CO2-enhanced Coalbed Methane Recovery Research Articles

Analysis of reservoir permeability evolution and influencing factors during CO2-Enhanced coalbed methane recovery

CO2-ECBM is an effective energy saving and emission reduction technology. Among them, the permeability of coal seam will affect the CO2 injection efficiency, which is the key factor affecting the CH4 production. In order to reveal the evolution of reservoir permeability, effective stress, adsorption/desorption effect, and binary gas seepage and diffusion are considered. The research results indicate that the evolution of permeability can be summarized into two stages. The permeability during the CO2 undisturbed period is mainly controlled by the effective stress, which exhibits a parabolic relationship with permeability. The CO2 disturbed period is mainly controlled by the strain, which exhibits a negative correlation with permeability. Increasing the injection pressure, during the CO2 undisturbed period, the effective stress increases, the strain decreases, and the higher the permeability. During the CO2 disturbed period, the lower the effective stress, the higher the strain, and the lower the permeability. Increasing the injection temperature, the greater the effective stress and permeability in the undisturbed period. In the CO2 disturbed period, the competitive adsorption intensifies, the larger the strain, and the faster the permeability decreases. Increasing the initial water saturation, the slower the gas transportation rate. The slower the effective stress rises in the CO2 undisturbed period, the lower the permeability. In the CO2 disturbed period, the smaller the strain, and the higher the permeability. In summary, injection pressure, temperature, initial water saturation will have an impact on the permeability, in engineering practice, it is not the case that the higher the injection pressure, the higher the injection temperature, the smaller the initial water saturation is the better.

Experimental investigation into coal wettability changes caused by reactions with scCO2-H2O

Geological CO2 sequestration (GCS) can help mitigate global warming and enhance methane recovery from coal beds. However, few studies have linked the effects of CO2 to surface chemistry changes controlling wetting behavior in deep coal beds. Contact angles (CAs) of CO2/N2-high volatile bituminous coal-water systems were measured under different temperatures and pressures. The surface chemistry and physical structure of coals were characterized to investigate changes in physicochemical properties and their relations with wettability after reactions. For N2 treatment, the time-dependence of static and dynamic CAs were insignificant, ranging within 4°. For gaseous CO2 treatment, the static CAs and the average advancing angles increased slightly. With supercritical (sc) CO2, both the static and dynamic CAs increased significantly, and θadv changed to intermediate-wet (92°). Reactions with minerals exposed to scCO2 resulted in greater surface roughness and heterogeneity, greater contact angle hysteresis and more surface sites occupied by scCO2 rather than H2O. Increases in hydrophobic functional groups and decreases in hydrophilicity were shown by FTIR spectra, reflecting the shedding of polar oxygen-containing functional groups, reduction of hydrogen bonds, and increasing percentage of hydrocarbons. XRD patterns obtained following scCO2-treatment showed that crystallite growth and molecular polymerization were higher toward graphite-like. The calculated structural parameters of functional groups and crystallites both showed elevated coal rank. Changes in crystallite structure, notably higher carbon content and decreased negative surface charge, are unfavorable for water-wetting. This study contributes to understanding surface chemistry changes responsible for decreased wettability during CO2-enhanced coal bed methane recovery and GCS in coal reservoirs.

Effects of CO2 injection pressure and confining pressure on CO2-enhanced coalbed methane recovery: Experimental observations at various simulated geological conditions

Systematic understanding of the characteristics of fluid transport and occurrence in conventional Coalbed Methane (CBM) recovery and Injecting CO2 enhancing CBM recovery (CO2-ECBM) engineering plays an essential role in improving the recovery rate of CBM and mitigating the greenhouse effect. In this study, the displacement adsorption time, fluid flow rate and cumulative fluid flow rate of different coal rank samples under conventional and CO2 injection conditions were obtained. The efficiency of exchange adsorption was evaluated based on sweep efficiency and displaced volume. It is found that: (a) In conventional CBM recovery, there is no good correlation between the CBM recovery amount and the gas adsorption capacity of different coal rank samples, but CO2 injection can effectively improve the CBM recovery amount of coal reservoir. (b) In CO2-ECBM, the overall flow rate of the mixed gas decreases rapidly in the initial stage, and then when CO2 is detected at the end, the flow rate of the mixed gas gradually increases and becomes stable until the displacement is completed. (c) Due to the fluid velocity sensitivity effect of coal reservoirs, excessive CO2 injection pressure will have a negative effect on sweep efficiency, but confining pressure will weaken this negative effect. (d) Confining pressure changes the pore-fracture structure of coal reservoirs, strongly influencing conventional CBM recovery. However, it does not change the gas occurrence mechanism in the coal reservoir, it only causes a larger number of gas molecules in the coal matrix instead of the pores, which has little influence on the effectiveness parameters of CO2-ECBM. This research is of great significance for the systematic understanding of the occurrence mechanism of multiple fluids in coal reservoirs, the safe and efficient development of CBM resources, and the realization of carbon neutrality.

