Definition of geological scenarios for the implementation of the CEEGS technology for underground energy storage and CO₂ sequestration

 

by Jesús García-Crespo¹*, Paula Canteli¹, Julio Carneiro², Dounia Behnous², Márton Pál Farkas³, Eleni Gianni⁴, Pavlos Tyrologou⁴, Nikolaos Koukouzas⁴, Ricardo Chacartegui⁵, Andrés Carro⁵, Edgar Berrezueta¹

¹  CN Instituto Geológico y Minero de España-Consejo Superior de Investigaciones Científicas

²  Converge!, Lda; Universidade de Évora

³  GFZ Helmholtz Centre for Geosciences Potsdam ;

⁴  Centre for Research and Technology-Hellas (CERTH) ;

⁵  University of Seville;

* Corresponding author: garcia.crespo@igme.es

Abstract

The de-carbonization energy transition requires solutions to store energy on a large scale while reducing CO₂ emissions. The CEEGS project integrates the transcritical CO₂ cycle with geological storage and heat extraction, combining electrothermal storage and CO₂ sequestration. This work aims to identify the most suitable geological scenarios for future implementation and establish the fundamental criteria that must be met. To this end, the properties of different geological media were reviewed and evaluated, based on previous knowledge about geological CO₂ and energy storage and geothermal applications. As a result, a preliminary set of criteria is proposed, based on thermodynamic and geochemical considerations and subsurface conditions. It constitutes a first approximation, which can be updated as the development of the CEEGS concept progresses.

Keywords

CO₂, Energy storage, Geothermal energy, Renewable energy, CCUS, Carbon capture, Energy transition

Note:

Papers published in this special issue of the European Geologist journal have undergone a thorough peer-review process but have not been copy-edited. Authors bear full responsibility for the linguistic accuracy of their contributions.

This work is licensed under a Creative Commons Attribution 4.0 International License.

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1. Introduction

The transition to a low-carbon energy system demands innovative solutions that can simultaneously address energy storage and greenhouse gas mitigation. Among emerging options, the CO₂-based Electrothermal Energy and Geological Storage (CEEGS) system represents a promising multi-functional concept that integrates three processes: energy storage via a transcritical CO₂ power cycle, geological CO₂ sequestration and, in some geological environments, geothermal heat extraction [1]. The effective deployment of this technology relies on identifying geological scenarios capable of supporting cyclic operation, thermal exchange, and long-term CO₂ trapping.

The foundational concept of coupling geologic carbon sequestration with geothermal energy recovery was proposed by Karsten Pruess [2], based on work from Brown, D. [3], though only in the context of engineered or enhanced geothermal systems. Randolph and Saar (e.g. [4─6]) introduced the concept of CO₂-Plume Geothermal (CPG) system, which involves injecting supercritical CO₂ into deep, naturally porous, permeable geologic reservoirs. Their model demonstrated that CO₂, due to its lower viscosity and higher expansivity compared to water, can enhance subsurface heat extraction and act as a buoyant plume for sustained energy production. This thermosiphon-driven system, initially modelled for saline aquifers, also provides a potential mechanism for permanent CO₂ storage [7].

Further contributions have validated the feasibility of such systems under dynamic subsurface conditions. Norouzi et al. [8] simulated transcritical CO₂ cycles in fluvial aquifers and identified key factors influencing energy output, such as geological heterogeneity, permeability anisotropy, and thermal conductivity variations. Their results reinforce the significance of reservoir-specific characterization and underscore the need to account for cap rock integrity and hydraulic continuity issues also highlighted in earlier works on CO₂ injection behaviour [9, 10].

Carro et al. [11] extended the concept of utilisation of CO₂ as a working fluid to underground storage of energy, with a focus on porous media environments and later expanded to salt cavities [1]. Their techno-economic analysis highlighted that cavern geometry, cycle frequency, and thermal gradient control system efficiency, which can reach up to 73% under optimized conditions. Notably, they emphasized the importance of the storage media’s thermophysical properties and insulation performance, in line with broader evaluations of underground thermal energy storage [12, 13].

