CO2 Sequestration In Coal Seams Research Articles

Real-time pore structure evolution difference between deep and shallow coal during gas adsorption: A study based on synchrotron radiation small-angle X-ray scattering

Coal seam CO2 sequestration is an important option to address global warming. A better knowledge on coal pore structure evolution during gas adsorption can provide guidance for coal seams CO2 sequestration. However, few investigations on the pore structure evolution differences between the deep and shallow coal were conducted during gas adsorption. In this study, based on the real-time synchrotron radiation small-angle X-ray scattering (SAXS) observation, the average pore diameter and pore surface fractal dimension evolution differences between deep and shallow coal were investigated from the aspects of coal compositions and stress history. Two types of coal deformation (inner-swelling and outer-swelling) coexist during gas adsorption. Coal compositions have significant impact on the dominance of deformation type. The dominance of inner-swelling in deep coal is induced by the higher ash contents, and there is the decrease of average pore diameter during gas adsorption. The impact of stress-history (burial depth) on adsorption-induced deformation is more prominent than that of gas adsorption capacity. In deep coal, the surface fractal dimension evolution presents a negative correlation with the evolution of pore diameters. In shallow coal, the surface fractal dimension evolution presents a Langmuir-type correlation with the adsorption time.

Impact of supercritical CO2 saturation temperature on anthracite microstructures: Implications for CO2 sequestration

CO2 sequestration in coal seams is one of promising methods to achieve the goal of carbon neutrality, which has attracted the worldwide attention. Reservoir temperature plays a vital role in supercritical CO2 (ScCO2)-coal interactions and it even serves as a crucial factor in determining the CO2 trapping capacity and the feasibility of CO2 geological storage in these coal formations. The impact of saturation temperature on the interactions between coal and ScCO2 at geo-storage conditions is rarely reported in the literature, which directly affects the recovery of CO2-enhanced coalbed methane and the sealing integrity of structural trap provided by the caprocks. The microstructural changes of coal is also essential in prevention of upwards migration and CO2 leakages. In this paper, changes in coal microstructures including functional groups, mineral compositions and multiscale pore distribution at the saturation pressure of 10 MPa and various temperatures of 35–85 ℃ were comprehensively investigated using the combination of low-pressure CO2/N2 adsorption, X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR). The results show that micropores and mesopores are enhanced by 24.2–42.7% and 25.8–49.6%, respectively, while macropores are decreased by 4.9–19.5%. The carbonate minerals are most sensitive to ScCO2, and temperature rise accelerates the mineral dissolution to form more new pores within coal matrix. ScCO2 has strong extraction capacity for hydrocarbons, and approximately 80% of aliphatic structures are extracted from coal macromolecules. The higher saturation temperature, the more aliphatic groups are mobilized. Some isolated organic pores or closed pores are also extracted and damaged to become accessible for gas flow, which directly enhances the CO2 storage capacity in coal reservoirs. Once the temperature exceeds 65 ℃, the further temperature rise has less influence on coal microstructures. The results of this study may play a significant role in assessing the hydrocarbon recovery and the trapping of CO2 in coal reservoirs.

Experimental Study on the Effect of CO2 Injection Pressure on the Migration Characteristics and Extraction Effects of Replacement CH4.

To study the effect of CO2 injection pressure on gas migration characteristics and coalbed methane (CBM) extraction, a platform for the experimental replacement of CH4 with CO2 was used to conduct experiments on the replacement of CH4 under different CO2 injection pressures and analyze the gas transport characteristics and CH4 extraction during the experiment. The results reveal that the rate of gas migration out of the coal seam accelerates with increasing gas injection pressure, as determined by comparisons of the migration rates between adjacent monitoring points. The change trend of the CH4 desorption rate under different gas injection pressures is divided into slow decline, sharp decline, and stability stages, and the maximum value of the effective diffusion coefficient increases from 2.3 × 10-5 to 3.4 × 10-5 and 4.6 × 10-5 cm2/s as the gas injection pressure increases from 0.6 to 0.8 and 1.0 MPa. Similarly, the change pattern of coal seam permeability can be divided into slow decline, sharp decline, and stability stages. After the gas injection pressure was increased from 0.6 to 0.8 and 1.0 MPa, the CH4 desorption volume increased from 90.2 to 94.1 and 97.8 L, whereas the coal seam CO2 sequestration volume increased from 269.2 to 274.2 and 322.8 L, respectively. In contrast, the CH4 extraction efficiency increased from 76.9 to 80.2 and 82.9%, respectively. The research results have important reference value and practical significance for optimizing the CO2 injection pressure and improving the CBM extraction.

