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Article

Evaluation of the CO2 Storage Capacity in Sandstone Formations from the Southeast Mesohellenic trough (Greece)

by
Marina A. Christopoulou
1,
Petros Koutsovitis
1,*,
Nikolaos Kostoglou
2,
Chrysothemis Paraskevopoulou
3,
Alkiviadis Sideridis
1,
Petros Petrounias
1,4,
Aikaterini Rogkala
1,
Sebastian Stock
5 and
Nikolaos Koukouzas
4,*
1
Section of Earth Materials, Department of Geology, University of Patras, 265 04 Patras, Greece
2
Department of Materials Science, Montanuniversität Leoben, 8700 Leoben, Austria
3
School of Earth and Environment, University of Leeds, Leeds LS2 9JT, UK
4
Centre for Research & Technology Hellas (CERTH), Chemical Process & Energy Resources Institute, 151 25 Athens, Greece
5
Institute of Physics, Montanuniversität Leoben, 8700 Leoben, Austria
*
Authors to whom correspondence should be addressed.
Energies 2022, 15(10), 3491; https://doi.org/10.3390/en15103491
Submission received: 6 April 2022 / Revised: 4 May 2022 / Accepted: 6 May 2022 / Published: 10 May 2022
(This article belongs to the Section B3: Carbon Emission and Utilization)
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Versions Notes

Abstract

:
This study investigates the capability of the Southeast Mesohellenic Trough (SE MHT) sandstone formations to serve as a potential reservoir for CO2 storage in response to the emerging climate change issues by promoting environmentally friendly mineral sequestration applications. Sandstone samples, for the first time, were evaluated for their petrographic characteristics, mineral chemistry, geochemical properties, as well as their petrophysical and gas adsorption properties through tests. The sandstones were tested and classified into distinct groups. The most promising site to be considered for pilot CO2 storage testing is the Pentalofos Formation locality since its sandstones display specific mineral phases with the proper modal composition to conceivably react with injected CO2, leading to the development of newly formed and stable secondary mineral phases. The gas adsorption results are also more encouraging for sandstones from this sedimentary formation. All the measured UCS (uniaxial compressive strength), Ei (bending stiffness), and ν (Poisson’s ratio) results are above those dictated by international standards to perform CO2 storage practices safely. Furthermore, the specified targeted locality from the Pentalofos Formation holds the geological advantage of being overlaid by an impermeable cap-rock formation, making it suitable for deploying CO2 mineralization practices. The demarcated area could permanently store a calculated amount of ~50 × 105 tons of CO2 within the geological reservoir by reacting with the specified mineral phases, as specified through the proposed petrographic PrP index (potential reactive phases).

1. Introduction

Climate change has become one of the most significant challenges that humanity is facing, and it poses a threat to a sustainable future. The rapid increase in carbon dioxide emissions in Earth’s atmosphere due to excessive burning of fossil fuels and intense industrial activity worldwide has contributed to the observed temperature increase resulting from the greenhouse effect [1,2,3,4]. The United Nations Framework Convention on Climate Change (UNFCCC) and the Paris Climate Conference (COP21) call for immediate actions to address global climate change, aiming to vastly reduce CO2 emissions. Despite the ambition to achieve net-zero CO2 emissions, CO2 will continue to be emitted in the atmosphere in rather vast amounts. Therefore, carbon capture and storage (CCS) practices constitute a vital and sustainable solution to safely capture and permanently store CO2 and other greenhouse gas emissions (GHG), for which permeable geologic formations are already being considered environmentally safe and economically viable solutions [5,6]. Moreover, throughout lab and field experiments, the potential of CO2 use for geological carbon storage (GCS) and enhanced oil/gas recovery in shales, as well as the partial replacement of CH4 with CO2, has been confirmed [7,8,9]. In recent years, research has been conducted focusing on determining the most appropriate geological formations as natural reservoirs for CO2, either for utilization or long-term storage via sequestration, with the formation of new stable mineral phases. Mineral trapping of CO2 is considered as the safest and most stable geologic CO2 long-term storage mechanism [10,11,12].
Geological formations that comprise sandstones have been suggested as suitable reservoirs for capturing and storing CO2, mainly due to their widespread distribution, high permeability, and preferable mineralogical and geochemical characteristics (i.e., better long-term pH buffer capacity, the reaction of feldspars with CO2 [13,14,15], and presence of cap-rocks to retain the buoyant CO2 within the reservoir rock [16]. Experimental studies that have been conducted upon sandstones, both on a laboratory scale [15,16,17,18,19,20] and in situ under simulated conditions [21], have shown results that vary from fair to promising, highly depending on the nature of the sandstone examined (e.g., mineral composition and participation percentage, presence of micropores).
The Mesohellenic Trough (MHT) is located in central Greece, constituting a large molassic sedimentary basin of considerable thickness (~5 km). Its main advantages for deploying CCS practices are related to the geological parameters of the hosted formations, i.e., thick sandstone formations often overlaid by relatively impermeable cap-rock formations (mainly pelites). These formations are also considered tectonically stable [16], corresponding to a substantial factor favoring the storage of CO2. Most studies in the area have focused their efforts on determining the CO2 amounts that can be potentially stored through numerical computational modeling [22] but without undertaking petrophysical parameters for permanent storage purposes. Additionally, Koukouzas et al. [16] obtained results that were conducted upon sandstone samples from the northeastern part of the MHT with the use of laboratory techniques. To the best of our knowledge, sandstone formations from the southern part of the MHT have not been examined at a laboratory scale to date; thus, it remains unresolved as to whether this specific locality holds a similar potential.
In this scope, the aim of the present study is to provide a mineralogical, petrological, and mechanical characterization of the sandstones cropping out along the southeastern MHT region and to determine the suitability of the geological formations for deploying CO2 storage practices. Other highlights of this research are to provide storage capacity estimations, mineral sequestration reactions, as well as insights regarding the ability of these formations to act as safe reservoirs.

2. Study Area

The NW–SE-trending Mesohellenic Trough is the largest sedimentary basin (200 km length, 30–40 km width) developed during the last orogenic stage (molasse-type basin) of the Hellenides. The basin was formed between the Mid-Upper Eocene and the Mid-Upper Miocene, at back-arc settings, superimposed on the Olonos–Pindos external and Pelagonian internal geotectonic units [23,24,25,26]. This sedimentary basin extends from Albania to the Thessaly region in Greece [26,27] and is located between the Apulian (non-metamorphic) and the Pelagonian (partly metamorphic) microcontinental plates [28].
The Mesohellenic Trough displays a significant lateral extent, and the thickness of the sedimentary formations reach about 4 km in vertical sections. It is also characterized by complicated sedimentary structures and phases [22]. Τhe thicknesses of the deposits, both longitudinally and transversally to the axis of the basin, vary significantly [26]. The sedimentary phases include deltaic conglomerates, alluvial scree, sandstones, and clays belonging to turbiditic sequences and deltaic, floodplain, and sandy shelf sediments.
This study is focused on the southeastern part of the Mesohellenic Trough, which was thoroughly studied and mapped using petrographical criteria, and those results, along with the sampling spots, are presented in the renewed map using the ArcGIS software (Figure 1). The main localities include the Meteora, Mourgani, Oxinia, and Asproklisia. As illustrated in the map, the main lithostratigraphic formations that host the sandstones are, namely, Tsotyli, Pentalofos, and Eptachori. In particular, the stratigraphically lower parts of the Eptachori Formation comprise the clastic Upper Eocene to Lower Oligocene sediments (conglomerates, sandstones). The upper part of the Eptachori Formation is located at the Taliaros Mountain (Grevena–Kastoria; West Macedonia), appearing in the form of local sedimentary phases that mainly include sandstones, marls, and limestones. The sequence’s thickness varies from 1 to 1.50 km. The Pentalofos Formation (Upper Oligocene to Lower Miocene) includes two types of clastic sedimentary rocks separated by marl–sandstone intercalations. Their thickness ranges from 2.3 to 4 km. The Tsotyli Formation (Lower to Middle Miocene) consists of marls accompanied by conglomerates, sandstones, and limestones of variable thickness (0.2 to 1.0 km) [29].

