CBM Recovery Research Articles

Investigation on the influence of the macropores in coal on CBM recovery

As an important component of coal structure, the macropores have a great influence on CBM recovery. In this paper, the macropores characteristic of two coal samples collected from 3# coal seam in the south of Qinshui Basin, China was analyzed on the basic of mercury intrusion porosimetry, and a fully coupled triple-porosity/double permeability mathematical model for CBM recovery was established according to the physical structure of coal and the non-Darcy flow of methane in macropores. Then, the various factors affecting the macropores permeability were discussed and the influence of size distribution and connectivity of macropores on CBM recovery was investigated. The following conclusions have been drawn from these efforts: (1) in 3# coal seam of the south of Qinshui Basin, the macropores have an extremely heterogeneous pore size distribution with the high variation, and their connectivity is not good because they are mainly composed of the conical and cylindrical pores with one dead end and the open pores, the structure characteristics of macropores are not conducive to CBM recovery; (2) the fully coupled triple-porosity/double permeability mathematical model containing the non-Darcy flow of methane in the macropores includes the methane occurrence-migration field, hydraulic field, thermal field and stress field as well as the complex intercoupling between them, and the model was verified by the fitting of methane production history, with an average error of 3.24%; (3) the macropores permeability is closely related to the Knudsen number controlled by methane pressure and temperature in macropores and the intrinsic permeability which is an internal attribute of macropores affected by size distribution and connectivity of pore; (4) the pressure drop of reservoir plays a major role in the macropores permeability, which promotes the increase of macropores permeability with time, and the high intrinsic permeability of macropores corresponding to the good pore size distribution and connectivity is more conducive to the improvement of fracture permeability and methane production rate of coal reservoir during CBM recovery. It is recommended that coal seams including the macropores with uniform size distribution and good connectivity should be preferentially used for the development of CBM.

Experimental study of coal rank effect on carbon dioxide injection to enhance CBM recovery

Coal seam gas extraction is an important part of green mining. At the same time gas injection for enhanced coalbed methane recovery (ECBM) has been proposed as an environmentally friendly, low-carbon and effective technology to increase methane production. To study the influence of coal rank on carbon dioxide (CO2)-ECBM efficiency, lignite, bituminous coal, and anthracite were tested using the Gas Flow and Displacement Testing Apparatus (GFDTA). Data indicated that: (1) in lignite and anthracite, outflow CO2 and CH4 fractions changed at higher rates as compared with bituminous coal. At the end of the experiments, lignite and anthracite displayed CH4 outflow fractions of 0.19% and 7.73%, respectively; (2) at the initial stage of the experiments, permeability rapidly decreased. Later, a fast increase was followed by a stable phase. In lignite and anthracite, the increment in permeability during the rapid increase stage was significantly larger than that of bituminous coal. (3) The CH4 flow rate of the three coal ranks showed a decreasing trend. Before the breakthrough, CH4 flow rate decreased rapidly and slowed down. After the breakthrough, CO2 flow displayed a linear increasing trend and later reached stability. (4) In general, CH4 production increased with the increase in coal rank. The variation of CH4 production is manifested in two phases with different dominant factors. In the first stage, CH4 production was mainly affected by permeability, and followed the order lignite > anthracite > bituminous. In the second stage, CH4 production was mainly controlled by the displacement effect, following the order anthracite > bituminous coal > lignite.

Research on gas injection to increase coalbed methane production based on thermo-hydro-mechanical coupling

Gas injection to increase coalbed methane production is an effective energy saving and emission reduction technology, with triple benefits of clean energy production, environmental protection and safety production. In this paper, a coupled thermo-hydro-mechanical flow (THM) model is developed, the effects of different injected gases on ECBM are considered, the effect of initial water saturation on CO2/N2-ECBM is quantified using the control variable method. Exploring changes in storage and production during the injection gas mixture enhanced CBM recovery (GM-ECBM). The results show that: The breakthrough time of CO2 injection was much later than N2 injection. The higher the initial water saturation, the lower the CH4 production and gas storage. CO2 injection will reduce the permeability of coal seam and damage it, while N2 injection can effectively restore the permeability. The higher the initial water saturation, the slower the permeability decrease during CO2-ECBM and the slower the permeability increase during N2-ECBM. Increasing the N2 injection concentration before the N2 breakthrough time can promote CH4 production, and increasing the CO2 injection concentration near the N2 breakthrough time will delay the N2 breakthrough time and provide more time for CH4 to migration in the pore space, which will increase the CH4 production. When injecting gas mixture into water-bearing coal seam, the higher the initial water saturation, the slower the coal seam permeability increases when N2 is disturbed, and the slower the coal seam permeability decreases when CO2 and N2 are co-disturbed, while both production and storage will decrease.