Lower reservoir permeability and temperature in CO₂-enhanced geothermal systems have been associated with improved thermal recovery and reduced CO₂ loss, highlighting the dual role of geological formations in both heat extraction efficiency and carbon retention [14]. CO₂ plume geothermal (CPG) systems leverage deep geological formations, such as saline aquifers, basaltic reservoirs, or salt caverns, where supercritical or subcritical CO₂ serve both as a heat transport fluid and a geological storage medium, enhancing thermal extraction efficiency compared to conventional water-based geothermal operations [4, 5].

In this context, defining geological scenarios suitable for CEEGS implementation becomes essential. This study builds upon previous advances by developing a high-level geological screening framework tailored to the characteristics of CEEGS. The analysis focuses on petrophysical and structural criteria relevant to deep saline aquifers (DSA) and salt caverns, with particular attention to the effects of cyclic operation on reservoir behaviour and system efficiency. In particular, one of the scenarios of interest studies a geothermal system in a carbonate aquifer. Given the early technology readiness level of CEEGS (TRL 2), the proposed criteria and site conditions are based on existing knowledge from CO₂ storage assessment and support future refinements as technical understanding progresses.

2. Materials and Methods

The definition of the geological scenarios (understood as sets of properties and initial – pristine – conditions in an underground site) and the evaluation of their behaviour in the operation of the CEEGS concept was carried out through a combined approach of analytical, semi-analytical and numerical modelling. The methodological procedure was structured in three components: (i) construction of realistic geological scenarios and study of their technological efficiency in order to balance CO₂ injection/production cycles and CO₂ permanent storage; (ii) study of the behaviour of the reservoir in each scenario under cyclic storage conditions, including transient behaviour of pressure, temperature and hydrogeochemistry of the reservoir; and (iii) screening and ranking criteria definition for the selection of reservoirs for implementing the technology.

2.1. Geological configurations

One of the differentiating aspects of the CEEGS technology, with respect to other energy storage systems, is the use of underground geological storage, which adds an improvement in the efficiency of the system. Finding suitable geological formations is crucial for the success of this technology, as it will condition the design and lifetime of a future development. For this reason, special care has been put in establishing which characteristics a geological site must accomplish, through the study of similar proven technologies like geothermal energy storage and supercritical CO₂ (scCO₂) storage.

Multiple geological configurations, representative of deep saline aquifers (DSA), depleted hydrocarbon fields (DHF) and saline cavities, were defined on parameters documented in real formations of Portugal, Spain and Germany sites. For porous media (DSA and DHF), systematic variations of:

• depth (850–3000 m),
• reservoir injectivity (classified as Type I, II and III, after [15]),
• porosity and permeability,
• heterogeneity and anisotropy,
• degree of confinement (open and closed structures),
• configuration of one or two aquifers at different depths.

For saline cavities, depths of 500, 1,000 and 1,500 m were simulated, evaluating the admissible pressures and thermodynamic stability during the loading and unloading cycles.

Each of these variants was incorporated into the semi-analytical models to obtain the transient response of pressure, temperature, saturation and plume extension. These metrics were subsequently used to compare scenarios and derive suitability criteria.

2.2 Whole operational cycle – Physical model and computation

The effective deployment of GEEGS technology also relies on the definition of cyclical operations that combine surface and subsurface components. An initial process involves the setup, when CO₂ is continuously injected in a well A to ensure there is a large CO₂ plume in supercritical state. The whole cycle consists of two operational stages: 1) charge, when renewable energy surplus is utilised to compress and inject CO₂ in the geological reservoir again in well A, while providing thermal energy storage by transferring heat to a surface high-temperature tank; and 2) discharge, when CO₂ is produced and energy is recovered through a heat engine and a turbine. Extracted CO₂ is then reinjected underground, in liquid phase, through well B (Figure 1).

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The transient flow of CO₂ was represented by the radial diffusion equation for pressure, under a mass conservation scheme in porous media. Fluid properties were calculated as a function of pressure and temperature to maintain consistency with supercritical conditions at depth [16]. The thermal evolution was described through an energy balance that allowed estimating the injected thermal load and the temperature of the fluid produced in each cycle. For salt cavities, the behaviour was approximated by pressure- and temperature-dependent thermodynamic models under rigid wall conditions [17].

