The growth in science, technology, and productivity during the industrial age of the nineteenth century was concurrent with wealth generation based upon cheap energy from fossil fuels and primarily coal (Asif et al. 2019c). This growth and the subsequent shift to petroleum for energy have also carried many challenges for human beings (Longinos et al. 2022, 2021). Anthropogenic climate change, or change in long-term climate patterns induced by human activity, is the greatest challenge facing us in the wake of this development (Asif et al. 2018a, b). Particularly, global warming, or the rise in average temperatures in the earth’s atmosphere, is a critical phenomenon that is impacting and will continue to affect our planet in the foreseeable future (Longinos and Parlaktuna 2021). Global warming results from an accumulation of certain gases, e.g., CH₄ and CO_2, accumulating in the atmosphere beyond the natural balance in the carbon cycle. These gases trap the heat radiating from the earth that was generated from incident sunlight, thus increasing the average temperature of the earth’s atmosphere. Since temperature plays a critical role in weather patterns, agriculture, and the security of economic systems, it is necessary to control the problem of human-induced global warming (Asif et al. 2022c; Serikov et al. 2022). According to the Intergovernmental Panel on Climate Change (IPCC), Global warming has caused the temperature to rise 1 °C above preindustrial levels and may increase to 1.5 °C between 2030 and 2052 (IPCC 2018). The United Nations also commissioned a report in 2019 with a clear warning that the current rate of temperature rise will take a huge toll on lives, natural systems, and the economy (United Nations 2019).

The proposed goal to remove 1000 Giga tons of CO₂ from the earth’s atmosphere by 2100 demands additional carbon dioxide removal (CDR) techniques and air capture plants. About 53 Giga tons of greenhouse gases are emitted each year, and the world would need to deploy enough CDRs to counteract more than 18 years of total global carbon emissions, as suggested by World Bank in 2016 (WBG 2016). The emission of CO₂ in the atmosphere has been increased to 130 ppm (280 to 410 ppm) from the preindustrial period to the present time (IPCC 2018). Five methods are suggested to counter the increasing emission of CO₂ in the atmosphere, e.g., switching to low-carbon generation fuel, increasing conservation, deploying energy efficient methods, improving environmental protection, and sequestering CO₂ (Meer 2005).

The sequestration of CO₂ in geological formations is presently the most feasible and effective method. Geological formations, e.g., the porous basalts, saline aquifers, depleted oil and gas reservoirs, or unmineable coal seams offer viable options for the longtime storage of CO₂ (Bachu and Adams 2003). A schematic diagram for the geological storage options of CO₂ has been shown in Fig. 1.

Geologic storage options for carbon dioxide (Perera 2014)

This paper provides a detailed geological review of the carboniferous coal basins of Kazakhstan and assesses their viability for CO₂ storage. The paper is divided into four parts: (1) A description of the energy sector using coal in Kazakhstan, (2) A short review of CO₂-ECBM recovery, (3) A detailed geological review of carboniferous coal basins in Kazakhstan, and (4) An assessment of Kazakhstan coal basins as for potential CO₂ storage.

Coal has experienced significant growth as a cheap and available energy source worldwide, surpassing the growth rates of gas, oil, nuclear, hydro, and other renewable sources (Muhammed et al. 2022). Similarly, in Kazakhstan, a country with a thriving economy, coal deposits play a vital role in meeting its energy demands. Additionally, Kazakhstan stands out for its extensive use of coal for household heating, ranking among the highest in the world. According to a report by The World-Watch Institute in 2020, Kazakhstan's carbon emissions amounted to approximately 272 million tons in 2019. Of these emissions, 235.3 million tons were attributed to energy-related activities, accounting for 0.55% of global emissions in terms of CO₂ equivalent. The sources of CO₂ emissions in Kazakhstan include fossil fuel combustion in various sectors, e.g., power generation industries, heating in residential complexes, and fuel for vehicles. Notably, coal-fired plants were responsible for an estimated 176 million tons of CO₂ emissions in 2019, comprising 70% of the country's annual emissions (Khoyashov et al. 2024). This high percentage can be attributed to the dominant use of coal as a fuel for power generation, contributing to over 70% of Kazakhstan's electricity production in 2022 (Agency 2022).