Enhancement of biomethane production from coal by supercritical CO2 extraction

The recalcitrant nature of organic matter in coal presents a significant challenge to biomethane production. Supercritical CO2 has gained considerable attention for its unique extraction capabilities. This study uses four coal samples of varying ranks to investigate the promotion mechanism of biomethane production by supercritical CO2 extraction in combination with anaerobic digestion (Sc-AD). Methanogenic potential oxygenated functional groups, intermediate liquid phase products, microbial community structure, and methanogenic metabolic pathways were analyzed between Sc-AD and AD systems. The study found that microorganisms in the Sc-AD system produce more extracellular polymers that enhance the content of aromatic proteins in the anaerobic digestion system. The liquid-phase compounds extracted by supercritical CO2 were consumed by fermentative acidogenic bacteria, significantly increasing the metabolic efficiency of intermediates such as indole and catechol. The reduction in the relative content of carbonyl and carboxyl groups on the coal surface in the Sc-AD system was attributed to the successive attacks by supercritical CO2 and microorganisms on the coal surface. This resulted in the shedding of carbonyl and carboxyl groups from the coal surface, which were then utilized to synthesize small molecule acids. In contrast, hydroxyl groups were retained in the coal. The Sc-AD system exhibited a more stable and efficient microbial community structure in methane production than the AD system. The study demonstrated that supercritical CO2 extraction is a potential method for enhancing CBM recovery and CO2 reduction.

Micropore Structure Changes in Response to H2O2 Treatment of Coals with Different Ranks: Implications for Oxidant Stimulation Enhancing CBM Recovery

Micropore Structure Changes in Response to H2O2 Treatment of Coals with Different Ranks: Implications for Oxidant Stimulation Enhancing CBM Recovery

Numerical analysis of matrix swelling and its effect on microstructure of digital coal and its associated permeability during CO2-ECBM process based on X-ray CT data

Matrix swelling effect will cause the change of microstructure of coal reservoir and its permeability, which is the key factor affecting the engineering effect of CO2-ECBM technology. The Sihe and Yuwu collieries are taken as research objects. Firstly, visualization reconstruction of coal reservoir is realized. Secondly, the evolution of the pore/fracture structures under different swelling contents is discussed. Then, the influence of matrix phase with different swelling contents on permeability is discussed. Finally, the mechanism of swelling effect during the CO2-ECBM process is further discussed. The results show that the intra-matrix pores and matrix-edge fractures are the focus of this study, and the contacting area between matrix and pore/fracture is the core area of matrix swelling. The number of matrix particles decreases with the increase of size, and the distribution of which is isolated with small size and interconnected with large size. The swelling effect of matrix particles with larger size has a great influence on the pore/fracture structures. The number of connected pores/fractures is limited and only interconnected in a certain direction. With the increase of matrix swelling content, the number, porosity, width, fractal dimension, surface area and volume of pores/fractures decrease, and their negative contribution to absolute permeability increases from 0.368% to 0.633% and 0.868%–1.404%, respectively. With the increase of swelling content, the number of intra-matrix pores gradually decreases and the pore radius becomes shorter during the CO2-ECBM process. The matrix continuously expands to the connected fractures, and the width of connected fractures gradually shorten. Under the influence of matrix swelling, the bending degree of fluid flow increases gradually, so the resistance of fluid migration increases and the permeability gradually decreases. This study shows that the matrix swelling effect is the key factor affecting CBM recovery, and the application of this effect in CO2-ECBM process can be discussed.