3. Materials and Methods

3.1. Materials

The study of the SE MHT was conducted upon a range of lithotypes within different geological formations in order for their CO2 sequestration capacity to be optimally assessed. Thus, after high scrutiny, twelve samples from different types of sandstones (coarse-grained, medium-grained, and fine-grained) were selected from four localities (see Figure 1) as representative of the studied area.

3.2. Methods

The mineralogical and textural characteristics of the studied samples were identified petrographically by examining polished thin sections using of a polarizing petrographic microscope (Leitz Ortholux II POL-BK Ltd., Midland, ON, Canada) following the EN-932-3 [30] standard for the petrographic description of aggregates, emphasizing their mean grain size and grain shape. The bulk mineral composition of the studied samples was also determined through X-ray Diffraction (XRD) facility, using a Bruker D8 advance diffractometer, with Ni-filtered CuKα radiation. Random powder mounts were prepared by gently pressing the powder into the cavity holder. The scanning angle for bulk mineralogy of specimens covered the 2θ interval 2–70°, with an angular step size of 0.015° and a time step of 0.1 s. The mineral phases were determined using the DIFFRACplus EVA 12® software (Bruker-AXS GmbH, Karlsruhe, Germany) based on the ICDD Powder Diffraction File of PDF-2 2006. The semi-quantitative analyses were performed by TOPAS 3.0® software (TOPAS MC Inc., Oakland, CA, USA) based on the Rietveld method refinement routine. The routine is based on the calculation of a single mineral-phase pattern according to the crystalline structure of the respective mineral, and the refinement of the pattern using a non-linear least-squares routine. Quantitative errors for each phase were calculated according to the procedures set out in Ref. [31].
Mineral chemistry analyses were performed using a JEOL JSM-6300 SEM (Tokyo, Japan) equipped with energy-dispersive and wavelength dispersive spectrometers and INCA software at the Laboratory of Electron Microscopy and Microanalysis, University of Patras, Greece. Operating conditions were accelerating voltage 25 kV and beam current 3.3 nA, with a 4 μm beam diameter. The total counting time was 60 s and dead-time 40%. Synthetic oxides and natural minerals were utilized as standards for our analyses. Detection limits are ~0.1%, and accuracy better than 5% was obtained. Further examination of the textural features and acquisition of three-dimensional images occurred through an energy-dispersive X-ray spectrometry (EDX) facility at the Department of Chemical Engineering, University of Patras, Greece. Loss on ignition (LOI) in the sample was determined according to the ASTM D7348-13 standard [32]. Whole-rock major and trace element chemistry analyses were derived by using an X-ray Fluorescence (XRF) spectrometer and a sequential spectrometer located at the Laboratory of Electron Microscopy and Microanalysis (University of Patras, Greece). For each sample, 0.8 g of dried ground sample was mixed with 0.2 g of wax (acting as binder) and was pressed to a pellet under 15 tones. Pressed pellets were analyzed with a RIGAKU ZSX PRIMUS II spectrometer (Tokyo, Japan), which is equipped with Rh-anode.
Gas adsorption tests were conducted on all 7 samples in both untreated (whole-rock) and solvent-extracted condition, which were crushed to 80 mesh. The testing was performed at the Montanuniversität Leoben, Austria. Low-pressure N2 adsorption and desorption isotherms were collected at −196 °C by a Quantachrome Autosorb iQ3 manometric gas sorption analyzer (Boynton Beach, FL, USA). Ultra-pure (99.999%) He and N2 gases were used for void volume calculations and gas sorption analysis, respectively. Prior to the tests, samples of ~1 g were degassed under high-vacuum (10−6 mbar) at 250 °C for 24 h to remove any physisorbed surface species (e.g., moisture, trapped air, etc.). The specific surface areas were calculated by the multi-point Brunauer–Emmet–Teller (BET) method following the pertinent consistency criteria (ISO 9277:2010). The specific pore volumes and average pore sizes were estimated by applying the Barret–Joyner–Halenda (BJH) method in the N2 adsorption data (average pore size ranges 0–50 nm). It should be noted that the BJH method is, in principle, not suitable for mesopore analysis in heterogeneous and low-surface area materials, such as the ones presented in this study. However, the BJH-derived data can be used in a qualitative or semi-quantitative manner for a relative comparison between samples showing similar adsorption characteristics. In addition, a low-pressure (0–1 bar) CO2 adsorption/desorption isotherm was recorded at 0 °C for the sample with the highest surface area using the same gas sorption analyzer coupled with an external temperature controlling device and a circulating bath filled with a mixture of water and antifreezing liquid. The CO2 gas was also ultra-pure (99.999%).
Baseline unconfined compressive strength (UCS) tests were conducted on seven cylindrical samples of MHT sandstones at the laboratory of the University of Leeds (Rock Mechanics, Engineering Geology and Geotechnical (RMEGG) Laboratories). A modified 2000 kN Walter and Bai servo-controlled rock testing device was utilized. Load was applied in such a way as to maintain a constant radial displacement rate of 0.01 mm/min. Radial displacement was measured using single-strain gauge that was attached to a chain wrapped around the mid height of the specimens, and the axial strain was measured on the sample surface using two-strain gauges at opposite sides of the specimen.

4. Results

4.1. Petrographic Features and Mineral Chemistry

The petrographic study of the examined samples reveals clear and distinctive characteristics amongst the various lithotypes that are directly dependent on the area from which the samples were collected and, therefore, the stratigraphic unit to which they belong.
More specifically, representative sandstones were considered that are derived from the geological Pentalofos Formation that extends throughout the western part of the study area (Figure 1). These are mostly medium- to slightly-coarse-grained sandstones, being moderately sorted, whereas their grains are sub-angular to sub-rounded. The mineralogical composition includes quartz, which forms monocrystalline and polycrystalline grains (mainly undulose—grain contacts generally straight to suture), K-feldspars, plagioclase feldspars, and calcite (Figure 2a). The cement material is mainly siliceous (Figure 2a). The K-feldspars are classified as orthoclase or microcline, whereas the plagioclase feldspars are mostly oligoclase and andesine (Table 1). Clay minerals are also present, represented mostly by halloysite, resulting from the alteration of feldspars (Figure 2b). An increase in the participation of serpentinite fragments within the sandstone matrix constitutes the major characteristic of this group. The main serpentine polymorph is antigorite, with minor amounts of lizardite. The antigorite is characterized by its relatively low Fe and Al amounts (FeO = 3.9–4.9 wt.%, Al2O3 = 0.5–2.08 wt.%). Petrographically, they appear in the form of flakes, and, also, XRD patterns confirm their presence (see Section 4.2). Chlorite is present either as diabantite or as ripidolite. Geothermometry calculations based on the thermometer of [33] reveal that these sandstones underwent diagenesis at low-temperature conditions ranging between 89 and 115 °C. The mica phases are represented by muscovite coupled with minor amounts of biotite. The epidote-group minerals comprise clinozoisite and allanite, with highly comparable mineral chemistry with those present in these sandstones. Clinozoisite is characterized by relatively high Al and Ca contents (i.e., Al2O3 = 23.2–26.5 wt.% and CaO = 19.7–22.8 wt.%). The allanite grains possess a concentrate CaO ranging from 13.5 to 13.8 wt.%, including considerable amounts of Rare Earth Elements (REE, i.e., Ce2O3 = 7.8–8.5 wt.% and La2O3 = 3.2–3.8 wt.%).
The sandstones from the Northern Eptachori Formation (Figure 1) tend to be coarser-grained compared to the sandstones from the Pentalofos Formation, comprising mostly sub-angular to angular grains (Figure 2b). They are generally moderate- to poor-sorted. The mineralogical composition mainly includes quartz, K-feldspars (orthoclase or microcline), plagioclase, calcite, and, in minor amounts, muscovite, chlorite, and serpentine, as well as lithic fragments (Table 2). Most plagioclase grains are classified as albites, although a few bear slightly higher Ca contents (Table 2). Mica minerals are classified as either biotite or muscovite, with the former being characterized by relatively low Fe-Mg contents (Table 2). According to the classification [34], chlorite is mainly classified as pseudothuringite. Chlorites geothermometry calculations provide important information regarding the conditions of deposition and the diagenesis. In particular, chlorites seem to have been crystallized at relatively elevated temperature conditions that range between 122 and 173 °C (under the calibration of Lanari et al. [33]). Most of the samples investigated comprise serpentine, which is predominantly lizardite, although minor antigorite has been identified. This has been confirmed through petrographic observations and XRD patterns (see Section 4.2). These lizardites are characterized by their relatively elevated Fe contents (i.e., FeO = 7.1–9.8 wt.%). The epidote-group minerals are represented by clinozoisite and allanite. Clinozoisite is characterized by relatively high Al and Ca contents (i.e., Al2O3 = 24.1–24.6 wt.% and CaO = 22.1–23.8 wt.%). The allanite grains possess a concentrated CaO ranging from 12.8 to 15.5 wt.%, and they include considerable amounts of REE (i.e., Ce2O3 = 8.8–11.3 wt.%). Monazite grains were also encountered, and they are characterized by high REE contents and other incompatible elements (REE + Y + Nd = 27.1–58.3 wt.%). The Northern Eptachori samples present clay cement. Quartz is mostly displayed as undulose monocrystalline and less as polycrystalline grains. The monocrystalline quartz grains (ranging from 0.3 to 0.5 mm) vary from sub-angular to angular, while the polycrystalline counterparts (0.5 to 1 mm) range from sub-angular to sub-rounded. The grain boundaries in most cases are present as sutures. The K-feldspars’ grains vary in size (0.4 to 0.7 mm), with angular to subangular shapes, whereas plagioclase is observed in smaller grains (<0.4 mm). In general, the fragments are sub-rounded and sub-angular to angular, and they are mainly comprised of clasts of quartz, feldspars, as well as by rock-fragments of granitic composition, maintaining their granular features.
Significant differences were noticed in the sandstones that were sampled from the Southern Eptachori Formation of the study area (Figure 1), comprising relatively fine-grained and well-sorted quartz framework grains that are mainly sub-angular to sub-rounded (Figure 2e), whereas their main characteristic is the development of a fine-grained cement, indicative of this region. The modal composition mostly comprises quartz, K-feldspars, calcite, and mica. Mineral chemistry analyses of the plagioclase grains indicate that they are mainly classified as oligoclase (Table 3). Feldspars are presented in lesser amounts, with the form of orthoclase or microcline. In these sandstones, clay minerals alteration was also noticed (Figure 2f). Muscovite (and, less frequently, biotite) is characterized by relatively low micro-roughness. Chlorite is classified as either as diabantite or ripidolite. The chlorite geothermometry calculations [33] show low temperature diagenetic conditions (i.e., 81–97 °C). Εpidote-group minerals comprise clinozoisite Al and Ca contents (i.e., Al2O3 = 22.0–22.5 wt.% and CaO = 23.1–23.5 wt.%). Serpentine grains comprise antigorite and lesser lizardite. The antigorite is characterized by its relatively low Fe and Al amounts (i.e., FeO = 4.8–4.9 wt.% and Al2O3 = 1.9–2.08 wt.%). Quartz is mostly displayed as undulose. The monocrystalline quartz grains vary from sub-angular to sub-rounded. The grain contacts presented generally as corroded by matrix (Figure 2e). The K-feldspars’ grains vary in size (70 to 200 μm), with angular to subangular shapes, whereas plagioclase is observed in smaller grains (<150 μm). The cementing material is mainly calcareous (Figure 2e).