Influence Factors and Feasibility Evaluation on Geological Sequestration of CO2 in Coal Seams: A Review.

The geological sequestration of CO2 in coal seams holds significant implications for coalbed methane development and greenhouse gas mitigation. This paper examines the principles, influencing factors, and evaluation methods for geological CO2 sequestration in coal seams by analyzing relevant domestic and international findings. Suitable geological conditions for CO2 sequestration include burial depths between 300 and 1300 m, permeability greater than 0.01 × 10-3 μm2, caprock and floor strata with water isolation capabilities, and high-rank bituminous coal or anthracite with low ash yield. Geological structures, shallow freshwater layers, and complex hydrological conditions should be avoided. Additionally, the engineering conditions of temperature, pressure, and storage time for CO2 sequestration should be given special attention. The feasibility evaluation of CO2 geological storage in coal seams necessitates a comprehensive understanding of coalfield geological factors. By integrating the evaluation principles of site selection feasibility, injection controllability, sequestration security, and development economy, various mathematical models and "one vote veto" power can optimize the sequestration area and provide recommendations for rational CO2 geological storage layout.

Characterizing mechanical heterogeneity of coal at nano-to-micro scale using combined nanoindentation and FESEM-EDS

The variations of mechanical properties of coals in nano-to-micro scale play vital roles in defining the coal deformative and failure behaviors with a wide range of engineering applications, including coal stimulation for gas recovery and/or CO2 sequestration in coal seams. Based on the assumption that the heterogeneity in the mechanical properties of coals is combinedly determined by the mineralogical/carbon compositions and microstructures, three coals were experimentally measured through combing nanoindentation test, X-ray diffraction (XRD) analysis and field emission scanning electron microscopy - energy dispersive spectroscopy (FESEM-EDS) imaging. Two sub-bituminous coals from Illinois basin and one bituminous coal from Pittsburgh No.8 seam from Pennsylvania were collected and prepared. The moduli, hardness, and energy dissipations were all analyzed for three coals. The mean Young's moduli and hardness of two sub-bituminous and bituminous coals are ~4.13 GPa, ~4.79 GPa, and ~ 5.56 GPa respectively and the mean hardness of which are ~0.38 GPa, ~0.49 GPa and ~ 0.58 GPa respectively. The results confirmed that the nanoscale mechanical properties are compositional and microstructural dependent. It was also observed that the heterogeneity of sub-bituminous coals is more apparent than the bituminous coal. Also, the bituminous coal is more harder and brittle than sub-bituminous coal, which can better withhold the mechanical response, but the deformations are relatively permanent due to brittle failure. The energy dissipation data showed that the irreversible work decreases with the increase in hardness, which confirmed that the plastic deformation is much more easily to be inducted. The mechanism-based phenomenon pop-in event on the load-displacement curve was also analyzed by linking with the characteristics of the morphology and composition beneath a pointed indenter or cracks formation due to the localized indent.

Effects of Pore Parameters and Functional Groups in Coal on CO2/CH4 Adsorption.