NMR investigation on gas desorption characteristics in CBM recovery during dewatering in deep and shallow coals

Abstract Coalbed methane (CBM) development requires dewatering until the reservoir pressure is less than the critical desorption pressure. Significant quantities of CBM in China are buried >1000 m deep. Therefore, the desorption characteristics of deep CBM reservoirs must be investigated for the further development of deep CBM. In this study, the variation laws of adsorbed and free CH4 during adsorption in dry samples and during desorption via dewatering are investigated using nuclear magnetic resonance. During CH4 adsorption in dry samples by increasing CH4 pressure and during CH4 desorption in water-injected samples by dewatering, a Langmuir relationship exists between the volume of adsorbed CH4 and the pressure in deep and shallow coals, and the volume of free CH4 and the pressure are linearly related. When the pressure is the same, the volume of adsorbed CH4 in the dry coal samples during adsorption is larger than that in the water-injected samples during desorption by dewatering. When the pressure is the same, for the difference in the adsorbed CH4 volume between adsorption and desorption isotherms, shallow coal is less significant than deep coal. The slopes of free CH4 in deep coal are lower than those in shallow coal during adsorption and desorption.

Coupled coal–gas interaction during CBM and CO2-ECBM recovery in coal seams: a critical review

Coupled coal–gas interaction during CBM and CO2-ECBM recovery in coal seams: a critical review

Laboratory experiments of CO2-enhanced coalbed methane recovery considering CO2 sequestration in a coal seam

CO2-enhanced coalbed methane (CO2-ECBM) recovery has the triple benefits of recovering clean resources, ensuring coal mine safety, and achieving CO2 geological sequestration. To study the synergistic effect of CH4 recovery and CO2 sequestration, we designed a large-scale multifunctional apparatus that can realistically simulate the environment of CO2-ECBM technology. The results showed that the reservoir pressure dropped rapidly during conventional CBM recovery, but remained at a high level for a long time during CO2-ECBM recovery. The reservoir temperature close to the injection well was dominated by CO2 adsorption, whereas that close to the production well was dominated by CH4 desorption. The recovery efficiency of CH4 increased from 66.67% during conventional CBM recovery to 90.86%–93.50% after CO2-ECBM recovery. However, the sequestration efficiency of CO2 decreased from 67.89% to 43.98% with increasing CO2 injection pressure from 1.0 to 1.6 MPa. We then proposed a displacement index to reflect the synergistic effect of CH4 recovery and CO2 sequestration. This index first increased then decreased during CO2-ECBM recovery, dropping significantly at the last stage under increased injection pressure. Therefore, we suggest that a dynamic pressure injection mode may be more effective for ensuring efficient CH4 recovery and CO2 sequestration effect.

Fracture processes in coal measures strata under liquid CO2 phase transition blasting

Liquid carbon dioxide phase-transition blasting (LCO2-PB) has been applied to improve the permeability coal seam and enhanced coalbed methane recovery (ECBM). However, the mechanical behavior of crack generation and propagation in LCO2-PB under true triaxial stresses, has not been studied in detail. In this paper, five small-scale experiments of LCO2-PB were carried out with different restraint stresses on concrete blocks (150 × 150 × 150 mm3). Three uniaxial strain-gauges were used to monitor the surface strain of the blocks. The results show that the dynamic shock and expansion exists in the process of LCO2-PB. The release pressure markedly affected the relationship between shock and expansion. The expansion of the borehole and the fracture zone decrease with the increase in the confining pressure stress, and it cannot affect the number and direction generated of the blast cracks. The release pressure plays a decisive role in the number of fractures. The propagation process of LCO2-PB fracture was affected by the combination of LCO2 volume and surrounding rock stress field. The increase in the volume of LCO2, mainly influenced the length of cracks rather than the number of those. The propagation direction of fractures can be determinate by the effects of confining pressure. The research results will help enhance CBM recovery and reduce the occurrence of the gas disaster.