The models for porous media (DSA and DHF) were implemented using numerical modelling tools CMG STARS and CMG GEM [16], solving the evolution of pressure, temperature and saturation in injection-extraction cycles. For the salt cavities, Python routines for analytical and semi-analytical formulations in MATLAB [18] were used that incorporate the Well-Cavity behaviour and the evolution of the P-T during charge and discharge. The modelling allowed to calculate the distribution of CO₂, the thickness of the plume, the thermal gradients, the transient evolution at the wellhead and different thermodynamic indicators of the cycle.

A sensitivity analysis was performed using Monte Carlo approaches to evaluate the influence of uncertain geological parameters on the evolution of P–T, plume extent, and saturation around the producing well. The variables analysed included depth, permeability, thickness, porosity, heterogeneity, anisotropy and geothermal gradient. Scenarios for salt cavities were evaluated through variations in depth, admissible pressure, and cavity geometry. This analysis allowed to identify the ranges of parameters that determine the stability of the supercritical conditions and the cyclic behaviour of the system.

2.3. Development of selection criteria

Based on physical models and technical literature on geological CO₂ storage, energy storage and geothermal applications, a structured set of geological and petrophysical parameters was developed. These parameters were classified into mandatory (exclusive) criteria and prioritisation criteria, assigning them threshold values ​​or operating intervals. Finally, the criteria were organized in a spreadsheet in two steps. First, a kind of checklist of mandatory features (such as minimum depth or permeability) to verify positive potential of the site (or to be discarded). The second step to apply prioritization criteria, based on expertise knowledge, which allows properties to be filtered. A comparative score is generated between different locations. Results of the previous tasks were also considered and weighted, and recommendations were incorporated into the compilation of criteria. Only geological and technical aspects have been considered, so environmental and social conditions were not included.

Among all the parameters that influence and constrain a storage site, those considered the most relevant in terms of efficiency and safety have been chosen. The starting point was the review of previous works on CO₂ storage [19─26] and geothermal energy [27], in which many parameters were included, depending on the scope and purpose of the type of site.

The main properties source were those studies focused on CO₂ storage, given that most limiting criteria are common with subsurface usage. The CO₂ containment in the supercritical state is crucial for the adequate performance of CEEGS, therefore, the priority was to find the criteria that most influence the behaviour of CO₂ in the whole cycle since it is injected until it is withdrawn for energy recovery.

Characteristics of the reservoir rocks are important since both the CO₂ plume and energy storage efficiency depend on intrinsic properties of the geological formation, such as porosity, permeability, brine salinity, clay content, lithology, and thermal properties. At a broader scale, the geological setting includes features such as depth, thickness, presence of seals and faults, seismicity, structural closure, and geothermal gradient.

A value or a range of values have been given to each parameter to consider it suitable or not, or for assigning a score which, in the end, will result in a total score for ranking a site. A justification of the proposed values is given.

Finally, criteria, values and options have been arranged in a simple excel spreadsheet to filter the appropriate parameters, leading to a ranking score [28].

3. Results and Interpretation

3.1. Characteristics of the geological scenarios

The geological scenarios must meet a priori a series of requirements to cover the needs of the CEEGS technology, such as a highly saturated reservoir of CO₂, which requires the creation of a low dispersion but sufficiently large plume. This requires injecting a large amount of CO₂ in a short time at first and a sufficient depth to keep the pressure high enough to maintain scCO₂.

Analytical and semi-analytical solutions allow for the analysis of the limitations that reservoir depth, petrophysical parameters, and hydraulic conditions may impose on the implementation of the CEEGS concept in these environments. This approach was applied to: i) porous media, whether DSA or DHF; ii) saline cavities.

The study encompassed both open and porous reservoirs and included the possibility of simultaneously using two porous reservoirs at different depths. In CO₂ storage technology it is recommended that, when selecting sites for porous reservoirs, priority be given to high-permeability, high-porosity reservoirs with significant thicknesses (Type I reservoirs) [15]. However, it is suggested that thinner reservoirs (Types II and III) may present more favourable conditions for CEEGS. It is also recommended that the minimum reservoir depth be greater than that used in the CO₂ storage industry, at least 1300 m, to avoid CO₂ phase transition in the wellbore.

