Adsorption Capacity Of Coal Research Articles

Modelling of the thermal history of the Carboniferous Lorraine Coal Basin: Consequences for coal bed methane

This paper proposes a new scenario for the thermal history of the Carboniferous Lorraine Coal Basin following the tectonic model developed by Averbuch et al. (2012) and establishes some consequences for the coal bed methane (CBM) exploitation. Organic matter maturation data (Vitrinite Reflectance) determined on eleven boreholes in the eastern Lorraine have been used to characterize the Lorraine Coal Basin evolution. Paleozoic and Mesozoic overburials have been calculated using a thermal modelling software (Petromod). Results show that (1) Paleozoic erosion may be estimated at a maximum of 1200m which represents a low amplitude event, (2) a little erosion occurred between Upper Paleozoic and Lower Mesozoic: paleotemperature offset is about 20°C and VR data range from 0.7 (Westphalian D) to 0.5 (Lower Triassic), (3) Cretaceous cover overburial reaches a maximum of 300m and decreases eastwards and (4) the variation of heat flows is in agreement with the compressive and extensive phases of Paleozoic and the extensive phase of Mesozoic. Consequences on the petroleum system are the following: organic matter is immature in the Jurassic and Triassic sedimentary rocks, mature (oil window) in the Permian, Stephanian and Westphalian C-D, highly mature (gas window) in Westphalian A-B-C and overmature in Namurian-Westphalian A. The high methane adsorption capacity of coal and the presence of natural fractures inside coal seams demonstrated by coal tomography allow a high CBM potential in this basin. The Lorraine Coal Basin is therefore a target for coal gas.

Influence of coal petrology on methane adsorption capacity of the Middle Jurassic coal in the Yuqia Coalfield, northern Qaidam Basin, China

The lower coalbed methane (CBM) adsorption capacity of the low rank coals is not only related to its lower maturity, but also determined by the coal maceral compositions. In this study, a total of 13 samples including 10 coals and 3 carbonaceous mudstones, were collected from the Middle Jurassic Dameigou Formation in the borehole YQ-1 of the Yuqia Coalfield, northern Qaidam Basin, NW China. Coal lithotypes, maceral compositions, coal ranks, coal facies and methane adsorption characteristics of these samples were investigated using microscopic observation, proximate analysis, porosity analysis, and isothermal adsorption experiments. The results show that the maceral composition has a great influence on the methane adsorption capacity, and the influence of vitrinite on the methane adsorption is generally stronger than that of inertinite at a similar coal rank. For most samples, there is a positive correlation between the vitrinite contents and the Langmuir volume (VL), a negative correlation between the inertinite contents and the VL, and no obvious correlation between the exinite contents and the VL. Furthermore, the vitrinite/inertinite (V/I) ratio also shows a positive relationship with the VL. However, some samples containing large amounts of unfilled fusinite and/or semifusinite have more pore spaces favoring methane adsorption and can also adsorb significant quantities of methane. Consequently, coals with higher vitrinite contents, in association with some unfilled fusinites or semifusinites, should have the greatest adsorption capacity. Coal maceral compositions vary with the types of coal facies, and thus the methane adsorption capacity of coals may be closely related to coal facies. It is found that the methane adsorption capacities of the coals in a wet forest swamp (Type I) and an intergradation forest swamp (Type II) are greater than those in a drained forest swamp (Type III) and an open water peat swamp (Type IV). It is suggested that the area developed with the wet forest swamp and in the intergradation forest swamp with high porosity should be the best target areas for the CBM exploration in the Yuqia Coalfield.