Analysis of permeability evolution mechanism during CO2 enhanced coalbed methane recovery based on impact factor method

To studying the influence of CO2 injection on CBM recovery and reservoir permeability, a hydraulic-mechanical-thermal coupling numerical model considering gas flow, competitive adsorption, temperature, and coal deformation was established. The model validation was carried out first, then the CO2 enhanced CBM recovery (CO2-ECBM) processes under different injection pressure and temperature were simulated, and the characteristics of CH4 output and reservoir permeability evolution were analyzed. Finally, the impact factor method for quantitative evaluating the contribution of critical parameters was proposed based on the porosity model. The results show that the production increases with the increase of injection pressure. When the injection pressure is 4 MPa, the production increases by 27,860 m3. When the injection pressure was not higher than the initial reservoir pressure, the coal matrix's deformation was dominated by CH4 desorption. When the injection pressure was higher than the initial reservoir pressure, the deformation was dominated by CH4 desorption, and the permeability increased at the preliminary stage. As time increased, the relative impact factor of CH4 desorption (RIFD) decreased, and the relative impact factor of CO2 adsorption (RIFA) increased; the deformation was dominated by CO2 adsorption, so the permeability decreased. The relative impact factor of pore pressure (RIFP) decreases with the increase of injection pressure, and its maximum value is only 0.014. The microscopic mechanism of permeability evolution was analyzed in this paper from the perspective of coal matrix deformation. The results can provide a theoretical reference for the parameter optimization of CO2-ECBM engineering applications.

Influence of coal deformation on the Knudsen number of gas flow in coal seams

The effective stress and pore pressure can change the fracture aperture, which in turn can change the methane Knudsen number and transitions between gas flow regimes. A novel Knudsen number (Kn) model for gas flow under coal deformation was established; a revised permeability model was formulated and used to obtain an equation for the Klinkenberg coefficient b. The predictions of the revised permeability model were in good agreement with experimental data, and the parameters used to determine the gas Kn were obtained. An increase in the pore pressure under coal deformation causes the fracture aperture to increase and the gas molecular mean free path to decrease, thereby decreasing the Kn. Thus, the critical pore pressure for the transition between gas flow regimes is reduced. The gas Kn for a deformed fracture is higher than that for a constant fracture aperture. The critical fracture aperture for the transition between gas flow regimes increases under coal deformation. The Klinkenberg coefficient b for methane is a variation value that increases as Kn decreases. The Kn variation is very important for determining the coal permeability and which can be used to resolve problems in CBM recovery.

Pore/fracture structure and gas permeability alterations induced by ultrasound treatment in coal and its application to enhanced coalbed methane recovery

Ultrasonic treatment is a highly potential method for reservoir permeability modification in enhanced coalbed methane recovery. Quantitative evaluations of ultrasonic induced changes in structure and permeability in coal are of great interest for potential field applications of ultrasound technology. In this study the scanning electron microscope (SEM) and low-temperature liquid nitrogen (LTLN) were used to examine the ultrasound induced alterations in coal pore/fracture, and further measured gas permeability in coal by coupling the ultrasound field and external stress. The SEM images show that ultrasound treatment can extend the original pore/fracture, increase the connectivity and remove barriers in gas seepage channel in coal. The LTLN data reveals that with the ultrasonic treatment (25 kHz, 18 kW, 2 h), the maximum N 2 adsorption quantity of the tested samples increased by 106.5%–258.4%, and the average pore diameter increased by 37.06%–81.40%; the specific surface area increased by 94.80%–118.15%, and the total pore volume increased by 87.12%–252.55%. It implies that coal pores in size ranging from 1 nm to 100 nm can be remarkably improved by ultrasonic treatment, which is significant beneficial to promote gas desorption/diffusion within coal matrix. Permeability tests demonstrate that gas permeability in stressed coal increased by 29.3%–57.1% with the implement of ultrasound field (25 kHz, 18 kW, 30 min). Ultrasonic performance on permeability improvement is proportional to the treatment time, but the function of ultrasonic fracturing has time threshold effect, and after a certain period of treatment, the ultrasonic fracturing performance becomes very limited in stressed coal. • Coal pore/fracture structure alterations induced by ultrasound treatment is examined using SEM and LTLN techniques . • Ultrasound treatment significantly increases the N 2 accessible specific surface area and pore volume in coal. • External stress and ultrasound treatment time influences gas permeability behavior in coal induced by ultrasonic fracturing. • A conceptual field application scheme of ultrasonic fracturing for enhanced CBM recovery is proposed.