Kazakhstan, like many other countries, must prioritize the precise monitoring and regulation of methane emissions originating from coal mines, as stated in the ESR Report of 2022. The country faces additional environmental challenges due to its outdated, inefficient, and highly polluting coal-fired thermal power plants, as highlighted in the same report. With a total of 14 major coal-fired thermal power plants, Kazakhstan's concentration of these facilities is primarily found in its northern power zones, where coal production takes place, as illustrated in Fig. 2. The energy transition in Kazakhstan will be challenging.

Significant emission sources of CO₂ and the energy operations of coal-fired, oil-fired, or gas-fired power and heating plants (modified after Abuov et al. 2020; National inventory report 2016))

In coal mining, methane poses a risk to the health and safety of workers and operations. While the energy released in burning coal drives its use as a fuel, coal also contains large volumes of adsorbed methane – an energy transition low carbon generation fuel. While coal has this natural affinity to store methane, it has an even stronger affinity for CO₂, presenting a unique opportunity to sequester CO₂, usually a pure operational cost, while providing a low carbon generation fuel as an income stream (Baines and Worden 2004; Tiyntayev et al. 2022). The process is named CO₂-ECBM, where ECBM stands for Enhanced Coal Bed Methane (Asif et al. 2021).

Unmineable coal seams have advantages over geo-sequestration methods, as they are economically attractive and efficient for the environment. Concurrent with CO₂ injection, the preferential adsorption of CO₂ over CH₄ in coal displaces the methane rapidly (Ahuja et al. 2021). The CO₂-ECBM process is illustrated in Fig. 3a. The CO₂-ECBM process offers a dual advantage for humans as CO₂ can be permanently stored in the coalfields while enhancing the coalbed methane recovery. Furthermore, it was identified that ~ 98% of CO₂ could remain in the coal in an adsorbed form (Asif et al. 2019b, 2017; Bustin and Clarkson 1998). The initial commercial ECBM field and the longest trial were performed between 1995–2001 at the Allison unit in San Juan, where approximately 5 billion cubic feet were injected, resulting in a profit of $2 per thousand cubic feet. Other than this pilot project, significant projects have been conducted such as Powder River has been conducted e.g., Powder River Basin, Black Warrior Basin, Fenn-Big Valley in Alberta, Canada, the Yuabri project in Hokkaido, Japan, and China (including the Qinshui Basin from 2004 to 2015 and the APP ECBM project from 2011 to 2012) (Fan et al. 2023; Godec et al. 2014).

Depiction of mechanism in CO₂-ECBM process (Asif et al. 2022c)

The work of a few researchers relevant to the CO₂-ECBM recovery has been summarized from Tables 1, 2 and 3 from the start of the research in 1998 till date (2024). Table 1 includes theoretical and modeling research and Table 2 includes some of the experimental research in the CO₂-ECBM process. The pilot test and commercial projects have been included in Table 3.

The mechanism of the CO₂-ECBM process lies in the adsorption phenomenon itself. When CH₄ or CO₂ interacts with coal, there is physisorption rather than chemisorption. Thus, this process alters the adsorption energy and molecular bonds between gas and coal molecules. Inside the coal structure, the coal molecules are intact, with equilibrium forces between them. However, a force imbalance occurs between the surface and inside molecules. Therefore, potential adsorption sites are created at the surface, which attract the gas molecules for adsorption (Asif et al. 2019a; Mohammad et al. 2012; Ottiger et al. 2008; Shimada et al. 2005; Yu et al. 2014). On the surface of the coal, the gas molecules adhere to the surface by weak van der Walls forces. The van der Walls equation has been given in Eq. (1):

