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1.
Sorption and desorption behaviour of methane, carbon dioxide, and mixtures of the two gases has been studied on a set of well-characterised coals from the Argonne Premium Coal Programme. The coal samples cover a maturity range from 0.25% to 1.68% vitrinite reflectance. The maceral compositions were dominated by vitrinite (85% to 91%). Inertinite contents ranged from 8% to 11% and liptinite contents around 1% with one exception (Illinois coal, 5%). All sorption experiments were performed on powdered (−100 mesh), dry coal samples.Single component sorption/desorption measurements were carried out at 22 °C up to final pressures around 51 bar (5.1 MPa) for CO2 (subcritical state) and 110 bar (11 MPa) for methane.The ratios of the final sorption capacities for pure CO2 and methane (in molar units) on the five coal samples vary between 1.15 and 3.16. The lowest ratio (1.15) was found for the North Dakota Beulah-Zap lignite (VRr=0.25%) and the highest ratios (2.7 and 3.16) were encountered for the low-rank coals (VRr 0.32% and 0.48%) while the ratio decreases to 1.6–1.7 for the highest rank coals in this series.Desorption isotherms for CH4 and CO2 were measured immediately after the corresponding sorption isotherms. They generally lie above the sorption isotherms. The degree of hysteresis, i.e. deviation of sorption and desorption isotherms, varies and shows no dependence on coal rank.Adsorption tests with CH4/CO2 mixtures were conducted to study the degree of preferential sorption of these two gases on coals of different rank. These experiments were performed on dry coals at 45 °C and pressures up to 180 bar (18 MPa). For the highest rank samples of this sequence preferential sorption behaviour was “as expected”, i.e. preferential adsorption of CO2 and preferential desorption of CH4 were observed. For the low rank samples, however, preferential adsorption of CH4 was found in the low pressure range and preferential desorption of CO2 over the entire pressure range.Follow-up tests for single gas CO2 sorption measurements consistently showed a significant increase in sorption capacity for re-runs on the same sample. This phenomenon could be due to extraction of volatile coal components by CO2 in the first experiment. Reproducibility tests with methane and CO2 using fresh sample material in each experiment did not show this effect.  相似文献   

2.
There is still no clear understanding of the specific interactions between coal and gas molecules. In this context sorption–desorption studies of methane and carbon dioxide, both in a single gas environment and gas mixtures, are of fundamental interest. This paper presents the results of unique simultaneous measurements of sorption kinetics, volumetric strain and acoustic emission (AE) on three tetragonal coal samples subjected to sorption of carbon dioxide and methane mixtures. The coal was a high volatile bituminous C coal taken from the Budryk mine in the Upper Silesia Basin, Poland. Three different gas mixtures were used in the sorption tests, with dominant CO2, with dominant CH4 and a 50/50 mixture.The experimental set-up was designed specially for this study. It consisted of three individual units working together: (i) a unit for gas sorption experiments using a volumetric method, (ii) an AE apparatus for detecting, recording and analysing AE, and (iii) a strain meter for measuring strains induced in the coal sample by gas sorption/desorption. All measurements were computer aided.The experiments indicated that the coal tested showed preferential sorption of CH4 at 2.6 MPa pressure and exhibited comparable affinities for CH4 and CO2 at higher pressures (4.0 MPa). The results of chromatographic analysis of the gas released on desorption suggested that the desorption of methane from the coal was favoured. The relationship between the volumetric strain and the amount of sorbed gas was found to be non-linear. These results were contrary to common opinions on the coal behaviour. Furthermore, it appeared that the swelling/shrinkage of coal was clearly influenced by the network of fractures. Besides, the AE and strain characteristics suggested common sources of sorption induced AE and strain.The present results may have implications for the sequestration of carbon dioxide in coal seams and enhanced coalbed methane recovery (ECBM).  相似文献   