Study on the Coal Damage and Fracture Mechanism under Multiple Actions of Blasting Stress Wave

Controlled blasting technology is an effective way to enhance coalbed methane recovery, but the theoretical research has fallen behind engineering practice seriously. Therefore, in this work, based on the damage and fracture mechanics theory, the coal damage process and mechanism under the multiple actions of the blasting stress wave (BSW) were analyzed firstly. Then, the simulation experiment of coal damage accumulation was designed and carried out; during the experiment, the ultrasonic velocity of the samples was measured and the damage values were calculated. Under the multiple actions of BSW, the initial radial cracks around the blast-hole wall or in the area near the explosion source are formed under the action of tangential tension of BSW and the internal pressure of the blast-hole, and under the combined actions of stress wave, gas, and in situ stress, the primary cracks in the middle and far area of the explosion source expand and form the mesoscopic cracks because of the damage accumulation at the crack tip. And then the mesoscopic cracks grow and intersect to form a macroscopic crack. Finally, the macroscopic cracks expand and intersect with the initial radial cracks, the samples break suddenly under the load far below their fracture toughness because of the damage accumulation effects. Hence, the research results have guiding significance for better understanding and application of controlled blasting technology to enhance the CBM recovery.

Effects of pore morphology and moisture on CBM‐related sorption‐induced coal deformation: An experimental investigation

AbstractCoalbed methane (CBM) is an important resource of energy. For CBM recovery, sorption‐induced coal deformation can cause significant reservoir permeability change. Moisture content and coal pore morphology affect the gas adsorption capacity and can alter the coal deformation of the coal seam. Therefore, it is crucial to establish a coal gas moisture‐coupled model for CBM production prediction. However, there are currently insufficient data available for quantitative analyses. In this paper, a series of typical sorption‐induced strain experiments were carried out during methane adsorption and desorption on coal samples with different metamorphic degrees and moisture content. The pore morphology and adsorption capacity of coals were measured to analyze the reason for different deformation of coals with various pore structures and moisture. Results show that the adsorption capacity and deformation is corresponding to the specific surface area of micropore, which first decreases and then increases with coal ranks. The deformation and adsorption gas content of dried coals is greater than that of natural moisture coals, which means that moisture can reduce the sorption capacity of coals, resulting in the decrease of gas adsorption‐induced coal deformation. There is also residual deformation of coal samples after gas desorption caused by the residual adsorbed gas in coals. This paper quantitatively investigates the effects of pore characteristics and moisture on coal deformation and the internal mechanism. This work will provide essential information for building out a fully cross‐coupled model of coal gas‐moisture relationships for CBM production prediction.

Experimental Investigation of CS2 Extraction to Enhance the Permeability of Coal

To examine the permeability enhancement effect of the carbon disulfide (CS2) extraction on block coal samples, in this study, different cylindrical samples of anthracite coal were selected to conduct CS2 extraction experiments. Comprehensive measurement techniques, including Fourier transform infrared spectroscopy, X-ray diffraction, low-temperature nitrogen adsorption, industrial computed tomography, and permeability measurements, were employed to analyze the variations of in the chemical structure, microcrystalline structure, pore–fracture structure, and coal permeability before and after the CS2 extraction treatment. The results showed that CS2 extraction can improve the connectivity of multi-scale migration channels, with which the coal permeability can be enhanced. Also, it was found that the permeability enhancement performance by the CS2 extraction is related to the coal anisotropy, as the coal permeability in the vertical bedding direction is smaller than that in the parallel bedding direction before and after CS2 extraction. Also, the CS2 extraction had a time effect on enhancing the coal permeability, and the permeability enhancement was more significant after a longer period of CS2 treatment. It can be predicted that the permeability can be enhanced up to more than 1 mD when the extraction time exceeds 6 h. Also, we concluded that CS2 fracturing is worth exploring by the on-going studies to enhance the permeability and CBM recovery in low-permeability reservoirs.