4.2. XRD Analyses

The mineralogical-related results reported in Section 4.1 are confirmed by the study of the samples through XRD analysis. In particular, the XRD patterns of random powder mounts from the studied sandstone rocks revealed the mineral assemblages of sandstones derived from the South MHT. Representative XRD patterns from each group of the samples are shown below (Figure 3). In short, the XRD pattern of the OX-3G sample (Pentalofos Formation) displays the mineralogical assemblage quartz (50 vol.%), microcline (20 vol.%), calcite (20 vol.%), and albite (10 vol.%), whereas the sample EPN-2B (Northern Eptachori Formation) consists of quartz (40 vol.%), albite (25 vol.%), calcite (20 vol.%), and muscovite (15 vol.%). On the contrary, the KAL-3B (Southern Eptachori Formation) is composed of quartz (30 vol.%), calcite (30 vol.%), albite (25 vol.%), and microcline (15 vol.%).

4.3. Clay Fraction

Aiming at the complete mineralogical characterization, the clay fractions of the studied rocks were analyzed since the clay mineral species seem to affect both the CO2 sequestration capacity and the mechanical characteristics of the studied rocks. Differences were noticed in regard to the clay mineral species participation. The Pentalofos Formation comprises low concentrations of expanded clay minerals (main component is a mixed layer of illite–smectite, whereas illite dominates) (Figure 4a). An important indication is the appearance of serpentine and chlorite, which predominate. On the contrary, the other two formations are enriched in expanded clay minerals; the Northern Eptachori Formation consists of greater amounts of smectite than illite and chlorite, whereas smectite is mainly noted within the Southern Eptachori Formation. The smectite, under the right conditions, can be greatly expanded, affecting the mechanical properties.

4.4. Geochemical Classification

According to the classification diagrams [35,36], the sandstones are distinguished into graywackes and litharenites (Figure 5). Both diagrams denote the same classifications without significant differences. In particular, the sandstones from the Pentalofos Formation are composed of litharenites, which also applies to the sandstones from the Northern Eptachori Formation despite the fact that the latter tend to be coarser grained. On the other hand, the fine-grained sandstones from the Southern Eptachori Formation consist of graywackes and litharenites. The relatively high Ca contents observed in the Southern Eptachori sandstones are attributed to the relatively high abundance of calcite compared to the other two formations, whereas the Ca contents in the other formations with sandstones mostly reflect the presence of plagioclase, epidote, and allanite. The Mg contents tend to be higher in the Pentalofos Formation sandstones and the samples from the Northern Eptachori Formation (i.e., MgO up to 5.3 and 8.8 wt.%, respectively) compared to those measured in the Southern Eptachori Formation (i.e., MgO up to 3.6 wt.%) (Table 4).

4.5. Gas Adsorption

The notable aspects are the results of the gas adsorption that the studied rocks displayed. Their adsorption capacity correlates with the lithotype distinction and the overall petrographic characteristics. N2 adsorption/desorption isotherms recorded at −196 °C for the degassed sandstone samples are presented in Figure 6. All the sandstones from all the studied formations showed a comparable N2 adsorption/desorption isotherm shape, combining features of type II (macroporous or non-porous) and type IV (mesoporous) isotherms based on the IUPAC classification [37], thus suggesting a similar pore structure. The hysteresis loop formed between the adsorption and desorption curves at P/P0 > 0.45 is usually attributed to capillary condensation in mesopores (i.e., pore widths of 2–50 nm). The hysteresis loop is classified as type H3 according to the IUPAC [37] and is demonstrated by non-rigid aggregates of plate-like particles but also if the pore network consists of macropores that are not completely filled with condensate. The lack of a saturation point at P/P0~1 describes condensation in macropores (i.e., pore widths > 50 nm) and/or adsorption onto external surfaces. It should be noted that the OX-3G sample from the Pentalofos Formation exhibited the highest adsorbed N2 volume both at lower (P/P0 < 0.1) and higher (P/P0~1) relative pressures. This was also confirmed by using the multi-point BET and BJH methods on the N2 adsorption data to calculate the BET areas and BJH pore volumes, respectively, as displayed in Table 5. The average mesopore diameter values also ranged from 6.6 to 14.9 nm for both fragments and powders (Table 5, Figure S1).
In general, the samples belonging to the Pentalofos Formation (i.e., OX-3G, OX-4B, and OX-6C) demonstrate higher surface areas and pore volumes compared to the rest of the formations (Table 5). Pore structure is an important factor that should be considered in studies concerning rock formations as possible CO2 reservoirs.
After completion of the previous test, the sample with the best performance in the N2 adsorption-desorption studies (i.e., OX-3G; Pentalofos Formation) was selected for CO2 sorption analysis at 0 °C; the results are presented in Figure 7.