Adsorption capacity, adsorption potential and surface free energy of different structure high rank coals

Methane adsorption capacity of coal is one of the major factors influencing total gas-in-place and it is linked to coal structure, while our knowledge of it is still limited. In this study, coal structures were classified into five categories, including integrated, blocky, cataclastic, granulated and mylonitized. The integrated and blocky coals are intact and the other three are deformed. To investigate the methane adsorption capacity, adsorption potential and surface free energy of different structure coals, four high rank coal samples were collected from No.3 coal seam of the Zhaozhuang coal mine in Southeastern Qinshui Basin China, and a series of methane isothermal adsorption experiments and pore distribution measurements were carried out in the laboratory, and then the adsorption potential and the surface free energy were analyzed. It turns out that, there are no obvious differences in the methane adsorption capacity for the various structure coals when the equilibrium pressure is below 2MPa; while there are significant differences when the pressure is above 2MPa. The higher the gas pressure is, the greater the difference is. Methane adsorption capacity of different structure coals can be arranged in a descending order: mylonitized>granulated>cataclastic>intact. That is, the methane adsorption capacity is higher for the deformed coal samples than for the intact ones. Both the specific surface area and the pore volume increase with the increase of deformation degree. Mesopores contribute most to the total pore ​​volume; while the adsorption pores have the greatest contribution to the total specific surface area. The adsorption potential and surface free energy for different-structure coals under the same adsorption space volume increase with the increase of deformation degree. Adsorption potential decreases with the increase of adsorption space, and the adsorption potential of micropore is considerably higher than that of mesopore and macropore. As the temperature rises, the cumulative reduction of surface free energy decreases and the surface free energy decreases significantly at each equilibrium pressure. Methane adsorption in coal is dominated by adsorption potential and surface free energy.

Experimental observations of matrix swelling area propagation on permeability evolution using natural and reconstituted samples

Many researchers have concluded that the permeability should be measured after the adsorption equilibrium state is reached and in many permeability models, it is normally assumed that pm (pressure in the coal matrix) is equalized with pf (pressure in the fractures). However, when the dynamic balance between pm and pf is broken which exists in most coalbed methane engineering, the differences between the two pressures induced by gas diffusion and matrix shape factor will generate nonuniform matrix swelling which have not been well learned to date, thus influence the permeability evolution. It is of great importance to investigate this issue on account of it will play a more important role in CO2-enhanced coalbed methane and CO2 sequestration engineering as the coal adsorption capacity for CO2 is considerably higher than that for CH4. To achieve this target, a series of novel coal permeability tests and simultaneous strain measurements were conducted using both natural coal and reconstituted coal. In these experiments, the radial strain evolution characteristics and the permeability evolution characteristics as a function of the adsorption equilibrium time and confining pressure were obtained. Based on the experimental results, the effects of the differences between pm and pf on radial strain evolution and coal permeability evolution were discussed, and a new matrix swelling area propagation theory was achieved. In addition, the differences between natural coal and reconstituted coal in terms of their matrix swelling and stress sensitivity properties were obtained.

Theoretical insight into the enhanced CH4 desorption via H2O adsorption on different rank coal surfaces

The density functional theory was used to investigate the adsorption of CH4 and H2O on different rank coal surfaces. The coal rank is the dominant factor in affecting the adsorption capacity of coal. In order to better understand gas and water interaction with coal of different maturity, we developed fourteen coal models to represent the different rank coal. The interactions of CH4 and H2O with coal surfaces were studied and characterized by their adsorption energies, Mulliken charges and electrostatic potential surfaces. The results revealed that the interaction between coal and CH4 was weak physical adsorption, and that the interaction between coal and H2O consisted of physical and chemical adsorption. Adsorption energy of coal–H2O system was larger than that of coal–CH4 on all rank coals, suggesting that the adsorption priority in the coal models is H2O>CH4. Consequently, the injection of H2O into the different rank coal could effectively enhance the coal bed methane (CBM) recovery.