The mechanisms of CO2/CH4 adsorption in coal are the theoretical foundation for CO2 sequestration in coal seams targeted for enhanced coalbed methane recovery. Herein, by changing the model (low rank coal: WMC, middle rank coal: XM and high rank coal: CZ) with plenty of side aliphatic chains and functional groups established in the literature, the influence and mechanism of pore parameters and functional groups(–CH3, –OH, –C2O, –C=O) on the adsorption of CO2 and CH4 in different rank coals are systematically studied. Using the Connolly surface algorithm to calculate the pore volume (VF) and the specific surface area (SSA) of coal with different functional groups, it can be seen that the influence of the functional group change on the pore structure is related to the coal rank. Changing the various functional groups in the original coal structure to a unified functional group (−CH3, −OH, −C2O, or −C=O) will increase the accessible pore volume (VF) and the specific surface area (SSA), except in low-rank and middle-rank coal, where the ordered arrangement of −C=O will decrease VF and SSA. The adsorption capacities of different pore parameters and functional groups were calculated by Grand Canonical Monte Carlo simulation and density functional theory. On pure adsorption, the pore parameters exert greater influence than the functional groups. By comparing the adsorption energy of the original pore structure containing functional groups and that of modified pores without functional groups, the contributions of the pore structure and original functional groups on CO2/CH4 adsorption are 71 and 29% and 83 and 17%, respectively. Small-diameter pores and −C2O have a strong adsorption capacity. In terms of competitive adsorption, the −C=O functional groups and pore diameters ranging from 1.0 to 2.0 nm can significantly enhance the selectivity of CO2 over CH4. The CH4 and CO2 adsorption does not occur via rigorous monolayer adsorption; multilayer adsorption can occur for CH4 and CO2 with pore diameters of 1.0–2.0 and 1.0–2.2 nm, respectively, thus causing micropore filling. These quantitative results establish a foundation for the development of adsorption theory for CO2/CH4 in coal.

Interactions of dynamic supercritical CO2 fluid with different rank moisture-equilibrated coals: Implications for CO2 sequestration in coal seams

The interactions between CO2 and coals during CO2-ECBM (CO2 sequestration in deep coal seams with enhanced coal-bed methane recovery) could change pore morphology and chemistry property of coals, thereby affecting adsorption, diffusion and flow capability of CO2 and CH4 within coal reservoirs. To simulate CO2-ECBM process more practically, the dynamic interactions of supercritical CO2 (scCO2) and moisture-equilibrated coals were performed at temperature of 318.15 K, pressure of 12.00 MPa, and duration of 12.00 h. The impacts of the interactions on physicochemical properties of coals were investigated. Results indicate that scCO2/H2O exposure shows minor effect on micropores of coals. However, the exposure significantly decreases the mesopore surface area of bituminous coals, while increases that of anthracites. The mesopore volume and the average mesopore diameter of all the coals after scCO2/H2O exposure decrease. The multi-fractal analysis verifies that the scCO2 exposure can enhance the pore connectivity of various rank coals. Apart from the pore morphology, the exposure of scCO2/H2O also affects the oxygenic functional groups on coal surface. Particularly, the exposure of scCO2/H2O reduces the content of CO and CO of coals. The content of COOH of low rank coals including Hehua-M2# coal, Zhongqiang-4# coal, Buliangou-9# coal and Tashan-5# coal decreases, while the high rank Laochang-11# coal and Kaiyuan-9# coal witness a growth in COOH. The content of total oxygenic functional groups of all coals after interaction with scCO2/H2O decreases; on the contrary, that of CC/CH of all coals after scCO2/H2O exposure increases. In summary, the interaction with scCO2/H2O significantly changes the pore system and oxygenic functional groups of various rank coals, which needs further attention regarding CO2-ECBM.

Numerical analysis of permeability rebound and recovery during coalbed methane extraction: Implications for CO2 injection methods

The coal’s permeability plays a crucial role in coalbed methane (CBM) extraction and coal seam CO2 sequestration. An accurate understanding of permeability rebound and recovery is therefore essential. This study establishes an improved fully coupled gas migration model for CBM extraction. The permeability rebound and recovery times as well as rebound values are proposed to accurately quantify permeability evolution during CBM extraction. The evolution of these three parameters under the influence of different factors are evaluated in detail, such as initial gas pressure, the diffusion coefficient, and the permeability. The results show that the permeability rebound and recovery times increase along with initial gas pressure and the amount over time rises rapidly under high gas pressures. As the initial gas pressure increases, the permeability rebound value decreases. However, initial diffusion coefficient and permeability have a negative trend in permeability rebound, recovery time, and rebound value. These tendencies are particularly large for low initial permeabilities and diffusion coefficients, yet the change in rebound time is smaller than the one in recovery time. Finally, inspired by the relationship between permeability rebound and gas pressure change during CBM extraction, the evolution of coal seam permeability under different CO2 injection method is discussed. A stepwise increasing-pressure CO2 injection method is also proposed, which could effectively increase the volume of CO2 sequestered and reduce project costs. Therefore, our findings shall shed light on improving coal mine safety production and reducing greenhouse gas emission.

