Coalbed Methane Recovery Research Articles

Anisotropic strain of anthracite induced by different phase CO2 injection and its effect on permeability

To study the swelling/shrinkage strain effects of different phases of CO2 when injected into anthracite and the influence that has on the dynamic evolution of coal permeability, an experimental study of coal anisotropic adsorption of CO2 was carried out by using a triaxial compression test device, and the relationship between the swelling/shrinkage strain and permeability of the coal was discussed. The results show that the adsorption strain and time of gas exposure throughout different phase states of the CO2 injected show three stages of evolutionary behavior. The adsorption swelling of the coal sample increases with rising gas pressure. The adsorption strain of coals under the action of supercritical CO2 increased by 16.28% and 17.36% against the action of subcritical CO2, respectively. The coal portrayed anisotropic characteristics during the process of adsorption-caused swelling deformation and desorption-caused contraction, and the deformation in the vertical layer direction was greater than that in the parallel layer direction, and the anisotropy gradually weakened with increasing pressure. The permeability of the coal and the temperature is a quadratic function relationship under different effective stress conditions, and there is a zone where the changes occur. The study provides a theoretical basis CO2- Enhanced coalbed methane recovery.

Insights into influence of CH4 on CO2/N2 injection into bituminous coal matrix with biaxial compression strain in molecular terms

Under the influence of strong mining disturbances in coal mines, the coal gradually deforms, which affects the extraction of coalbed methane (CBM). In this study, the microscopic mechanism of CH4 affecting CO2/N2 injection into a bituminous coal matrix with biaxial compression strain was investigated by analyzing its microporous structure, adsorption isotherm, CH4 content effect, strain effect, adsorption density, intermolecular adsorption potential energy and adsorption energy using grand canonical Monte Carlo simulation, molecular dynamic simulation and density functional theory. The results show that with increasing (negative) compression strain (0.00 → −0.20), the distribution range of the gas adsorption potential energy field decreases, with a deviation order of CO2 > CH4 > N2. Also the Henry constant of CO2 increases while those of CH4 and N2 stabilize. The orders of influence of CH4 content and strain on adsorption are N2 > CO2 > CO2/N2 and CO2 > CO2/N2 > N2, respectively. Additionally, the potential energy field distribution ranges of CO2 and N2 injections decrease with increasing CH4 content. More importantly, CH4 is adsorbed in the Up orientation, while CO2 and N2 are adsorbed parallel to the aromatic groups on the pore surface of coal matrix. Also, pre-adsorbed CH4 increases the negative adsorption energy of CO2 and N2 by 45.37–50.26%, thereby effectively inhibiting their injections. This research lays the foundation for improving CBM recovery efficiency in the deformation zone of CBM reservoirs with realistic exploitation.

Experimental study on erosion mechanism and pore structure evolution of bituminous and anthracite coal under matrix acidification and its significance to coalbed methane recovery

Acid fracturing is a potential method to resolve the mineralization restriction around wellbore and enhance the permeability in coalbed methane extraction. The previous study mainly focused on exploring the effect on permeability enhancement for oil reservoir. However, the performance of acid fracturing fluid in coal reservoir is still unclear. In this paper, the surface morphology and internal pore structure of coal were characterized by erosion rate test, scanning electron microscope-energy dispersive spectrum (SEM-EDS) and low-temperature nitrogen adsorption test (LT-N2GA), and the relationship between erosion rate and pore structure index of bituminous and anthracite coal under matrix acidification was obtained. The results show that the bituminous coal presented higher acid sensitivity than anthracite from no matter aspects of erosion rate, surface morphology and internal pore structure variation. The increment of erosion rate and pore structure index is increase with the coal ranks under matrix acidification, which can provide reference for the selection of acid pad fluids for acid fracturing in different coal seams.

Effect of reservoir temperatures on the stabilization and flowback of CO2 foam fracturing fluid containing nano-SiO2 particles: An experimental study