Effects of temperature and pressure on gas desorption in coal in an enclosed system: a theoretical and experimental study

The desorption of a gas from any type of coal depends on the gas's pressure and temperature. The adsorption capacity of coal decreases as temperature increase and gas pressure decrease. Desorption capacity increases as temperature rise, and then gas pressure of system also rises in a closed system. The objective of this paper was to determine the relationship between desorption temperature and gas pressure in a closed coal-gas system. At temperatures between 30°C and 240°C, three different types of coal samples (such as lean coal, anthracite and coke coal) were prepared for desorption experiments with methane and nitrogen. Curves describing the relationship between desorption pressure and temperature were obtained. Based on the Boltzmann energy distribution law and the ideal gas state equation, a theoretical model of the desorption pressure and temperature in the system considered was established. The Dubinin-Radushkevich equation was also introduced into the new model. Finally, the gas adsorption capacities of coal at any temperature and pressure could be forecast using the new mathematical model.

Characteristics of coal re-oxidation based on microstructural and spectral observation

In order to investigate the characteristics of re-oxidation of residual coal in goafs in close coal seam mining, scanning electron microscope and infrared spectrometer are used to study the changes of coal microstructure and chemical reaction of functional groups of eight coal samples at different ranks. Result shows that after initial oxidation, the surface morphology of pore are different, and the porosity of coal is increased and the oxygen adsorption capacity of coal is improved. The change of coal molecular structure and presence of a large amount of active oxygen-containing functional groups lead to increasing tendency of coal to further oxidation. In addition, the higher lever of the initial oxidation is, the easier the re-oxidation occurs.

흡착에 의한 석탄암체의 부피변화가 고려된 흡착모델 개선 및 부피변형률 예측

Abstract - We proposed the improved Langmuir adsorption relations considering volume change effect of coal matrix during primary production of CBM and Enhanced-CBM with injection of carbon dioxide or CCS in coalseam but also volumetric strain. To verify this model, experimental data of pure gas adsorption such as CO 2 , CH 4 , and N 2 on coals were used to compare conventional Langmuir model with this model. From the re-sults, we obtained that the larger adsorption capacity of coal and the higher adsorption affinity of gas, the larger error occur with Langmuir model. Using this model, however, we found not only substantially better fit in all condition but also reasonable volumetric strain of the coal matrix. We also applied this volume modified pure gas adsorption model to the IAS model to describe gas adsorption and volumetric strain for mixed gas. This modified-IAS model fitting experimental data by Hall et al(1994) improved accuracy of mixed gas adsorption calculation compared with conventional model.Key words : volume modified adsorption isotherm, volumetric strain, pure gas, mixture gas, ECBMKIGAS Vol. 19, No. 2, pp 37~44, 2015 http://dx.doi.org/10.7842/kigas. 2015.19.2.37(Journal of the Korean Institute of Gas)

Deformation and reservoir properties of tectonically deformed coals

Tectonic deformation and pore structure characteristics of tectonically deformed coals (TDC) collected from the Hancheng area, Weibei block, eastern Ordos Basin were characterized through scanning electron microscope, micro-CT, mercury porosimetry and low-temperature nitrogen adsorption experiments. The isothermal adsorption experiment of lumpy TDC was also performed to investigate the gas storage behavior. The results show that the primary layer structure can be observed in cataclastic coals and granulated coals. These coals show sub-angular particles of 1–3 cm. For scaled coals and mylonitic coals, the primary structure and coal bedding have been destroyed, and the coal is sheared with directional arrangement of grains. The pore volume, pore surface area, micro-pore volume and pore connectivity increase with the increase in deformation extent. The CH4 adsorption capacity of coals shows an increasing trend from brittle deformation to ductile deformation. It is concluded that the coal structure, pore and fracture characteristics of TDC control the gas adsorption capacity of coals.