4.6. Mechanical Tests

Aiming to locate the best rock formation in Greece to act as a successful and safe CO2 reservoir, we also considered the study of the mechanical properties of the reported sandstones. The test results are presented in Table 6 and Figure 8, Figure 9, Figure 10 and Figure 11. The physical–mechanical characteristics of the examined samples, as observed in Table 6, show significant variations in their values and are directly dependent on their particular mineral–petrographic characteristics even in the same study areas. For example, the sample OX-3G has a much higher mechanical strength than the sample OX-6C, although they belong to the same formation and have received similar alteration effects; however, the present difference is due to the different percentage of quartz, with the samples showing increased rates of quartz and increased mechanical properties. The correlation between the strength of the rock mass and the percentage of quartz has been reported by Sabatakakis et al. [38]. The mechanical properties of the Pentalofos Formation demonstrate the best results; however, even the rest of the sandstone samples display acceptable mechanical characteristics. In particular, uniaxial static load tests were performed to determine and investigate the behavior of the brittle sandstone rocks under constant (controlled) stress-state conditions. Stress–strain curves for brittle rocks can be used to determine: (1) UCS; (2) Ei; and (3) ν [39]. The latter is defined as the ratio of the change in lateral width per unit width to change in axial length per unit length caused by the axial stretching or stressing of a material. As illustrated in Figure 8 and Figure 9, it is evident that the samples developed a significant brittle cracking; however, they demonstrated moderate to significant resistance, as shown in the binary strain versus axial stress plots (Figure 10 and Figure 11). In these graphs, three variables are plotted that include the axial stress, the radial strain, and the volumetric strain. The projected plots define curves that yield UCS, Ei, and ν results for each sample, as shown in Table 6. Generally, the UCS results of the sandstones vary from 38 to 272 MPa. More specifically, the sandstone from the Pentalofos Formation displays strength values ranging from 64 to 271 MPa. The Pentalofos Formation displays the highest reported values, meaning that these sandstones are described by high strength, being able to support high uniaxial forces. Concerning the sandstone specimens from the Northern Eptachori Formation, these display a variety of strength values (88 to 108 MPa), whereas the sandstones specimens from the Southern Eptachori Formation showed high compressive strength values (~38 MPa). The Ei values seem to be generally aligned and consistent with the UCS values. On the other hand, the ν values seem to behave more independently from the above-mentioned parameters due to the fact that, in some cases, the axial stress and radial strain compression may not significantly affect the elasticity, which depends highly upon the mineral constituents and their mode.

5. Discussion

5.1. Suitability of Sandstones for CO2 Mineralization

A possible means of reducing carbon dioxide (CO2) emissions to the atmosphere is the injection of CO2 into structural reservoirs in deep permeable geologic formations [5]. Numerical modeling of geochemical processes is necessary to investigate long-term CO2 injection in deep geologic formations. The present study was based upon the combination of two important pillars: (a) the detailed mineralogical and petrographical study, and (b) the mechanical properties of the studied rocks in order to test the capability of the MHT-hosted sandstones in Greece to act as potential CO2 reservoirs.
Through the detailed mineral and petrographical study, the MHT can be divided into three groups based on the distinctions that correspond to local geological formations. In the context of the present study, a normalized approach regarding the modal composition of the studied samples is provided in Table 7, which resulted by deploying the TOPAS software coupled with the petrographic study results.
As shown in Table 7, for the quantification of the mineralogical components, the samples are clearly distinguished into three different groups, corresponding to the three distinct regional geological formations. More specifically, Group 1 consists of the Pentalofos Formation sandstone samples, Group 2 contains the samples of the Northern Eptachori Formation, whereas Group 3 comprises the sandstone samples of the Southern Eptachori Formation. Group 1 demonstrates the least amount of matrix material and quartz content. Moreover, the sandstones of this group are characterized by the presence of noticeable amounts of serpentinite fragments, which are also present in the Group 2 sandstones but in smaller amounts. On the other hand, compared to Group 1, Groups 2 and 3 tend to display higher amounts of quartz, calcite, and matrix material, as well as lower amounts of serpentine, feldspars, and mica. These characteristics are also identified through the geochemical and mineral chemistry analyses listed in Section 4.1, Section 4.2, Section 4.3 and Section 4.4. It is worth noting that Group 3 consists of smaller amounts of serpentine and chlorite compared to the other two sandstone groups, whereas the total alkali contents of Groups 1 and 2 remain rather stable without significant differences; this denotes the highly comparable amounts of feldspars present within the sandstone samples of our study.
Based upon the experimental results of the capacity of the studied rocks to adsorb N2 gas, as shown in Figure 6, in combination with the pore structure properties (Table 5), it is evident that the initial division into three distinct groups corresponding to the three regional geological formation units is justified. All the studied formations showed a comparable N2 adsorption/desorption isotherm shape (see Figure 6), combining features of type II (macroporous or non-porous) and type IV (mesoporous) isotherms based on the IUPAC classification [37], thus suggesting a similar pore structure.
The three groups, apart from being characterized by distinctive mineral assemblages, are also corelated with their specific surface areas and specific pore volumes. The combination of these two factors seems to have an important effect upon the ability of rocks to infuse N2 aided by adsorption-related processes upon the surfaces of the participating mineral phases, combined with the different types and degrees of porosity. Group 1 generally presents the greatest capacity for N2 adsorption, followed successively by Group 2 and Group 3 in order of capacity. It should be noted that the OX-3G sample (Group 1—Pentalofos Formation) exhibited the highest adsorbed N2 volume both at lower (P/P0 < 0.1) and higher (P/P0~1) relative pressures, which is attributed to its highest surface area and pore volume amongst all the samples, pointing to a formation capable of reserving increased amounts of CO2 since N2 exhibits similar kinetic behavior within the rock-building units. This was also confirmed by using the multi-point BET method on the N2 adsorption data. The calculated BET areas for the degassed sandstones (see Table 5) range between 2 and 12 m2/g and 2 and 15 m2/g for the as-received fragments and the processed powders, respectively, with the OX-3G samples having more than double the value compared to the other samples. A similar trend was observed for the calculated BJH pore volumes, with the highest value (~0.02 cm3/g) shown for the OX-3G powder sample. The BET area values presented in this study are comparable to the ones reported for organic-matter-rich shales [41]. The OX-3G sandstone powder sample from the Pentalofos Formation was also investigated using CO2 adsorption. As shown in Figure 7, the OX-3G sample shows a particular uptake curve with specifically encouraging results regarding the ability of this sandstone to physically retain CO2 on its porous structure (i.e., a CO2 uptake of ~0.03 mmol/g was recorded at 0 °C and ~1 bar). The OX-3G outperforms the rest of the sandstone samples in terms of N2 and CO2 uptake. However, its full potential for practical applications can only be deduced by high-pressure (up to 50 bar) CO2 studies. This fact indicates that the sandstone rocks of the Pentalofos unit (Group 1) are highly associated with their mineralogical composition and their pore structure properties to successfully achieve CO2 sequestration (Figure S1). More specifically, there are particular mineral phases that are considered critical in relation to the capacity of a rock to sequestrate CO2. These minerals that seem to have a positive effect on sequestration are serpentine, epidote, K-feldspar, plagioclase, and mica-group minerals. On the contrary, quartz, the matrix (given the fact that it is not composed of clay minerals), dolomite, and calcite are reported as inhibitory components. Based on the above and in an effort to combine the presented results with those reported in other research, we propose for the first time a new petrographical index that could represent the capacity of a rock to sequestrate CO2:
