Potential evaluation and favorable zone optimization of CO(2) geological sequestration in deep coal reservoirs.

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Learn more: PMC Disclaimer | PMC Copyright Notice Sci Rep . 2026 Mar 7;16:12208. doi: 10.1038/s41598-026-42680-z Search in PMC Search in PubMed View in NLM Catalog Add to search Potential evaluation and favorable zone optimization of CO 2 geological sequestration in deep coal reservoirs Zhengzheng Xue Zhengzheng Xue 1 School of Energy Science and Engineering, Henan Polytechnic University, Jiaozuo, 454003 China Find articles by Zhengzheng Xue 1 , Xiaokai Xu Xiaokai Xu 1 School of Energy Science and Engineering, Henan Polytechnic University, Jiaozuo, 454003 China Find articles by Xiaokai Xu 1, ✉ , Lin Tian Lin Tian 2 School of Resources and Environment, Henan Polytechnic University, Jiaozuo, 454003 China Find articles by Lin Tian 2 , Kuo Jian Kuo Jian 3 College of Energy and Materials Engineering, Taiyuan University of Science and Technology, Taiyuan, 030024 China Find articles by Kuo Jian 3 , Shuo Zhang Shuo Zhang 1 School of Energy Science and Engineering, Henan Polytechnic University, Jiaozuo, 454003 China Find articles by Shuo Zhang 1 , Liangwei Xu Liangwei Xu 2 School of Resources and Environment, Henan Polytechnic University, Jiaozuo, 454003 China Find articles by Liangwei Xu 2 , Jian Li Jian Li 4 School of Safety Science and Engineering, Henan Polytechnic University, Jiaozuo, 454003 China Find articles by Jian Li 4 , Zehua Zhang Zehua Zhang 1 School of Energy Science and Engineering, Henan Polytechnic University, Jiaozuo, 454003 China Find articles by Zehua Zhang 1 , Yue Xin Yue Xin 1 School of Energy Science and Engineering, Henan Polytechnic University, Jiaozuo, 454003 China Find articles by Yue Xin 1 , Yixuan Yao Yixuan Yao 1 School of Energy Science and Engineering, Henan Polytechnic University, Jiaozuo, 454003 China Find articles by Yixuan Yao 1 Author information Article notes Copyright and License information 1 School of Energy Science and Engineering, Henan Polytechnic University, Jiaozuo, 454003 China 2 School of Resources and Environment, Henan Polytechnic University, Jiaozuo, 454003 China 3 College of Energy and Materials Engineering, Taiyuan University of Science and Technology, Taiyuan, 030024 China 4 School of Safety Science and Engineering, Henan Polytechnic University, Jiaozuo, 454003 China ✉ Corresponding author. Received 2025 Dec 9; Accepted 2026 Feb 26; Collection date 2026. © The Author(s) 2026 Open Access This article is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License, which permits any non-commercial use, sharing, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if you modified the licensed material. You do not have permission under this licence to share adapted material derived from this article or parts of it. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by-nc-nd/4.0/ . PMC Copyright notice PMCID: PMC13076859  PMID: 41794932 Abstract CO 2 geological sequestration in deep, unmineable coal seams is a key technology for carbon capture, utilization, and storage (CCUS). This study focuses on the No. 3 major high-rank coal reservoir in the southern Qinshui Basin, utilizing high-pressure and high-temperature CO 2 isothermal adsorption experiments coupled with multiple adsorption models (Langmuir, BET, D–R) to establish a sequestration capacity calculation model. The study investigates CO 2 sequestration mechanisms and evaluates potential and favorable zones for sequestration. The results show that CO 2 sequestration mechanisms vary with temperature and pressure conditions. Specifically, adsorption dominates in the middle–deep subcritical zones but decreases with depth, while free-phase sequestration increases in the deep supercritical zone. Regarding model applicability, the BET model best fits supercritical CO 2 adsorption, effectively capturing the sharp adsorption increase near the critical point. Quantitative assessments indicate that the optimal sequestration depth is 800–1100 m, with a total sequestration potential of 575.5 Mt, 65.4% of which is in the deep supercritical zone. The supercritical zone’s sequestration abundance reaches 956.1 × 10 3 t/km 2 , representing a 53.5% increase over the subcritical zone. Furthermore, adsorption and free-phase sequestration account for over 99% of the total potential, while dissolution and mineralization are negligible. Based on these evaluations, two deep coal reservoir units in the northern region are identified as optimal sequestration zones, coinciding with areas of high CBM potential, which could facilitate integrated CBM development and CO 2 sequestration. This study provides a theoretical and methodological framework for evaluating CO 2 sequestration potential and identifying favorable zones in deep coal reservoirs. Keywords: CO 2 geological sequestration, Deep coal reservoir, Adsorption model, Sequestration potential, Favorable zones optimization Subject terms: Energy science and technology, Environmental sciences, Solid Earth sciences Introduction One of China’s main initiatives to improve the global atmospheric environment is reaching the targets of carbon peak and carbon neutrality (referred to as the “dual carbon” goals) 1 . The most direct and efficient way to lower carbon emissions at the moment is through Carbon Capture, Utilization, and Storage (CCUS) technology 2 , which is also one of the major technological routes to reaching the “dual carbon” goals 3 . The CO 2 geological sequestration potential is a critical indicator for CCUS technology, directly limiting its development potential and application scale 4 , 5 . Among these, sequestration capacity and geological sequestration suitability respectively represent the magnitude and quality of CO 2 geological sequestration potential, and serve as the core criteria for site selection in CO 2 geological sequestration 6 . One of the main strategies for quickly reducing CO 2 emissions in the near future is large-scale and effective CO 2 geological sequestration. Deep coal seams, utilizing robust CO 2 adsorption for sequestration, not only improve coalbed methane (CBM) extraction but also offer significant advantages in terms of high sequestration stability 7 , 8 , rendering them appropriate for extensive carbon sequestration applications 9 . Thus, they have become ideal target formations for CO 2 sequestration 10 . Upon the injection of CO 2 into coal reservoirs, a pronounced matrix swelling effect occurs during the CO 2 adsorption process 11 – 13 , resulting in a reduction of pore quantity, a diminishment in pore size, and an augmentation of curvature, which subsequently elevates fluid migration resistance and induces a gradual decrease in permeability 14 – 17 . This demonstrates its favorable sequestration potential in coal reservoirs. The CO 2 sequestration capability of a coal seam is contingent upon various elements, including coal rank and quality, seam depth, pressure, thickness, and the total surface area accessible for adsorption 18 – 20 . Additionally, the type and content of primary mineral components in the reservoir also affect the sequestration capacity of different CO 2 capture mechanisms 21 – 23 . Moreover, geochemical interactions between supercritical CO 2 and reservoir minerals can induce differential porosity evolution 24 . These variations are significantly influenced by the coal’s moisture content, ultimately promoting the expansion/development of the pore network 25 – 27 . In the southern region of the Qinshui Basin, pilot-scale experiments have demonstrated that CO 2 injection simultaneously enhances CBM production and storage capacity 28 . CO 2 injection can promote CBM recovery across a range of coal seam burial depths; however, the most notable production enhancement is generally reported for seams at intermediate depths of approximately 500–950 m under comparable development durations 29 . Numerical simulations of CO 2 -driven CBM sequestration in the Huainan coalfield show that increasing CO 2 injection pressure helps to enhance the cumulative methane yield and CO 2 sequestration during the simulation period 30 . Studies have shown that in the Shizhuang North coalbed methane experimental area, both CO 2 injection and sequestration processes primarily occur in the supercritical state, indicating that maintaining CO 2 in the supercritical state is a key factor for optimizing CO 2 geological sequestration effectiveness 31 . Under elevated