Numerical analysis of dual porosity coupled thermo-hydro-mechanical behaviour during CO2 sequestration in coal

This study presents a coupled dual porosity thermal-hydraulic-mechanical (THM) model of non-isothermal gas flow during CO2 sequestration in coal seams. Thermal behaviour is part of the disturbed physical and chemical condition of a coal seam caused by CO2 injection, and must be understood for accurate prediction of CO2 flow and storage. A new porosity-permeability model is included for consideration of the fracture-matrix compartment interaction. The new model is verified against an analytical solution and validated against experimental measurements, before being used to analyse coupled THM effects during CO2 sequestration in coal. A simulation of CO2 injection at a fixed rate shows the development of a cooling region within the coal seam due to the Joule-Thomson effect, with the temperature in the vicinity of the well declining sharply before recovering slowly. The temperature disturbance further from the well is more gradual by comparison. Under the simulation conditions studied, CO2 injection increases coal matrix porosity and decreases the porosity and permeability of the natural fracture network, especially in the vicinity of the injection well, due to adsorption-induced coal swelling. Compared with the effects of gas pressure and temperature, the matrix-fracture compartment interaction plays an important role in changes of porosity and permeability. Considering the temperature disturbance caused by CO2 injection under the set of representative conditions studied, the coupled model can provide an insight into the associated effects on CO2 flow and storage during its sequestration in coal seams.

Isotherms and kinetics of deformation of coal during carbon dioxide sequestration and their relationship to sorption

An in-depth understanding of the carbon dioxide (CO2) sorption behavior on coal is critical for CO2 sequestration in coal seams and enhanced coalbed methane recovery. However, CO2 induces the deformation of coal through physical and/or chemical reactions during the CO2 injection process, which has a significant effect on the gas flow and stability of the coal seams. This study investigates the isotherms and kinetics of CO2 sorption and hence deformation of a bituminous coal sample during isotherm tests at three different temperatures. The isotherms and kinetics curves of sorption and deformation can be described by the Langmuir and the pseudo-second-order (PSO) models, respectively. The experiment results and PSO model parameters indicate that a pore-filling effect at low pressure results in an overestimated amount of excess sorption, leading to a mismatched phenomenon between the sorption and swelling, that is the maximum amount of sorption fails to induce corresponding maximum swelling. The CO2-induced deformation is anisotropic and heterogeneous during the whole sorption process, which can be attributed to the bedding structure of coal and heterogeneous maceral distribution in coal. Because of the difference in the sorption characteristic between different macerals, the clay + inertinite-rich regions first rapidly swell and then suffer from compression through the later swelling of the vitrinite + liptinite -rich regions in coal, showing an “overshoot” behavior. The dissolution of CO2 induces a structural rearrangement of coal, leading to strains in different directions maintain changes without further CO2 intake. This phenomenon is observed in the first few sorption steps until the structure of coal reaches a stable state. The kinetic relationships between sorption and deformation in individual pressure steps are nearly linear or are linear up to a certain point but slopes of these curves are different, resulting in a simple quadratic polynomial relationship between sorption and deformation isothermals during the whole sorption process.

Dynamic fluid interactions during CO2-ECBM and CO2 sequestration in coal seams. Part 2: CO2-H2O wettability