3.
Supercritical gas sorption on moist coals   总被引：2，自引：1，他引：1  
The effect of moisture on the CO2 and CH4 sorption capacity of three bituminous coals from Australia and China was investigated at 55 °C and at pressures up to 20 MPa. A gravimetric apparatus was used to measure the gas adsorption isotherms of coal with moisture contents ranging from 0 to about 8%. A modified Dubinin–Radushkevich (DR) adsorption model was found to fit the experimental data under all conditions. Moisture adsorption isotherms of these coals were measured at 21 °C. The Guggenheim–Anderson–de Boer (GAB) model was capable of accurately representing the moisture isotherms over the full range of relative pressures.Moist coal had a significantly lower maximum sorption capacity for both CO2 and CH4 than dry coal. However, the extent to which the capacity was reduced was dependent upon the rank of the coal. Higher rank coals were less affected by the presence of moisture than low rank coals. All coals exhibited a certain moisture content beyond which further moisture did not affect the sorption capacity. This limiting moisture content was dependent on the rank of the coal and the sorbate gas and, for these coals, corresponded approximately to the equilibrium moisture content that would be attained by exposing the coal to about 40–80% relative humidity. The experimental results indicate that the loss of sorption capacity by the coal in the presence of water can be simply explained by volumetric displacement of the CO2 and CH4 by the water. Below the limiting moisture content, the CO2 sorption capacity reduced by about 7.3 kg t− 1 for each 1% increase in moisture. For CH4, sorption capacity was reduced by about 1.8 kg t− 1 for each 1% increase in moisture.The heat of sorption calculated from the DR model decreased slightly on addition of moisture. One explanation is that water is preferentially attracted to high energy adsorption sites (that have high energy by virtue of their electrostatic nature), expelling CO2 and CH4 molecules.  相似文献   

4.
CO2 injection in unmineable coal seams could be one interesting option for both storage and methane recovery processes. The objective of this study is to compare and model pure gas sorption isotherms (CO2 and CH4) for well-characterised coals of different maturities to determine the most suitable coal for CO2 storage. Carbon dioxide and methane adsorption on several coals have been investigated using a gravimetric adsorption method. The experiments were carried out using both CO2 and CH4 pure gases at 25 °C from 0.1 to 5 MPa (1 to 50 bar). The experimental results were fitted using Temkin's approach but also with the corrected Langmuir's and the corrected Tóth's equations. The two last approaches are more accurate from a thermodynamical point of view, and have the advantage of taking into account the fact that experimental data (isotherms) correspond to excess adsorption capacities. These approaches allow better quantification of the adsorbed gas. Determined CO2 adsorption capacities are from 0.5 to 2 mmol/g of dry coal. Modelling provides also the affinity parameters of the two gases for the different coals. We have shown these parameters determined with adsorption models could be used for classification and first selection of coals for CO2 storage. The affinity ratio ranges from a value close to 1 for immature coals to 41 for high rank coals like anthracites. This ratio allows selecting coals having high CO2 adsorption capacities. In our case, the modelling study of a significant number of coals from various ranks shows that anthracites seem to have the highest CO2 storage capacities. Our study provides high quality affinity parameters and values of CO2 and CH4 adsorption capacities on various coals for the future modelling of CO2 injection in coal seams.  相似文献   

5.
Gas adsorption isotherms of Akabira coals were established for pure carbon dioxide (CO2), methane (CH4), and nitrogen (N2). Experimental data fit well into the Langmuir model. The ratio of sorption capacity of CO2, CH4, and N2 is 8.5:3.5:1 at a lower pressure (1.2 MPa) regime and becomes 5.5:2:1 when gas pressure increases to 6.0 MPa. The difference in sorption capacity of these three gases is explained by differences in the density of the three gases with increasing pressure. A coal–methane system partially saturated with CH4 at 2.4 MPa adsorption pressure was experimentally studied. Desorption behavior of CH4 by injecting pure CO2 (at 3.0, 4.0, 5.0, and 6.0 MPa), and by injecting the CO2–N2 mixture and pure N2 (at 3.0 and 6.0 MPa) were evaluated. Results indicate that the preferential sorption property of coal for CO2 is significantly higher than that for CH4 or N2. CO2 injection can displace almost all of the CH4 adsorbed on coal. When modeling the CH4–CO2 binary and CH2–CO2–N2 ternary adsorption system by using the extended Langmuir (EL) equation, the EL model always over-predicted the sorbed CO2 value with a lower error, while under-predicting the sorbed CH4 with a higher error. A part of CO2 may dissolve into the solid organic structure of coal, besides its competitive adsorption with other gases. According to this explanation, the EL coefficients of CO2 in EL equation were revised. The revised EL model proved to be very accurate in predicting sorbed ratio of multi-component gases on coals.  相似文献   