When CO₂ is injected in coal seams, it decreases the partial pressure of CH₄ without decreasing the overall reservoir pressure, which leads to fast desorption of CH₄ and adsorption of CO₂ in its place (Norouzbahari et al. 2015; Perera et al. 2012; J. Q. Shi and Durucan 2005). However, the desorption of CH₄ is incomplete. A competitive adsorption phenomenon between CO₂ and CH4 is a function of concentration (Asif et al. 2018a, b). It may be noted from Eq. (1) that the measure of intermolecular forces represented by the ‘a’ value of CO₂ is more than CH₄ (Table 4), which suggests that CO₂ would be preferentially adsorbed over CH₄.

Due to the distinct properties of CO_2, as shown in Table 4, the preferential adsorption leads to adsorption capacity differences between 2 to 10 times (Asif et al. 2022a). Therefore, storage of CO₂ in coalfield affords long-term stability with reduced risk of back migration of CO₂ into the atmosphere.

Furthermore, it is identified that the preferred depth for CO₂ sequestration in coal seams is over 800 m since, at this depth, CO₂ is a supercritical phase with liquid-like density (Pashin and Mcintyre 2003), thus high storage capacity, and efficient use of coal porosity. It may be observed from Fig. 3b that CO₂ is injected at point 1, where both pressure and temperature are at surface conditions. Supercritical CO₂ also possesses higher adsorption capacity than subcritical CO₂ and has more potential to displace methane (Fan et al. 2020). The competitive adsorption for adsorption sites between CO₂ and CH₄ is depicted in Fig. 3c, with a co-adsorption adsorption isotherm shown in Fig. 3d.

Kazakhstan, the ninth largest country in the world by land mass, possesses substantial coal reserves in the central and northeastern parts of the country, estimated to be 170 Bt, with coal production of 103 Mt/year from 400 operating mines (Ammosov et al. 1973a, b, Azizov et al. 2013). Two-thirds of Kazakhstan's coal is bituminous and sub-bituminous coal. Four carboniferous coal formations are described here, notably the Karagandy, Teniz-Korzhinkol, Ekibustuz, and Chu coal basins (Fig. 4). Except for the Chu basin, the three remaining are major coal-producing coalfields of Kazakhstan.

1. (1)

   Karagandy coal basin is the country’s main coal-producing basin (around 37 Mt/year) in the Karagandy region. The basin enriches all types of coal (from lignite to anthracite) in the main seven coal formations (Shahan, Tentek, Dolyn, Nadkaragnda, Karaganda, Ashlyar, and Akkuduk) of Lower-Middle-Upper Carboniferous age with a thickness between 350–850 m.

2. (2)

   Teniz-Korzhynkol coal basin is located in the Akmola region, which is 200 km southwest of Astana and includes four Lower Carboniferous coal-producing deposits (Qosmurun in the west, Qyzylsor in the south, Bozshasor in the north and Saryadyr in the southeast). This coal basin includes all coal types from lignite to anthracite in the Karagandy, Ashlyar, and Akkuduk coal formations of thickness between 430 to 450 m). This coal basin produces 3 Mt/year.

3. (3)

   Ekibastuz coal basin is located in the Pavlodar region and contains Lower Carboniferous coal formations. Coal is being produced by three giant open-pit mines (Bogatyr, Severniy, and Vostochniy) in the basin. The basin includes all coal types, from lignite to anthracite, in Nadkaragandy, Karagandy, Ashlyar, and Akkuduk coal formations. The coal production from the basin is about 30 Mt/year.

4. (4)

   The Chu coal basin is in the Zhambyl region. It has all coal types (from lignite to anthracite) in the Lower Carboniferous coal formations (Visean and Tunisian) ranging between 350 and 850 m thick.

The Carboniferous coal basins of Kazakhstan

The stratigraphic column of the four coal basins is shown in Fig. 5.