pressure conditions in middle-to-deep coal seams, the progressive occupation of available adsorption sites on the coal matrix leads to a saturation state, beyond which further increases in pressure result in only marginal changes in the adsorbed CO 2 quantity. Consequently, the adsorption capacity approaches an upper limit and tends to remain nearly constant 32 . The theoretical sequestration capacity of free-phase CO 2 in deep coal seams changes with depth. By analyzing the basic data from coal sample tests in the southern sector of the Qinshui Basin, Shanxi Province, North China, the sequestration capacity for free -phase CO 2 in coal seams was calculated, showing its dependence on factors such as reservoir porosity, gas saturation, formation temperature, and formation pressure 33 . In line with the trend of deep CBM development, CBM extraction projects contribute to the engineering exploration of deep CO 2 sequestration. Once the in-situ temperature and pressure of deep coal seams exceed the critical thresholds of CO 2 (31.1 °C and 7.38 MPa, respectively), the injected CO 2 enters a supercritical phase, characterized by fluid properties intermediate between those of a gas and a liquid 34 . To achieve coal seam CO 2 geological sequestration, two conditions should be met: a sequestration carrier and a geological cap layer 5 . In areas with developed coal seams, the goaf, coal seams, and aquifers should be considered as a unified system for staged CO 2 sequestration 3 , fully considering the sequestration situation of different forms in the same carrier. Based on regional geothermal pressure and structural development conditions, a burial depth of 800 m is commonly used to determine when CO 2 reaches the supercritical state. The Qinshui Basin is characterized by a relatively simple structural framework. Within the stratigraphic succession, the sandstone units bounding the principal coal seams are laterally discontinuous yet densely compacted, whereas the associated mudstone strata exhibit greater continuity and mechanical stability, providing effective sealing capacity. As a result, the potential for CO 2 migration and leakage is considered minimal, supporting the geological security of CO 2 sequestration in deep coal reservoirs of this basin 29 . Previous studies have predominantly focused on CO 2 -ECBM applications in relatively shallow coal seams, whereas integrated assessments of CO 2 sequestration potential in deep coal reservoirs—particularly on a block-scale basis—are largely lacking. Existing studies largely focus on adsorption mechanisms or laboratory-scale observations, while integrated evaluation frameworks applicable to deep coal reservoirs are still limited. To fill this research gap, the present work targets deep coal reservoirs within the Zhengzhuang development block of the southern Qinshui Basin. Building upon previous research on CO 2 -induced alterations of coal pore–fracture systems and physical properties, the present work shifts the emphasis from mechanism-oriented analysis to sequestration-oriented evaluation. Adsorption behaviors are examined using Langmuir, BET, and Dubinin–Radushkevich (D–R) models, representing single-layer, multilayer, and micropore adsorption mechanisms, respectively, and their applicability to deep coal reservoirs is systematically compared. Based on the selected models, the theoretical CO 2 sequestration capacities associated with different storage modes are quantitatively estimated. Based on this approach, a static assessment framework is developed to evaluate CO 2 geological storage potential in deep coal seams and is further implemented at the block scale for the delineation of favorable sequestration zones. This framework provides a transferable methodological reference for assessing CO 2 sequestration potential in deep coal reservoirs and supports future applications of CO 2 geological storage and CO 2 -ECBM coupling. Geological setting General geology Northern China hosts the Qinshui Basin, a coal-rich sedimentary basin distributed across central and southern Shanxi Province. The basin is surrounded by uplift zones, and the overall structure is a large, complex anticline. Surrounded by the Taihang, Lvliang, Wutai, and Zhongtiao mountain ranges, the basin exhibits a relatively stable tectonic setting conducive to coal formation. The coal-bearing sequences were primarily developed during the Carboniferous–Permian interval, forming one of the major coal resource bases in the region. The coal ranks are primarily medium- to high-rank bituminous coal and anthracite, with generally high coalbed methane content. Across the majority of the basin, coal seams are distributed at depths of less than 1500 m, and these favorable reservoir conditions not only support the formation of large CBM fields but also provide favorable sealing conditions for CO 2 sequestration. The Zhengzhuang Block, located in the southern Qinshui Basin, is selected as the study area, and its geographic location is illustrated in Fig. 1 . Fig. 1. Open in a new tab Geographical location of the Zhengzhuang Block within the Qinshui Basin. The study area features a "horseshoe-shaped" slope structure, with gently inclined strata. The dip angles generally range from 3 to 7°. Tectonic activity is minimal, resulting in relatively simple structural deformation. Faults are underdeveloped, while fold structures are more prominent (see Fig. 2 ). The area has not experienced magmatic activity, but collapse columns are widely distributed. Coal resources are predominantly hosted within stratigraphic units of Carboniferous–Permian age, which exhibit broad regional continuity and stable geological development. The No. 3 coal seam, which constitutes the principal target horizon, occurs at depths between approximately 256 and 1337 m, with a mean burial depth of about 729 m, while seam thickness ranges from 2.2 to 7.4 m and averages 5.4 m 35 . Fig. 2. Open in a new tab Structural distribution of the principal coal seam in the study area (modified according to Zhang 36 ). The principal coal seams within the study area were formed in a deltaic plain sedimentary system, with a distribution pattern characterized by a higher southern region and a lower northern region. The coal seams are vertically enclosed by compact and mechanically competent mudstone units deposited in a marine–continental transitional deltaic environment, with individual layers commonly exceeding 10 m in thickness. The stratigraphic sequence is dominated by mechanically stable quartz-rich sandstones, interbedded with dark gray to nearly black mudstones and coal horizons exhibiting variable thickness (see Fig. 3 ). Fig. 3. Open in a new tab Geological stratigraphy and aquifer subdivision of the study region. The presence of unconformities is denoted by dashed as well as wavy line symbols. (modified according to Zhang 36 ). Among the deep coal seams that are unsuitable for mining in the southern Qinshui Basin, the No. 3 seam was selected as the target reservoir. It represents the principal contributor to regional deep coal resources, accounting for more than 80% of the total, and is characterized by relatively uniform burial depth, coalification degree, and seam thickness across the study area. Existing geological and coal petrological analyses indicate that the No.3 coal seam shares key compositional and structural attributes—such as maceral composition, pore architecture, and maturity level—with other deep, high-rank coal seams across the region, thereby supporting the similarity of their CO 2 adsorption responses. Notably, the present study focuses on a single representative coal seam due to experimental constraints, and therefore the conclusions are primarily applicable to deep high-rank coal reservoirs with similar geological conditions. Future studies will incorporate multiple coal seams and samples from different regions to further validate and generalize the proposed framework. Hydrogeological characteristics From shallow to deep, five major aquifer units can be identified within the study area. These include the Quaternary aquifer, the Shihezi Formation sandstone fracture aquifer, the Shanxi Formation sandstone fracture aquifer, the Taiyuan Formation limestone fracture aquifer, and the Ordovician limestone karst fracture aquifer 37 . The individual aquifer units are hydraulically isolated by intervening mudstone and bauxite-bearing mudstone layers. Two principal regional aquitards are recognized: one developed within