6.
CBM and CO2-ECBM related sorption processes in coal: A review   总被引：1，自引：0，他引：1  
This article reviews the state of research on sorption of gases (CO2, CH4) and water on coal for primary recovery of coalbed methane (CBM), secondary recovery by an enhancement with carbon dioxide injection (CO2-ECBM), and for permanent storage of CO2 in coal seams.Especially in the last decade a large amount of data has been published characterizing coals from various coal basins world-wide for their gas sorption capacity. This research was either related to commercial CBM production or to the usage of coal seams as a permanent sink for anthropogenic CO2 emissions. Presently, producing methane from coal beds is an attractive option and operations are under way or planned in many coal basins around the globe. Gas-in-place determinations using canister desorption tests and CH4 isotherms are performed routinely and have provided large datasets for correlating gas transport and sorption properties with coal characteristic parameters.Publicly funded research projects have produced large datasets on the interaction of CO2 with coals. The determination of sorption isotherms, sorption capacities and rates has meanwhile become a standard approach.In this study we discuss and compare the manometric, volumetric and gravimetric methods for recording sorption isotherms and provide an uncertainty analysis. Using published datasets and theoretical considerations, water sorption is discussed in detail as an important mechanisms controlling gas sorption on coal. Most sorption isotherms are still recorded for dry coals, which usually do not represent in-seam conditions, and water present in the coal has a significant control on CBM gas contents and CO2 storage potential. This section is followed by considerations of the interdependence of sorption capacity and coal properties like coal rank, maceral composition or ash content. For assessment of the most suitable coal rank for CO2 storage data on the CO2/CH4 sorption ratio data have been collected and compared with coal rank.Finally, we discuss sorption rates and gas diffusion in the coal matrix as well as the different unipore or bidisperse models used for describing these processes.This review does not include information on low-pressure sorption measurements (BET approach) to characterize pore sizes or pore volume since this would be a review of its own. We also do not consider sorption of gas mixtures since the data base is still limited and measurement techniques are associated with large uncertainties.  相似文献   

7.
Numerical modelling of the processes of CO2 storage in coal and enhanced coalbed methane (ECBM) production requires information on the kinetics of adsorption and desorption processes. In order to address this issue, the sorption kinetics of CO2 and CH4 were studied on a high volatile bituminous Pennsylvanian (Upper Carboniferous) coal (VRr=0.68%) from the Upper Silesian Basin of Poland in the dry and moisture-equilibrated states. The experiments were conducted on six different grain size fractions, ranging from <0.063 to 3 mm at temperatures of 45 and 32 °C, using a volumetric experimental setup. CO2 sorption was consistently faster than CH4 sorption under all experimental conditions. For moist coals, sorption rates of both gases were reduced by a factor of more than 2 with respect to dry coals and the sorption rate was found to be positively correlated with temperature. Generally, adsorption rates decreased with increasing grain size for all experimental conditions.Based on the experimental results, simple bidisperse modelling approaches are proposed for the sorption kinetics of CO2 and CH4 that may be readily implemented into reservoir simulators. These approaches consider the combination of two first-order reactions and provide, in contrast to the unipore model, a perfect fit of the experimental pressure decay curves. The results of this modeling approach show that the experimental data can be interpreted in terms of a fast and a slow sorption process. Half-life sorption times as well as the percentage of sorption capacity attributed to each of the two individual steps have been calculated.Further, it was shown that an upscaling of the experimental and modelling results for CO2 and CH4 can be achieved by performing experiments on different grain size fractions under the same experimental conditions.In addition to the sorption kinetics, sorption isotherms of the samples with different grain size fractions have been related to the variations in ash and maceral composition of the different grain size fractions.  相似文献   