CO2 foam fracturing technology shows promise concerning enhancing coalbed methane (CBM) recovery and realizing CO2 geo-sequestration in deep un-mineable coal seams. However, one of the main challenges for successful CO2 foam fracturing is the foam destabilization and rupture that are caused by reservoir temperatures. The application of nanotechnology can improve foam stability. This paper aims to investigate the mechanism of CO2 foam stabilization by nano-SiO2 particles and the effect on fracturing fluid flowback at different reservoir temperatures. The foaming performance, interfacial rheological properties, and wettability of sodium dodecyl sulfate (SDS) and SDS/SiO2 dispersions in distilled water were examined using the foam scanner and interfacial rheometer. The results show that the foaming capacity of a CO2 foam fracturing fluid is found to increase with increases in temperature while the foam half-life is decreased. With an increase in the 0.2 mass% SDS + 0.3 mass% nano-SiO2 solution temperature from 20 °C to 60 °C, the foaming capacity increases by 24.42%, from 0.86 to 1.07, while the foam half-life decreases by 94.14%, from 7254 s to 425 s. The addition of nano-SiO2 particles reduces the foaming capacity of the SDS solution but substantially increases the foam half-life. When the temperature is 30 °C, the foaming capacity of 0.2 mass% SDS + 0.3 mass% nano-SiO2 solution is 8.33% lower than that of 0.2% SDS solution, but the foam half-life increases by 100.29%. The adsorption of nano-SiO2 particles at the gas-liquid interfaces increases both the viscoelastic and elastic moduli, which in turn enhances the ability of the foam to resist external disturbances. The viscoelastic and elastic moduli of the foam are both observed to decrease with increasing temperature, while both high temperatures and the presence of nano-SiO2 particles increase the capillary resistance of the fluid, which will degrade flowback. This study provides basic theoretical support for the application of CO2 foam fracturing technology to CBM production enhancement.

The variations and mechanisms of coal wettability affected by fracturing fluids with different concentrations of Ca2+ during fracturing

In low-porosity, low-permeability, and high-adsorption coalbed methane (CBM) reservoirs, the flow of fracturing fluid and methane gas is strongly affected by the wettability of coal. Ca2+ in mine water can affect the wettability of coal by changing the properties of the fracturing fluid, thus affecting the recovery efficiency of CBM. To investigate the mechanism of the wetting properties of bituminous coal by fracturing fluid with different concentrations of Ca2+, contact angle and T2 distribution curves were tested using an interfacial rheometer and low-field nuclear magnetic resonance (LF-NMR), and changes in water absorption and functional groups of coal soaked by fracturing fluid with different concentrations of Ca2+ were described using a precision electronic balance, X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR) to evaluate the wetting properties of coal. Molecular dynamics simulation was used to characterize the microscopic wetting properties of water molecules on the coal surface. Results show that when the concentration of Ca2+ is lower than 50 mg/L, the surface tension of the fracturing fluid gradually decreases, and the lowest value is 27.66 mN/m at 50 mg/L. The overall change in the contact angle and T2g value was lower than that of the control group (no Ca2+). The content of oxygen-containing functional groups on the surface of coal increased by 22.30%, and the content of C-O functional groups inside coal increased by 3.55%. The relative concentration and density of water molecules on the coal surface increase with increasing molecular diffusion ability. These results indicate that when the Ca2+ concentration is lower than 50 mg/L, the wettability of coal is enhanced, and thus, the fracturing fluid and methane gas flow capacity is improved. However, with increasing Ca2+ concentration, this phenomenon is reversed, and the water absorption of coal decreases, indicating that the wetting ability of coal decreases. The results of this study provide theoretical guidance that can be used to improve the recovery of CBM by fracturing and permeability enhancement measures, and preparing fracturing fluid in mine water to strengthen the protection of freshwater resources.

Reasonable start time of carbon dioxide injection in enhanced coalbed methane recovery involving thermal-hydraulic-mechanical couplings

Reasonable start time of carbon dioxide injection in enhanced coalbed methane recovery involving thermal-hydraulic-mechanical couplings

Systematic Pore Characterization of Sub-Bituminous Coal from Sohagpur Coalfield, Central India Using Gas Adsorption Coupled with X-ray Scattering and High-Resolution Imaging

Pore characterization helps to estimate the coalbed methane recovery and carbon storage potential of the reservoir. Earlier research on the characteristics of coal pores has shown that coal has high hydrocarbon storage potential in the adsorbed state, but few studies have shown the influence of chemical heterogeneities and depth on the adsorption potential of the coal. With the objective of studying the effect of chemical variation, depth, and surface roughness on gas adsorption potential, this study combines coal composition analysis and adsorption-based pore characterization of coal and shale samples coupled with high-resolution imaging and X-ray scattering measurements. Variation in pore features is correlated with varying depth and composition. A decrease in the mesopore volume and surface area is observed with an increase in the depth and total organic content and inverse behavior is observed for micropores. Scanning electron microscopy images depict the change in the pore shape from semi-spherical OM pores to elongated pores with depth, and samples with high mineral content show a dominance of inter- and intraparticle pores. Fractal dimension values estimated from SAXS are notably higher than N2-LPGA-derived values (i.e.,─DS > DN) due to the incorporation of inaccessible pores, which reflects an increase of up to 62% in SAXS estimated mesopore volume and surface area. This study will provide a better approach to understand the impact of composition, depth, and surface roughness over the gas storage potential in coal reservoirs.