A schematic stratigraphic column showing coal types and formations (with thickness in meters) of the selected four coal basins of Kazakhstan (modified after Ammosov et al. 1973a, b, Azizov et al. 2013)

4.1 Karagandy coal basin coal basin

The Karagandy coal basin, a deep synclinal sub-latitudinal structure covering approximately 3600 km², has the largest area in Kazakhstan with a width of approximately 30 km east to west and a length of 120 km measured north to south. It hosts the three synclinal trough separated by the Alabas and Maikuduk uplifts of Tentek, Churubay-Nurun, and Dubov (Fig. 6). The basin contains mainly Devonian strata of volcanic-sedimentary and sedimentary rocks of sandstone (36%), siltstone (30%), mudstone (27%) and others (10%) of tuffs, carbonate rocks and conglomerate. While the Carboniferous sequence in the basin contains sandstone and clay, it ranges up to 4500 m and includes 80 coal seams. The depth of the Carboniferous coal formation is down to 1800 m, while the Jurassic coal formation is found in depths from 300 to 450 m. The Lower-Middle Carboniferous coal formations consist of deep coal seams in the Karagandy, Ashlyar, and Akkuduk formations, while other coal seam formations are shallow. These three coal basins host all coal varieties, from lignite to anthracite. The seams of interest here lie in the carboniferous formation. The deeper coal seams were highlighted in the red dotted line as a potential target for CO₂ storage.

Geological cross-section of the Karagandy coal basin showing Lower Jurassic coal-bearing strata : 1. Middle Jurassic Mihailov Formation (J₂mh, mudstone); 2. Lower Jurassic Kumuskuduk Formation (J₁km, siltstone); 3. Lower Jurassic Dubov Formation (J₁db, Sandstone); 4. Upper Triassic-Lower Jurassic Saran Formation (T₃–J₁sr, gravels, conglomerate); Carboniferous formations from 5.Upper Carboniferous Shahan Formation (C₃sh), 6. Middle Carboniferous Tentek Formation (C₂te), 7. Middle Carboniferous Dolyn Formation (C₂do), 8. Lower Carboniferous Nadkaragndy Formation (C₁na), 9. Lower Carboniferous Karagandy Formation (C₁ka) to 10. Lower Carboniferous Ashlyar Formation (C₁ash); 11. Upper Devonian; 12. Middle Devonian; 13. Lower Devonian; 14. Faults; 15. Coal seams; 16. A cross-sectional profile (modified after Azizov et al. 2013); Mining area: I-Tentek; II- Churubay-Nuryn; III-Karagandy; IV-Verhnesokur; Underground mines: 1-Tentek; 2-Kazakhstan; 3-Shahtins; 4-Lenin; 5-Abay; 6-Saran; 7-Kuzembayev; and 8-Kostenko) (Sabitova 2015; Junussov et al. 2024)

The coal has many fractures ranging between 2 mm × 2 mm and 6 mm × 6 mm. The coal has a low sulfur content of up to 1.5% and a moisture content of up to 14%. The basin contains one of the largest concentrations of methane gas, ranging between 22–25 m³/t. The ash content of coals varies from 20%–30%, while vitrinite reflectance (Ro) ranges from 1.80%–2.03%. The total coal reserves of the basin are 43.1 billion tons (Azizov et al. 2013). Nine mineable underground coal mines are shown in Fig. 6.

4.2 Teniz-Korzhinkol coal basin

The Teniz-Korzhynkol basin occurs as a large brachy synclinal structure in the northern part of the Shydertyn megasyncline. It includes the four small syncline structures of Qosmurun, Qyzylsor, Bozshasor, and Saryadyr, formed as coal deposits and mined only by open-pit from the Saradyr coal deposit (Fig. 7).