8.
Complete sorption isotherm characteristics of methane and CO2 were studied on fourteen sub-bituminous to high-volatile bituminous Indian Gondwana coals. The mean vitrinite reflectance values of the coal samples are within the range of 0.64% to 1.30% with varying maceral composition. All isotherms were conducted at 30 °C on dry, powdered coal samples up to a maximum experimental pressure of ~ 7.8 MPa and 5.8 MPa for methane and CO2, respectively.The nature of the isotherms varied widely within the experimental pressure range with some of the samples remained under-saturated while the others attained saturation. The CO2 to methane adsorption ratios decreased with the increase in experimental pressure and the overall variation was between 4:1 and 1.5:1 for most of the coals. For both methane and CO2, the lower-ranked coal samples generally exhibited higher sorption affinity compared to the higher-ranked coals. However, sorption capacity indicates a U-shaped trend with rank. Significant hysteresis was observed between the ad/desorption isotherms for CO2. However, with methane, hysteresis was either absent or insignificant. It was also observed that the coal maceral compositions had a significant impact on the sorption capacities for both methane and CO2. Coals with higher vitrinite contents showed higher capacities while internite content indicated a negative impact on the sorption capacity.  相似文献   

9.
Interpretation of carbon dioxide diffusion behavior in coals   总被引：3，自引：1，他引：3  
Storage of carbon dioxide in geological formations is for many countries one of the options to reduce greenhouse gas emissions and thus to satisfy the Kyoto agreements. The CO2 storage in unminable coal seams has the advantage that it stores CO2 emissions from industrial processes and can be used to enhance coalbed methane recovery (CO2-ECBM). For this purpose, the storage capacity of coal is an important reservoir parameter. While the amount of CO2 sorption data on various natural coals has increased in recent years, only few measurements have been performed to estimate the rate of CO2 sorption under reservoir conditions. An understanding of gas transport is crucial for processes associated with CO2 injection, storage and enhanced coalbed methane (ECBM) production.A volumetric experimental set-up has been used to determine the rate of sorption of carbon dioxide in coal particles at various pressures and various grain size fractions. The pressure history during each pressure step was measured. The measurements are interpreted in terms of temperature relaxation and transport/sorption processes within the coal particles. The characteristic times of sorption increase with increasing pressure. No clear dependence of the characteristic time with respect to the particle size was found. At low pressures (below 1 MPa) fast gas diffusion is the prevailing mechanism for sorption, whereas at higher pressures, the slow diffusion process controls the gas uptake by the coal.  相似文献   

10.
The cylindrical coal samples were subjected to three successive cycles of sorption–desorption processes of a single gas (CO2, CH4). Acoustic emission (AE) and strains were simultaneously recorded during the sorption and desorption processes.Tests were conducted on medium-rank coal from the Upper Silesia Basin, Poland. Follow-up tests for gas sorption–desorption consistently showed significant changes of AE characteristics for re-runs on the same sample. The AE level decreased in each successive test. The most spectacular differences were observed between AE generated during the first cycle of gas sorption and the subsequent cycle. This phenomenon could be due to structural changes in the coal taking place substantially on its first exposure to the sorbate. The AE results indicate, that each cycle of gas sorption–desorption was run on the same coal though with a somewhat different structure.In those tests, the swelling of coal by CO2 or/and CH4 was anisotropic (greater in the direction perpendicular to the bedding plane than parallel) in each cycle of the gas sorption–desorption process.  相似文献   

