection phase that, according to the results of scenario (1) delayed significant production; and (3) a second hypothetical scenario with cold-water injection and modified feed zone permeabilities (enhanced permeability at 4400 m and reduced at 3400 m) to assess if reservoir engineering measures could enhance energy production by favoring inflow from the superhot part of the reservoir. For the first scenario, resembling the actual evolution of the well, we find that by 2018 the formation had cooled significantly in all feed zones due to the extensive cold-water injection. A consequence is that the well flow rate is initially very small (less than 2 kg.s−1) and only slowly rises to its maximum (about 50 kg.s−1) after more than 6 years, which is the time it takes for the formation to recover its initial temperature. This is in good agreement with early flow observations and a well flow test conducted in 2022 that shows a gradual increase in flow rate. The results of the second scenario, which simulates the case where cold-water injection did not occur, are similar to the first one, the only difference being that the reservoir does not need to warm up and that the well achieves its maximum production potential without delay. The third scenario, which considers the case of enhanced permeability of the deepest feed zone and reduced permeability of the intermediate feed zone results in a worse outcome in terms of energy production because of halite scaling and well blockage occurring at the deepest feed zone. These results show that advanced well-reservoir modeling is an essential tool to devise geothermal reservoir production strategies for superhot resources, namely in the case of saline systems.ISSN:0375-650

Thermodynamic Properties of Aqueous Species Calculated Using the HKF Model: How Do Different Thermodynamic and Electrostatic Models for Solvent Water Affect Calculated Aqueous Properties?

Thermodynamic properties of aqueous species are essential for modeling of fluid-rock interaction processes. The Helgeson-Kirkham-Flowers (HKF) model is widely used for calculating standard state thermodynamic properties of ions and complexes over a wide range of temperatures and pressures. To do this, the HKF model requires thermodynamic and electrostatic models of water solvent. In this study, we investigate and quantify the impact of choosing different models for calculating water solvent volumetric and dielectric properties, on the properties of aqueous species calculated using the HKF model. We identify temperature and pressure conditions at which the choice of different models can have a considerable effect on the properties of aqueous species and on fluid mineral equilibrium calculations. The investigated temperature and pressure intervals are 25–1000°C and 1–5 kbar, representative of upper to middle crustal levels, and of interest for modeling ore-forming processes. The thermodynamic and electrostatic models for water solvent considered are: Haar, Gallagher and Kell (1984), Wagner and Pruß (2002), and Zhang and Duan (2005), to calculate water volumetric properties, and Johnson and Norton (1991), Fernandez and others (1997), and Sverjensky and others (2014), to calculate water dielectric properties. We observe only small discrepancies in the calculated standard partial molal properties of aqueous species resulting from using different water thermodynamic models. However, large differences in the properties of charged species can be observed at higher temperatures (above 500°C) as a result of using different electrostatic models. Depending on the aqueous speciation and the reactions that control the chemical composition, the observed differences can vary. The discrepancy between various electrostatic models is attributed to the scarcity of experimental data at high temperatures. These discrepancies restrict the reliability of the geochemical modeling of hydrothermal and ore formation processes, and the retrieval of thermodynamic parameters from experimental data at elevated temperatures and pressures

A Peaceman-type well model for the 3D Control Volume Finite Element Method and numerical simulations of supercritical geothermal resource utilization

While supercritical geothermal resources receive increasing attention, it has so far remained unclear if and how they could be utilized. In order to provide a tool that can simulate both the natural transient evolution of a supercritical resource near a magma intrusion and its response to possible operation schemes, we augmented the CSMP++ simulation platform with a Peaceman-type well model. For the purpose of generic porous medium simulations of the supercritical resource's response to direct production, injection or other operation schemes, only a single in-/outflow interval per well is considered and flow in the wellbore is not simulated. The model's semi-analytical source/sink function accounts for the rapid change in pressure near the wells in the reservoir simulation. As the implementation is based on the 3D Control Volume Finite Element Method it allows the use of unstructured computational grids in order to be able to include geologically realistic geometries. We validate that the well model provides robust solutions of well pressures and rates. Such robust solutions are also obtained for supercritical conditions where water is highly compressible and, therefore, does not strictly fulfil the assumptions used in the derivation of the Peaceman model. To illustrate how a magma-related supercritical resource responds to well operation, we first simulate the evolution of a geothermal system around and above a 2 km deep, explicitly represented magma body. During the system's hottest phase, ca. 2700 years after its initiation and during progressive inward cooling of the magma body, a well completion interval is activated at 2.1 to 2.2 km depth where temperatures then reach 420 to 490 °C. For the relatively low host rock permeability used in the model (10−15 m2), injection and production rates remain quite limited at several kg/s. Yet, the evolving patterns of temperature, enthalpy, density and flow rates provide some first insights into supercritical resource response to operation.ISSN:0375-650

Cold water injection near the magmatic heat source can enhance production from high-enthalpy geothermal fields

Although existing deep geothermal wells have confirmed that shallow magmatic intrusions heat circulating hydrothermal fluids to supercritical temperatures, successfully utilizing such fluids for power generation has proven challenging. As an alternative, using deep geothermal wells drilled into the vicinity of intrusions for injection rather than production has the potential to enhance heat extraction from the intrusion, while simultaneously providing fluid pressure support to the overlying geothermal system. However, the dynamic response of the heat source and overlying geothermal system to deep injection has not previously been investigated using numerical modelling. Here, we present numerical models showing that injection in the near-vicinity of a magmatic heat source increases the production potential of shallower geothermal wells. By locally cooling the hot rock near an intrusion, deep injection increases permeability and the rate of heat transfer across the brittle–ductile transition. The injected fluids ascend in upflow zones above the heat source and provide pressure support to shallower geothermal wells, without significantly affecting the temperature and enthalpy of produced fluids. We investigate a variety of scenarios involving only a single deep injector and a single shallower producer. Our models suggest that deep injection could increase the production potential of shallower geothermal wells by up to 10 percent after 30 years of well operation and by 20 percent after theoretical 100 years, with the magnitude of the impact increasing with continued injection over longer timescales.ISSN:0375-650