Quantitative characterization of methane adsorption on coal using a low-field NMR relaxation method

To develop a nuclear magnetic resonance (NMR) method for characterizing the methane adsorption capacity of coals, we built an NMR transparent isotherm adsorption experimental setup. Proton (1H) NMR measurements were first performed with bulk methane to obtain a hydrogen amplitude index reflecting the methane volume concentration. Then, dry coals were pressurized with methane to assess the quantity of adsorbed methane at pressures up to 6.1MPa. The adsorption isotherms obtained by this procedure were compared with the corresponding sorption isotherms determined by the traditional volumetric method under the same experimental conditions. The methane transverse relaxation time (T2) spectra have three distinct peaks at T2<7ms, T2=7–240ms, and T2=240–2000ms. The peaks at T2<7ms and T2=7–240ms both correspond to the surface relaxation mechanism, and individually, they are interpreted as “coal-adsorbed methane” and “porous medium-confined methane”, respectively. The peak at T2=240–2000ms represents the relaxation of bulk methane. The integrated amplitude of the “porous medium-confined methane” peak has a positive linear relationship with pressure, whereas the integrated amplitude of the “coal-adsorbed methane” peak with increasing pressure follows the Langmuir equation. The adsorption isotherms from the NMR and volumetric methods are nearly identical. The absolute deviations of the experimental data points from the two methods fall within ±2m3/t, and the calculated Langmuir volumes have absolute deviations of <0.38m3/t and relative deviations of <1.24%. We suggest that the low-field NMR may be applied as a noninvasive analytical technique to characterize the methane adsorption capacity of coals.

Effects of igneous intrusion on microporosity and gas adsorption capacity of coals in the Haizi Mine, China.

This paper describes the effects of igneous intrusions on pore structure and adsorption capacity of the Permian coals in the Huaibei Coalfield, China. Twelve coal samples were obtained at different distances from a ~120 m extremely thick sill. Comparisons were made between unaltered and heat-affected coals using geochemical data, pore-fracture characteristics, and adsorption properties. Thermal alteration occurs down to ~1.3 × sill thickness. Approaching the sill, the vitrinite reflectance (R o) increased from 2.30% to 2.78%, forming devolatilization vacuoles and a fine mosaic texture. Volatile matter (VM) decreased from 17.6% to 10.0% and the moisture decreased from 3.0% to 1.6%. With decreasing distance to the sill, the micropore volumes initially increased from 0.0054 cm3/g to a maximum of 0.0146 cm3/g and then decreased to 0.0079 cm3/g. The results show that the thermal evolution of the sill obviously changed the coal geochemistry and increased the micropore volume and adsorption capacity of heat-affected coal (60–160 m from the sill) compared with the unaltered coals. The trap effect of the sill prevented the high-pressure gas from being released, forming gas pocket. Mining activities near the sill created a low pressure zone leading to the rapid accumulation of methane and gas outbursts in the Haizi Mine.

Influences of temperature and moisture on coal sorption characteristics of a bituminous coal from the Sydney Basin, Australia

Accurate testing of coal isotherms plays a significant role in the areas of coal seam gas drainage, outburst control, CO2 geo-sequestration, coalbed methane (CBM) and enhanced coalbed methane recovery (ECBM). Coal isotherms were tested with CO2 and CH4 gases with a modified indirect gravimetric method in the University of Wollongong (UOW), Australia. The adsorption capacity of coal shows a linear decrease with increasing temperature. The decreasing rate of adsorption capacity is greater for CO2 than CH4, and which is also higher for dry coals. The moist coal sample isotherms for both CO2 and CH4 demonstrate a much lower sorption capacity. The average reduction ratio of coal adsorption capacity of CO2 and CH4 with moisture is 11.1% and 14.0%, respectively in the testing temperature range. The experimental results show that moisture content in coal has greater effect on the adsorption capacity of CH4 than that of CO2.

