Corrigendum to “CO₂ utilization and sequestration potential in deep coal seams: A case study on Carboniferous coals from the Karaganda Basin, Kazakhstan”(Journal of CO2 Utilization, (2025), 101, C, (103204), (S221298202500188X), 10.1016/j.jcou.2025.103204)

Safaei-Farouji M. Misch D. Sachsenhofer R.F. Kostoglou N. Gaus G. Bauersachs T. Junussov M. Fustic M.
2026 Elsevier Ltd

Journal of CO2 Utilization
2026

The authors regret the necessary changes regarding the calculation of CO₂ storage capacities. The amended sections are marked in red. It should be noted that the essential content of the article remains unchanged. The authors apologise for any inconvenience caused. Abstract Kazakhstan is a major coal producer and emitter of carbon dioxide (CO₂), presenting both a challenge and an opportunity for CO₂ utilization and storage. The main goal of this work is to study the feasibility of CO₂ as a feedstock for enhanced coalbed methane recovery (CO₂-ECBM), as well as the associated geological storage potential of the D6 coal seam in the Karaganda Basin. For this purpose, coal samples were investigated using elemental analysis, Rock-Eval pyrolysis (RE), organic petrography as well as low-pressure (LP: N₂, CO₂), and high-pressure (HP: CO₂, CH₄) sorption tests. Vitrinite reflectance values show that seam D6 reached the medium-volatile bituminous rank. Higher organic matter content significantly increases the LP CO₂ sorption capacity. The adsorption-desorption isotherms of CO₂ recorded under both LP and HP conditions show a hysteresis loop. This is probably due to interactions between CO₂ and functional groups leading to enhanced physisorption at LP and chemisorption and matrix swelling at HP conditions. This effect is favorable for storage purposes as it implies safe CO₂ trapping even at reduced reservoir pressure. The CBM potential of seam D6 is estimated at 9 billion m³ initial gas and 2.2 billion m³ producible gas in place. Estimates of the adsorptive and total CO₂ storage capacity yielded 6.9 and 22.3 megatons (Mt), respectively. This study highlights how CO₂ can be effectively utilized as a feedstock to enhance methane recovery while achieving long-term CO₂ sequestration. 3.5 CCS and CO₂-ECBM estimations To estimate the CO₂-ECBM and resulting CCS potentials of the D6 seam, first the initial CH₄ in place (IGIP; Eq. 7) and the producible CH₄ in place (PGIP; standard conditions [cm³]; Eq. 8) [76,81] have been calculated following the equations below:(7)IGIP = A × TH × ρ_coal × GC With: A= areal extent of the coal seam (m²) TH = cumulative coal thickness (m) ρ_coal= coal skeletal density (t/m³) GC= CBM content in the seam (m³/t) It should be noted that IGIP represents the total volumetric estimate of methane initially stored within the coal seam before any production and recovery. IGIP therefore represents an estimate of maximum CH₄ in place and does not account for practical CH₄ extraction efficiency.(8)PGIP = RF × CF × IGIP With: RF = recovery factor (proportion of gas fraction that can be extracted by reducing reservoir pressure via water production) CF = completion factor (fraction of area that can be accessed through drilling operations; [81]). The producible gas in place (PGIP) here estimates the fraction of initial CH₄ in place that can be produced considering common efficiency factors. Considering the prime target to store CO₂ during gas production, RF also denotes the storage efficiency of CO₂ and typically varies from 0.2 to 0.6 (mainly 0.5; [34,76,81]). Here, CF is considered 0.5 [76]. Hendriks et al. [34] estimated the global CO₂ storage potential in different types of underground reservoirs. For coal basins under CO₂-ECBM operations, the CO₂ storage capacity (S_CO2) was calculated using the following equation:(9)S_CO2 = PGIP × ER × ρ_CO2 With: ER = exchange ratio between CO₂ and CH₄ ρ_CO2 = CO₂ density at standard temperature and pressure (1.977 kg/m³) 5.5 CCS and CO₂-ECBM potentials of the D6 seam The CO₂-ECBM potential and resulting CCS capacity of the D6 seam in the Dolin Formation were calculated using Eqs. 7–9 and applying an RF of 0.5 and a CF value of 0.5 as used by Hendriks et al. [34] and Weniger et al. [81]. The areal extent of the D6 seam applying an upper depth cut-off of 800 m is estimated as 40 km². The average thickness of the seam is assumed as 5.5 m according to available mining data. Laboratory measurements yielded an average coal density of 1.37 t/m³ . Based on these parameters, the total mass of coal in the D6 seam is calculated at 301,400,000 (∼ 300 million tons) and the corresponding coal volume at 220 million m³ . According to the average supercritical CH₄ excess sorption capacity, the mean gas content of the D6 coal seam is 0.65 mmol/g (14.66 m³/t). However, significantly higher gas contents (∼ 30–32 m³/t) have been observed during coal exploitation at 600–900 m depth. The discrepancy between the theoretical and the measured gas content may be in part to free gas (e.g., in cleat systems) or to gas stored in the rocks above and below the seam. Accepting a gas content of 30 m³ /t, the initial gas in place (IGIP) is estimated as 9042,000,000 m³ (∼ 9 billion m³). With an RF of 0.5 and a CF of 0.5, the producible gas in place (PGIP), which represents the CO₂-ECBM potential of the D6 seam in the area, is calculated at 2260,500,000 m³ (∼ 2.2 billion m³). For the assessment of the resulting CCS potential by adsorption, the exchange ratio (ER) between CO₂ and CH₄ is important. The maximum Langmuir (n_L) CO₂ and CH₄ adsorption data from D6 samples (Table 4) indicate a mean ER of 2.05. However, Liu et al. [54] showed that the measurement of the exchange ratio of CO₂ and CH₄ in coal based on pure gas measurements leads to an underestimation of ER, as the effect of competitive sorption is