the upper and lower members of the Shihezi Formation, and another associated with the lower Taiyuan Formation and the underlying Benxi Formation. The No.3 coal seams are located within the Shanxi Formation sandstone fracture aquifer 38 , with sandstone aquifers both above and below. The coal seams primarily establish hydrodynamic connections with the overlying sandstone aquifer through macroscopic fractures and joints. Groundwater flows along the bedding planes but has weak vertical flow capacity 39 , and the underlying sandstone aquifer is effectively isolated from the coal-bearing strata by a laterally continuous mudstone unit with substantial thickness. Therefore, the Shanxi Formation sandstone aquifer system where the coal seams are located is isolated from other aquifers and forms an independent hydrological unit, reducing the risk of CO 2 lateral migration. Sealing structures, exemplified by the Sitou Fault, display limited hydraulic transmissivity, while the northern segment of the eastern Jinhuo fault belt functions as an effective lateral groundwater barrier, while the southern section has very weak groundwater flow. In the southern part of the western boundary, only a limited number of faults exhibit significant water-conducting behavior, but their impact is limited, creating a stable hydrogeological environment. Definition of deep coal reservoir According to previous studies 33 , the dominant coal seams are marked by strong lateral variations in burial depth and display non-uniform, elongated distribution patterns across the study area. Depth measurements indicate that burial depths extend from 255.5 m up to 1337 m, with an overall average of 728.7 m. Overall, there is a trend of decreasing depth from the southeast to the northwest. The constant temperature zone in the study area is 9 °C at a depth of 20 m, with an average geothermal gradient of 3.53 °C/100 m and a pressure coefficient of 0.91 MPa/100 m. In this study, the term “deep coal reservoir” is defined operationally based on the thermodynamic state of CO 2 under in-situ conditions, rather than by burial depth alone. This is because CO 2 sequestration behavior in coal is strongly influenced by whether CO 2 is present in a supercritical state, which alters its density and transport properties and, consequently, affects storage mechanisms in the coal matrix. Using the CO 2 critical conditions together with the local geothermal and pressure gradients, the onset of supercritical CO 2 is estimated to occur at approximately 800 m depth in the study area. However, this depth is not treated as a rigid threshold, because spatial variability in the geothermal gradient and pressure coefficient can shift the supercritical transition by several tens to more than one hundred meters. Therefore, we adopt 800 m as a practical reference depth to delineate “deep” conditions for subsequent capacity evaluation and favorable-zone optimization in this specific study area, while acknowledging the associated uncertainty. Importantly, the tectonically stable setting suggests that coal seams below this reference depth can maintain effective sealing and experience limited lateral groundwater disturbance, providing a comparatively secure geological context for deep CO 2 sequestration. Experimental methods Coal sample preparation and physical property test Physical characterization was conducted on a representative coal sample collected from the Zhengzhuang block of the southern Qinshui Basin, from the No. 3 coal seam at a burial depth of approximately 900 m. The sample was then ground and sieved to obtain 60–80 mesh coal powders for subsequent CO 2 adsorption experiments and sequestration-capacity analyses (Fig. 4 ). The laboratory adsorption results from this sample are used to constrain adsorption behavior and model parameters, whereas the regional-scale evaluation integrates spatial datasets (e.g., depth/pressure/temperature and coal seam thickness) to account for basin-scale heterogeneity across the study area. Fig. 4. Open in a new tab Representative coal specimens and prepared experimental samples were obtained for analysis: ( a ) Raw coal sample; ( b ) Pulverized coal sample. Proximate analysis and vitrinite reflectance determinations were conducted to characterize the fundamental physical properties of the coal samples, with the corresponding results summarized in Table 1 . Table 1. Proximate analytical results and vitrinite reflectance data of the No. 3 coal. M ad , Moisture on an air-dry basis; A ad , Ash on an air-dry basis; V ad , Volatile on an air-dry basis; FC ad , Fixed carbon on an air-dry basis; R o ,max , vitrinite reflectance. Coal samples Moisture M ad (%) Ash A ad (%) Volatile V ad (%) Fixed carbon FC ad (%) Maximum vitrinite reflectance R o ,max (%) 3 # 1.29 8.65 6.26 85.6 3.9320 Open in a new tab To examine the microscopic pore structure of the experimental coal samples from the study area, mercury intrusion porosimetry (MIP) and low-temperature nitrogen adsorption (LTNA) experiments were conducted to characterize the pore parameters of the coal. The derived pore structure parameters are summarized in Table 2 . Table 2. Pore structure characterization of the No. 3 coal based on MIP and LTNA. Coal samples Sample porosity(MIP) (%) Total pore volume(MIP) (cm 3 /g) Micropore volume(LTNA) (cm 3 /g) Specific surface area of micropore(LTNA) (m 2 /g) 3 # 4.67 0.054 0.04 127.33 Open in a new tab The metamorphic rank significantly influences the evolution of the pore structure and CO 2 sequestration mechanisms. As coal rank increases, the pore network typically transitions toward micropore dominance, thereby establishing adsorption as the primary storage mode. As indicated in Tables 1 and 2 , the coal specimens collected from the investigated area exhibit a high degree of coalification, with the maximum vitrinite reflectance ( R o ,max ) reaching 3.93%, classifying them as typical high-rank anthracite. Experimental results indicate that, although the samples exhibit relatively small total pore volumes and low reservoir porosity, the pore architecture is predominantly controlled by micropores. This aligns with the evolutionary pattern of high-rank coal, characterized by developed micropores and abundant adsorption sites. This dense micropore structure endows the coal reservoirs with strong adsorption capacity, serving as the primary carrier for long-term sequestration; however, the limited development of mesopores and macropores results in increased fluid migration resistance. Therefore, despite the considerable theoretical potential for adsorption-based storage, effective field-scale application depends on exploiting the high density of supercritical CO 2 together with elevated injection pressures to improve displacement and pore-filling performance, thereby mitigating constraints imposed by limited effective storage volume and low permeability. Experimental system and scheme Geological CO 2 sequestration mainly occurs through four forms: adsorption, free-phase sequestration, dissolution, and mineralization 3 . The sequestration characteristics and capacity of coal for CO 2 vary significantly under different temperatures. Therefore, this study concentrates on the adsorption characteristics of high-rank coal for CO 2 by conducting isothermal adsorption experiments. The experimental work is primarily based on the critical temperature of CO 2 , investigating the CO 2 adsorption and sequestration characteristics under both supercritical and subcritical conditions. The specific experimental plan is as follows: The semi-automatic single-component high-pressure isothermal gas adsorption experimental setup consists of a FINESORB-3120 high-pressure isothermal adsorption instrument, a booster pump, a high-pressure gas supply tank, and a computer detection and control system (Fig. 5 ). Based on a comprehensive investigation of the geothermal gradient, reservoir pressure gradient, and isothermal zone temperature in the study area, three temperatures of 20, 30, and 40 °C—spaced at 10 °C intervals around the critical temperature of CO 2 (31.1 °C)—were selected to represent the temperature conditions at burial depths of 300, 600, and 900 m, respectively, for CO 2 isothermal adsorption experiments. The experiments were designed to explore the adsorption and storage characteristics of CO 2 in high-rank coal reservoirs under different reservoir pressures and temperatures, thereby characterizing their adsorption capacity. The experimental