In addition to CO2-CH4 interactions (Part 1), the success of CO2 enhanced coalbed methane (CO2-ECBM) and geological sequestration are significantly affected by the CO2-H2O wettability. Wettability controls both gas desorption and transport and is influenced by injection pressure, reservoir temperature and the state of water that is present – as either adsorbed- or free-water. Dynamic changes in wettability remains poorly constrained – due to the innate difficulty and invasive nature of conventional measurements (e.g., captive gas bubble and pendent drop tilted plate methods). In part 2, we use nuclear magnetic resonance (NMR) as a non-invasive method to explore the mechanisms of these factors (pressure, temperature, water-state) on CO2-H2O wettability during CO2-ECBM. Results for contrasting subbituminous coal and anthracite show that the CO2 wettability of coals significantly increases with increasing CO2 injection pressure up to 5 MPa before stabilizing to a limiting value. This suggests that the most economically-suitable injection pressure is ~5 MPa. CO2 wettability also increases with a decrease in temperature suggesting that shallower reservoirs may be marginally improved in this trend. Additionally, the presence of non-adsorbed water in coals significantly reduces both the sensitivity of CO2 wettability to pressure and the absolute magnitude of wettability relative to the case where free-water is absent. Thus, draining free-water from the reservoir will serve the dual purposes of both increasing gas transport and the potential for desorption from the perspective of CO2-H2O wettability. The far-reaching results in this study, together with the companion paper (Part 1) are significant for evaluating CO2-ECBM improvement both in enhancing methane recovery and CO2 utilization in coals.

Potential impact of CO2 injection into coal matrix in molecular terms

Coal seams are one of the targeted geological bodies for CO2 sequestration. Coal structure deformation is a significant feature of the interaction between CO2 and coal seams. The rearrangement of macromolecular structure is a recognized deformation mechanism whose process has not been quantified. We used a high-order coal macromolecular model to reveal specific processes of coal macromolecular rearrangement caused by CO2 injection through molecular dynamics. In addition, the potential impact of this injection on the sequestration and mechanical properties of CO2 in the coal matrix was investigated. The rearrangement of the coal macromolecular structure is accomplished through the mutual coupling of the pore structure and macromolecular ordering. The compression of closed pores and the expansion of open pores together cause the swelling of macromolecular volumes. The concentration of closed pores was an important indicator for the quantification of the rearrangement of macromolecular structure. The greater the content of closed pores in the coal, the higher the potential for macromolecular structure transformation after CO2 injection. Furthermore, the more disordered the direction of the coal macromolecular layer, the lower the peak strength of the coal matrix. The amount of CO2 sequestration in the coal matrix is gradually saturated near the critical pressure point. Our findings contribute to the study of CO2 sequestration in coal seams and CO2 leakage risk prediction.

Effects of cyclic saturation of supercritical CO2 on the pore structures and mechanical properties of bituminous coal: An experimental study

The geological sequestration of CO2 in unmineable coal seams has gradually become one of the most effective means of responding to the global greenhouse effect. Presently, the cyclic injection of CO2 into coal seams is utilized in some enhanced CBM recovery projects with CO2 sequestration; therefore, a proper understanding of the effects of the cyclic saturation of supercritical CO2 (ScCO2) on coal is essential. Here, we present a series of uniaxial compressive strength (UCS) tests on bituminous coal subjected to both sustained and cyclic ScCO2 saturation. Nuclear magnetic resonance and acoustic emission (AE) are used to investigate changes in the pore structure distribution and porosity, as well as the fracture propagation. The results indicate that ScCO2 saturation enhances the continuity of the pore volume distribution, while cyclic saturation has a stronger influence on the pore structure. Furthermore, samples subjected to cyclic saturation exhibit significantly greater decreases in UCS and elastic modulus than the sustained-saturation samples, owing to mechanical fatigue caused by the cyclic saturation. The AE results show that cyclic saturation produces multiple signal releases in the form of unstable crack propagation, reducing crack closure and enhancing stable crack propagation. We also analyze the failure mechanism of coal samples under cyclic ScCO2 saturation in terms of the pore structure and mechanical changes experienced and discuss the influence of the cyclic injection of CO2 into coal seams. Hence, the results of this study are expected to provide a reference for the selection of appropriate CO2 injection methods and safety assessments for field projects involving coal-seam CO2 sequestration.