neglected. Using mixed-gas experiments, Liu et al. [54] showed that the actual selectivity of CO₂ over CH₄ adsorption in coal is 1.5–2.5 times higher than the ratio determined based on a comparison of measurements with pure gases. Based on these findings and considering a conservative increase of 1.5 times, a modified ER value of 3.07 is used in the present study. To assess the CCS potential of the D6 seam, two approaches have been used. First, the average Langmuir sorption capacity below 800 m (39.02 kg/t [Fig. 9]; corresponding to 18.57 m³/t and 0.83 mmol/g), corrected for the moisture content of the coal, was used to calculate the adsorptive storage capacity. The moisture content of the D6 coal ranges from 0.8 to 0.9 wt% (e.g., [68]). Considering the molar mass of H₂O (18.015 g/mol), the seam contains nearly 0.5 mmol H₂O per gram of coal. Busch and Gensterblum [10] suggested that in bituminous coal, each water molecule displaces ∼0.3 molecules of CO₂. Consequently, the moisture content reduces the CO₂ adsorption capacity by 0.15 mmol/g, from 0.83 to 0.68 mmol/g (15.24 m³/t). By adopting this value in Eq. 7 (for GC) and Eq. 8 (for PGIP) and applying Eq. 9, the adsorptive CCS potential is estimated at 6.9 Mt. In the second approach, the total storage capacity below 800 m (96.64 kg/t; 48.88 m³/t, Fig. 9), including additional volume filling of pores by free, supercritical CO₂, was considered. Again, by adopting the values in Eq. 7 (for GC) and Eq. 8 (for PGIP) and using Eq. 9, the total CCS potential is estimated at 22.3 Mt. Similar storage capacities are expected for coal seams in the Karaganda Formation. This shows that CCS in deeply buried coal seams is a highly promising technique for decarbonization of hard-to-abate industry and energy sectors in Kazakhstan. 6. Conclusions The Upper Carboniferous D6 seam, a major coal resource in the Karaganda Basin, was characterized in detail with respect to bulk geochemical and compositional parameters, as well as its rank. The main study aim was then to assess its CCS and CO₂-ECBM potential by LP and HP gas adsorption tests. The most important findings regarding depositional environment, resource and storage potential are summarized below: •The D6 seam in the Lenin mine reached the medium-volatile bituminous coal rank. The D6 seam in the Lenin mine reached the medium-volatile bituminous coal rank. •Variable, but locally relatively high ash yields together with very low sulfur contents suggest deposition in a low-lying mire without a brackish/marine influence. Upward increasing fusinite contents suggest increasingly drier conditions during proceeding peat accumulation. Variable, but locally relatively high ash yields together with very low sulfur contents suggest deposition in a low-lying mire without a brackish/marine influence. Upward increasing fusinite contents suggest increasingly drier conditions during proceeding peat accumulation. •LP N₂ adsorption measurements classify the coals as mesoporous-to- macroporous with BET-SSA and BJH-SPV ranging from 1.33 to 7.55 m²/g and 0.004–0.028 cm³/g, respectively. LP N₂ adsorption measurements classify the coals as mesoporous-to- macroporous with BET-SSA and BJH-SPV ranging from 1.33 to 7.55 m²/g and 0.004–0.028 cm³/g, respectively. •The average selectivity of CO₂ over CH₄ under HP conditions was calculated at 2.05. The average selectivity of CO₂ over CH₄ under HP conditions was calculated at 2.05. •At LP conditions, CO₂ adsorption capacity increases with increasing TOC content, but decreases with increasing mineral matter content (ash yield). However, under HP conditions, the effect of TOC and ash yield on CO₂ adsorption capacity is minor. In contrast, CH₄ HP adsorption increases considerably with increasing TOC content. At LP conditions, CO₂ adsorption capacity increases with increasing TOC content, but decreases with increasing mineral matter content (ash yield). However, under HP conditions, the effect of TOC and ash yield on CO₂ adsorption capacity is minor. In contrast, CH₄ HP adsorption increases considerably with increasing TOC content. •The observed hysteresis loop at LP conditions may result from electrostatic interactions between the injected CO₂ and surface functional groups in the organic matter fraction leading to an enhanced physisorption. The observed hysteresis loop at LP conditions may result from electrostatic interactions between the injected CO₂ and surface functional groups in the organic matter fraction leading to an enhanced physisorption. •The observed hysteresis loop at HP conditions may result from CO₂ chemisorption and coal matrix swelling and shrinkage. This behavior is beneficial to storage safety as it suggests stable adsorption even in case of a reservoir pressure reduction. The observed hysteresis loop at HP conditions may result from CO₂ chemisorption and coal matrix swelling and shrinkage. This behavior is beneficial to storage safety as it suggests stable adsorption even in case of a reservoir pressure reduction. •The CBM potential of the D6 seam is estimated at 9 billion m³ initial gas in place and 2.2 billion m³ producible gas in place. The CBM potential of the D6 seam is estimated at 9 billion m³ initial gas in place and 2.2 billion m³ producible gas in place. •The total CO₂ storage capacity during CO₂-ECBM operations is estimated at 22.3 Mt. The total CO₂ storage capacity during CO₂-ECBM operations is estimated at 22.3 Mt. Using the D6 seam as an example, this study shows that coal seams in the Karaganda Basin have very high CO₂-ECBM and substantial CCS potential. Detailed reservoir development simulations need to confirm if this storage resource can be tapped by the hard-to-abate industry emitters in the region. > . The authors would like to apologise for any inconvenience caused.