pressure range was 0–20 MPa, with an equilibrium point set approximately every 1 MPa. The adsorption amount at each equilibrium point was measured, and the cumulative adsorption at all points was calculated to compare and elucidate the temperature-dependent adsorption characteristics of CO 2 . Fig. 5. Open in a new tab The schematic diagram of isothermal experimental system. Experimental results and analysis The results of the CO 2 isothermal adsorption experiments on coal at 20 °C, 30 °C, and 40 °C are shown in Fig. 6 . Fig. 6. Open in a new tab Relationship between CO 2 isothermal adsorption capacity and pressure (Experimental temperatures: 20, 30, and 40 °C). As shown in Fig. 6 , at 20 °C, 30 °C, and 40 °C, the excess adsorption of CO 2 exhibits similar evolutionary characteristics with changing pressure. Overall, the amount of CO 2 adsorbed increases sharply with rising pressure at the initial stage, followed by a gradual decline and eventual stabilization. Specifically, within the initial low-pressure adsorption regime, the amount of CO 2 adsorbed rises rapidly as pressure increases, but the rate of increase gradually slows as pressure rises, reaching the highest adsorption value near the supercritical pressure. After reaching the peak adsorption value, the curve no longer rises and instead begins to decrease and fluctuate, with the initial decline after the peak being the most significant. At identical pressure levels, increasing the temperature from 20 to 40 °C results in a systematic reduction in excess CO 2 adsorption, suggesting that elevated temperatures inhibit the adsorption process. According to classical theories such as monolayer adsorption theory, adsorption potential theory, and Gibbs excess adsorption 40 , under conditions where gas condensation does not occur, the adsorbate’s adsorption amount should continuously increase with pressure until it reaches its maximum adsorption capacity 41 , which is the absolute adsorption amount. However, the experimentally observed values are excess adsorption amounts, and a decrease in the curve is observed in the high-pressure section, which is governed by the coupled influence of pressure and temperature on CO 2 density variations 42 . Based on adsorption intensity, the coal surface can be subdivided into regions exhibiting strong adsorption and those dominated by weaker adsorption. The region within one molecular layer from the coal surface is identified as the strong adsorption region, while areas greater than one molecular layer are defined as the weak adsorption region. A schematic illustration of gas adsorption is shown in Fig. 7 . Within the strong adsorption regime, gas uptake is predominantly governed by the interaction intensity between the gas molecules and the solid matrix, whereas in the weak adsorption regime, adsorption behavior is mainly controlled by pressure. During the gas molecule adsorption process on the solid surface, the strong adsorption region is filled first, followed by the filling of the weak adsorption region. In the strong adsorption region, the gas density is high, and the adsorption amount changes little with variations in temperature and pressure. Within the weak adsorption regime, the density of the adsorbed CO 2 phase approaches that of the free gas. Under such conditions, variations in temperature and pressure exert a pronounced influence on the density of the adsorbed phase, which in turn results in marked fluctuations in the overall adsorption capacity. Fig. 7. Open in a new tab Gas adsorption diagram of coal surface ( modified according to Wu 41 ). The absolute gas adsorption capacity V per unit area of coal pore surface is calculated by the following equation 1 where δ is the adsorption layer thickness and ρ ads is the adsorption gas density. According to the definition of excess adsorption (Fig. 7 ), the excess adsorption amount, V ex , can be expressed as: 2 where ρ g is the free-phase CO 2 density. Combining Eqs. 1 and 2 yields Eq. 3 . 3 As shown in Fig. 8 , The pressure-dependent evolution of CO 2 density demonstrates a distinct two-stage behavior, characterized by: in the initial low-pressure stage, the density increases rapidly with pressure, while in the later high-pressure stage, the change in density with pressure becomes gradual 43 . Within the low-pressure regime, the density variation in the strong adsorption region is minimal, while the density in the weak adsorption region increases with pressure. Compared with the adsorption region, the free region is characterized by a lower density of free-phase CO 2 . At this point, based on the analysis of Eq. 3 , it can be concluded that the excess adsorption capacity is approximately equal to the true adsorption capacity, specifically indicating that the adsorption capacity in the low-pressure region increases with the pressure. As pressure increases and surpasses the CO 2 supercritical pressure threshold, entering the high-pressure regime, the density in the strong adsorption regime also increases slowly with pressure. The density in the weak adsorption region increases, and the gap between it and the free-phase CO 2 density gradually narrows. Due to the weakened binding capacity of supercritical gas with the solid phase, the number of adsorption layers (δ) decreases, the growth rate of the adsorption region density slows down, and the gap between the free-phase gas density in the free region and the adsorption region density narrows. Based on the analysis of Eqs. 3 , it can be concluded that the excess adsorption capacity either decreases or remains stable. At this point, the excess adsorption capacity is less than the true adsorption capacity, and as shown in Fig. 6 , this is represented as a decline in adsorption capacity after the peak is reached, stabilizing at a lower value. Fig. 8. Open in a new tab Pressure dependence of CO 2 density. Isothermal adsorption experiments indicate that temperature significantly affects CO 2 adsorption. At the three temperature gradients of 20 °C, 30 °C, and 40 °C, as the temperature increases, the CO 2 adsorption capacity shows a decreasing trend, and the pressure required to reach the adsorption peak also increases accordingly. At the same pressure range, the rate of increase in adsorption with pressure decreases as the temperature rises. Model development Model construction of different sequestration mechanisms Adsorption sequestration model fitting and optimization Adsorption onto the coal matrix surface represents the principal mode of CO 2 storage in coal seam sequestration systems. Consequently, the accurate estimation of CO 2 sequestration capacity critically depends on the selection of an appropriate adsorption model. This study systematically explores three classical adsorption theory frameworks for the coal-CO 2 adsorption system: the Langmuir model founded on a homogeneous monolayer adsorption assumption, the BET model representing multilayer adsorption mechanisms, and the D–R model derived from micropore filling and adsorption potential concepts. By analyzing the degree of fit between these models and isothermal adsorption data under laboratory-simulated formation conditions, the model with the highest fit to the experimental data is determined, thus selecting the appropriate adsorption model for calculating the CO 2 injection capacity in deep coal reservoirs. The Langmuir isotherm adsorption model is a classical model that describes the monolayer adsorption behavior at the gas–solid interface. It has a solid theoretical foundation and describes the dynamic equilibrium process of monolayer adsorption on a homogeneous surface. This model is well suited for simplified representations of monolayer adsorption and has been extensively applied in studies of CBM and CO 2 geological sequestration 44 . 4 where V is the excess adsorption amount (cm 3 /g); P L is the Langmuir pressure (MPa); V L is the Langmuir volume(cm 3 /g) and p is the equilibrium pressure (MPa). The BET adsorption theory (Brunauer–Emmett–Teller, 1938) is a classical model that characterizes multilayer adsorption behavior. By establishing a multilayer adsorption mechanism, this theory overcomes the theoretical framework of Langmuir’s monolayer adsorption, and serves as a robust tool for characterizing gas physical adsorption under medium-to-high pressure conditions. 