Pore structure and its impact on CH4 adsorption capacity and flow capability of bituminous and subbituminous coals from Northeast China

Adsorption-pore (pore size less than 100nm) and seepage-pore (pore size greater than 100nm) structures have great effects on gas adsorption/diffusion and gas flow in coal seam, respectively. Pore properties, including porosity, size/volume distribution, volumes, surface fractals, specific surface area, and connectivity, for four coals from Northeast China were acquired through mercury porosimetry, N2 adsorption at 77K, small angle X-ray scattering (SAXS) and their relationships with CH4 adsorption capacity and permeability are investigated. The roughness of pore surface was analyzed with surface fractal dimensions. Obtained values of fractal dimensions from mercury porosimetry and N2 adsorption at 77K were comparable with values determined by SAXS measurement. The surface fractals results show that the more irregular surface, the more inhomogeneous pore structures is, meaning more surface area and then stronger adsorption capability, especially for the micropores with sizes in the range of 2–10nm and the mesopores. Moreover, with the data of petrographic, proximate and ultimate analyses, the ratio of C/H, moisture content, ash yield also have great effects on CH4 adsorption capacity of coals. For bituminous and subbituminous coals, macropores have significant impacts on gas flow. The coals with high contents of macroporosity generally have good gas flow capability. Therefore, they may have significant implications for coalbed methane (CBM) exploitation.

Recoverable Potential of Coalbed Methane in the Wangying-Liujia Depression, Fuxin Basin, China

The Wangying-Liujia depression in the Fuxin basin covers a 20 km2 area and has abundant coalbed methane resource with in-place gas resource 51.81 × 108 m3. Compared with the basins where coalbed methane (CBM) development is successful, the recoverable potential of CBM is evaluated in the investigated district. In the Lower Cretaceous Fuxin Formation, the coal seams are thick (averagely 35.21 m – 42.96 m), the coal adsorption capacity is comparatively high with about 18∼34 m3/t, and the gas content is relatively high (8 m3/t – 10 m3/t). Vertical lithological sealing, lateral fault sealing and hydrodynamics are favorable for the preservation of CBM. Thermogenic gas generated at the end of early Cretaceous and Himalayan Orogeny is the main gas source. Secondary biogenic gas generated from the end of the Cretaceous to now increases gas content. All these provide resource basis for CBM development, and the recoverable resources is 15.543 × 108 m3. Though the permeability is low (well test 0.323 md – 0.469 md), coal can be fractured to improve the reservoir permeability. The coal reservoir is saturated gas or oversaturated, and the ratio of critical pressure to reservoir pressure is high (&gt;0.74), which indicates that a little pressure need be decreased to make CBM desorbed from coal matrix and gas production. In one word, there are preferable recoverable potential in Wangying-Liujia depression, especially in the area limited by Piangan F2 fault, Liujia F2 and β5–2 dike. At present, 17 CBM production wells with 50 000 m3 per day operate in this area.

Estimation of Gas Adsorption Capacity in Coal: A Review and an Analytical Study

Adsorption plays an important role in carbon dioxide sequestration and methane recovery processes in deep coal seams. If the effects of coal physical properties on its gas adsorption capacity are considered, it increases with vitrinite content and pressure and reduces with liptinite, mineral matter, moisture contents, and temperature and follows a U-shaped variation with carbon content. Furthermore, CO2 has higher adsorption capacity compared to other gases. There are two main methods to estimate the amount of gas adsorbed in a coal seam, which are called direct and indirect methods. The latter method is more common for coal. In the direct method, the volume of gas released from a coal mass into a sealed desorption canister is measured, and under the indirect method, adsorption isotherms and empirical relations are used. However, any of those methods are unable to count the effect of swelling-induced strain and, therefore, fail in measuring the absolute adsorption in coal. Moreover, among various adsorption models, Langmuir and Dubnin-Radushkevich (D-R) equations are the most widely using models. However, since gas can be absorbed through both adsorption and absorption processes for coal, it is important to have an additional term to count the missing absorption mechanism in both the Langmuir and D-R models. Finally, a new descriptive model for gas adsorption capacity of coal as a function of effective factors is proposed. The new model is based on the existing D-R equation, which was modified by inserting a new expression for the term of micropore capacity. Two types (CO2 and N2) of gas adsorption data for coal from five different locations (British Colombia and Alberta in Canada and Victoria, Sydney and Bowen in Australia) at three different temperatures (273, 296.5, and 318 K) were considered for the model development. According to the model results, new gas adsorption equations can fairly well accurately predict the adsorption capacity in coal.