11.
Enhanced coalbed methane (ECBM) involves the injection of a gas, such as nitrogen or carbon dioxide, into the coal reservoir to displace the methane present. Potentially this strategy can offer greater recovery of the coal seam methane and higher rates of recovery due to pressure maintenance of the reservoir. While reservoir simulation forms an important part of the planning and assessment of ECBM, a key question is the accuracy of existing approaches to characterising and representing the gas migration process. Laboratory core flooding allows the gas displacement process to be investigated on intact coal core samples under conditions analogous to those in the reservoir. In this paper a series of enhanced drainage core floods are presented and history matched using an established coal seam gas reservoir simulator, SIMED II. The core floods were performed at two pore pressures, 2 MPa and 10 MPa, and involve either nitrogen or flue gas (90% nitrogen and 10% CO2) flooding of core samples initially saturated with methane. At the end of the nitrogen floods the core flood was reversed by flooding with methane to investigate the potential for hysteresis in the gas displacement process. Prior to the core flooding an independent characterisation programme was performed on the core sample where the adsorption isotherm, swelling with gas adsorption, cleat compressibility and geomechanical properties were measured. This information was used in the history matching of the core floods; the properties adjusted in the history matching were related to the affect of sorption strain on coal permeability and the transfer of gas between cleat and matrix. Excellent agreement was obtained between simulated and observed gas rates, breakthrough times and total mass balances for the nitrogen/methane floods. It was found that a triple porosity model improved the agreement with observed gas migration over the standard dual porosity Warren-Root model. The Connell, Lu and Pan hydrostatic permeability model was used in the simulations and improved history match results by representing the contrast between pore and bulk sorption strains for the 10 MPa cases but this effect was not apparent for the 2 MPa cases. There were significant differences between the simulations and observations for CO2 flow rates and mass balances for the flue gas core floods. A possible explanation for these results could be that there may be inaccuracy in the representation of mixed gas adsorption using the extended Langmuir adsorption model.  相似文献   

12.
Laboratory experiments were conducted to investigate the adsorption kinetic behavior of pure and mixed gases (CO2, CH4, approximately equimolar CO2 + CH4 mixtures, and He) on a coal sample obtained from the Black Warrior Basin at the Littleton Mine (Twin Pine Coal Company), Jefferson County, west-central Alabama. The sample was from the Mary Lee coal zone of the Pottsville Formation (Lower Pennsylvanian). Experiments with three size fractions (45–150 µm, 1–2 mm, and 5–10 mm) of crushed coal were performed at 40 °C and 35 °C over a pressure range of 1.4–6.9 MPa to simulate coalbed methane reservoir conditions in the Black Warrior Basin and provide data relevant for enhanced coalbed methane recovery operations. The following key observations were made: (1) CO2 adsorption on both dry and water-saturated coal is much more rapid than CH4 adsorption; (2) water saturation decreases the rates of CO2 and CH4 adsorption on coal surfaces, but it appears to have minimal effects on the final magnitude of CO2 or CH4 adsorption if the coal is not previously exposed to CO2; (3) retention of adsorbed CO2 on coal surfaces is significant even with extreme pressure cycling; and (4) adsorption is significantly faster for the 45–150 μm size fraction compared to the two coarser fractions.  相似文献   

13.
CO2, CH4, and N2 adsorption and gas-induced swelling were quantified for block Blind Canyon, Pittsburgh #8 and Pocahontas Argonne Premium coals that were dried and structurally relaxed at 75 °C in vacuum. Strain measurements were made perpendicular and parallel to the bedding plane on ~ 7 × 7 × 7 mm3 coal blocks and gravimetric sorption measurements were obtained simultaneously on companion coal blocks exposed to the same gaseous environment. The adsorption amount and strain were determined after equilibration at P  ≤ 1.8 MPa. There is a strong non-linear correlation between strain and the quantity of gas adsorbed and the results for all gases and coals studied follow a common pattern. The dependence of the coal matrix shrinkage/swelling coefficient (Cgc) on the type and quantity of gas adsorbed is seen by plotting the ratio between the strain and the adsorbate concentration against the adsorbate concentration. In general, Cgc increases with increasing adsorbate concentration over the range of ~ 0.1 to 1.4 mmol/g. Results from the dried block coals are compared to CO2 experiments using native coals with an inherent level of moisture as received. The amount of CO2 adsorbed using native coals (assuming no displacement of H2O by CO2) is significantly less than the dried coals. The gas-induced strain (S) and adsorption amount (M) were measured as a function of time following step changes in CO2, CH4, and N2 pressure from vacuum to 1.8 MPa. An empirical diffusion equation was applied to the kinetic data to obtain the exponent (n) for time dependence for each experiment. The data for all coals were pooled and the exponent (n) evaluated using an ANOVA statistical analysis method. Values for (n) near 0.5 were found to be independent on the coal, the gas or type of measurement (e.g., parallel strain, perpendicular strain, and gas uptake). These data support the use of a Fickian diffusion model framework for kinetic analysis. The kinetic constant k was determined using a unipore diffusion model for each experiment and the data were pooled for ANOVA analysis. For dry coal, statistically significant differences for k were found for the gases (CO2 > N2 > CH4) and coals (Pocahontas >Blind Canyon > Pittsburgh #8) but not for the method of the kinetic measurement (e.g., strain or gas uptake). For Blind Canyon and Pittsburgh #8 coal, the rate of CO2 adsorption and gas-induced strain for dry coal was significantly greater than that of the corresponding native coal. For Pocahontas coal the rates of CO2 adsorption and gas-induced strain for dry and native coal were indistinguishable and may be related to its low native moisture and minimal amount of created porosity upon drying.  相似文献   