5 where V m is the monolayer saturation capacity (cm 2 /g) and C is the constant related to the adsorption heat and adsorbate and P 0 is the saturation vapor pressure (MPa). Where 6 P c is the CO 2 critical pressure, with a value of 7.38 MPa; T c is the CO 2 critical temperature, with a value of 304.25 K and T is the experimental temperature (K). The D-R adsorption theory (Dubinin-Radushkevich, 1947) is a classical theoretical model that describes the micropore filling adsorption behavior. This theory, through adsorption potential theory and the micropore filling mechanism, addresses the problem of characterizing the adsorption of supercritical gases in microporous materials. It is particularly suitable for the adsorption behavior of supercritical gases in microporous media, such as coal. 7 where V 0 is the micropore volume (cm 3 g) and D is the constant related to the net adsorption heat. Based on the excess adsorption data of CO 2 at 20 °C, 30 °C, and 40 °C, the results of the systematic fitting of the Langmuir, BET, and D-R theoretical models are presented in Fig. 9 . Fig. 9. Open in a new tab Fitting results of the experimental data: ( a ) Langmuir model fitting; ( b )BET model fitting; ( c )D-R model fitting. Table 3 presents the fitting degrees of the three models at different temperatures, quantified by the coefficient of determination ( R 2 ), where higher values indicate a better goodness of fit between the experimental data and model predictions. Table 3. At 20 ℃, 30 ℃, 40 ℃ adsorption model fitting results table. temperature \ model Langmuir model ( R 2 ) BET model ( R 2 ) D-R model ( R 2 ) 20℃ 0.98978 0.99607 0.96457 30℃ 0.97873 0.9787 0.95872 40℃ 0.973 0.97498 0.89483 Open in a new tab From the fitting results, it can be concluded that the fitting degree, as quantified by the coefficient of determination ( R 2 ), of the Langmuir, BET, and D–R models to the adsorption data decreases with increasing temperature, with the highest R 2 values observed for the adsorption curves at 20 °C, indicating the best overall fitting performance at this temperature. As shown in Fig. 9 , in the low-pressure region, all three models exhibit good adaptability. However, at elevated pressures where CO 2 transitions into the supercritical state, gas–solid interaction strength weakens, the effective number of adsorbed molecular layers on coal pore surfaces decreases, and the density of the free phase progressively approaches that of the adsorbed phase. This results in a significant change in the adsorption behavior, leading to a noticeable decline in the model fitting performance. The adsorption behavior of CO 2 on coal matrix surfaces exhibits pronounced multilayer adsorption characteristics at elevated relative pressures. Using the Langmuir model, which is based on monolayer adsorption, or the D-R model, based on micropore filling theory, cannot fully and accurately describe CO 2 adsorption behavior across the full pressure range. Although, as discussed in Section " Experimental results and analysis ", the decline in excess adsorption after the peak is primarily attributed to the rapid increase in free-phase density under supercritical conditions, within the scope of engineering calculations, the BET model—by incorporating a multilayer adsorption mechanism—can mathematically reproduce these high-pressure adsorption characteristics more accurately than the Langmuir monolayer model. This provides a more accurate empirical formula for estimating sequestration potential. From the perspective of theoretical suitability, the BET equation framework accounts for the multilayer physical adsorption process, making it more applicable for characterizing such complex adsorption behaviors in coal reservoirs. Particularly under supercritical conditions, treating the BET equation as a semi-empirical model effectively covers the behavior across the entire pressure range, from micropore filling at low pressures to liquid-like compression at high pressures. Consequently, this paper adopts the BET model to characterize the CO 2 adsorption in the study area. Calculation model of free-phase sequestration capacity When calculating the free-phase CO 2 sequestration amount, it is important to consider that the adsorbed CO 2 occupies part of the pore space on the pore walls, reducing the space that would otherwise be occupied by free-phase gas, thereby affecting the prediction of free-phase CO 2 sequestration. Considering the impact of adsorbed gas occupying various pore spaces, the reservoir porosity occupied by free-phase CO 2 can be evaluated based on the following Eq. 45 : 8 In the equation, ϕ f is the reservoir porosity occupied by free-phase CO 2 ; ϕ i is the reservoir porosity; ϕ ad is the reservoir porosity occupied by adsorbed CO 2 , and the calculation formula is: 9 where V ad is the adsorbed gas content in the reservoir (cm 3 /g); ρ c is the coal density (g/cm 3 ); ρ g,STP is the free-phase CO 2 density at standard conditions (0.001977 g/cm 3 ); ρ ad is the density of adsorbed methane in the reservoir (g/cm 3 ), It is an empirical formula calculation based on the adsorption experiment of carbon molecular sieve 46 . 10 In the equation, ρ b is the boiling point density of CO 2 at atmospheric pressure (0.00281 g/cm 3 ;) T b is the boiling point temperature of CO 2 at atmospheric pressure (194.65 K) and C 1 is the thermal expansion coefficient, in 1/T. The volume of free-phase CO 2 can be calculated using Eq. 11 : 11 where ρ c is the apparent coal density (g/cm 3 ); S w is the water saturation (%) and B g is the ratio of the gas volume under formation conditions to the volume at standard conditions, dimensionless. B g can be calculated using Eq. 12 : 12 where Z is the CO 2 compressibility factor under specific temperature and pressure conditions, which can be calculated using the NIST REFPROP fluid property software; p STP is the pressure at standard conditions (0.101325 MPa); T STP is the temperature at standard conditions (273.15 K); T is the reservoir temperature (K); p is the reservoir pressure (MPa) and Z 0 is the gas compressibility factor at standard conditions, with a value of 1. The volume of free-phase CO 2 can be calculated using the formula derived from Eqs. ( 8 – 12 ): 13 Calculation model of dissolution sequestration capacity In coal seams, the CO 2 dissolution sequestration capacity is defined as the volume of CO 2 that is dissolved in pore and fracture water throughout the sequestration process. The dissolution-based sequestration capacity of CO 2 is governed by several reservoir parameters, including the solubility of CO 2 in formation water, coal reservoir porosity, water saturation, and coal density. Among these factors, the solubility of CO 2 in pore water is mainly controlled by thermodynamic conditions, particularly temperature, pressure, and the degree of water mineralization 47 . Previous studies have conducted numerous CO 2 dissolution experiments under various temperature, pressure, and mineralization conditions. By analyzing the experimental and model calculation results from prior research 48 , the CO 2 solubility in the study area was obtained. The water-soluble gas content in a unit mass of coal can be calculated using the following equation: 14 where V s is the water-soluble gas content (cm 3 /g); S w is the water saturation of the coal reservoir (%) and S CO2 is the solubility of CO 2 in the coal seam water (mol/L). Total sequestration calculation model In coal seam CO 2 geological sequestration, adsorbed and free-phase CO 2 together constitute the core storage mechanisms. Adsorbed CO 2 determines the upper bound and long-term storage potential, typically accounting for more than 80% of the total sequestration capacity, whereas free-phase CO 2 primarily controls injectivity and short-term storage dynamics. Both mechanisms are quantifiable and operationally relevant, and thus represent the key storage components in current coal seam CO 2 sequestration projects. Compared with other sequestration mechanisms, the amount of CO 2 stored in dissolved form is limited by its relatively low solubility in coal seam water. Simulation results further show that this storage component typically represents less than 1% of the overall sequestration capacity 38 , rendering its overall impact minor. Mineral trapping is commonly neglected in engineering-scale evaluations due to several limiting factors 49 : (1) the generally low mineral content of coal seams—specifically, the No. 3 coal seam in the southern Qinshui Basin possesses limited mineral sources available for geochemical reactions; (2) extremely slow geochemical reaction kinetics, with