Salt cavities scenarios were addressed, including a single cavity with surface CO₂ storage in a tank, as well as two salt cavities at different depths (or managed at different pressures). The range of admissible depths for the cavities and the expected wellhead pressures were studied for simplified cases.

Resulting scenarios include various configurations of DSA (open structure, closed structure, and two aquifers), depleted hydrocarbon reservoirs (closed structure), geothermal sedimentary (carbonate) systems, and salt cavities (a single cavity with surface CO₂ storage and two salt cavities). The numerical modelling studies address the transient behaviour of the reservoirs to understand the cyclical and long-term evolution of T and P in the reservoir and at the wellheads, as well as the chemical changes that can occur during injection and back-production cycles due to the interaction between CO₂ and the brine and mineral components of the reservoir.

The geological characteristics evaluated in this section, by their nature, are the basis for evaluating the efficiency of the system.

3.2. Impact of the geological conditions on the efficiency of the system

Analysing the transient behaviour of CO₂ in geological reservoirs and wells is essential for understanding the efficiency of the subsurface component of the CEEGS concept and the sustainability of the energy storage system. A summary of the most favourable geological properties is presented in Figure 2. This was tested in various real-world scenarios in porous media reservoirs, including DSA and geothermal reservoirs. Different variations and configurations were considered for the DSA scenarios, including bounded (closed) and not bounded (open) aquifers, the influence of heterogeneity and anisotropy, and the use of a single aquifer for CO₂ injection and production or the use of two aquifers at different depths [29, 30]. Criteria for DSA selection are defined in Tables 1 and 2, and for salt caverns in Table 3.

The analysis was based on numerical simulations of the subsurface component of the CEEGS under the geological conditions considered for CO₂ storage in Portugal and Spain, as well as for geothermal development in Germany. The selection of these real-world cases ensured that the scenarios considered realistic conditions, but they were complemented by a comprehensive sensitivity analysis to identify the parameters that would significantly impact the viability and efficiency of this technology.

The feasibility of the CEEGS concept was lower for shallow aquifers, with the scenario having a maximum reservoir depth of 850 m exhibiting the lowest gross efficiencies, since the produced CO₂ is in gas phase. The same conclusion was reached for the scenario with two aquifers, in which the shallower aquifer, which allows for the storage of CO₂ in gaseous form, proved ineffective and did not warrant further study. Another issue favouring deeper reservoirs is the decrease in the proportion of brine produced as reservoir depth increases.

The deep, closed-bound saline aquifer raised concerns about the amount of brine produced with CO₂, which increased throughout the six charge-discharge cycles analysed, although it always remained a low proportion of the total flow (e.g. from 0.02 kg/s in the first cycle to 0.59 kg/s in the sixth cycle). Furthermore, the closed-bound situation implies (as expected) lower mass flow rates due to pressure build-up restrictions.

The open-boundaries DSA scenario, with considerable reservoir depth, and the geothermal scenario yielded the most interesting gross efficiencies and sustainability, as measured by well injectivity and productivity. This reflects the importance of reservoir temperature for system efficiency, since higher depths, even with normal geothermal gradients (35-45 °C), allow for higher wellhead temperatures during the discharge phase.

Scenarios where the geological formation is a salt cavity were studied using a semi-analytical approach [31]. In the tested scenarios, for three different cavity depths (500 m, 1000 m, and 1500 m), the efficiency ranged from 47.2% to 55.2%.

Several important concerns were also addressed. It is necessary to understand the effects of the fluid’s chemical composition at the wellhead due to the chemical reactions between CO₂, the rock, and the brine in the reservoir. Furthermore, the challenges posed by the intermittent nature of CO₂ injection and production must be carefully considered. This can be partially resolved by a strategy in which one well continuously injects CO₂ produced from a stationary source (charge phase) or a mixture of CO₂ from the stationary source and CO₂ produced by the second well (discharge phase). This strategy would maximize the amount of CO₂ permanently sequestered, but intermittency would persist at the producing well.

The scenarios assessing reservoir heterogeneity and permeability anisotropy also raised concerns about the degree of CO₂ saturation that could be reached around production wells.

The mechanisms governing CO₂ trapping, as well as the effect of impurities on well performance in three selected geological scenarios were investigated [32]. This includes assessing the CO₂ sequestration potential and the impact of charge/discharge cycles on the chemical composition of the CO₂ stream.