14.
Carbon dioxide (CO2) is considered to be the most important greenhouse gas in terms of overall effect. CO2 geological storage in coal beds is of academic and industrial interest because of economic synergies between greenhouse gas sequestration and coal bed methane (CH4) recovery by displacement/adsorption. Previously, most work focused on either theoretical analyses and mathematical simulations or gas adsorption?Cdesorption experiments using coal particles of millimeter size or smaller. Those studies provided basic understanding of CH4 recovery by CO2 displacement in coal fragments, but more relevant and realistic investigations are still rare. To study the processes more realistically, we conducted experimental CH4 displacement by CO2 and CO2 sequestration with intact 100?×?100?×?200?mm coal specimens. The coal specimen permeability was measured first, and results show that the permeability of the specimen is different for CH4 and CO2; the CO2 permeability was found to be at least two orders of magnitude greater than that for CH4. Simultaneously, a negative exponential relationship between the permeability and the applied mean stress on the specimen was found. Under the experimental stress conditions, 17.5?C28.0 volumes CO2 can be stored in one volume of coal, and the displacement ratio CO2?CCH4 is as much as 7.0?C13.9. The process of injection, adsorption and desorption, displacement, and output of gases proceeds smoothly under an applied constant pressure differential, and the CH4 content in the output gas amounted to 20?C50% at early stages, persisting to 10?C16% during the last stage of the experiments. Production rate and CH4 fraction are governed by complex factors including initial CH4 content, the pore and fissure fabric of the coal, the changes in this fabric as the result of differential adsorption of CO2, the applied stress, and so on. During CO2 injection and CH4 displacement, the coal can swell from effects of gas adsorption and desorption, leading to changes in the microstructure of the coal itself. Artificial stimulation (e.g. hydraulic fracturing) to improve coalbed transport properties for either CO2 sequestration or enhanced coal bed methane recovery will be necessary. The interactions of large-scale induced fractures with the fabric at the scale of observable fissures and fractures in the laboratory specimens, as well as to the pore scale processes associated with adsorption and desorption, remain of profound interest and a great challenge.  相似文献   

15.
The paper reports the results of experiments concerning the sorption/desorption processes, observed under laboratory conditions, in two types of coal extracted from operational coal-mines in Poland, using CH4 and CO2 to observe their relative inter-reaction with the coal samples when introduced in varying proportions and conditions. Numerous studies concerning the sorption/desorption phenomena have described the operational mechanisms and the relationship of mine gases to the organically-created coal-body in mines. The differences in the behaviour of certain gases is twofold: firstly the essentially different characteristics of CO2 and CH4, and secondly the structure of the coal-bed itself: its degree of metamorphism and content of macerals. From the results yielded, it was observed that the divergence of the isotherms of sorption of CH4 and other gases in comparison with the isotherms of sorption of CO2 and a CO2/CH4 mixture differed and that the curve on the sorption isotherm was more clearly distinct after the introduction of CO2 molecules to the system: coal with a higher degree of metamorphism—CH4, which is closely related to the rigidity of the structure according to the level of metamorphism. Since coals with higher carbon content exhibit lower molecular bonding than low-carbonised coals, the characteristic feature of the bonds in the first case is their mobility. Knowledge of the physical and chemical properties of hard coals, as well as their interaction with mining gases, is of great use in solving problems concerned with the extraction of methane from mines or its storage in goafs.  相似文献   