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Chair of Energy Geosciences, Department of Applied Geosciences and Geophysics, Montanuniversität Leoben, Leoben, 8700, Austria
Department of Materials Science, Montanuniversität Leoben, Leoben, 8700, Austria
Energy and Mineral Resources Group (EMR), Institute of Organic Biogeochemistry in GeoSystems, RWTH Aachen University, Lochnerstraße 4-20, Aachen, 52056, Germany
Fraunhofer Research Institution for Energy Infrastructures and Geotechnologies, Aachen, 52062, Germany
School of Mining and Geosciences, Nazarbayev University, Astana, Kazakhstan
Department of Earth, Energy, and Environment, University of Calgary, 2500 University Drive NW, Calgary, T2N 1N4, AB, Canada

Chair of Energy Geosciences
Department of Materials Science
Energy and Mineral Resources Group (EMR)
Fraunhofer Research Institution for Energy Infrastructures and Geotechnologies
School of Mining and Geosciences
Department of Earth

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Corrigendum to “CO2 utilization and sequestration potential in deep coal seams: A case study on Carboniferous coals from the Karaganda Basin, Kazakhstan”(Journal of CO2 Utilization, (2025), 101, C, (103204), (S221298202500188X), 10.1016/j.jcou.2025.103204)

Corrigendum to “CO₂ utilization and sequestration potential in deep coal seams: A case study on Carboniferous coals from the Karaganda Basin, Kazakhstan”(Journal of CO2 Utilization, (2025), 101, C, (103204), (S221298202500188X), 10.1016/j.jcou.2025.103204)