This study indicates that the three geological scenarios (DSA, DHF, and geothermal reservoir) provide valuable information on the CO₂ sequestration process in the reservoir, the impact of energy storage cycles on the chemical composition of the CO₂ stream, and the effect of impurities on system performance. In addition, this is also representative for the potential CEEGS host rock formations. It was demonstrated that a sufficiently long CO₂ plume establishment phase, allowing for negligible water production during energy storage cycles, is essential to minimize the impact of intraday charge/discharge cycles on the chemical composition of the CO₂ stream and on system efficiency during the study period.

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Table 1: Mandatory criteria for deep saline aquifers (DSA) selection. Optimum values are indicated in the right column.

Parameters	Criteria	Values
Storage site definition	DEPTH of storage formation	>1,300m
	SEAL thickness	>50m
	BRINE salinity	>10,000 ppm TSD
Storage site Lithology	% CLAY (mass)	<30%
Petrophysical properties	Absolute Porosity (intergranular)	<15%
	Seal permeability	<0.02 mD
Risks (complex area) (indentified or expected)	Open Faults	NO
	Active faults	NO
	Orphan wells	NO

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Table 2: Prioritisation criteria for deep saline aquifers (DSA) selection. Optimum values are indicated in the right column.

Parameters	Criteria	Values
Storage site definition	Structure closure	Closed
	Thickness	10-100m
	Brine salinity	>100,000 ppm TDS
Lithology	Homogeneity and lateral continuity	Homogeneous
Petrophysical properties	Permeability	10-1,000 mD
	Absolute Porosity	10-15%
Geomechanical properties	Fractures networks	Null/low
Thermal properties	Heat Flow Density	>70mW/m2
	Rock thermal capacity	>800 J/(kg.K)
Complex Area Features	Natural seismicity	Low/null

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Table 3: Prioritisation criteria for salt caverns selection and optimal values

Parameters	Criteria	Values
Storage site definition	Type of formation	Dome
	Thickness	>100m
	Depth of storage formation	800-1,700m
Thermal properties	Geothermal gradient	>33 °C/km
Petrophysical properties	Halite content	>95%

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4. Geological screening criteria

Based on the criteria and scoring of mentioned works, a subset of criteria has been compiled under the conditions of a safe CO₂ storage, energy storage and geothermal resources potential, and considering the geological settings proposed originally in the CEEGS project (DSA, DHF, and salt cavities). These criteria only refer to geological and safety aspects, and do not address social acceptance, economic risks, environmental limitations, interaction with other resources, social impact or distance to CO₂ capture or renewable energy facilities. The decision to focus on geological and safety aspects relates to the low TRL of the CEEGS technology and the acknowledgement that further development of the technology is required before issues such as social acceptance and environmental concerns can be included as selection criteria.

Upon the previous results of the tasks explained in the sections above, and following their final recommendations, porous media (a single DSA and geothermal reservoir) and salt cavities cases have been considered as geological settings with the highest potential for CEEGS technology application. Scenarios with two DSAs and DHFs are also regarded as possible for CEEGS, but specific complexities exist for each such environment that imposes additional challenges to be addressed at later stages of the development of CEEGS.

In order to be able to compare different settings, the decision process should not be based only on geological parameters but also on the efficiency of the system, as the capacity for (1) store energy, (2) permanently sequester CO₂, and 3) producing heat. Therefore, some common criteria for assessing efficiency in terms of recovered heat over injected CO₂ volume should be foreseen in future refinements of the methodology.

Given that geothermal sites typically depend on a more limited number of parameters to be viable than CO₂ storage sites, the proposal includes mostly criteria frequently considered in selecting suitable CO₂ reservoirs [24, 33].

A literature review was conducted on the most relevant criteria for CO₂ conditioning, energy storage, and geothermal energy [34]. The compiled properties, along with the results of simulations carried out in the CEEGS project, were used to develop a list of the most important criteria for selecting suitable sites for CEEGS.

A list of 19 criteria, grouped into 10 classes and divided into two types (exclusionary and non-exclusionary), is proposed for the selection of sites for DSA. A value or range of values has been assigned to each parameter to determine its suitability or to assign it a score. Porous media and salt cavities are not directly comparable; therefore, a separate list of criteria has been developed for the latter [28].6.