characteristic timescales on the order of millions of years; (3) the small fraction of CO 2 that can realistically participate in mineralization, with studies indicating that mineralized CO 2 accounts for only about 18% of the dissolved fraction, which itself is limited 50 ; and (4) pronounced mineralogical heterogeneity within coal, such that mineral assemblages and abundances can vary substantially even at comparable coal ranks and burial depths, rendering quantitative estimation of mineral trapping highly uncertain. Consequently, mineral trapping contributes negligibly to short- to medium-term CO 2 storage assessments 50 – 52 and is therefore commonly excluded from engineering-scale evaluations of coal seam CO 2 geological sequestration capacity. Accordingly, this study evaluates CO 2 sequestration capacity by explicitly quantifying the dominant and engineering-relevant storage mechanisms, namely adsorption, free-phase, and dissolution sequestration, using a static capacity assessment framework. The analysis is conducted under the assumptions of local thermodynamic equilibrium, laterally averaged coal reservoir properties within each evaluation unit, and pressure–temperature conditions primarily governed by burial depth. For free-phase storage, effective porosity is assumed to be constant within each depth interval, and adsorption-induced coal matrix swelling and associated permeability evolution are not explicitly coupled, as the objective is to provide a screening-level and upper-bound estimation of sequestration capacity rather than to resolve dynamic injection or flow behavior. These assumptions are considered appropriate for deep coal reservoirs under supercritical CO 2 conditions, where sequestration capacity is predominantly controlled by depth-dependent pressure–temperature regimes and CO 2 phase behavior, while mineral trapping mechanisms contribute only marginally within engineering-relevant timescales. Therefore, this study focuses on calculating the total CO 2 sequestration capacity based on the three mechanisms that play a decisive role in sequestration potential and engineering implementation—free, adsorbed, and dissolved states. The total inventory can be obtained by Eq. 15 . 15 where V t is the CO 2 sequestration capacity per unit mass of coal (cm 3 /g). By combining Eqs. 5 , 13 , and 14 , Eq. 16 can be derived. 16 Accordingly, the total sequestration capacity can be calculated by applying the derived Eq. 15 to the geological model of the study area. The proposed CO 2 sequestration capacity model is a static, algebraic framework and is therefore sensitive to key parameters such as porosity, water saturation, coal density, adsorption density, and the compressibility factor (Z) 53 , which directly control pore volume and supercritical CO 2 properties. Representative average values are adopted at the block scale, inevitably smoothing local heterogeneity and introducing uncertainty, particularly near the critical pressure–temperature transition. Accordingly, the model is intended for screening-level and comparative evaluation of sequestration potential rather than precise prediction of site-specific injection performance. Discussion Construction of geological model in the study zone The evaluation of CO 2 geological sequestration potential in coal seams is fundamentally based on geological conditions. The more detailed the geological characterization, the more accurate the constructed geological model. By extracting and simplifying key geological model parameters, a sequestration model of the study area can be developed. Based on a systematic classification and summary of the geological and petrophysical characteristics of the coal reservoirs, a parameter system for calculating CO 2 sequestration capacity under different sequestration states within the region is established. According to previous studies 33 , The study area exhibits a geothermal baseline of 9 °C at 20 m depth, with an average geothermal gradient of 3.53 °C/100 m and a pressure gradient of 0.91 MPa/100 m. The burial depth of the main coal seam varies considerably, ranging from 255.5 to 1337 m, with an average depth of 728.7 m, and exhibits an overall trend of thinning toward the southeast and thickening toward the northwest, forming an irregular belt-like distribution. Permeability within the primary coal seam is relatively homogeneous, typically exceeding 0.01 mD, with a mean value of approximately 0.16 mD. The gas saturation ranges from 0.02 to 1.04%, with an average of 0.54%, indicating that most reservoirs are undersaturated. The strata in the study area are relatively stable and flat, with dips generally between 3 and 7°. For the purpose of calculation, an average dip angle of 5° was adopted. Considering the background information of the study area and the established geological parameters, the relationship between coal seam burial depth and reservoir pressure and temperature can be expressed as follows: 17 where P is the reservoir pressure at a given burial depth (MPa), and H is the coal seam burial depth (m). 18 where T is the reservoir temperature at a given burial depth (K), and H is the coal seam burial depth (m). Based on the calculated geothermal and pressure gradients, the reservoir temperature at a burial depth of 650 m is approximately 31.1 °C, which has already reached the critical temperature. However, the reservoir pressure at this depth has not yet reached the critical pressure (7.38 MPa). When the burial depth of the coal seam reaches 800 m, the in-situ temperature–pressure conditions meet the CO 2 critical point. Therefore, a subcritical adsorption–sequestration model is applied for coal seams shallower than 800 m, while a supercritical adsorption–sequestration model is adopted for coal seams deeper than 800 m. The position of the temperature–pressure gradient of the study area in the CO 2 phase diagram is shown in Fig. 10 . Fig. 10. Open in a new tab CO 2 three-phase diagram (red solid line is the temperature and pressure change curve of the study area). Evaluation of sequestration potential in the study area By substituting the geological model parameters of the study area (Table 4 ) into Eq. 15 , the CO 2 sequestration capacity per unit mass of coal seam can be calculated, and the corresponding results are presented in Fig. 11 . Table 4. CO 2 sequestration geological model parameters. Parameter Subcritical adsorption sequestration Supercritical adsorption sequestration Buried depth(m) 300–800 m 800–1300 m Pressure gradient(MPa/100 m) 0.91 Reservoir pressure(MPa) 2.8–7.3 7.3–11.9 Temperature gradient(℃/100 m) 3.53 Reservoir porosity (%) 4.84 4.22 Average moisture content (%) 1.42 1.31 Solubility ( mol/L) 1.13 1.17 Pore volume of mercury intrusion method ( cm 3 /g) 0.0384 0.0325 Pore volume of low temperature CO 2 adsorption method ( cm 3 /g) 0.076 0.069 Apparent density(g/cm 3 ) 1.26 1.3 Average coal thickness(m) 5 5.5 Adsorptive capacity(cm 3 /g) 38.5 25.55 Area(km 2 ) 319.36 393.66 Open in a new tab Fig. 11. Open in a new tab Comparison of sequestration potential of different CO 2 storage forms (relationship between sequestration amount and depth variation). As shown in Fig. 11 , the total CO 2 sequestration capacity per unit mass of coal in the study area exhibits an exponential growth trend with burial depth. In the subcritical zone (shallower burial depths), the growth rate of sequestration capacity remains relatively moderate; however, once the supercritical zone is reached, the growth rate increases significantly, with the sequestration capacity per unit depth rising approximately 50% faster than in the subcritical zone. From the perspective of different sequestration mechanisms, adsorption sequestration is highly sensitive to pressure variations at shallower depths, where it increases rapidly with burial depth before stabilizing and becoming nearly unaffected at greater depths. Dissolution sequestration contributes only a minor fraction of the total sequestration (less than 1%) and is essentially independent of burial depth. free-phase sequestration, however, is the dominant component driving the increase in total sequestration capacity at greater depths, exhibiting an approximately linear growth trend with burial depth. Notably, the growth rate of free-phase sequestration increases significantly once the critical depth of approximately 800 m is reached, but begins to decelerate beyond approximately 1100 m. This change in growth behavior within the supercritical zone (acceleration after 800 m and deceleration beyond 1100 m) is primarily attributed to the phase characteristics of CO 2 : depths around 1100 m correspond to the transition from gas-like supercritical CO 2 to liquid-like supercritical CO 2 54 , 55 , resulting in variations in the state and sequestration capacity of free CO 2 . Overall, the total sequestration curve is controlled by adsorption sequestration at shallower depths, where sequestration capacity increases rapidly. Beyond this stage, free-phase sequestration becomes the main driver: between 300–800 m the increase is slow, between 800 and 1100 m it is rapid, while between 1100 and 1300 m the growth rate declines, with a tendency toward stabilization at greater depths. Based on the calculation method of coalbed geological reserves, the CO 2 geological sequestration capacity of the main coal seams in the study area can be estimated, as shown in Eq. 19 . 