Influences of igneous intrusions on coal rank, coal quality and adsorption capacity in Hongyang, Handan and Huaibei coalfields, North China

article i nfo Localized igneous intrusions with varying types of intrusion patterns were found in the Pennsylvanian-Permian coals in north China. Five typical patterns, including dike cut-through (pattern-I), dike cut-in (pattern-II), floor intru- sion by sill (pattern-III), roof intrusion by sill (pattern-IV) and dual intrusions of roof and floor by sills (pattern-V), were investigated at five different underground profiles. It was found that the influence of localized intrusions on rank, petrology and coal quality characteristics are mainly related to the emplacement temperature, the style of heat transfer (convection or conduction), the intrusion forms (dike or sill) and size, the distance from the contact and the thermal properties of the surrounding rocks at the contact of the intrusion. Among the five patterns, only pattern-V was found to have two distinct contact metamorphic aureoles within a distance of 1-2timesofthethick- ness of the intrusion, whereas patterns I through IV show wave-like profiles of vitrinite-reflectance, ash and volatile matter. This resulted from the typical characteristics of multiphasic and superimposed thermal metamorphic evo- lution of north China coals. Except for the heat conduction by intrusion contact, the hydrothermal convection and tectonic-heat played important roles in heat transfer away from dike/sill. Intrusion-induced coal changes including coal rank, organic/inorganic composition and pore properties work together to influence the adsorption capacity of coals. The effect of intrusion upon the adsorption capacity of altered coals is related to the values of their altered coal ranks. Adsorption capacity is elevated from the Langmuir volume (VL) of 7.6 (pre-intrusion) to 17.5 m 3 /t (post-intrusion) for altered bituminous coals and semi-anthracites with VRrb2.1%. In contrast, the adsorption ca- pacity is moderately reduced from background levels of about 27.2 m

Petrographic and geochemical effects of sill intrusions on coal and their implications for gas outbursts in the Wolonghu Mine, Huaibei Coalfield, China

Since the early 1980s, fifteen outbursts have occurred in the Huaibei Coalfield of China. These outbursts were reported to be associated with sills. To study the effect of sill intrusions on coal seam and their relationship to methane outbursts, eleven samples from the No. 10 coal seam were taken from the Wolonghu Mine at various distances from a diorite sill. Comparisons were made between unaltered and heat-affected coals using petrographic and chemical data, micropore characteristics, adsorption properties of coal, and gas outburst indexes from field. Approaching the intrusion, vitrinite reflectance levels increased from 2.74% to 5.03%, and the thermal aureole of the sill ~ 60 m (from the sill boundary to sample 9). Three zones along this gradient were identified as corresponding to (1) thermal evolution zone No. 1 (0–5 m from sill), (2) thermal evolution zone No. 2 (5–60 m from sill), and (3) unaltered zone. The methane adsorption capacity of coal samples in the thermal evolution zone No. 2 was generally higher than in the two other zones, and the unaltered zone higher than the thermal evolution zone No. 1. It is concluded that the contact-metamorphism decreased the adsorption capacity of coal and the thermal evolution of sill increased it. The trap effect of sill, combined with the mudstone and siltstone roof and floor of the No. 10 coal seam, provided a seal for the formation of a gas pocket. Abnormally high formation pressures at the No. 10 coal seam led to two outbursts.