5. Discussion

5.1 Subsurface performance of CEEGS

The simulation results show that the performance of the CEEGS system critically depends on depth conditions, reservoir architecture, and petrophysical properties. Deep aquifers present the best performance because the higher pressures and temperatures guarantee the stability of CO₂ in the supercritical state and allow higher temperatures to be recovered during the discharge phase. In contrast, shallow aquifers show low efficiency due to pressure drop, significant brine production and the possible appearance of gaseous CO₂, which alters flow and reduces energy recovery [26]. Likewise, heterogeneity and anisotropy decrease the saturation reached around the producing well, fragmenting the plume and reducing the thermal efficiency of the cycle. In general, the most favourable scenarios are those with great depth (i.e. depths >1,300 m), intermediate permeabilities and porosities, low heterogeneity and moderate geothermal gradients, which allow a stable evolution of pressure and temperature during the operation cycles [29─31] (Figure 2).

5.2 Implications for geological suitability and operational feasibility

The results obtained coincide with previous observations in geological CO₂ storage and in geothermal systems based on CO₂, which highlight the need for deep, homogeneous reservoirs with good lateral continuity to maintain supercritical conditions and ensure stable operation. In this context, DSA emerge as the most viable scenarios, offering adequate pressures and temperatures, as well as a more predictable response during repeated injection and extraction cycles. In contrast, DHF and carbonate geothermal systems present greater complexity associated with structural heterogeneity, presence of fracturing and petrophysical variations that can affect CO₂ saturation and the thermal efficiency of the cycle [30]. Salt cavities, on the other hand, present clear operational advantages by allowing precise pressure control and minimizing geochemical interaction, although they require a specific design to manage thermal variations and avoid deformations of the salt rock [1, 31].

At the operational level, the main limitations identified include the production of brine in shallow aquifers; the possible phase transition of CO₂ under low pressure conditions; the loss of injectivity in heterogeneous media and the sensitivity of the system to the salinity of the fluid. All these factors must be considered in the selection of sites and in the design of a future demonstrator.

5.3 Proposed screening framework for site selection

The criteria defined for site selection are based directly on the results obtained and on the physical principles that control the behaviour of CO₂ in the subsurface. The mandatory criteria reflect the minimum conditions necessary to ensure the stability of CO₂ in the supercritical state, structural containment and safe operation of the system, justifying the requirement of sufficient depth and the presence of a competent seal (Table 1). Likewise, the results show that excessively high porosities and permeabilities favour plume dispersion, while intermediate values allow more stable saturation and greater thermal efficiency; therefore, the ranges proposed as optimal derive directly from the behaviour observed in the simulated scenarios.

The prioritisation of characteristics such as homogeneity, adequate thickness and high salinity responds to their proven influence on plume stability and the reduction of brine co-production [29] (Table 2). On the other hand, saline cavities stand out for offering a practically ideal environment from a thermodynamic and operational point of view: they allow pressure control, minimizing geochemical interaction and maintaining a more stable cyclic behaviour than in porous media, which justifies a set of specific criteria for their evaluation (Table 3). This framework coherently integrates the physical conditions identified in the models together with the accumulated experience in geological storage and geothermal energy, providing a structured basis to discriminate between potential sites and guide the design of future CEEGS pilots.

5.4 Practical implementation of the screening framework

The practical application of the proposed criteria framework involves recognizing its limitations and uncertainties, which derive from both the natural geological variability and the degree of technological maturity of the CEEGS concept. Although the criteria allow a first discrimination between potential sites, some parameters, such as fine heterogeneity, structural connectivity, geomechanical response and geochemical evolution over multiple cycles, require additional research to reduce uncertainty in complex scenarios. The tool developed in Excel format facilitates comparative evaluation by integrating mandatory and prioritisation criteria, allowing geological formations to be filtered and assigning scores in a transparent and reproducible manner [28] (Figure 3). However, its use should be considered as an iterative process subject to refinement, especially as new field data, experimental results, and higher resolution simulations are incorporated. Since the technology is still at a low level of TRL maturity, this framework constitutes preliminary decision support to identify candidate sites and guide the design of future characterization campaigns and pilot tests, which will be essential to validate and fine-tune the established criteria.