Dynamic effects of rough elements in fractures on coal permeability during different stages of methane extraction

Fractures in coal seams are channels for gas migration, and their geometric characteristics can affect permeability, thereby affecting the effectiveness of coalbed methane (CBM) extraction and carbon dioxide sequestration. The surface of natural fractures is rough and composed of minerals with complex shapes. At present, research on rough fracture is mainly focused on surface characteristics, with less attention given to the dynamic effects of rough structure in fractures on coal permeability considering effective stress and matrix adsorption expansion during different stages of methane extraction. In this work, we construct the complex geometric structure in the fracture surface into simplified rough elements. First, a physical model of fracture rough elements was constructed by using the analytical geometry method; meanwhile, the roughness applicable to this model was proposed, and the characterization of roughness degree of different-shaped rough elements was unified. Subsequently, a definition and an approach for selecting and computing effective fracture aperture were proposed. By taking into account the competition between effective stress and matrix adsorption expansion, the change of effective fracture aperture related to pore pressure was determined. Additionally, a novel permeability evolution model was formulated according to the cubic law and compared with the numerical simulation of other researchers for verification. The variation patterns of permeability, gas pressure and fracture aperture under diverse characteristic fracture roughness elements during CBM extraction were obtained by utilizing the proposed model. The findings indicate that the fractures with smaller roughness, higher stagger degree and gentler inclination angle of rough elements are more favorable for CBM extraction. This has inspired a new understanding of the mechanism of CO2-enhanced coal bed methane recovery (CO2-ECBM) technology in deep coal seams with regard to fracture rough element characteristics, which offers valuable theoretical support for the practical implementation of this technology.

Experimental and numerical model of anisotropic permeability and CO2 injectivity of coal during CO2 enhanced coalbed methane recovery process

Guaranteeing CO2 injectivity has been the precondition for implementing the CO2-enhanced coalbed methane recovery (CO2-ECBM), however, it dramatically decreases during the CO2 injection process because it is influenced by the dynamic change of the anisotropic permeability of coal. To reveal and evaluate the CO2 injectivity of coal, the anisotropic permeability test and the CO2 injectivity simulation test were first conducted, then the corresponding numerical models were established and verified by the experimental data. The results show that the permeability of coal in parallel face cleat direction is the largest, followed by the permeability of coal in parallel butt cleat direction and that in vertical bedding direction is the minimum. The peak value of the instantaneous injectivity rate is enhanced and the injection time is prolonged with the increase of the CO2 injection pressure. The total CO2 injectivity rate is nonlinearly increased from 13.61 to 311.87 cm2/MPa min when the CO2 injection pressure raises from 2 to 10 MPa. The anisotropic permeability model is appropriate to describe the dynamic evolution of permeability under different boundary conditions, the CO2 injectivity prediction model can be used to evaluate the CO2 injectivity during the CO2 injection process. Increasing the CO2 injection pressure may temporarily promote CO2 injectivity, while the CO2 injection increment is limited. The CO2 fracturing by phase transition may be an available reservoir stimulation method for enhancing the CO2 injection and should be focused on in the future.

Evaluation of carbon dioxide geological sequestration potential in coal mining area