16.
The Huntly coalfield has significant coal deposits that contain biogenically-sourced methane. The coals are subbituminous in rank and Eocene in age and have been previously characterised with relatively low to moderate measured gas (CH4) contents (2–4 m3/ton). The CO2 holding capacity is relatively high (18.0 m3/ton) compared with that of CH4 (2.6 m3/ton) and N2 (0.7 m3/ton) at the same pressure (4 MPa; all as received basis). The geothermal gradient is also quite high at 55 °C/km.A study has been conducted which simulates enhancement of methane recovery (ECBM) from these deposits using a new version of the TOUGH2 (version 2) reservoir simulator (ECBM-TOUGH2) that can handle non-isothermal, multi-phase flows of mixtures of water, CH4, CO2 and N2. The initial phase of the simulation is CH4 production for the first 5 years of the field history. The model indicates that methane production can be significantly improved (from less than 80% recovery to nearly 90%) through injection of CO2. However, although an increase in the rate of CO2 injection increases the amount of CO2 sequestered, the methane recovery (because of earlier breakthrough with increasing injection rate) decreases. Modeling of pure N2 injection produced little enhanced CH4 production. The injection of a hypothetical flue gas mixture (CO2 and N2) also produced little increase in CH4 production. This is related to the low adsorption capacity of the Huntly coal to N2 which results in almost instantaneous breakthrough into the production well.  相似文献   

17.
We report laboratory experiments that investigate the permeability evolution of an anthracite coal as a function of applied stress and pore pressure at room temperature as an analog to other coal types. Experiments are conducted on 2.5 cm diameter, 2.5-5 cm long cylindrical samples at confining stresses of 6 to 12 MPa. Permeability and sorption characteristics are measured by pulse transient methods, together with axial and volumetric strains for both inert (helium (He)) and strongly adsorbing (methane (CH4) and carbon dioxide (CO2)) gases. To explore the interaction of swelling and fracture geometry we measure the evolution of mechanical and transport characteristics for three separate geometries — sample A containing multiple small embedded fractures, sample B containing a single longitudinal through-going fracture and sample C containing a single radial through-going fracture. Experiments are conducted at constant total stress and with varied pore pressure — increases in pore pressure represent concomitant (but not necessarily equivalent) decreases in effective stress. For the samples with embedded fractures (A and C) the permeability first decreases with an increase in pressure (due to swelling and fracture constraint) and then increases near-linearly (due to the over-riding influence of effective stresses). Conversely, this turnaround in permeability from decreasing to increasing with increasing pore pressure is absent in the discretely fractured sample (B) — the influence of the constraint of the connecting fracture bridges in limiting fracture deformation is importantly absent as supported by theoretical considerations. Under water saturated conditions, the initial permeabilities to all gases are nearly two orders of magnitude lower than for dry coal and permeabilities increase with increasing pore pressure for all samples and at all gas pressures. We also find that the sorption capacities and swelling strains are significantly reduced for water saturated samples — maybe identifying the lack of swelling as the primary reason for the lack of permeability decrease. Finally, we report the weakening effects of gas sorption on the strength of coal samples by loading the cores to failure. Results surprisingly show that the strength of the intact coal (sample A) is smaller than that of the axially fractured coal (sample B) due to the extended duration of exposure to CH4 and CO2. Average post-failure particle size for the weakest intact sample (A) is found to be three times larger than that of the sample B, based on the sieve analyses from the samples after failure. We observe that fracture network geometry and saturation state exert important influences on the permeability evolution and strength of coal under in situ conditions.  相似文献   