Impact of thermal processes on CO2 injectivity into a coal seam

The objective of this study is to investigate how thermal gradients, caused by CO2 injection, expansion and adsorption, affect the permeability and adsorption capacity of coal during CO2 sequestration. A new permeability model is developed in which the concept of elastic modulus reduction ratio is introduced to partition the effective strain between coal matrix and fracture. This model is implemented into a fully coupled mechanical deformation, gas flow and heat transport finite element simulator. To predict the amount of CO2 sequested, the extended Langmuir sorption model is used, with parameters values taken from the literature. The coupled heat and gas flow equations, are solved in COMSOL using the finite element method. The simulation results for a constant volume reservoir demostrate that thermal strain acts to significantly reduce both CO2 injectivity and adsorption capacity. These impacts need to be considered in the calculation of the optimum injection rate and the total sequestration capacity.

Gas Transport in Bidisperse Coal Particles: Investigation for an Effective Diffusion Coefficient in Coalbeds

Abstract Much work has been carried out on adsorption capacity of coals. Diffusive transport processes within the coal matrix blocks could be the rate-limiting step for adsorption during gas injection and production operations. Identifying these processes and determining their contributions to overall mass transport is a complex and time consuming procedure. The paper presents numerical diffusion models in varying coal particles and investigates transport mechanisms. For this purpose, the coal particle is represented as a microporous solid penetrated by a network of larger interconnected macropores. The solid adsorbs the bulk of the gas. A simple relationship between the apparent and intrinsic Fickian diffusion coefficients is derived in the case of single-component (methane) transport and Langmuir-type adsorption. Mass transport in the bidisperse coal particle is significantly influenced by the adsorption in the microporous solid. The investigation is then extended to study concentration dependence of the microporous solid diffusion for binary (methane-CO2) mixtures using the Maxwell-Stefan formulation. It is found that co-diffusion of the gas molecules enhances the gas mass transport in the solid in the presence of competitive sorption dynamics, while counter-diffusion may diminish the gas mass transport. The influence of lateral interactions among the adsorbed molecules in the solid phase is discussed. The work finds application in modelling CBM and CO2-ECBM processes. Introduction As an unconventional natural gas resource, coalbed methane receives worldwide attention. Deep coal seams that are not accessible for mining are suitable for in situ gas production using conventional drilling, well completion and gas recovery technologies. Hence, a vast amount of natural gas is globally available. Unlike conventional gas resources, however, the gas storage, flow and transport processes in coalbeds are quite complex mainly due to the intricate nature of coal(1–6). Coalbeds are porous media often characterized by a bimodal pore structure: a primary structure consisting of micro- and meso-scale pores, and a secondary structure with macropores and interconnected natural fractures. The microporous coal has an extremely large internal surface area and a strong affinity for certain naturally occurring chemical species such as methane, carbon dioxide, nitrogen and water. At high coalbed pressures, therefore the majority of the natural gas-in-place, in particular, methane, exists abundantly at an adsorbed liquid-like state in the microporous solid(1). Depressurizing the coalbed may yield a significant volume of natural gas. However, the initial stage of recovery is often dominated by water production, whereas, the latter stage is under the influence of diffusional resistances of the primary pores of the coal matrix. Treatment and disposal of the produced water is expensive and its long-term environmental impact has not been clearly understood yet. Injection of a second gas with much higher adsorption capacity, on the other hand, could possibly enhance recovery by maintaining the overall reservoir pressure, thus, keeping the water production at a minimum level. Additionally, injecting a second gas would promote methane desorption by lowering the partial pressure of the coalbed methane in the migrating gas phase.

Adsorption mechanism of different coal ranks under variable temperature and pressure conditions

Variable temperature and pressure adsorption tests were conducted on four coal samples with different coal ranks, under simulated temperatures and pressures corresponding to coal reservoirs at different depths. The regularity of the variation in the amounts of adsorption by coals under variable temperature and pressure and 30° C isothermal conditions are compared and the adsorption characteristics of coal under the composite effect of temperature and pressure were obtained. The adsorption test and data processing method of coal under variable temperature and pressure are presented and the effect of the mechanism of temperature and pressure on the adsorption capacity of coal has been studied. The research results are of significant importance in the investigation of coalbed methane storage mechanism and for the prediction of the amounts of coalbed methane at various depths.