Effect of microwave-assisted acidification on the microstructure of coal: XRD, 1H-NMR, and SEM studies

Microwave heating contributes to coal fracturing and gas desorption. However, problems of low penetration depth, local overheating and fracture closure exist. Coal demineralisation by acids has advantages in coal unblocking and permeability improvement, while it is difficult for acid to enter microcracks. Microwave-asisted acidification may offer an alternative. In this work, XRD, 1H-NMR, and SEM were used to evaluate the effect of microwave-assisted acidification on the microstructure of coal. Results show that kaolinite, calcite, and dolomite can be dissolved by acid. After microwave irradiation, the graphitization of microcrystalline structure of carbon improves. Microwave-assisted acidification erodes minerals in coal and enhances the graphitization degree of microcrystalline structure. Compared to individual microwave irradiation or acidification, the pore volume and pore connectivity can be greatly enhanced by microwave-assisted acidification. The NMR permeability of coal increased by 28.05%. This study demonstrates the potential of microwave-assisted acidification for coalbed methane recovery.

Influence of uniaxial strain loading on the adsorption-diffusion properties of binary components of CH4/CO2 in micropores of bituminous coal by macromolecular simulation

Heterogeneous and anisotropic coal tends to deform under strong mining disturbances, affecting the microscopic adsorption and diffusion of binary CH4-CO2 mixtures in micropores. Therefore, the grand canonical Monte Carlo and molecular dynamics methods were used to study the influence of the uniaxial strain loading on the adsorption-diffusion properties of binary components of CH4/CO2 in coal micropores. The results show that compression strain has significant effects on adsorption selectivity and thermodynamic factor compared to tension strain, and CH4 displacement efficiency is greater when the strain is less than −0.12. Both compression and tension strains have dual effects on promotion and inhibition of volumetric swelling of the gas-containing coal matrix, micropore development and gas diffusion. Furthermore, the diffusivity of CH4 is significantly larger than that of CO2, and Einstein diffusion is dominated by low-velocity motion. Results provide an important basis for the optimal coalbed methane recovery design in deformed coal reservoirs.

Effects and influencing factors of the CO2-alkaline-water two-phase displacing gas and wetting coal

To prevent the risk of coal and gas outburst and improve the coalbed methane (CBM) recovery, the CO2-alkaline-water two-phase displacing gas and wetting coal technique (CADW) was researched. Effects of the CADW in the residual CH4 and CO2, pore pressure, moisture content and permeability in coal under different conditions were studied detailedly. The results showed that, compared with the CO2 displacing CH4, after adopting the CADW, the amount of residual gas in the Qincheng (QC), Liangshun (LS) and Zhongxing (ZX) coals increased significantly, with the increment of 203.99%, 75.15%, 61.35%. The desorbed CH4 and CO2 of the three coals accounted for 86.37%, 69.12%, 64.60% and 4.30%, 3.38%, 13.43% of the injected CH4 and CO2 respectively. Compared to the conventional water injection, after using the CADW, the pore pressure, water injection volume and moisture content in coal all increased obviously. The maximum pore pressure of QC, LS and ZX coals increased by 2.05–6.96%, 4.37–5.44% and 2.88–9.25% respectively. The water injection volume of QC, LS and ZX coals increased by 10.69%, 33.51%, 43.45% respectively. The moisture content increased by 2.81–13.01%, 20.00–42.58%, 16.69–114.34% radially, and its increment increased with distancing the borehole. Besides, the permeability in three coal samples all decreased by one order of magnitude, with the reduction of 84.21%, 86.49% and 89.47%. Additionally, increasing the water injection pressure and time can distinctly improve effects of the CADW, which will reduce with rising ground stress and reducing temperature. The results can provide important reference for better preventing the coal and gas outburst risk and increasing the CBM recovery.