Geological cross-section of the Teniz-Korzhinkol coal basin showing Carboniferous and Devonian sequences: 1. Upper Carboniferous (Vladimirov Formation); 2. Middle Carboniferous (Kirey Formation); 3. Middle-Lower Carboniferous (Karagandy coal Formation); 4. Lower Carboniferous Middle-Lower Visean (Ashlyar and Akkuduk coal Formations); 5. Upper Devonian (Famennian)-Lower Carboniferous (Tournaisian); 6. Middle Devonian; 7. Lower Devonian; 8. Cambrian; 9. Permian–Triassic magmatic intrusions; 10. Faults. 11. Coal seams; 12. A cross-sectional profile (modified after Ammosov et al. 1973a; Azizov et al. 2013). Coal deposits: I-Bozshasor; II- Saryadyr (currently coal mined); III- Qyzylsor and IV- Qosmurun

The Basin area is approximately 400 km², with a width of 20 km east to west and a length of 20 km north to south. The coal basin hosts about 60–70 coal seams. The thickness of coal seams fluctuates from 1.5 m to 50 m, buried at a depth of up to 1800 m. Coal seams are found in the Lower Carboniferous formations of Karagandy, Ashlyar, and Akkuduk and occur within sedimentary rocks of grey to dark clay, marl, mudstone, sandstone, and siltstone. Cambrian and Devonian basements primarily originate in the northwest part of the basin. The primary sediments of the basements are limestone, shale, and marl, with a thickness of 200 m. Magmatic intrusions are mainly composed of granite-porphyry, found dikes, sills, and a deeply buried batholith in contact with highly metamorphosed coals. The coal seams usually have bituminous coal to anthracite (probably graphite), give high ash yields of 20%–45%, contain between 0.2%–1.8% sulfur, and have up to 10% moisture content. Total coal reserves in the basin are estimated to be about 2.6 billion tons (Ammosov et al. 1973a). Hence, this coal basin is a potential target for CO₂ storage in deeper coal seams.

4.3 Ekisbastuz coal basin

The Ekibastuz coal basin is restricted to the asymmetrical graben-syncline covering over 155 km² with a width of 8.5 km and a length of 24 km, extending in the northwestern direction. It is bounded in the northeast and southeast by significant faults with amplitudes of 1000 m and 150 m, respectively, as shown in Fig. 8. The coal basin is stratified with a Devonian basement and volcanic shale deposits of mudstone and sandstone. The Lower Carboniferous sequence shows coal-bearing formations of Nadkaraganda, Karaganda, Ashlyar, and Akkuduk, deposited with dark grey mudstone, siltstone, and green, fine-grained sandstones and containing around 6 mineable coal seams and 15 unmineable coal seams. The coal seams range from 8–30 m in thickness, buried at a 530–680 m depth. The coal seams in the basin are mainly characterized from lignite to anthracite. Ash content of the coal seams ranges from 30% to 41%, while sulfur and moisture content range between 0.4% to 1% and up to 14.2%, respectively. The coal seams contain methane and nitrogen gases from 4–10 m³/t, with total coal reserves of the basin at around 9.7 billion tons (Ammosov et al. 1973a).

Geological cross-section of the Ekibastuz coal basin showing the Lower Carboniferous coal-bearing formations of 1. Nadkaragndy (C₁ ndk), 2. Karagandy (C₁ krg), and 3. Ashlyar (C₁v_1-2 as, Middle-Lower Visean); 4. Akkuduk; Devonian basements of 5. Middle Devonian, 1. Lower Devonian; 7. Faults; 8. Coal seams; 9. A cross-sectional profile (Modified after Azizov et al. 2013).