Feasibility study of enhanced biogenic coalbed methane production by super-critical CO2 extraction

Super-critical CO2 enhanced coalbed methane (Sc-CO2-ECBM) and microbially enhanced coalbed methane (MECBM) are environment-friendly technologies that can improve CBM recovery and generation. In this study, a new approach of MECBM based on Sc-CO2 extraction is presented. The Sc-CO2 pretreatment experiments of anthracite and bituminous coal were conducted to produce biomethane. The extracted organics and the changes of coal structure caused by Sc-CO2 were also analyzed to discuss the mechanism of methane stimulation. The results indicated that methane yields have been greatly improved after Sc-CO2 extraction by 734.85% and 148.15% for anthracite and bituminous coal, respectively. The extractions observed by GC-MS analysis is also favored to generate methane by microorganisms. The little increment of methane production from coal treated by subcritical CO2 indicated that the special characteristics of Sc-CO2 were critical for the stimulation of methane production. More functional groups and even new functional groups were formed to increase coal bioavailability after Sc-CO2 extraction. The specific surface area and total pore volume of bituminous coal increase after Sc-CO2 extraction that could provide more action sites for microorganisms and enzymes. These results strongly proved the feasibility of enhancing CBM by microbial degradation based on Sc-CO2 extraction.

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.

Methane Adsorption Interpreting with Adsorption Potential and Its Controlling Factors in Various Rank Coals

Water content, metamorphism (coal rank) particle size, and especially pore structure, strongly influence the adsorption capacity of coal to methane. To understand the mechanism of methane adsorption in different rank coals, and its controlling factors, isothermal adsorption experiments with different coal ranks, moisture contents and particle sizes at the temperature of 303.15 K were conducted. In addition, the pore structures of coals were investigated through N2 adsorption/desorption experiments at the low-temperature of 77 K for selected coals from the Junggar Basin of NW China, Qinshui Basin and Ordos Basin of north China. Moreover, the adsorption potential of methane on the surface of the coal matrix was calculated, the controlling factors of which were discussed. The obtained methane isothermal adsorption result shows that the Langmuir volume (VL) of coal is independent of the particle size, and decreases with the increase of moisture content, which decreases first and then increases when the coal rank increases. Combined with the pore structure by the N2 adsorption at 77 K, VL increases with the increase of pore surface area and pore volume of coal pores. Besides, the adsorption potential of all selected coals decreased with the increase of the methane adsorption volume, showing a negative relationship. The interesting phenomena was found that the surface adsorption potential of the coal matrix decreases with the increase of moisture content, and increases with the decrease of particle size at the same pressure. With the same adsorption amount, the adsorption potential on the surface of coal matrix decreases first, and then increases with the increase of coal rank, reaching a minimum at Ro,m of 1.38%, and enlarging with the increase of pore surface area and the pore volume of coal pores. These findings may have significant implications for discovering CBM accumulation areas and enhancing CBM recovery.

Mechanistic insight into the optimal recovery efficiency of CBM in sub-bituminous coal through molecular simulation

In this study, a newly created Fortran program was used to assist in the constructing of a sub-bituminous coal model (SCM). Followed by grand canonical Monte Carlo simulations, the adsorption behaviors of CBM adsorption on dry-/moisture-SCM systems have been systematically studied at different pressure-temperature conditions. A geological model with typical geothermal and pressure gradients was established to investigate the effect of coupled pressure and temperature on the absorption and recovery. As a result, pure-N2 injection has been confirmed to paly a negative role on CH4 recovery, whereas CO2 could effectively improve the CO2 sequestration and CH4 displacement. Meanwhile, the adsorption percentage (δ) and the recovery efficiency (η) were first put forward in terms of theoretical calculations. The results showed that, the δ was determined by pore size distribution of the SCM, adsorption temperature/pressure, moisture-containing in coal and the types of adsorbates. The value of ηCH4 is 53.07% in dry coal seam, whereas 47.20% is evaluated in moisture coal seam, the corresponding optimum mining depths are 600 m and 800 m, respectively. Remarkably, water-containing in coal seam has an obviously inhibitory effect on simultaneous extraction of coal and gas, and it should be valued in the follow-up studies. Based on this principle, the optimum mining depth for CO2-ECBM was predicted to be ∼800 m in actual deep coal seams through our geological model. Our results provide a molecular-scale insight into CBM recovery efficiency in coal at different burial depths, and offer useful guidance for realistic exploitation.