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6. Conclusions

CO₂-based Electrothermal Energy and Geological Storage (CEEGS) system represents a concept that integrates energy storage via a transcritical CO₂ power cycle and geological CO₂ sequestration, and in some cases geothermal heat extraction, contributing to reduce greenhouse gas emissions [14, 1].

To help implement this technology, different geological conditions were analysed for several scenarios based on real sites. This included porous media in deep saline aquifers [26], depleted hydrocarbon fields [30] and salt caverns [31]. Several configurations and parameters were considered in terms of number of aquifers, depth, thickness, porosity, permeability, heterogeneity, anisotropy and degree of confinement. Semi-analytical and numerical models were used to analyse the response of the system [16, 18]. Results allowed comparison between scenarios and derive suitability criteria [28].

Numerical simulations were performed on selected scenarios, allowing to drive conclusions about the optimal conditions and characteristics for implementing CEEGS, along with sensitivity analysis for testing the influence of different parameters on the efficiency of the system. It can be concluded that the efficiency is lower in shallow aquifers and two aquifer configurations. Open deep reservoirs favour a less amount of brine produced, in contrast with closed deep reservoirs, which may produce more brine and lower mass flow rates. Depth of 800m is normally considered for scCO₂, but more than 1,300m is optimal for minimising water production. Reservoir temperature has an important role on the efficiency, as high temperature allows for higher wellhead temperatures during the discharge phase. Therefore, high geothermal gradient is good for heat gain, mainly in salt caverns, however in DSA it can favour dispersion of the plume. Dispersion can also occur with high values of porosity and permeability, so not very high values allow higher gas saturation, although these parameters are good for injectivity [29]. Salt caverns have fewer limitations; pressure is lithostatic and can be controlled by well operation. It only requires a suitable thickness and quality for a cavern to be created [31].

The technology under investigation is currently at Technology Readiness Level 4 (TRL 4) and is therefore far from implementation. As the level of readiness increases, it is natural that the constraints and criteria will be better understood and defined. Therefore, this initial list of criteria and classification methodology should necessarily be considered high-level, without aiming for a detailed analysis, given that it may change, as may the considered values, as new knowledge is acquired with the development of the CEEGS technology.

Other factors must be considered when selecting a site for CEEGS technology implementation in the selection and classification criteria. Competition for the use of the geological subsurface, proximity to the CO₂ emitter, or the point of demand are techno-economic factors that are beyond the scope of this screening, task and have been addressed in other tasks of the CEEGS project.

Future work should include extending the study period of the existing geological scenarios to include several years of seasonal energy storage and several decades, to obtain implications for salt precipitation and long-term trapping mechanisms. Furthermore, it is suggested that other potential CO₂ emitter sources be included, i.e., additional model variants with different CO₂ impurity levels in the injector. This would allow for the design of CO₂ purity requirements, both in the injector and the producer, with the greatest possible flexibility. Next steps for further research would be moving beyond TRL4. For this, a demonstrator should be built for validating the feasibility of this technology at scale.

Author Contributions: conceptualization, García-Crespo, J. Berrezueta, E .; methodology, Carneiro, J., Behnous, D., Farkas, M., Koukouzas, N. and Canteli, P.; investigation, Behnous, D., Gianni, E., Farkas, M. and García-Crespo, J.; writing—original draft preparation, García-Crespo, J. Berrezueta, E.; writing—review and editing, Chacartegui, R, Carro, A.Carneiro, J., Behnous, D., Farkas, M., Koukouzas, N., Tyrologou, P., Gianni, E., Canteli, P.; supervision, Chacartegui, R., Carneiro, J.; project administration, Chacartegui, R.; All authors have read and agreed to the published version of the manuscript.

Funding: Funded by the European Union HORIZON-CL5-2021-D3-03-02. Views and opinions expressed are however those of the authors only and do not necessarily reflect those of the European Union or CINEA. Neither the European Union nor the granting authority can be held responsible for them.

Conflicts of Interest: The authors declare no conflict of interest.

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This article has been published in European Geologist journal 60 – 5th IPGC Special Edition 1