4.4 Chu coal basin

The Chu coal-bearing basin occurs as a brachy synclinal structure in the Chu-Sarysu Basin, which is the second-largest sedimentary basin in Kazakhstan (Fig. 9). The southwest and northeast boundaries of the sedimentary basin are marked by the parallel Karatau and Zhalair-Naiman faults, respectively (Box et al. 2012). The coal-bearing formations as a coal basin originated between the tectonic faults. Mesozoic sediments mainly cover the coal basin as a sand desert (a width of 15–30 km), forming the Moyinkum deflection. Significant four coal deposits of Kamkaly, Karakol, Alexandrovskoe, and Kasymtobe are included in the basin. Coal deposits in the basin were traced along the strike for 100 km. More productive coal formations in the Lower Carboniferous host about 13 coal seams. All coal seams in lens shapes reach a thickness from 0.3–2.5 m to 4.5 m to 15 m and are considered mineable. The basin has deep coal seams at a depth of 2500 m. The coal seams are mainly bituminous coal to anthracite coal types in the Lower Carboniferous Turnaisian and Visean formations (Fig. 9). Furthermore, these are interbedded with carbonate and terrigenous sediments of grey limestone breccias and marl, conglomerate, greenish-grey fine-grained sandstone, and siltstones. The coal seams usually yield a high ash content between 40%–46%, have up to 2.3% sulfur content, and contain moisture content up to 11.4%. Total coal reserves are 3.3 billion tons (Ammosov et al. 1973a; Azizov et al. 2013). Therefore, this coal basin is a potential target for CO₂ storage in deep unmineable coal seams.

Geological cross-section of the Chu coal basin showing 1. Quaternary; 2. Eocene; 3. Middle-Upper Carboniferous; 4. Middle Carboniferous; 5. Lower Carboniferous; 6. Lower Carboniferous Middle-Upper Visean; 7. Lower Carboniferous Lower Visean and Turnaisian; 8. Upper Devonian; 9. Faults; 10. Coal seams; 11. Basin counter line; 12. A cross-sectional profile (Junussov and Mustapayeva 2024)

4.5 Paleogeographic evolution of the carboniferous coal basins for CO₂ storage

The paleogeographic evaluation of the four coal basins suggests that coal formation belongs to the Carboniferous age as per the literature of Ammosov et al., (1973a, b) and Azizov et al. (2013). According to the review of the paleogeographic evolution during the Carboniferous time in Kazakhstan, the most economical coal was deposited mainly in the Lower and Middle Carboniferous periods. The paleogeographic outline of the Lower and Middle Carboniferous, which inherited the characteristics of the deep marine basins in the western, southern, and eastern areas, developed in the northern region, extending between the northern and the central-southern upland. Extensive low-relief carbonate shelves were typical for central areas. On the Kazakhstan continent, behind the volcanic belt, large marsh-lacustrine systems accumulated thick coal beds of Karagandy, Ekibastuz, Tenyz-Korzhynkol, and Chu (Daukeev et al. 2002), as shown in Fig. 10. The Carboniferous period was the time for the climatic transition. The Lower Carboniferous generally had a more humid climate, which was more favorable for coal occurrence.

Paleogeographic evaluation of Kazakhstan showing the Lower Carboniferous (modified after (Korobkin and Buslov 2011))

In contrast, the Upper Carboniferous had a warming trend and generally became drier, unfavorable for coal deposits (Rowley et al. 1985). In the late Lower Carboniferous, the oceanic crust resumed subduction on all the margins of Kazakhstan. This resulted in forming the Valer’yanovka island arc and the Balkhash–Ile volcanic-plutonic belt, intense volcanic-arc activity, and the closure of some interarc basins. Vast lake bog plains with abundant coal accumulation existed at the back of the volcanic belt in Kazakhstan, occurring thick coal measures. A single marine shelf basin with terrigenous–carbonate sedimentation existed in most of Kazakhstan (Korobkin and Buslov 2011).

The required data has been collected from previous literature (Ammosov et al. 1973a, 1973b; Azizov et al. 2013). Information on four carboniferous coal deposits was shown in Table 5.

It may be noted from Table 5, that due to the unavailability of data on Kazakhstan coalfields and for the sake of brevity, only average values have been considered. The CO₂ storage capacity of the coal seam (S, Mt) was generally found using the formula proposed by the U.S. Department of Energy (Goodman et al. 2011):

where A_c is the area of the coal seam (km²), T_c is the thickness of the coal seam (m), ${\rho }_{\text{c}}$ is the density of coal (g/cc), $E_{\text{f}}$ is the storage efficiency of coal which was assumed as 57% (George Hall 2007), $V_{{\text{CO}}_2}$ is the adsorption capacity of coal seams concerning CO₂ (Langmuir constant, m³/t), ${\rho }_g$ is the density of CO₂ at different depth (cc/g_).