Interactions of CO2–H2O-coal and its impact on micro mechanical strength of coal

High temperature flue gas enhanced coalbed methane (CBM) recovery is a potential technique for improving CBM recovery efficiency and carbon sequestration, and it is of significance to carbon emission reduction in coal mine area. Previous research has demonstrated that the injection of CO2 could markedly alter the mechanical properties of coal seam, which further changes the gas flow and long-term safety of CO2 storage. Aiming at this issue, we studied the coupling effect of CO2–H2O on micro mechanical properties of coal with nanoindentation method. And the pore-fracture structure and minerals of the target sample was systematically characterized with LF-NMR and SEM to explore the influence mechanism of CO2–H2O on coal. The results show that (1) the micro strength of coal is obviously weakened after the treatment of CO2–H2O, and with an increase of the time, both the hardness and reduced modulus decreases, while the indentation depth shows an opposite trend. (2) Compared with the control group, the volumes of micropore, minipore, mesopore and macropore/microfracture in coal treated by CO2–H2O all increase in varying degrees. The increase of micropore volume is generally less than 20%. While the increases of the volumes of mesopore and macropore/microfractures are significant, with the maximum increase of 44.13% for the mesopore, and 299.58% for the macropore/microfracture. (3) Ulan coal contains a large number of calcareous minerals, which undergo chemical reaction after encountering the acid solution formed by CO2–H2O, resulting in dissolution of part of the minerals, which can be evidenced by the corrosion pits observed on the coal surface. (4) The injection of CO2 results in nonuniform swelling of the coal, which further leads to the formation of new cracks and the resultant coal strength deterioration. In addition, the acid solution formed by CO2–H2O reacts with the minerals in coal, resulting in dissolution of carbonate and other minerals in the coal, thus weakening the coal strength. The research results in this work provide basic support for the evaluation of the effectiveness of high temperature flue gas enhanced CBM and the safety of long-term CO2 storage.

Development of linear-element boundary element method for inverse solution from induced far-field displacements to reservoir loading source

An inverse solution strategy using linear element of Boundary Element Method(BEM)was developed, which will become a novel application base to potentially reflect the equivalent mechanical effect around reservoir corresponding to CO2 spreading during its injection process. Firstly, a linear-element BEM code was studied to improve the accuracy in analyzing the connection between induced ground minute deformations and underground inner loading. Secondly, the space regularization was adopted to deal with the ill-posed problem during the optimization to inverse problem solution. The boundary conditions of a near-field structure as active source were reversely figured out in an efficient numerical way based on its passive deformation that was sampled at a few specific positions at far field. Meanwhile, the effectiveness of linear-element BEM in solving inverse problems was demonstrated. Finally, based on the implementation of hydraulic fracturing in a coal seam in Alberta, Canada, the monitored data of its induced surface displacements was input to invert the equivalent internal pressure onto are-opened fracture within reservoir. The results show that the proposed inverse solution workflow and programmed algorithm are applicable to effectively guiding hydraulic fracturing interpretation so as to further facilitating coal bed methane recovery.

Effects of steam treatment on the internal moisture and physicochemical structure of coal and their implications for coalbed methane recovery

Effects of steam treatment on the internal moisture and physicochemical structure of coal and their implications for coalbed methane recovery

Evolution of the pore structure and fractal characteristics of coal under microwave-assisted acidification

Coal reservoir usually requires stimulation to enhance coalbed methane (CBM) production. As a novel stimulation method, microwave heating contributes to methane desorption and coalbed fracturing. However, the microwave-induced fractures are easily closed under in-situ stresses. Acidic stimulation is a process of demineralisation in the coal reservoir using hydrochloric, acetic, or formic acids, resulting in opening and interconnecting pores and fractures to flow gas. However, it is difficult for acids to enter microfissures under normal pressures. Microwave-asisted acidification may offer an alternative. In this work, the evolution of the pore structure and fractal characteristics of coal under microwave-assisted acidification was evaluated using low-temperature nitrogen adsorption and Scanning electron microscope. Compared to microwave heating or acidification, microwave-assisted acidification greatly increases the pore volume of coal and extends the length and width and connectivity of fractures. In addition, the isolated pores were interconnected and the methane migration was facilitated. This work demonstrates the potential of microwave-assisted acidification for CBM recovery.