19 where M is the theoretical sequestration capacity (m 3 ); S is the gas-bearing area of the main coal seams in the study area (km 2 ); θ is the dip angle of the coal reservoir and h is the average coal seam thickness (m). Based on the structural outline of the main coal seams and CO 2 sequestration geological model parameters in the study area, the sequestration range of the study area is shown in Fig. 12 . The subcritical sequestration zone is located in the southwestern part of the study area, while the supercritical sequestration zone is located in the northern and southern parts, with structural closure serving as the boundary. In the study area, faults such as the Sitou Fault, which act as sealing faults, have poor water conductivity. The northern section of the eastern Jinhuo fault zone exhibits lateral water-blocking characteristics, while the southern section has very weak groundwater flow. Only a few water-conducting faults exist in the southern section of the western boundary, with limited influence. These features form multiple barriers that effectively limit the diffusion of CO 2 and provide a stable hydrogeological environment for CO 2 geological sequestration. The evaluation results of the study area indicate that the total theoretical CO 2 sequestration capacity of the main coal seams is 575.5 million tons. Of this, the supercritical zone accounts for the majority of the sequestration capacity (376.3 million tons, 65.4%), which is significantly higher than that of the subcritical zone (199.2 million tons, 34.6%). Fig. 12. Open in a new tab Schematic diagram of the sequestration range in the study area. The dark green area represents the subcritical sequestration zone, while the brown area represents the supercritical sequestration zone. The calculations show that the theoretical CO 2 sequestration abundance (i.e., the storage capacity per unit area) in the subcritical and supercritical zones of the study area are 623.2 × 10 3 t /km 2 and 956.1 × 10 3 t /km 2 , respectively. In each cubic meter of coal in the subcritical and supercritical zones, 113 kg and 173 kg of CO 2 can be fixed, respectively. This indicates that the deep coal reservoirs in the study area have a higher CO 2 geological sequestration potential compared to the shallower coal seams. From the sequestration depth distribution shown in Fig. 11 , it is evident that the sequestration amount increases with burial depth. Adsorption and free-phase sequestration together account for over 99% of the total CO 2 sequestration, while dissolution sequestration has a very limited potential. As the burial depth of the coal seams increases, the adsorption sequestration amount tends to stabilize after reaching a certain depth, while the free-phase sequestration continues to rise. As a result, the contribution of adsorption-based sequestration decreases, whereas the proportion of free-phase sequestration increases. Within the burial depth interval of 300–800 m, the total CO 2 sequestration capacity shows a gradual upward trend. After reaching the critical depth of 800 m, the sequestration rate increases rapidly in the 800–1100 m range. Between 1100 and 1300 m, the sequestration rate slows down again, and the amount tends to stabilize as the depth increases further. Therefore, for CO 2 sequestration in the study area, considering both sequestration efficiency and economic benefits, it is recommended to prioritize the 800–1100 m depth range, as it offers significant sequestration potential and rapid gains. For coal seams deeper than 1300 m, the increase in sequestration is limited, and the engineering difficulty increases, making further depth extension less economically viable. Division and evaluation of favorable sequestration areas The CO 2 theoretical sequestration abundance in the supercritical zone of the study area is relatively high. In the actual geological sequestration process, geological structures also have a significant impact on sequestration effectiveness. Among these, faults play a particularly complex and critical role. Faults can potentially damage the integrity of the cap rock, becoming leakage pathways, leading to the escape of stored gases, sequestration failure, and potentially causing environmental issues. Folds, on the other hand, are typically favorable for sequestration, providing natural trap structures, increasing sequestration space, and offering a stable structural environment. This study primarily divides the study area into favorable zones based on sequestration state and geological structural distribution. Based on different sequestration states, and considering the spatial distribution of faults and folds, the study area is categorized into favorable, relatively favorable, and potential favorable zones. Folds and block boundaries serve as boundaries, and the area is divided according to the concentration of faults. The distribution of these zones is shown in Fig. 13 . Fig. 13. Open in a new tab Evaluation results of favorable zone for CO 2 sequestration in the study area. Units I and II are located in the northern supercritical zone of the study area, with the fold in the central part of the block serving as the boundary. Due to the relatively high theoretical CO 2 sequestration abundance in the supercritical zone and the relatively few faults in the region, the geological conditions for sequestration are favorable. Therefore, these units are selected as favorable CO 2 geological sequestration zones for the block. Units IV and V are located in the central subcritical zone of the study area, with the fold in the central part of the block serving as the boundary. Although the theoretical CO 2 sequestration abundance in the subcritical zone is not as high, the central region of the block is structurally stable, with fewer faults. These units have greater sequestration potential than the potential favorable zones and are thus selected as relatively favorable CO 2 geological sequestration zones for the block. Units III, VI, and VII are located in the southern and northeastern parts of the study area. Unit VI is in the subcritical zone, while Units III and VII are in the supercritical zone. These three units have relatively more faults, which are not conducive to forming trapping conditions and may lead to gas leakage, making CO 2 sequestration unfavorable. Therefore, these units are designated as potential favorable CO 2 geological sequestration zones for the block. In summary, the favorable zones include Units I and II; the relatively favorable zones include Units IV and V; and the potential favorable zones include Units III, VI, and VII. Therefore, the CO 2 geological sequestration favorable zones should focus on the structurally stable areas within Units I and II, which are located in the supercritical sequestration zone. Feasibility analysis of CO 2 sequestration based on coalbed methane development The location of the optimal favorable zone aligns with the areas of high enrichment and production potential for CBM development in the middle-deep coal seams of the block. This alignment facilitates the future implementation of CO 2 geological sequestration in coal reservoirs based on CBM development projects. Combining CO 2 geological sequestration with deep coalbed methane development projects, and implementing CO 2 -ECBM 56 , is one of the most realistic and economically feasible technological pathways 57 . The comprehensive evaluation in this study indicates that this technological pathway has high feasibility in the study area, as evidenced by the following aspects: Technical Mechanism