PrP = (Serp + Ep + Kfs + Pl + Mica)/(Qtz + Cement + Cc + Dol),
The petrographical findings depending on the results of the PrP index (potential reactive phases) are presented in Table 7, and these demonstrate a clear correlation per studied sandstone group and, therefore, per lithological formation. More specifically, the numerical results of Table 7 and the gas sorption data (Figure 6 and Figure 7) reveal a direct correlation between them, with the apparent trend of the index tending to the unit of “1” as the rock becomes more suitable to retain CO2 within its structure. The proposed petrographic index of potential binding suitability exclusively for sandstone rocks (PrP) and its correlation depicted in Figure 6 and Figure 7 indicates that mineral phases susceptible to active reaction with injected CO2 dissolved with water were identified. These minerals include K-feldspars, plagioclase, epidote, chlorite, and serpentine. Some of the sandstones considered in this study contain rather high amounts of the aforementioned minerals, which are occasionally found in other lithological rock types (e.g., basalts and ophiolitic serpentinites) that are considered as suitable for CO2 mineralization purposes [42,43,44,45,46,47]. The most common perception is that, in order to achieve CO2 storage via mineralization, there must be sufficient quantities of Ca-rich plagioclase grains to be partly consumed via alteration reactions so as to successfully lead to the formation of calcite and thus achieve CO2 mineralization [48]. In recent years, research studies are also considering other mineral phases for reaction with CO2, forming other mineral phases, such as dolomite, talc, and clay minerals [14,49,50].
Based upon the between-groups distinctions of the sandstones considered in the present study, as well as regarding the mineral chemistry and the determined modal composition, the Group I sandstones incorporate sufficient amounts of K-feldspars that possess the potential of reacting with supercritical CO2 through the following reactions [14]:
2KAlSi3O8 (K-feldspar) + CO2 + 2H2O ⇒ Al2(Si2O5) (OH)4 (kaolinite) + 4SiO2 + K2CO3
3KAlSi3O8 (K-feldspar) + CO2 + H2O ⇒ KAl3Si3O10(OH)2 (illite) + 6SiO2 + K2CO3
The results from our experimental data (see Section 4.1) suggest that these clay minerals that are standard from Group 1 and the Pentalofos Formation have already been formed in relatively restricted amounts as a natural process of feldspar dissolution; thus, the presence of CO2 is expected to further facilitate clay mineral mineralization. The Group 3 sandstones that were subjected to more extensive alteration and weathering processes encompass slightly higher amounts of clay minerals, as shown through the clay fraction results. For this reason, despite the fact that plagioclase grains are present in all the sandstone samples considered in our study, they are present in higher amounts in the Group 1 and 2 sandstones. In the Group 1 sandstones, the mineral chemistry of most plagioclase grains is classified as compositionally intermediate, which can serve for the development of both clay minerals as well as calcite. According to Hangx and Spiers [48], it is expected that plagioclase of intermediate or basic composition will form both types of secondary minerals, although it is expected that the clay precipitation will precede that of carbonate. On the other hand, some of the plagioclase minerals in the Group 2 sandstones are albitic, and, therefore, their alteration is expected to lead to the formation of kaolinite and dawsonite, depending on the rock/water/CO2 proportions, through the following reactions [14,49]:
2NaAlSi3O8 (albite) + 2CO2 + 3H2O ⇒ Al2(Si2O5)(OH)4 (kaolinite) + 4SiO2 + 2Na+ + 2HCO−3
NaAlSi3O8 (albite) + CO2 + H2O ⇒ NaAlCO3(OH)2 (dawsonite) + 3SiO2 (chalcedony)
The majority of the sandstones considered include chlorite ranging between ~3 and 5 vol.%. Even though these are not regarded as high amounts, they possess the ability to interact with CO2 and store it as siderite and dolomite [49]. Mineral chemistry analyses of the studied sandstones revealed that most of the chlorite is classified as Mg-rich; hence, it is expected to form through the reaction:
Fe2.5Mg2.5Al2Si3O10(OH)8 (chlorite) + 2.5CaCO3 (calcite) +5CO2 ⇒ 2.5FeCO3 (siderite) + 2.5MgCa(CO3)2 (dolomite) + Al2Si2O5(OH)4 (kaolinite) + SiO2 (quartz) + 2H2O
In rocks such as basalts and serpentinites, significant mineralization of water-dissolved CO2 can be achieved through chemical reactions with the Mg- or Ca-bearing participating mineral phases [44,50,51,52]. In particular, greenschist facies serpentinites comprise significant amounts of the lizardite serpentine polymorph, which can react with CO2 to form Mg-rich carbonate minerals [43]:
Mg3Si2O5(OH)4 (lizardite) + 3CO2 = 3MgCO3 (magnesite) + 2SiO2 + 2H2O
The PrP index shows that the Group 1 sandstone samples include serpentine in the particular form of lizardite, thus suggesting that CO2 mineralization can also be achieved through decomposition of this mineral phase. All the sandstone samples that comprise serpentine in their mineral assemblage have been partially replaced by dolomite through relatively restricted alteration processes. Mica minerals comprising both biotite and muscovite are present in amounts in the general range of ~5 to 9 vol.%. Research studies during the past decade have noted the importance of these minerals in sandstones for capturing CO2, providing further support for the usage of the proposed PrP petrographic index.
In addition, epidote seems to play an important role in the PrP index since epidote has also been considered as a mineral phase that can potentially react with CO2-charged water injections and since it is a Ca-bearing silicate mineral that can lead to crystallization of relatively high amounts of calcite [53]. The studied sandstones host both Ca-rich and REE-rich epidote-group minerals; these minerals are slightly more frequently included in the Group 1 and 2 sandstones, and partial replacement by calcite has been identified. It is expected that, by injecting CO2 into the host sandstone rock formations, it will initially result in the decrease in the pH values; the rate depends highly upon the available dissolved CO2, the alkalinity of the solution, injection period, and reaction conditions [29,54,55]. However, during carbonate precipitation, pH values tend to become neutral [56,57]. According to the findings of Ref. [48], epidote can be partly dissolved under these conditions, releasing the necessary Ca and leading to the crystallization of calcite, starting at temperatures as low as 25 °C, although higher temperatures are expected to further promote epidote dissolution.
Based on the above, it is evident that Group 1 from the Pentalofos Formation possesses the capacity to effectively react and withhold CO2 via mineralization-related processes, whereas the effect of the mineralogical composition upon the capacity of the sandstone rocks to react with CO2 is calculated for the first time through the proposed petrographic index, PrP, which ranges between 0.0 and 1.0 depending on the capacity to achieve CO2 mineralization per lithotype. Despite the fact that the mineralogical assemblages of the Group 1 sandstones display some modal variabilities, the aforementioned reactive mineral phases are, in all cases, present in relatively considerable amounts. Furthermore, the rather restricted presence of typical sandstone amounts of calcite, clay minerals, and quartz suggests that these rocks can be characterized as susceptible to react with CO2, contrary to the Group 3 sandstones from the Southern Eptachori Formation (Meteora site), hindering the ability for CO2 storage compared to the sandstone formations represented by the other two groups.