Kinetic evaluation of hydrate-based coalbed methane recovery process promoted by structure II thermodynamic promoters and amino acids

Coalbed methane recovery is crucial to coal mine safety, greenhouse gas emission reduction and economic benefits. Gas hydrate technology can be employed to separate CH4 from the N2-rich coal mine gas. To enhance hydrate formation kinetics, separation performance and CH4 recovery, we examined the synergistic effect of structure II (sII) hydrate promoters and amino acids on hydrate formation of 30 mol% CH4/70 mol% N2 mixture, involving three thermodynamic promoters (tetrahydrofuran (THF), cyclopentane (CP) and propane) and four amino acids (l-cysteine, l-methionine, l-tryptophan and l-lysine). Amino acids exhibited extraordinary promotion effects on the propane system with over 10 times improvement in kinetics. CP-amino acid systems excelled in separation performance by enriching CH4 content from 30 mol% to 70 mol%. THF-amino acid systems showed moderate performance on kinetics and separation but achieved the highest CH4 recovery up to 50.27%. The impacts of amino acids were system-dependent: they did not alter the hydrate growth pattern of the THF system but completely changed the hydrate morphology of CP and propane systems. Two possible mechanisms were discussed including interfacial tension alteration and hydrophobicity of amino acids. These insights would provide a basis for further optimizing the hydrate process promoted by sII promoters and amino acids for applications including coalbed methane recovery and beyond.

Mechanism of supercritical CO2 on the chemical structure and composition of high-rank coals with different damage degrees

The interaction between carbon dioxide (CO2) and coal plays a vital role in CO2 geological storage enhanced coalbed methane recovery (CO2-ECBM). To investigate the mechanism of supercritical carbon dioxide (SC-CO2) extraction in technically deformed coals under deep coal seam conditions, the chemical composition and structure of technically deformed coals before and after SC-CO2 treatment were analyzed by Fourier transform infrared spectroscopy (FTIR), mass spectrometry (GC–MS) and X-ray diffraction (XRD). The results show that, after SC-CO2 extraction, the absorption peak intensities of weakly polar and non-polar compounds in coal are weakened. With the increase in tectonic deformation degree, the dissolution effect of aromatic compounds increases, while aliphatic compounds show a complementary evolution trend. The dissolution effect of SC-CO2 on carbonate minerals is the most significant, with the highest dissolution amount of 100 %, followed by clay minerals, with the dissolution amount between 1.3 % and 30.5 %, while oxide minerals are relatively stable and hardly react. With the increasing degree of tectonic deformation, the solubility of various minerals tends to increase. In summary, SC-CO2 has a dissolution effect on the organic functional groups and minerals of technically deformed coal, and the effect tends to be significant with the increase of coal tectonic deformation degrees. In view of the important role of mineral and organic composition on the porosity and adsorption of coal, when CO2 is injected into deep coal seams, the response characteristics of porosity, permeability and adsorption caused by the change in mineral and organic composition should also be considered.

Ultrasonic-Assisted Coalbed Methane Recovery: A Coupled Acoustic–Thermal–Mechanical–Hydrological Model

Ultrasonic propagation in coal seams is accompanied by heat transfer, which has the potential to increase coal seam permeability, enhance coalbed methane (CBM) recovery, and prevent coal and gas outbursts. Therefore, it is important to analyze the effects of heat transfer by ultrasonic vibration on CBM recovery and evaluate the prospects of engineering applications of ultrasonic heating technology. In this study, the factors affecting heat transfer by ultrasonic vibration were analyzed theoretically; then, CBM recovery under the condition of ultrasonic heating was simulated by establishing a coupled acoustic–thermal–mechanical–hydrological model. The correctness of acoustic–thermal model was validated by matching simulated data with experimental data. And the accuracy of the gas flow model was verified by comparing the production data from CBM extraction boreholes with the simulated data. The research results were as follows: heat transfer by ultrasonic vibration was affected by frequency and sound pressure. When the ultrasonic frequency varied from 30 to 40 kHz and the sound pressure varied from 0.1 to 0.12 MPa, the lower the frequency, the higher the sound pressure, and the better the ultrasonic vibration heat transfer effect. In addition, the thermal expansion volumetric strain of the coal matrix caused by the ultrasonic heating of coal seams was weaker than the shrinkage volumetric strain of the coal matrix caused by gas desorption, improving the porosity and permeability of the coal seams. Furthermore, the gas drainage standard area increased by 20.8 m2 after 720 days of CBM recovery when replacing conventional CBM recovery with ultrasonic-assisted CBM recovery. With a production time of 720 days, the maximum production of CBM after ultrasonic excitation at a frequency of 40 kHz and a sound pressure of 0.10 MPa increases from 3744 to 9740 m3/day compared to conventional excitation. Our fully coupled acoustic–thermal–mechanical–hydrological model can improve current understandings of heat and mass transfer in thermal simulation of ultrasonic-enhanced CBM recovery.