Numerical Analysis of Improvements to CO2 Injectivity in Coal Seams Through Stimulated Fracture Connection to the Injection Well

This work presents a hybrid discrete fracture-dual porosity model of compressible fluid flow, adsorption and geomechanics during CO2 sequestration in coal seams. An application of the model considers the influence of hydraulic fractures on CO2 transport and the stress field of the coal. The low initial permeability of coal is compounded by the injectivity loss associated with adsorption-induced coal swelling, which is recognised as the major challenge limiting CO2 sequestration in coal seams. In this model, the natural fracture network and coal matrix are described by a dual porosity model, and a discrete fracture model with lower-dimensional interface elements explicitly represents any hydraulic fractures. The two models are coupled using the principle of superposition for fluid continuity with a local enrichment approximation for displacement discontinuity occurring at the surface of hydraulic fractures. The Galerkin finite element method is used to solve the coupled governing equations, with the model being verified against analytical solutions and validated against experimental data. The simulation results show that the presence of a hydraulic fracture influences the distribution of gas pressure and improves the gas flow rate, as expected. The stress field of a coal seam is disturbed by CO2 injection, especially the vertical stress, and the presence of a hydraulic fracture leads to a reduction in stress with permeability recovery starting earlier.

Predicting permeability changes with injecting CO2 in coal seams during CO2 geological sequestration: A comparative study among six SVM-based hybrid models

CO2 geological sequestration and enhanced coal bed methane extraction is a significant CO2 utilization approach with dual-meaning of energy and environment, and coal permeability is considered as one of the critical parameters for evaluating this method. To better predict permeability changes with injecting CO2 in coal seams, six SVM-based hybrid models integrating support vector machine (SVM) with intelligent optimization algorithms are proposed and compared, SVM is used for the relationship modelling between CO2 permeability and its influencing variables, and six intelligent optimization algorithms, including artificial bee colony (ABC), cuckoo search (CS), particle swarm optimization (PSO), differential evolution (DE), gray wolf optimizer (GWO), DE-GWO, are used for the hyper-parameters tuning. A total of 125 data samples for CO2 permeability are retrieved from the reported studies to train and verify the proposed models. The input variables for the predictive models include CO2 injection pressure, effective stress, temperature, buried depth and coal rank, and the corresponding output variable is CO2 permeability. The predictive model performance is evaluated and compared by correlation coefficient (R), root mean square error (RMSE) and mean absolute error (MAE). The predictive results denote that the prediction performance of the six hybrid models from high to low is DEGWO-SVM, GWO-SVM, PSO-SVM, CS-SVM, DE-SVM, ABC-SVM, and the DEGWO-SVM hybrid model is recommended to predict permeability changes with injecting CO2 in coal seams. At the same time, the mean impact value (MIV) is used to investigate the relative importance of each input variable. The relative importance scores of CO2 injection pressure, effective stress, temperature, buried depth and coal rank are 0.0248, 0.4617, 0.0211, 0.1102, and 0.3822, respectively. The research results have important guiding significance for CO2 permeability prediction and CO2 sequestration in coal seams.

Simulation study on the biological methanation of CO2 sequestered in coal seams

The technologies for the bioconversion of CO2 to CH4 by microorganisms are clean and safe and have attracted extensive attention in the field of energy conservation and emission reduction in recent years. A series of biological gas production experiments were carried out using bituminous coal D and indigenous bacteria from the Qianqiu coal mine, with the addition of NaHCO3 to simulate CO2 sequestration in coal seams, to explore the feasibility of biomethanation of CO2 sequestered in coal seams. Changes in the characteristics of both the gas and liquid phases during the fermentation process were studied. The results showed that the methane production follows a "low-high-low" trend with an increase in the bicarbonate concentration. During the fermentation process, the pH and chemical oxygen demand (COD) values of the fermentation solution initially increased and later decreased. In the early stage of the fermentation experiment, the microorganisms mainly degraded the ethers; the alcohols were degraded in the later stage. Identification of the microbial community structure showed that although bicarbonate inhibits the growth of some microorganisms, it increases the hydrolysis capacity of the fermentation process and also enriches the hydrogenotrophic methanogens associated with the bioconversion process of CO2 to CH4. This indicates that the composition of the archaea in the fermentation solution transforms to support the bioconversion of CO2 to CH4. The results show that the CO2 sequestered in the coal seams can be converted into CH4 by indigenous bacteria, which can reduce greenhouse gas emissions and the re-leakage risk of CO2 and also produce clean energy.