The area of the coal seam (A_c) may be found using the below-mentioned formula or Eq. (3):

In this equation, the coal seam is assumed to be a tabular shape. Therefore, a new formula was proposed and used in this paper for calculating the CO₂ storage capacity (S_t) of a particular coal seam, as shown in Eq. (4). The methodology adapted by US DOE for the volumetric calculation of CO₂ storage in unmineable coal seams was slightly modified. Instead of area, thickness, and density of the coal seam, coal reserve and number of coal seams was used. Simple geometry was applied and a formula for CO₂ storage in tabular coal seams was developed. This value was doubled since CO₂ has around 2 times more adsorption capacity than CH₄ in coal (Mohammad et al. 2012; Zhang et al. 2014). The formula given by the U.S. Department of Energy was slightly modified as:

The Langmuir volume constant was also used in this formula which was a modified version of the Langmuir volume correlation given by Mohanty et al. (Mohanty et al. 2018). In this research paper, authors have developed a statistical correlation between the chemical composition of 39 coal samples with their respective Langmuir volume constants of methane. This value was doubled since CO₂ has around 2 times more adsorption capacity than CH₄ in coal (Mohammad et al. 2012; Zhang et al. 2014). The formula for calculating the Langmuir volume constant for CO₂ has been shown in Eq. (5):

It is worth mentioning that adsorption capacity is a function of temperature and pressure which was already considered for the experimentation of adsorption capacity of methane (Mohanty et al. 2018). The Langmuir constant of CH₄ was utilized in this paper for finding the storage capacity of coal concerning CO₂.

The real gas equation has been used to calculate the variable density (Fan et al. 2023):

where P is pressure, M is molecular mass, Z is compressibility, R is gas constant and T is temperature.

In Eq. (6), the following relations, and values have been taken (Lu et al. 2021):

where D is depth in meters, P is in MPa and T is in ℃

The values of M, Z, and R have been taken as 44.01, 0.449, and 8.314 J/mol.K respectively. Using Eq. (6), (7), and (8), the density of CO₂ was derived as (Lu et al. 2021):

It may be noted that this formula has been derived for supercritical CO₂ of compressibility 0.499 (Lu et al. 2021). However, this formula may be used for the Ekibustuz coal field in which the known depth of unmineable coal seams does not reveal the supercritical condition of CO₂. In contrast, the depth at which supercritical conditions are met is highly variable and mainly contingent on depth, surface temperature, hydrostatics, and the geothermal gradient of the reservoir (Perera et al. 2011; Zhao et al. 2022). The basin temperature (T) and hydrostatic pressure (P_hyd) were defined in Eq. (10) and Eq. (11) respectively:

From Eq. (10) and Eq. (11), the basin pressure may be defined as:

where T_s, G, and g are the surface temperature, geothermal gradient, the density of water, and gravitational constant (9.8 m/s²) respectively. The basin pressure was similar to hydrostatic pressure in sedimentary basins such as coal and the geothermal gradient varies from 20 ℃/km to 60 ℃/km (Kolawole and Evenick 2023). However, due to the insufficiency of data on the Kazakhstani carboniferous coal basins, the average and common geothermal gradient of 27 ℃/km was presumed (Abuov et al. 2020). It must be noted that different compressibility has been already used for the calculation of the Langmuir volume based on pressure and temperature that was used in the proposed formula for computing the CO₂ storage capacity of coal seams.

The adsorption capacity of coal concerning CO₂ may be experimentally calculated using several methods e.g., Langmuir equation, Toth equation, UNILAN equation, Dubinin–Astakhov (D–A) and Dubinin–Radushkovich (D–R) equations, 2D equations of state, D–R equation and other methods (De Silva et al. 2012). However, one formula table as Table 6 was given for calculating the CO₂ storage capacity of coal seams.