Coal bases are the concentration areas of coal exploitation and use in China in which the carbon emission sources are concentrated, and carbon emission reduction is challenging. However, the carbon emission sources and geological sequestration sinks typically correlate well with coal bases, providing favorable conditions for clustering carbon capture, utilization, and storage (CCUS). CCUS is the only way for coal bases to achieve near-zero carbon emissions in the future. This study established the constitutive equations of CO2 sequestration capacity for unexploited deep coal seams, residual coal in producing and abandoned mines, and mined-out areas based on their CO2 sequestration form, geological background, and coal mining status. Then, the geological sequestration potential of one of the most important coal bases in east China was discussed according to the constitutive equations of CO2 sequestration capacity by considering matching relationships between sequestration sources (coal-fired power plants) and sinks. The results showed that the coal base has excellent CO2 geological sequestration potential. The total CO2 sequestration capacity in coal seams in the coal base is approximately 0.85 billion tons, including 0.76 billion tons in unexploited deep coal seams, 5.17 million tons in residual coal in producing and abandoned mines, and 82.45 million tons in mined-out areas. The source–sink matching relationships of CO2 geological sequestration in the coal base provide favorable conditions for CCUS, and the average distance between the CO2 sequestration sources and target sequestration sinks is only 10 km. The unexploited deep coal seams with buried depths of 1000–1500 m are primary reservoirs for CO2 geological sequestration in the coal base. In the next 30 years, CO2-enhanced coal bed methane recovery (ECBM) in unexploited deep coal seams and CO2 sequestration in mined-out areas should become a special CCUS technology in the coal base due to their great CO2 geological sequestration potential. CO2-ECBM and CO2 sequestration in mined-out areas are also significant for other coal bases in China.

Mechanisms in CO2-enhanced coalbed methane recovery process

Injection of CO2 and subsequent desorption of CH4 is considered to be the most efficient enhanced coalbed methane (ECBM) recovery technique to date. Meanwhile, CO2-ECBM is an excellent option for CO2 geo-sequestration for an extended period. Despite ongoing research efforts and several field applications of this technology, the mechanisms of the process have yet to be fully understood. The coalbed heterogeneity, the fluid interactions with coal, the CO2 induced swelling, and the continuous pressure and composition changes require outright insights for optimal application of the technique. Furthermore, intermolecular interactions of CO2 and CH4, their competitive adsorption on the dry/wet coal surface, and the dispersion and advection processes play an important role in defining the CO2-ECBM recovery process. An attempt has been made here to understand the key mechanisms of CO2-ECBM recovery in coalfields, particularly the adsorption of CO2 in the supercritical state at the recommended sequestration depth. Cited as: Asif, M., Wang, L., Wang, R., Wang, H., Hazlett, R. D. Mechanisms in CO2-enhanced coalbed methane recovery process. Advances in Geo-Energy Research, 2022, 6(6): 531-534 . https://doi.org/10.46690/ager.2022.06.09

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.

Improved Permeability Model of the Binary Gas Interaction within a Two-Phase Flow and its Application in CO2-Enhanced Coalbed Methane Recovery.

In this paper, we develop a dual-porosity dual-permeability model for binary gas migration to explore the permeability evolution in the matrix and fracture in the process of a gas–water two-phase flow during CO2-enhanced coalbed methane (CO2-ECBM) recovery in coal reservoirs. This mechanistic model accommodates the effects of elastic deformation caused by the effective stress change in the matrix and fracture, the swelling/shrinkage deformation of the matrix caused by adsorption/desorption, the convection and diffusion of gas, and the discharge of water. Specifically, the time-dependent matrix swelling, from initially completely reducing the fracture aperture to finally affecting the coal bulk volume, is considered by the invaded volume fraction involving binary gas intrusion. The model is validated through laboratory data and applied to examine the permeability evolution of CO2-ECBM recovery for 10 000 days. Furthermore, we analyze the sensitivity of some selected initial parameters to capture the key factors affecting CO2-ECBM recovery. Our modeling results show that the permeability evolution can be divided into two stages during the process, where stage I is dominated by effective stress and stage II is dominated by adsorption/desorption. Increasing the injection pressure or initial permeability advances the start of stage II. The decrease in initial water saturation causes the permeability to change more drastically and the time of stage II to appear earlier until a time long enough, after which little effect is seen on the permeability results.

Insight on microscopic mechanisms of CH4 and CO2 adsorption of coal with different ranks

By conducting research on the microscopic mechanisms of CH4 and CO2 adsorption in different rank coals, the CO2-enhanced coal bed methane recovery (CO2-ECBM) technique can be further developed. In this paper, low-pressure N2 adsorption (LPGA-N2), low-pressure CO2 adsorption (LPGA-CO2), isothermal adsorption, Fourier transform infrared (FTIR) experiments, and molecular simulations were applied to analyze the mechanisms. The results demonstrated that the pore volume and specific surface area (SSA) of coal showed a U-shaped trend with the increase of metamorphic. The volume and SSA of the micropore were positively correlated with the ultimate adsorption volume, indicating that micropores were the main sites for adsorption. With the increased metamorphic degree, the aliphatic chains and oxygenated functional groups shed, and the aromatic rings polymerized. These changes in chemical functional groups directly affected the adsorption capacity of coal and indirectly affected the adsorption capacity by affecting micropore evolution. Compared with the CH4 molecule, the CO2 molecule had a smaller structure and a stronger intermolecular force, shown as the larger adsorption energy and closer distance to the coal molecule in the molecular simulation. These findings can provide the data and theoretical basis for the CO2-ECBM technique, coal bed methane extraction, and effect evaluation of extraction.