5.2. Recommended CO2 Storage Site in the Mesohellenic Trough

Geological storage involves injecting CO2 into rock formations that can absorb and contain it for thousands of years. Rocks well-suited to this are found in sedimentary basins, i.e., areas of subsidence in the Earth crust in which sediments have accumulated over geological periods. Typically, these basins extend for thousands of kilometers. An overarching challenge in CCUS is that the existing sources of CO2 are very often not located in the vicinity of storage sites. To address this issue, efforts have been undertaken to map the geographical distributions of emission sources and storage formations. Studies [29,58] have examined the MHT for its CO2 storage potential. Pressure and temperature data, as well as formation depths, were provided in these studies, suggesting an area with high CO2 storage potential. The pressure gradient appears to have a value of 57 MPa at 2 km with a temperature of 80 °C.
Taking into consideration the petrographic and mineralogical characteristics, as analyzed in the first part of the present discussion and in combination with the satisfactory mechanical characteristics of almost all the rocks of our study, this leads us to propose a well-demarcated area with clear boundaries as a potential reservoir to implement CO2 storage via mineralization. The mechanical properties of the Pentalofos Formation demonstrate the best results; however, even the rest of the samples demonstrate acceptable mechanical characteristics. The mechanical testing procedures were performed as uniaxial static load tests, and the bending stiffness and Poisson’s ratio were performed and calculated to determine and investigate the behavior of the brittle sandstone rocks under constant (controlled) stress-state conditions. All the examined sandstones, and especially the samples of Group 1 and Group 2, show increased mechanical properties as a result of their mineralogical characteristics and the reduced degree of alteration of the formations. A particularly positive feature seems to be the uniform distribution of the secondary minerals within the rock structure as support units appear to be created from the main detrital minerals that tend to function precisely as mechanical support bonds within the sandstone rock structure. A typical example is the continuous alternation of quartz with low-relief phyllosilicate minerals, as described in the petrographic study.
Based on these observations, the Pentalofos and a small part of the Northern Eptachori sandstone formations can be regarded as proper CO2 reservoirs for permanent storage purposes, especially at localities in which the aforementioned formations hold the geological advantage of being overlaid by the relatively impermeable cap-rock formation of Tsotyli (Figure 12). The example referred to in this paper provides an important potential for storing CO2 from emitted sources in mainland Greece. These point sources can correspond either to energy production power stations or industrial units. Although the sandy Pentalofos Formation can be considered as a relatively open reservoir, dipping to the NE, the fact that it is sealed by high-stand impermeable shales makes it a suitable formation that can act as a CO2 storage reservoir.
Based on the above, we can provide preliminary calculations that estimate the CO2 that could be stored in the frames of a potential pilot project in the studied region of MHT. For this aim, we implement this function:
CO2 Storage Capacity = Σ(V × ϕ × ρ × ε)
with V symbolizing the sandstone reservoir volume in m3 (under the flysch cap rock); ϕ denoting the effective porosity (%); ρ is the CO2 specific gravity property (in kg/m3); whereas ε stands for the sCO2 storage efficiency factor. In the context of the present study, the proposed potential area that meets the necessary conditions to function as a safe CO2 reservoir has been accurately measured through GIS mapping and was calculated such that it covers an area of 118 km2, whereas the depth of the geological reservoir, as delimited from the results of the present study, is defined at about 0.5 km, and, thus, the volume of the proposed reservoir is 59 km3. The cap rock formation that overlies the potential reservoir is about 0.8 km and, therefore, the CO2 sequestration can be achieved at depths reaching a measure of 1.3 km depth from the surface, allowing for CO2 injection under supercritical state conditions, which is the most efficient for efficient geological storage. According to van den Meer et al. [59], such storage depths account for a conservative sCO2 specific gravity property value of 400 kg/m3. In addition, based upon the estimations of the authors of Ref. [14] and with reference to the statistical values of USGS (United States Geological Survey) modeling, we can consider the CO2 storage efficiency factor for sandstones to be 1%. The application of this discount factor is necessary in order to obtain a realistic estimation of the sandstone reservoir storage potential. Taking the aforementioned values into account, as well as additional parameters that include the average sandstone effective porosity values from our studied site (6%), it is assessed that the demarcated area could potentially store an amount of 156 × 105 tons of CO2. The actual amount that can in fact react with the selective mineral phases and be stored through mineralization processes can only be roughly estimated because they may react, apart from feldspars, additionally with serpentine, epidote, mica, and chlorite. However, by applying the equation of Jin et al. [14], the amount of CO2 that is expected to react with the feldspars (K-feldspar and plagioclase) is calculated at 36 × 105 tons of CO2. Thus, considering the other mineral phases that have the potential to react with injected CO2, we estimate a total amount of ~50 × 105 tons of CO2 to be permanently stored within the geological reservoir.
Combining the set of results provided from the present study, it is evident that the study area presents sufficient porosity, permeability, and thickness, as well as sufficient mineralogy, an impermeable flysch cap rock formation, and mechanical strength capable of operating as a safe and large-volume CO2 reservoir. Porosity and permeability properties that ensure the rock will absorb the CO2 and significant thickness are critical to containing a substantial volume; these are features enable to perform high rates of CO2 injection without pressure build-up.

6. Conclusions

In this study, sandstones of various petrographic characteristics derived from the Southeast Mesohellenic Trough were examined for the first time to classify their suitability for potential pilot applications of CO2 storage in response to the emerging climate change issues by promoting environmentally friendly mineral sequestration applications. In this context, three sandstone groups have been identified, corresponding to the geological formations of Pentalofos (Group I), Northern Eptachori (Group II), and Southern Eptachori (Group III). These groups have been distinguished mainly based upon their petrographic features, mineral chemistry, whole-rock geochemistry, gas adsorption capacity, as well as mechanical testing. The research findings led to the following concluding remarks:
  • The petrographic characteristics of the Group I sandstones reveal that they have a better potential for CO2 storage due to the fact that they contain K-feldspars, plagioclase, epidote, serpentine, chlorite, and mica in relatively significant amounts; these are expected to react with CO2 and develop newly formed calcite, dolomite, and clay minerals. The latter mineral phases are commonly found in natural soils and, therefore, pose no harm to the environment.
  • The proposed petrographic index, PrP, reveals the sandstones that incorporate relatively small amounts of mineral phases and that are susceptible to actively react with CO2.
  • The gas adsorption results seem to be more encouraging for the Group I and II sandstones, coinciding with those that displayed optimal BET analyses results, pore volume values, and mineral modal compositions that are reactive to CO2 phases.
  • The results of the mechanical strength testing of UCS, Ei, and ν revealed that these values highly depend upon the participating mineral phases but also on other parameters, such as their porosity values, as well as their textural features and participation of matrix material. The mechanical results are sufficiently in agreement with international standards to safely perform CO2 storage practices.
  • The most promising site to be considered for pilot CO2 storage testing from the Southeast Mesohellenic Trough is that located in the Pentalofos Formation since it additionally holds the geological advantage of being overlaid by an impermeable cap-rock formation.
  • The demarcated area could permanently store a calculated amount of ~50 × 105 tons of CO2 within the geological reservoir by reacting with the specified mineral phases.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/en15103491/s1.

Author Contributions

Conceptualization, M.A.C. and P.K.; methodology, P.K., N.K. (Nikolaos Kostoglou), C.P., A.S., A.R. and P.P.; software, M.A.C., P.K., A.S., P.K. and A.R.; investigation, M.A.C., P.K., N.K. (Nikolaos Kostoglou), P.P., N.K. (Nikolaos Koukouzas), C.P., and A.S.; resources, M.A.C., P.K., A.R. and A.S.; data curation, M.A.C., P.K., A.R., N.K. (Nikolaos Kostoglou) and S.S.; writing—original draft preparation, M.A.C., P.K., P.P. and A.R., writing—review and editing, M.A.C., P.K., P.P., A.R., N.K. (Nikolaos Kostoglou), C.P., A.S. and S.S.; visualization, M.A.C., P.K., A.R., N.K. (Nikolaos Koukouzas) and P.P.; supervision, M.A.C. and P.K. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Acknowledgments

We kindly thank A. Govatsi of the Laboratory of Electron Microscopy and Microanalysis, University of Patras for his assistance with the microanalyses and SEM micrographs. Many thanks are given to Quentin Fisher for his support for the laboratory testing (University of Leeds). We also thank M. Kalpogiannaki for her assistance in the construction of the geological map. N.K. and S.S. would like to thank Christian Mitterer and Oskar Paris from the Montanuniversität Leoben for providing resources and access to experimental equipment. The Editor and Reviewers are also thanked for their constructive comments and observations.

Conflicts of Interest

The authors declare no conflict of interest.