The CO₂ storage capacity for the coal seams was found for these coalfields using the proposed formula in this paper. The ideal depth of coal seams for CO₂ storage is roughly 800 m for reasons stated earlier. Table 7 shows the CO₂ storage capacity for the carboniferous coal basins with ${\rho }_{\text{g}}$ and $V_{{\text{CO}}_2}$ for the respective coalfields.

The Karagandy coal basins show the highest CO₂ storage capacity among Kazakhstan's three major coal basins that satisfy the 800 m depth criterion. Karagandy coal basin shows excellent adsorption capacity for CO₂ ($V_{{\text{CO}}_2}$) towards coal. The geology of the coal basin also plays an essential role as the Karagandy coalfield has low ash and moisture content among the three coalfields. Furthermore, Karagandy coalfields have a huge methane content of about 22–25 m³/t (Azizov et al. 2013). The unmineable coal seams of the Karagandy coal basins may be employed for ECBM recovery to offset the sequestration cost. The supercritical CO₂ has a higher storage capacity as the density of CO₂ increases in the supercritical phase (Zhao et al. 2022). However, Ekibustuz shows a relatively higher storage capacity except Karagandy coal basin. The reason for this may be understood with the help of the proposed formula and data of the Ekibustuz coal basin. Ekibustuz has a huge coal reserve (T_r) and less no of seams (N) as shown in Table 5. Therefore, despite the known depth not being related with CO₂ as supercritical fluid, a relatively high CO₂ storage capacity of 14.02 billion Bt has been calculated for the Ekibastuz coal basins using this formula.

The Teniz-Korzhinkol coal basin shows the lowest value for CO₂ storage, as this coal has high ash and moisture content. Thus, the adsorption capacity of CO₂ towards coal was reduced, and further storage amount in coalfield was substantially diminished. Furthermore, the low coal reserve was also the reason for the lowest storage capacity for CO₂.

CO₂-ECBM recovery aligns with two major sustainable development goals (SDG) of the United Nations i.e., goals 7 and 13. Goal No. 7 states that by 2030 it is recommended to provide affordable, clean, and modern energy to every household. Furthermore, goal No. 13 sheds light on the urgent action to be taken to combat climate change and its impact. CO₂ sequestration in unmineable coal seams is vital to climate action, and coalbed methane provides lower carbon content energy needs. Based on the current study, the following conclusions may be drawn:

1. (1)

   The detailed geological review of the coal basins reveals that all significant coal types are present in the studied basins: lignite, sub-bituminous, and anthracite coal.

2. (2)

   The unmineable coal seams lying below 800 m of three carboniferous coal may be employed for CO₂ storage to reduce the emission of anthropogenic CO₂ from Kazakhstan.

3. (3)

   The unmineable coal seams of Karagandy coalfields are ideal for CO₂ storage based on the limited available data on Kazakhstani coalfields. Furthermore, this coalfield shows good potential for CO₂-ECBM recovery.

4. (4)

   The proposed formula for CO₂ storage in coalfields may be utilized for tabular coal seams; however, it may extend to any other seam if the correct area of the seam is known.

This paper provides an essential and initial assessment of the CO₂ storage capacity of coal seams of Kazakhstani carboniferous coalfields. However, the same analogy may be used in Kazakhstan's remaining five Jurassic coalfields. Nevertheless, detailed experimental investigations of various parameters, e.g., vitrinite reflectance, proximate, and ultimate analysis, are warranted and significant for quantifying the rank and maturity of coal. The accurate measurement of the CO₂ adsorption capacity of coal using the high-pressure adsorption isotherm will give more insight into the CBM and ECBM recovery from the coalfields and the competitive adsorption process. One of the future works of this crucial study on carboniferous coal formations includes studying the fundamental properties of caprock.

Notably, this is the first study on CO₂ storage potential and a detailed geological review of the coal basins in the post-Soviet territory of Kazakhstan.