Adsorption Mechanisms of High-Pressure Methane and Carbon Dioxide on Coals

Carbon dioxide (CO2) geological sequestration and coal-bed methane (CBM) recovery in deep coal seams are usually operated with a pressure higher than 10 MPa. The adsorption mechanisms of methane (CH4) and CO2 on coals in such a situation, however, are not yet revealed. With the help of a high-pressure gas adsorption system, CH4 and CO2 adsorption isotherms were first conducted on two coal samples. Simplified local density (SLD) theory was then tailored and applied to describe the adsorption characteristics of specific CH4 and CO2 on coals. Next, the adsorption mechanisms of high-pressure CH4 and CO2 on coal samples were revealed on the basis of adsorbed and bulk density distributions within the matrix pores. The results show that the high-pressure gas adsorption on coals is different from that at low pressure. The excess adsorption capacity first increases and then decreases with pressure, and the maximum value occurs at a specific pressure referring to reverse pressure. The maximum excess adsorption capacity of CH4 is less than that of CO2, while its reverse pressure is greater than that of CO2. The reversal of excess adsorption with pressure depends on the relative increase in adsorption and bulk phases. However, the mechanisms of adsorption reversal vary with gas types. The reversal of CH4 excess adsorption is due to the saturation near the pore wall, while the CO2 excess adsorption reversal is due to the dramatic bulk density changes near the critical point. Advances in the mechanism of high-pressure gas adsorption on coals suggests that coal seams deeper than 1000 m have more recovery potential, and CO2-enhanced coal-bed methane recovery success depends on the optimal management of its injection pressure and the associated coal swelling.

Mechanism and Characteristics of CH4/CO2/H2O Adsorption in Lignite Molecules

Adsorption characteristics of coalbed methane (CBM) are significant to investigate the absorption of coal, shale, and porous media. In particular, adsorption characteristics of CH4, CO2, and H2O play an important role in predicting CBM output and geologic sequestration potentials of CO2 in research fields of CO2-enhanced CBM recovery (CO2-ECBM) and sequestration of CO2. In this work, adsorption characteristics of CH4, CO2, and H2O in lignite molecules were simulated through the grand canonical Monte Carlo (GCMC) method and molecular dynamics (MD) method. Research results demonstrated that given the same temperature and pressure, the ultimate adsorption capacity of lignite per unit to H2O is the highest, followed by those of CO2 and CH4 successively. All isothermal adsorption curves conform to the “I-type” characteristics. In the saturated molecular configuration, gas molecules show different distribution patterns at two sides of the lignite molecule chain. Lignite has typical physical adsorption to CH4 and CO2, with adsorption energy provided by nonbonding energy. However, lignite has both physical adsorption and chemical adsorption to H2O, with adsorption energy provided by both nonbonding energy and hydrogen bond energy. High temperature is against adsorption of CH4, CO2, and H2O. Temperature might inhibit adsorption of gas molecules. Research conclusions lay foundations for the exploitation and development of CBM and relevant studies on sequestration of CO2.

Experimental study on the softening effect and mechanism of anthracite with CO2 injection

The injection of supercritical CO2 into coal seam alters the mechanical properties of the coal, which should be paid close attention when considering security issues during CO2-Enhanced CoalBed Methane recovery (CO2-ECBM). In this study, the Qinshui Basin is considered as the research object, and mechanical parameters, such as the peak strength, elastic modulus, and Poisson's ratio of anthracites under the coupled influences of CO2 saturation pressure, CO2 saturation time, and water content were investigated. The results indicate a decrease in the peak strength and elastic modulus and an increase in the Poisson's ratio of the CO2-saturated coal. Increasing the CO2 saturation pressure, extending the CO2 saturation time, and increasing the water content contributes to the softening of the coal. The modified Langmuir equation and stretched exponential (SE) equation are used to accurately predict the peak strength and elastic modulus reduction with CO2 saturation pressure and CO2 saturation time, respectively. The failure patterns change from brittle shear failure for natural coal and transitional shear failure for supercritical CO2-saturated coal to the ductile non-dilatant barreling for the supercritical CO2+water-saturated coal. The softening effect of coal can be attributed to the chemical and physical reactions between coal and CO2, including the surface energy reduction of CO2-bearing coal, extraction and plasticization reactions, differential adsorption swelling and swelling stress, and dissolution of carbonate solution for the mineral filling pore/fracture. In view of the geo-mechanical changes caused by CO2 to coal, the optimization of the targeted reservoir is pivotal for the successful implementation of CO2-ECBM.