Effect of Supercritical CO2 on Various Rank Coals: Implications for CO2 Sequestration in Coal Seams with Enhanced Coalbed Methane Recovery

Sequestration of CO2 in coal seams with enhanced coalbed methane recovery (CO2-ECBM) can mitigate CO2 emissions. In this work, the effects of the supercritical CO2 fluid on the physicochemical property of coals were elucidated. The results show that CO2 interaction can mobilise the volatile hydrocarbons in the coal matrix and subsequently result in the decrease of volatile matter content. The volatile matter contents of coals decrease with temperature, pressure and injection rate of CO2 interaction. The extracted volatile matters are of biological toxicity, thus CO2-ECBM process should be operated at the optimum temperature, pressure and CO2 injection rate. The moisture contents of Zhangji coal and Liulin coal after CO2 interaction increase which implies that the pore morphology of the above two coals grows up. For low rank Bulianta coal and high rank Qinshui coal, the swelling effect induced by CO2 is dominant. Thus, the moisture content after CO2 interaction decreases in comparison with the raw state.

Coal Wettability After CO2 Injection

Increasing energy demand and associated global warming are unarguably the two major challenges that the world currently faces. One of the ideas to reduce the carbon footprint while increasing the efficiency of the energy extraction is CO2 sequestration in coal seams. This can additionally enhance the coal-bed methane production. However, this process depends on many factors, among which coal wettability is of particular importance especially because of its pressure and temperature dependency. To evaluate this process, coal wettability was tested by measuring the contact angle of CO2 and water as a function of pressure, temperature, and salinity (DI water and brine (5 wt % NaCl + 1 wt % KCl), i.e., wt % is the weight percentage of salt to water. The results show that the CO2–water contact angle increases significantly, with increasing pressure, temperature, and salinity indicating more-effective CO2 wetness of coal. This, in turn, can reduce the CO2 residual trapping capacities and increase methane recover...

Influence of Fluid Exposure on Surface Chemistry and Pore-Fracture Morphology of Various Rank Coals: Implications for Methane Recovery and CO2 Storage

The surface chemistry and pore-fracture morphology of coals are critical to the process of CO2 sequestration in coal seams with enhanced coalbed methane (CH4) recovery (CO2-ECBM). To assess the influence of deionized water–CO2 mixture (DH2O–CO2) exposure on these properties, the interaction of DH2O–CO2 with three rank coals, i.e., sub-bituminous coal (SBC), high volatile bituminous coal (HVBC), and anthracite, was conducted on a dynamic supercritical fluid extraction system with a temperature of 45 °C and an equilibrium pressure of 12 MPa. Characterization methods including proximate analysis (PA), Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), probe molecule (N2/CO2) adsorption, and low-field nuclear magnetic resonance (NMR) were adopted to fully address the changes in surface functional groups and pore-fracture characteristics. The results indicate that the geochemical interaction occurs between the mineral matters and DH2O–CO2 as demonstrated by the change in th...

The influence of methane and CO2 adsorption on the functional groups of coals: Insights from a Fourier transform infrared investigation

Understanding the mechanism of CO2 and CH4 storage in coals is critical for CO2 sequestration in coal seams with enhanced coalbed methane recovery (CO2-ECBM) and the prevention of mine gas disasters. The primary objective of this study is to investigate the potential chemical interactions of CO2 and CH4 with coal due to high-pressure adsorption. The functional groups of two different-rank coals before and after high-pressure CH4 and CO2 adsorption treatments were studied using Fourier transform infrared spectroscopy (FTIR) spectrometry and curve fitting. The changes in the chemical structure of coal were evaluated using structural parameters derived from FTIR. The coal adsorption treatments were conducted at 40 °C and 6 MPa. The results indicate that both coal rank samples underwent functional group and structural parameter changes after high-pressure CO2 and supercritical CH4 adsorption and thus that chemical interactions occur at high pressure. Moreover, the variation law of functional groups is complex due to its dependence on the coal rank and gas type. FTIR spectra of long-flame coal and anthracite coal have clear functional group distribution differences. The amount of oxygen-containing functional groups decreases with the increase in coal rank. This study demonstrates the utility of FTIR spectrometry and curve fitting to quantitatively evaluate the chemical structure of coal samples. This preliminary study is intended to enrich the theories of gas adsorption in coals, guide applications of CO2-ECBM, and assist in the prevention of mine gas disasters.