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Energies 15 03491 g001 550
Figure 1. Geological map of the Southeast Mesohellenic Trough (northwest Greece) region (modified after fieldwork mapping).
Figure 1. Geological map of the Southeast Mesohellenic Trough (northwest Greece) region (modified after fieldwork mapping).
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Figure 2. Photomicrographs from optical microscopy showing the textural characteristics of the SE Mesohellenic Trough sandstones and backscattered images (BSI) from SEM: (a) photomicrograph of clastic texture presented in medium-grained sandstone with quartz (Qz), plagioclase (Plg), muscovite (Ms), and calcite (Cc); (b) BSI of smectite formation over K-feldspar; (c) photomicrograph of clastic texture presented in a coarse-grained sandstone with quartz (Qz), K-feldspars (K-Fs), and calcite (Cc); (d) BSI showing alteration of K-feldspar into clays; (e) photomicrograph of clastic texture represented in fine-grained sandstone with quartz (Qz), K-feldspars (K-Fs), muscovite (Ms), and calcite (Cc); (f) BSI showing alteration of K-feldspars and plagioclase into clay minerals.
Figure 2. Photomicrographs from optical microscopy showing the textural characteristics of the SE Mesohellenic Trough sandstones and backscattered images (BSI) from SEM: (a) photomicrograph of clastic texture presented in medium-grained sandstone with quartz (Qz), plagioclase (Plg), muscovite (Ms), and calcite (Cc); (b) BSI of smectite formation over K-feldspar; (c) photomicrograph of clastic texture presented in a coarse-grained sandstone with quartz (Qz), K-feldspars (K-Fs), and calcite (Cc); (d) BSI showing alteration of K-feldspar into clays; (e) photomicrograph of clastic texture represented in fine-grained sandstone with quartz (Qz), K-feldspars (K-Fs), muscovite (Ms), and calcite (Cc); (f) BSI showing alteration of K-feldspars and plagioclase into clay minerals.
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Figure 3. XRD analysis of samples EPN-2B (Northern Eptachori Formation; red-colored), OX-3G (Pentalofos Formation; green-colored), and KAL-3B (Southern Eptachori Formation; blue-colored).
Figure 3. XRD analysis of samples EPN-2B (Northern Eptachori Formation; red-colored), OX-3G (Pentalofos Formation; green-colored), and KAL-3B (Southern Eptachori Formation; blue-colored).
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Figure 4. X-ray diffractograms of the clay fraction of representative examined samples: (a) Pentalofos Formation sample, (b) Northern Eptachori Formation sample, (c) Southern Eptachori Formation sample. (Black pattern: air dried; blue pattern: glycolated; red pattern: heated).
Figure 4. X-ray diffractograms of the clay fraction of representative examined samples: (a) Pentalofos Formation sample, (b) Northern Eptachori Formation sample, (c) Southern Eptachori Formation sample. (Black pattern: air dried; blue pattern: glycolated; red pattern: heated).
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Figure 5. Binary classification plots of (A) log (SiO2/Al2O3) vs. log (Na2O/K2O) of Pettijohn et al. [35], and (B) log (SiO2/Al2O3) vs. log (Fe2O3/K2O) diagram of Herron [36].
Figure 5. Binary classification plots of (A) log (SiO2/Al2O3) vs. log (Na2O/K2O) of Pettijohn et al. [35], and (B) log (SiO2/Al2O3) vs. log (Fe2O3/K2O) diagram of Herron [36].
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Energies 15 03491 g006 550
Figure 6. N2 adsorption (solid symbols) and desorption (open symbols) isotherms recorded at −196 °C for the degassed sandstones as fragments (a) and fine powders (b).
Figure 6. N2 adsorption (solid symbols) and desorption (open symbols) isotherms recorded at −196 °C for the degassed sandstones as fragments (a) and fine powders (b).
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Energies 15 03491 g007 550
Figure 7. Gas sorption analysis with CO2 at 0 °C for powder OX-3G sandstone sample from Pentalofos Formation as representative sample. Showing a potential for CO2 storage within the relevant sandstone sample. Adsorption (solid symbols) and desorption (open symbols).
Figure 7. Gas sorption analysis with CO2 at 0 °C for powder OX-3G sandstone sample from Pentalofos Formation as representative sample. Showing a potential for CO2 storage within the relevant sandstone sample. Adsorption (solid symbols) and desorption (open symbols).
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Figure 8. Typical breaks in cylindrical specimens under uniaxial compressive strength before and after the testing of the corresponding samples; (a1,a2) and (b1,b2) are from the Northern Eptachori Formation sandstones (EPN-2B and EPN-5A, respectively), (c1,c2) are from Pentalofos Formation sandstones, and (d1,d2) are from the Southern Eptachori Formation sandstones (samples: OX-3G and KAL-2C, respectively). Note the strain gauge sensors that were emplaced for the measurements in vertical and horizontal orientations.
Figure 8. Typical breaks in cylindrical specimens under uniaxial compressive strength before and after the testing of the corresponding samples; (a1,a2) and (b1,b2) are from the Northern Eptachori Formation sandstones (EPN-2B and EPN-5A, respectively), (c1,c2) are from Pentalofos Formation sandstones, and (d1,d2) are from the Southern Eptachori Formation sandstones (samples: OX-3G and KAL-2C, respectively). Note the strain gauge sensors that were emplaced for the measurements in vertical and horizontal orientations.
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Figure 9. Typical breaks in cylindrical specimens under uniaxial compressive strength before and after the testing of the corresponding samples. (a1,a2) (sample OX-6C), (b1,b2) (sample OX-4B) are from Pentalofos Formation sandstones; (c1,c2) (sample KAL-3B) from Southern Eptachori Formation sandstones. Note the strain gauge sensors that were emplaced for the measurements in vertical and horizontal orientations.
Figure 9. Typical breaks in cylindrical specimens under uniaxial compressive strength before and after the testing of the corresponding samples. (a1,a2) (sample OX-6C), (b1,b2) (sample OX-4B) are from Pentalofos Formation sandstones; (c1,c2) (sample KAL-3B) from Southern Eptachori Formation sandstones. Note the strain gauge sensors that were emplaced for the measurements in vertical and horizontal orientations.
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Figure 10. Schematic representations of the stress–strain response and stages of brittle rock fracture process and evolution of the short-term strength of the material to its long-term strength when subjected to a constant stress condition; (a) OX-3G, (b) OX-4B, (c) OX-6C, (d) EPN-2B.
Figure 10. Schematic representations of the stress–strain response and stages of brittle rock fracture process and evolution of the short-term strength of the material to its long-term strength when subjected to a constant stress condition; (a) OX-3G, (b) OX-4B, (c) OX-6C, (d) EPN-2B.
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Figure 11. Schematic representations of the stress–strain response and stages of brittle rock fracture process and evolution of the short-term strength of the material to its long-term strength when subjected to a constant stress condition; (a) KAL-2C, (b) KAL-3B, (c) EPN-5A.
Figure 11. Schematic representations of the stress–strain response and stages of brittle rock fracture process and evolution of the short-term strength of the material to its long-term strength when subjected to a constant stress condition; (a) KAL-2C, (b) KAL-3B, (c) EPN-5A.
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Figure 12. Geological map of the Southeast Mesohellenic Trough (northwest Greece) region and modified model for capacity after fieldwork mapping and experimental study (red star symbols in the map represent sampling areas).
Figure 12. Geological map of the Southeast Mesohellenic Trough (northwest Greece) region and modified model for capacity after fieldwork mapping and experimental study (red star symbols in the map represent sampling areas).
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Table