Multiscale model for flow and transport in CO2-enhanced coalbed methane recovery incorporating gas mixture adsorption effects

In this work we develop a multiscale model for flow and transport problem in CO2-enhanced coalbed methane recovery. The coalbed methane reservoir is characterized by two levels of porosity associated with nanopores in the matrix and cleat network. Mass conservation equations for fluid mixture (CH4 and CO2) in the matrix at the microscale are rigorously derived by using the formal homogenization technique taking into account the gas mixture adsorption in the nanopores. The Density Functional Theory (DFT) is used to compute the gas adsorption isotherms and the solvation force acting on the nanopore wall, showing a much more pronounced adsorption capacity of CO2 compared to CH4. The average transport equations in the matrix together with the multiphase flow problem in fracture network (gas mixture and water) are homogenized giving rise to a macroscopic model ruled by the effective conductivities, partition and transfer coefficients. The cleat permeability evolution due to deformation is taken into account through a three-scale poromechanical model reported in a previous work. Computational simulations illustrate the macroscopic laws underlying the gas pressure distributions, cleat closure phenomena and CH4 production curve enhanced by CO2 injection.

Dynamic Fluid Interactions during CO2-Enhanced Coalbed Methane and CO2 Sequestration in Coal Seams. Part 1: CO2–CH4 Interactions

The injection of CO2 into coalbed methane (CBM) reservoirs to enhance methane recovery has a second desirable benefit in simultaneously sequestering CO2. However, the real-time dynamic evolution of native adsorbed and rejected non-adsorbed methane during the process of CO2-enhanced coalbed methane (CO2-ECBM) production remains poorly constrained as a result of the nonlinear and hysteretic response of both CO2–CH4 interactions (part 1) and CO2–H2O wettability (part 2) of the coal under recreated reservoir conditions. In part 1, we apply calibrated nuclear magnetic resonance (NMR) to explore mechanisms of methane desorption and CO2 replacement during multiple cycles of CO2-ECBM flooding under recreated in situ conditions. Results for contrasting sub-bituminous coal and anthracite indicate that the adsorbed methane sweep efficiency is improved by ∼16–26% with a single injection of CO2 over mere in situ desorption. Furthermore, CO2–CH4 displacement rates evolve during each CO2 injection cycle, first declining rapidly and then stabilizing with a long desorptive tail. Importantly, the cumulative methane sweep efficiency increases monotonically with successive cycles of CO2 injection, albeit at a reducing incremental efficiency, identifying the utility of cyclic CO2-ECBM as an effective method in both CO2 sequestration and enhanced gas recovery. Observed ratios of CO2 sorption capacities to CH4 recovery are 5.0 and 2.2 for sub-bituminous coal and anthracite, respectively, demonstrating an elevated potential for CO2 sequestration in sub-bituminous coals and more favorable CO2-ECBM recovery in anthracite, per unit mass of CO2 injected.

Molecular Simulation Study on the Effect of Coal Rank and Moisture on CO2/CH4Competitive Adsorption

The CO2-enhanced coalbed methane recovery (CO2-ECBM) technique is based on competitive adsorption. In this study, three models of different coal ranks were established using the molecular dynamics ...

The coupling mechanism of the thermal-hydraulic-mechanical fields in CH4-bearing coal and its application in the CO2-enhanced coalbed methane recovery

The injection of CO2 into deep coal seams can not only contribute to the CO2 geological sequestration but also increase the CH4 recovery (CO2-ECBM). Aimed at revealing the coupling mechanism of the thermal-hydraulic-mechanical (THM) fields during the CO2-ECBM process, the numerical models of the THM fields were established. Using COMSOL software, the performances of direct exploitation and CO2-ECBM production are compared. The effects of CO2 pressure injected and production well temperature are provided. The CO2 injection has enhancement effects on gas pressure and CH4 cumulative output. Increasing the CO2 pressure injected can raise the gas pressure and energy in a short time, which improves the efficiency of CO2 injection and CH4 production, and has positive effects on improving CH4 cumulative output and CO2 cumulative storage. The convective heat transfer and solid heat transfer are not obvious due to the low thermal conductivity coefficient and the slow gas migration velocity. Although increasing the production well temperature can raise the CH4 cumulative output and CO2 cumulative storage, the increasing amplitude is small. The differential pressure between the reservoir and the boundaries, and the competitive adsorption of gas are the major factors affecting the permeability. When CO2 doesn't reach the production well, the permeability near the injection well and the production well is mainly affected by the gas competitive adsorption of gas, and the differential pressure, respectively. When CO2 spreads to the production well, the competitive adsorption effect will gradually replace the differential pressure effect on permeability near the production well.