18.
A theoretical model for gas adsorption-induced coal swelling   总被引：6，自引：2，他引：6  
Swelling and shrinkage (volumetric change) of coal during adsorption and desorption of gas is a well-known phenomenon. For coalbed methane recovery and carbon sequestration in deep, unminable coal beds, adsorption-induced coal volumetric change may cause significant reservoir permeability change. In this work, a theoretical model is derived to describe adsorption-induced coal swelling at adsorption and strain equilibrium. This model applies an energy balance approach, which assumes that the surface energy change caused by adsorption is equal to the elastic energy change of the coal solid. The elastic modulus of the coal, gas adsorption isotherm, and other measurable parameters, including coal density and porosity, are required in this model. Results from the model agree well with experimental observations of swelling. It is shown that the model is able to describe the differences in swelling behaviour with respect to gas species and at very high gas pressures, where the coal swelling ratio reaches a maximum then decreases. Furthermore, this model can be used to describe mixed-gas adsorption induced-coal swelling, and can thus be applied to CO2-enhanced coalbed methane recovery.  相似文献   

19.
The sensitivity of coal permeability to the effective stress means that changes in stress as well as pore pressure within a coal seam lead to changes in permeability. In addition coal swells with gas adsorption and shrinks with desorption; these sorption strains impact on the coal stress state and thus the permeability. Therefore the consideration of gas migration in coal requires an appreciation of the coupled geomechanical behaviour. A number of approaches to representing coal permeability incorporate the geomechanical response and have found widespread use in reservoir simulation. However these approaches are based on two simplifying assumptions; uniaxial strain (i.e. zero strain in the horizontal plane) and constant vertical stress. This paper investigates the accuracy of these assumptions for reservoir simulation of enhanced coalbed methane through CO2 sequestration. A coupled simulation approach is used where the coalbed methane simulator SIMED II is coupled with the geomechanical model FLAC3D. This model is applied to three simulation case studies assembled from information presented in the literature. Two of these are for 100% CO2 injection, while the final example is where a flue gas (12.5% CO2 and 87.5% N2) is injected. It was found that the horizontal contrast in sorption strain within the coal seam caused by spatial differences in the total gas content leads to vertical stress variation. Thus the permeability calculated from the coupled simulation and that using an existing coal permeability model, the Shi–Durucan model, are significantly different; for the region in the vicinity of the production well the coupled permeability is greater than the Shi–Durucan model. In the vicinity of the injection well the permeability is less than that calculated using the Shi–Durucan model. This response is a function of the magnitude of the strain contrast within the seam and dissipates as these contrasts diminish.  相似文献   

20.
India recognizes the strategic importance for developing shale gas resources like other countries in the world. Shale gas reservoirs are known to be difficult for extracting gas in comparison to conventional reservoirs. Recently, due to high prices of gas, rising demand and enhancement in recovery technologies has attracted the Indian energy industries to explore the shale gas resource. Coal and lignite are the prime source of energy in India and these resources are well explored, while shale is ignored, despite it being associated with coal and lignite bearing formations. The paper presents reservoir characteristics of shale horizons in Barren Measures and Barakar formations of north and south Karanpura coalfields. Shale core samples were collected from exploratory boreholes in air tight canisters. In-situ gas content and adsorption capacities ascertained to be 0.51–1.69 m3/t and 3.90–5.82 m3/trespectively. Desorbed gas derived from canisters contains CH4, C2H6, C3H8, CO2, N2 and O2 and varies from 76.19–82.63, 0.38–0.76, 0.10–0.50, 8.65–12.34, 9.89–19.34 and 0.56–2.24 vol. % respectively. The permeability and porosity determined under reservoir simulated confining pressure is varying from 0.41–0.75 mD and 0.89–2.28 % respectively. The plots of Rock Eval S2vs TOC and HI against Calc. VRo% indicates that all shale samples belong to Type III kerogen, which is prone to generate gas. It is evaluated that insitu gas content, sorption capacity, saturation level and low permeability of shale beds are critical parameters for development of shale gas resource in the studied area.  相似文献   

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