Synergy: The adsorption capacity of CO 2 on the coal matrix is stronger than that of CH 4 (typically 2–4 times higher), allowing for the effective competitive replacement of adsorbed CH 4 42 . However, while this strong interaction enhances adsorption efficacy, it simultaneously intensifies the matrix swelling effect 58 , 59 . Although matrix swelling enhances sequestration stability and reduces leakage risk by compressing pore spaces, increasing fluid flow tortuosity, and reducing pore connectivity, it also increases fluid migration resistance and leads to a gradual decline in reservoir permeability, potentially hindering the migration of CO 2 to deeper reservoir regions. This permeability decay is qualitatively and quantitatively controlled by complex interactions involving coal composition, pore characteristics, and the supercritical CO 2 (SCO 2 ) environment. To balance sequestration stability with injectivity in engineering applications, future research should focus on establishing dynamic permeability models that integrate swelling strain with adsorption isotherms. Furthermore, engineering methods such as cyclic injection, mixed gas injection (e.g., CO 2 -N 2 ), or hydraulic stimulation should be explored to bridge the gap between theoretical capacity and actual injection capability. Geological and Engineering Compatibility: The main coal seams in the study area exhibit significant thickness and stable distribution, providing a solid foundation for commercial development. Existing CBM engineering technologies can be adapted for CO 2 injection, reducing both costs and risks. From an engineering feasibility perspective, the favorable zones are characterized by a stable structural and hydrogeological sealing system, which guarantees sequestration safety. Specifically, the Sitou Fault exhibits extremely low water and gas conductivity ; the eastern Jinhuo fault zone shows lateral water-blocking characteristics in the north and very weak groundwater flow in the south; and water-conducting faults along the western boundary are limited and have minor influence 60 . These features indicate excellent caprock sealing performance and a low risk of CO 2 leakage. Additionally, in the supercritical state, CO 2 possesses high density and low viscosity, which significantly improves displacement efficiency and sequestration capacity. Economic Viability and Synergy Benefits: Although CO 2 sequestration costs are currently influenced by multiple factors and remain uncertain, CCUS technologies are widely regarded as an effective pathway for reducing the overall cost of achieving global climate mitigation targets 61 . Economically, the study area demonstrates promising preliminary feasibility. Utilizing existing well networks, surface facilities, and geological data can significantly reduce initial investment costs 1 , 62 , 63 . Furthermore, the revenue generated from additional CH 4 production can partially offset the operational costs of capture and injection. Existing field pilot results indicate that injecting CO 2 into deep coal seams can enhance the coalbed methane recovery rate by 4.98% 57 . In the favorable zones, the coupling of existing infrastructure with production enhancement potential significantly improves practical economic value. Under the support of the “dual carbon” policy, opportunities for carbon credit trading and tax incentives 64 will further enhance the project’s economic viability. Conclusion This study integrates high-temperature and high-pressure isothermal CO 2 adsorption experiments, multi-model adsorption fitting (Langmuir, BET, and D–R), and total sequestration-capacity calculations to evaluate CO 2 storage potential and favorable zones in the main high-rank coal reservoirs of the middle–deep southern Qinshui Basin. The key conclusions are: Integrated evaluation framework: The BET model best represents supercritical CO 2 adsorption. Accordingly, a comprehensive capacity-calculation framework incorporating adsorbed, free-phase, and dissolved CO 2 was established for deep coal reservoirs. Sequestration potential and depth dependence: The total theoretical CO 2 sequestration capacity of the main coal seams is approximately 575.5 million tons. Sequestration increases with burial depth, with the supercritical zone (> 800 m) contributing 65.4% of the total. CO 2 storage is dominated by adsorption and free-phase mechanisms, whereas the dissolution contribution is negligible. The highest sequestration rate occurs within 800–1100 m, indicating the optimal depth range for CO 2 storage in the study area. Favorable zones: Structural and geological evaluation divides the coal reservoirs into four favorable units. The optimal zones are concentrated in Units I and II in the northern block, while relatively favorable zones mainly occur in Units IV and V in the middle–deep section. These zones broadly coincide with CBM-enriched areas, supporting future CO 2 -ECBM deployment and efficient sequestration. Future work should incorporate key reservoir complexities (especially in-situ water effects 65 and supercritical CO 2 phase behavior) and validate the framework using digital core analysis and field-scale injection tests, with extension to other blocks and coal ranks. Abbreviations CCUS Carbon capture, utilization and storage CBM Coalbed methane CO 2 -ECBM CO 2 -enhanced coalbed methane recovery BET Brunauer–emmett–teller adsorption model D–R Dubinin–radushkevich adsorption model LTNA Low-Temperature Nitrogen Adsorption MIP Mercury intrusion porosimetry NIST REFPROP NIST reference fluid thermodynamic and transport properties database STP Standard temperature and pressure Symbols (variables and parameters) H Burial depth (m) P Reservoir pressure at depth (MPa) T Reservoir/experimental temperature (K) V Excess adsorption amount (cm 3 /g) V ex Excess adsorption amount (cm 3 /g) δ Adsorption layer thickness (–) ρ ads Density of adsorbed CO 2 phase (g/cm 3 ) ρ g Density of free-phase CO 2 (g/cm 3 ) V L Langmuir volume (cm 3 /g) P L Langmuir pressure (MPa) p Equilibrium pressure (MPa) V m Monolayer saturation capacity (cm 2 /g) C BET constant (related to adsorption heat) (–) P 0 Saturation vapor pressure (MPa) P c Critical pressure of CO 2 (MPa) T c Critical temperature of CO 2 (K) V 0 Micropore volume (D–R) (cm 3 /g) D D–R constant (related to adsorption energy) (–) ϕ f Porosity occupied by free-phase CO 2 (%) ϕ i Initial/effective reservoir porosity (%) ϕ ad Porosity occupied by adsorbed CO 2 (%) V ad Adsorbed gas content in reservoir (cm 3 /g) ρ c Apparent coal density (g/cm 3 ) ρ g , STP Free-phase CO 2 density at STP (g/cm 3 ) ρ b CO 2 density at boiling point (1 atm) (g/cm 3 ) T b Boiling point temperature of CO 2 (1 atm) (K) C 1 Thermal expansion coefficient (1/K) S w Water saturation (%) B g Gas formation volume factor (ratio) (1) Z CO 2 compressibility factor at P , T (1) Z 0 Compressibility factor at STP (1) p STP Pressure at STP (MPa) T STP Temperature at STP (K) V s Dissolved (water-soluble) CO 2 content (cm 3 /g) S CO2 Solubility of CO 2 in formation water (mol/L) V t Total CO 2 sequestration capacity per unit coal mass (cm 3 /g) M Theoretical sequestration capacity (total inventory) (m 3 ) S Gas-bearing area / sequestration area (km 2 ) θ Coal seam dip angle (°) h Average coal seam thickness (m) Author contributions Conceptualization, Zhengzheng Xue and Xiaokai Xu; methodology, Xiaokai Xu and Lin Tian; validation, Kuo Jian ,Shuo Zhang and Liangwei Xu; formal analysis, Zhengzheng Xue and Jian Li; resources, Jian Li and Zehua Zhang; data curation, Yue Xin and Yixuan Yao; writing—original draft preparation, Zhengzheng Xue and Zehua Zhang; writing—review and editing, Xiaokai Xu and Liangwei Xu; funding acquisition, Xiaokai Xu and Kuo Jian. All authors have read and agreed to the published version of the manuscript. Funding The work was financially supported by the National Natural Science Foundation of China (41904118 and 42102218). Data availability The data presented in this study are available upon reasonable request from the corresponding author. Declarations Competing interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. The authors declare no competing interests. Footnotes Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. References 1. Davoodi, S. et al. Review of technological progress in carbon dioxide capture, storage, and utilization. Gas Sci. Eng. 117 , 205070 (2023). [ Google Scholar ] 2. Wang, G. et al. 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