Reservoir Permeability Evolution during the Process of CO2-Enhanced Coalbed Methane Recovery

In this study, we have built a dual porosity/permeability model through accurately expressing the volumetric strain of matrix and fracture from a three-dimensional method which aims to reveal the reservoir permeability evolution during the process of CO2-enhanced coalbed methane (CO2-ECBM) recovery. This model has accommodated the key competing processes of mechanical deformation and adsorption/desorption induced swelling/shrinkage, and it also considered the effect of fracture aperture and effective stress difference between each medium (fracture and matrix). We then numerically solve the permeability model using a group of multi-field coupling equations with the finite element method (FEM) to understand how permeability evolves temporally and spatially. We further conduct multifaceted analyses to reveal that permeability evolution near the wells is the most dramatic. This study shows that the farther away from the well, the gentler the evolution of permeability. The evolution of reservoir permeability near the injection well (IW) and the production well (PW) are very different, due to the combined effects of effective stress changes and gas adsorption and desorption. Furthermore, adsorption is the main controlling factor for the change of permeability for regions near the IW, while the change in effective stress is the main cause for the change in permeability near the PW. Increasing the injection pressure of CO2 will cause the reservoir permeability to evolve more quickly and dynamically.

Molecular simulation of the preferential adsorption of CH4 and CO2 in middle-rank coal

ABSTRACTThe adsorption of the CO2/CH4 mixture in coal affects the CO2-enhanced coalbed methane recovery project. To gain a better understanding of CH4 and CO2 interaction with middle-rank coal, we developed a molecular concept with support for the sorption of CH4 and CO2 on Ximing-8 coal (XM-8) (1.8% vitrinite reflectance). A XM-8 coal model was built by using molecular dynamic (MD) simulations. The molecular simulations were established by the Grand Canonical Monte Carlo and MD methods to study the effects of the temperature, pressure, and species bulk mole fraction on the pure component adsorption isotherms, isosteric heat and adsorption selectivity. It turns out that the CO2 selectivity decreases as the pressure and its own bulk mole fraction increases, but it increases as temperature increases, and the selectivity values are not always greater than 1. The interactions between the small molecules and XM-8 were determined by using density functional theory. It was found that the interactions between the CO2 and XM-8 surface is greater, particularly for the heteroatoms than CH4. The adsorption selectivity and interaction were simultaneously used to reveal that the advantageously substituted range is high temperature, low pressure and a high content of heteroatoms.

Numerical study of CO2-enhanced coalbed methane recovery

Although CO2-enhanced coalbed methane (CO2-ECBM) recovery has been comprehensively investigated, fewer scholars have taken the effect of temperature into account, which brought a large deviation for the study of the influence of CO2-ECBM. In this work, a hydraulic-mechanical-thermal coupled model of CO2-ECBM is established, it combines binary gases (CO2 and CH4) infiltration and diffusion, where non-isothermal adsorption is also considered. The effect of injection pressure and reservoir initial temperature on CO2-ECBM was simulated by the finite element simulation software COMSOL Multiphysics, results show that: the injection of CO2 into coalbed has a good effect on enhancing the production of CH4, and both the CO2 storage rate and CH4 production rate increase with the increase of injection pressure. The effect of initial reservoir temperature on CO2-ECBM is obvious. Since the amount of adsorbed gas in coal decreases with the increase of temperature, the CO2 storage rate and CH4 production rate decrease with the increase of initial reservoir temperature. In the gas extraction process without CO2 injection, the variation of permeability is competition result of two types of factors: the coal matrix shrinkage caused by temperature reduction and gas desorption increase, the other is the coal matrix expansion caused by gas pressure decrease, so the permeability follows the rule of first decreasing and then increasing with the extraction time. The injection of CO2 has a great influence on the permeability of coalbed, adsorption of CO2 by the coal matrix causes the permeability to drop rapidly.