Table 1. Average and 1σ standard deviation mineral chemistry values of selected sandstone samples from the geological Pentalofos Formation. Abbreviations: Plg: plagioclase, Ab: albite, Olg: oligoclase, Andes: Andesine, Kfs: K-feldspar, Ep: epidote, Srp: serpentine, Bt: biotite, Chl: chlorite, Aln: alanite, Mnz: monazite.
Table 1. Average and 1σ standard deviation mineral chemistry values of selected sandstone samples from the geological Pentalofos Formation. Abbreviations: Plg: plagioclase, Ab: albite, Olg: oligoclase, Andes: Andesine, Kfs: K-feldspar, Ep: epidote, Srp: serpentine, Bt: biotite, Chl: chlorite, Aln: alanite, Mnz: monazite.
MineralP|g (Ab)Plg (Olg-Andes)KfsEpSrpBtChlAlnMnz
n:7518869734
SiO267.3662.8467.0439.1145.6251.3337.8340.9010.74
1σ0.841.371.211.171.101.942.740.950.89
TiO2-----1.45 --
1σ-----0.87---
AI2O319.9223.7217.4423.841.4621.5415.3122.356.18
1σ1.091.640.671.160.742.281.220.914.85
FeO0.160.080.169.416.347.4114.6112.322.07
1σ0.090.040.051.480.812.343.181.581.95
MgO---0.3833.495.2718.64-1.17
1σ----1.631.453.26-
CaO1.244.320.2524.170.59-0.7913.693.21
1σ0.681.300.071.420.12-0.200.193.14
Na2O10.768.620.81------
1σ0.920.820.62------
K2O0.570.2914.21--9.57---
1σ0.230.110.54--0.82---
P2O5--------29.82
1σ--------4.99
La2O3-------3.8513.90
1σ-------0.431.72
Ce2O3-------7.8729.47
1σ-------0.092.52
Y2O3--------27.08
1σ--------2.37
Nd2O3--------10.48
1σ--------1.49
Total99.8599.8799.9196.9187.1996.5787.1899.0597.63
Table
Table 2. Average and 1σ standard deviation mineral chemistry values of selected sandstone samples from the Northern Eptachori Formation. Abbreviations: Plg: plagioclase, Ab: albite, Olg: oligoclase, Kfs: K-feldspar, Ep: epidote, Srp: serpentine, Bt: biotite, Chl: chlorite, Aln: alanite.
Table 2. Average and 1σ standard deviation mineral chemistry values of selected sandstone samples from the Northern Eptachori Formation. Abbreviations: Plg: plagioclase, Ab: albite, Olg: oligoclase, Kfs: K-feldspar, Ep: epidote, Srp: serpentine, Bt: biotite, Chl: chlorite, Aln: alanite.
MineralPlg (Ab)Plg (Olg)KfsEpSrpBtChlAln
n:871152652
SiO267.5563.4566.6238.5247.952.4839.2440.09
1σ1.031.451.280.730.560.732.160.95
TiO2-----1.22--
1σ-----0.36--
AI2O319.8723.3817.7824.322.0321.1214.8422.35
1σ1.311.551.160.440.072.862.330.91
FeO0.190.120.189.694.897.1215.7612.32
1σ0.080.060.110.980.021.833.41.58
MgO----31.585.9316.43-
1σ----0.672.262.75-
CaO1.113.550.2823.760.32-0.7713.68
1σ0.880.800.120.760.05-0.420.19
Na2O10.689.150.83-----
1σ0.710.610.42-----
K2O0.460.2414.27--9.19--
1σ0.500.221.35--1.13--
La2O3-------3.85
1σ-------0.58
Ce2O3-------7.86
1σ-------0.09
Total99.8699.8999.9696.2986.7297.0687.04100.15
Table
Table 3. Average and 1σ standard deviation mineral chemistry values of selected sandstone samples from the Southern Eptachori Formation. Abbreviations: Plg: plagioclase, Ab: albite, Kfs: K-feldspar, Ep: epidote, Srp: serpentine, Bt: biotite, Chl: chlorite.
Table 3. Average and 1σ standard deviation mineral chemistry values of selected sandstone samples from the Southern Eptachori Formation. Abbreviations: Plg: plagioclase, Ab: albite, Kfs: K-feldspar, Ep: epidote, Srp: serpentine, Bt: biotite, Chl: chlorite.
MineralPlg (Ab)KfsEpBtChl
n:55344
SiO267.7566.8639.6753.7239.86
1σ1.160.630.412.231.47
TiO2---0.78-
1σ---0.39-
AI2O319.6517.5623.2123.7215.03
1σ1.020.410.332.630.84
FeO0.140.2110.416.5116.61
1σ0.070.090.252.530.36
MgO---4.7114.82
1σ---1.610.39
CaO1.180.1523.23-0.47
1σ0.630.060.39-0.15
Na2O10.550.59---
1σ0.610.32---
K2O0.6014.53-10.32-
1σ0.390.33-1.07-
Total99.8799.996.5299.7686.79
Table
Table 4. Whole-rock major (wt.%) and trace element (ppm) compositions of the studied sandstone samples.
Table 4. Whole-rock major (wt.%) and trace element (ppm) compositions of the studied sandstone samples.
RegionPentalofosNorthern EptachoriSouthern Eptachori
Rock SampleOX-6COX-4BOX-3GEPN-2BEPN-5AKAL-3BKAL-2C
SiO259.8259.4649.5568.6565.9847.1248.18
Ti020.240.520.260.140.180.240.348
Al2O35.9912.995.226.215.826.8110.8
FeO *3.433.834.462.171.733.723.23
MnO0.180.110.140.050.090.140.15
MgO5.3153.638.803.031.922.821.60
CaO14.147.7915.578.8112.8318.5316.63
Na200.762.050.741.311.370.911.93
K201.082.550.981.401.431.882.81
P2050.050.070.040.030.040.160.09
LOI8.785.8914.124.378.5116.3912.61
Total99.8098.9099.9296.2099.9498.7598.41
Scb.d.l.6b.d.l.4b.d.l.b.d.l.b.d.l.
V56856534405758
Cr65028054612816219865
Ni57615477212114121882
Cu242538791217
Zn27513416212834
Rb281053752413963
Sr154139142252215273176
Y1421111191420
Zr75144577698155119
Nbb.d.l.1020b.d.l.b.d.l.b.d.l.b.d.l.
Bab.d.l.301b.d.l.137113b.d.l.153
Hfb.d.l.b.d.l.b.d.l.b.d.l.b.d.l.4420
Pb112b.d.l.2214b.d.l.4
La621b.d.l.124713
Ce3 b.d.l.372212115939
Co2328421919b.d.l.5
Wb.d.l.1b.d.l.11b.d.l.1
Ommit symbol *, b.d.l.: below detection limit.
Table
Table 5. Pore structure properties deduced by N2 adsorption data recorded at 77 K.
Table 5. Pore structure properties deduced by N2 adsorption data recorded at 77 K.
BET Area
(m2/g)		BJH Pore Volume
(cm3/g)		Average Mesopore Diameter
(nm)
SandstonesFragmentsPowdersFragmentsPowdersFragmentsPowders
OX-3G11.814.80.0180.0227.27.1
OΧ-4Β 4.85.90.0080.0107.97.7
OΧ-6C 6.17.70.0080.0166.69.4
EPN-2B 1.72.40.0030.0058.48.9
EPN-5A 1.52.30.0040.00510.310.6
KAL-3B 1.92.70.0040.0068.014.9
KAL-2C 2.42.50.0040.0098.810.0
Table
Table 6. UCS, Ei, and v values derived from mechanical tests at the University of Leeds (Rock Mechanics, Engineering Geology and Geotechnical (RMEGG) Laboratories).
Table 6. UCS, Ei, and v values derived from mechanical tests at the University of Leeds (Rock Mechanics, Engineering Geology and Geotechnical (RMEGG) Laboratories).
SampleUCSEiv
MPaMPa
OX-3G181.8564,6480.19
OX-4B271.89142,8500.25
OX-6C64.6653,5840.22
EPN-2B88.7334,9380.22
EPN-5A108.3561,9230.27
KAL-3B37.727,4440.11
KAL-2C30.4934,9960.15
Table
Table 7. Quantification of modal composition of the representative investigated groups of sandstones. Mineral abbreviations according to Whitney & Evans [40] (Qz: quartz, Kfs: K-feldspar, Pl: plagioclase, Cc: calcite, Dol: dolomite, Srp: serpentine, Chl: chlorite, Ep: epidote).
Table 7. Quantification of modal composition of the representative investigated groups of sandstones. Mineral abbreviations according to Whitney & Evans [40] (Qz: quartz, Kfs: K-feldspar, Pl: plagioclase, Cc: calcite, Dol: dolomite, Srp: serpentine, Chl: chlorite, Ep: epidote).
Modal Composition
Group 1 (Pentalofos)SamplesQzKfsPICcDolMicaSrpChlEpCementP.r.P
OX-3G28.820.69.97.32.74.48.24.95.18.11.03
OX-4B29.922.912.28.92.26.37.23.66.09.71.08
OX-4C30.719.210.79.02.43.87.43.22.810.80.83
OX-6C26.822.610.99.22.04.26.23.94.110.11.00
Group 2 (Eptachori north)EPN-1C32.816.612.812.30.52.61.41.92.716.40.58
EPN-2B33.213.414.613.20.83.11.52.22.815.20.57
EPN-4A31.714.816.214.90.52.60.72.81.913.90.59
EPN-5A31.616.415.811.60.93.81.52.01.514.90.66
Group 3 (Eptachori south)KAL-1B31.111.711.422.7-1.5-2.00.818.80.35
KAL-2C34.010.910.719.8-2.8-2.5-19.30.33
KAL-3B33.211.712.417.2-1.6-3.01.319.60.39
KAL-3C32.910.911.718.4-2.4-3.5-20.20.35
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Christopoulou, M.A.; Koutsovitis, P.; Kostoglou, N.; Paraskevopoulou, C.; Sideridis, A.; Petrounias, P.; Rogkala, A.; Stock, S.; Koukouzas, N. Evaluation of the CO2 Storage Capacity in Sandstone Formations from the Southeast Mesohellenic trough (Greece). Energies 2022, 15, 3491. https://doi.org/10.3390/en15103491

AMA Style

Christopoulou MA, Koutsovitis P, Kostoglou N, Paraskevopoulou C, Sideridis A, Petrounias P, Rogkala A, Stock S, Koukouzas N. Evaluation of the CO2 Storage Capacity in Sandstone Formations from the Southeast Mesohellenic trough (Greece). Energies. 2022; 15(10):3491. https://doi.org/10.3390/en15103491

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Christopoulou, Marina A., Petros Koutsovitis, Nikolaos Kostoglou, Chrysothemis Paraskevopoulou, Alkiviadis Sideridis, Petros Petrounias, Aikaterini Rogkala, Sebastian Stock, and Nikolaos Koukouzas. 2022. "Evaluation of the CO2 Storage Capacity in Sandstone Formations from the Southeast Mesohellenic trough (Greece)" Energies 15, no. 10: 3491. https://doi.org/10.3390/en15103491

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