Experimental study of the effect of ClO2 on coal: Implication for coalbed methane recovery with oxidant stimulation

Oxidation stimulation is a potential method to improve coalbed methane (CBM) recovery. Three different rank coals were treated with ClO2 to investigate the effect of ClO2 on molecular structure, pore structure and gas migration capacity, and the mechanism of ClO2 stimulation on CBM recovery was analyzed. The results show that the contents of aromatic CC and aromatic C–H decrease, while those of polar oxygen functional groups increase. The change in pore structure mainly includes the increasing macropore volume and the decreasing micropore specific surface area and mesopore fractal dimension. The desorption, diffusion and seepage capacities of coal are improved. The enhancement ranges of maximum equivalent desorption rate and permeability are 1.74%–26.19% and 20%–80%, respectively, and the reduction range of tortuosity is 22.73–80.11%. Furthermore, the capillary pressure of coal is reduced, which means less water block damage in gas migration. Based on the analysis of the relationship between pore/macromolecular structure and gas migration capacity, a model of ClO2 stimulation for enhanced CBM recovery is established, and high-volatile bituminous coal is most conducive for ClO2 stimulation.

Thermal-Hydraulic-Mechanical Coupling Simulation of CO2 Enhanced Coalbed Methane Recovery with Regards to Low-Rank but Relatively Shallow Coal Seams

CO2 injection technology into coal seams to enhance CH4 recovery (CO2-ECBM), therefore presenting the dual benefit of greenhouse gas emission reduction and clean fossil energy development. In order to gaze into the features of CO2 injection’s influence on reservoir pressure and permeability, the Thermal-Hydraulic-Mechanical coupling mechanism of CO2 injection into the coal seam is considered for investigation. The competitive adsorption, diffusion, and seepage flowing of CO2 and CH4 as well as the dynamic evolution of fracture porosity of coal seams are considered. Fluid physical parameters are obtained by the fitting equation using MATLAB to call EOS software Refprop. Based on the Canadian CO2-ECBM project CSEMP, the numerical simulation targeting shallow low-rank coal is carried out, and the finite element method is used in the software COMSOL Multiphysics. Firstly, the direct recovery (CBM) and CO2-ECBM are compared, and it is confirmed that the injection of CO2 has a significant improvement effect on methane production. Secondly, the influence of injection pressure and temperature is discussed. Increasing the injection pressure can increase the pressure difference in the reservoir in a short time, so as to improve the CH4 production and CO2 storage. However, the increase in gas injection pressure will also lead to the rapid attenuation of near-well reservoir permeability, resulting in the weakening of injection capacity. Also, when the injection temperature increases, the CO2 concentration is relatively reduced, and the replacement effect on CH4 is weakened, resulting in a slight decrease in CBM production and CO2 storage.

Recent Advances and Perspectives of CO2-Enhanced Coalbed Methane: Experimental, Modeling, and Technological Development

Carbon dioxide (CO2)-enhanced coalbed methane recovery (CO2-ECBM) is a critical way to increase methane production and reduce greenhouse gas (CO2 and CH4) emissions. As captured CO2 is continuously injected in the coal seams, a low cost of CO2 sequestration and high efficiency of CH4 recovery can be achieved via the flooding and replacing effects driven by the injected CO2 flow. Scientific insights into the complex process of CO2-ECBM in experiments, modelings, and technological developments need to be made to propose appropriate countermeasures. This review first highlights the progress of CO2-ECBM under laboratory conditions, e.g., the binary gas competitive adsorption and gas displacement experiments in the macroscale and porous structure tests using technologies of nuclear magnetic resonance (NMR), scanning electron microscopy (SEM), and computed tomography (CT) in the microscale. Then, the advances of mathematical models for changing in coal permeability and porosity during CO2-ECBM are reviewed, accompanying with the multi-field and multi-phase coupling responses of competitive sorption, diffusion, gas–water seepage, heat transfer, and solid deformation. Furthermore, the field pilot tests of CO2-ECBM in various countries and regions are also covered to reveal the key technical challenges confronted with the development of CO2-ECBM technology. The perspectives in experiments, models, and field pilots of CO2-ECBM are made, which include but are not limited to the following: conducting a core CH4/CO2 flooding test under in situ conditions, modeling CO2-ECBM with real fractures/faults and coal failure, developing a new method for gas migration and leakage monitoring in the field, and enacting relevant standards, laws, and regulations to promote CO2-ECBM.