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1.
Presently many research projects focus on the reduction of anthropogenic CO2 emissions. It is intended to apply underground storage techniques such as flue gas injection in unminable coal seams. In this context, an experimental study has been performed on the adsorption of pure CO2 and preferential sorption behavior of flue gas. A coal sample from the Silesian Basin in Poland (0.68% V Rr), measured in the dry and wet state at 353 K has been chosen for this approach. The flue gas used was a custom class industrial flue gas with 10.9% of CO2, 0.01% of CO, 9% of H2, 3.01% of CH4, 3.0% of O2, 0.106% of SO2 and nitrogen as balance.Adsorption isotherms of CO2 and flue gas were measured upto a maximum of 11 MPa using a volumetric method. Total excess sorption capacities for CO2 on dry and wet Silesia coal ranged between 1.9 and 1.3 mmol/g, respectively. Flue gas sorption capacities on dry and wet Silesia coal were much lower and ranged between 0.45 and 0.2 mmol/g, respectively, at pressures of 8 MPa. The low sorption capacity of wet coal has resulted from water occupying some of the more active adsorption sites and hence reducing the heterogeneity of adsorption sites relative to that of dry coal. Desorption tests with flue gas were conducted to study the degree of preferential sorption of the individual components. These experiments indicate that CO2 is by far the prefered sorbing component under both wet and dry conditions. This is followed by CH4. N2 adsorbs very little on the coal in the presence of CO2 and CH4. It is also observed that the adsorption of CO2 onto coal is not significantly hindered by the addition of other gases, other than dilution effect of the pressure.In addition to the sorption experiments, the density of the flue gas mixture has been determined up to 18 MPa at 318 K. A very good precision of these measurements were documented by volumetric methods.  相似文献   

2.
To stabilize the atmospheric concentration of greenhouse gases (GHG), a huge reduction of carbon dioxide (CO2) emissions is required. Although some people believe that this necessitates a considerable reduction in the use of fossil fuels or fuel switching, other options are available that allow the use of fossil fuels and reduce atmospheric emissions of CO2. Sequestration of CO2 from fossil fuel combustion in the subsurface could prevent the CO2 from reaching the surface for millions of years. Geological sequestration of CO2 in deep aquifers or in depleted oil and gas reservoirs is a mature technology. Despite the huge quantities of CO2 that can be sequestered in this way, this approach does not provide any economic benefit. This paper discusses a third option, which consists of injecting CO2 in deep coal seams to sequester the carbon and enhance the recovery of coalbed methane (CBM). Waste CO2 from CBM-fueled power plants could be injected into CBM reservoirs to produce more methane (CH4) for the power plant. The 2:1 coal-sorption selectivity for CO2 over CH4 supports the feasibility of operating fossil-fueled power plants without atmospheric CO2 emissions. Other CO2 sequestration technologies, such as ocean disposal and biofixation, are briefly discussed and the suitability of these approaches is evaluated for use in Alberta, Canada.  相似文献   

3.
This paper reports on the performance comparison for different CO2-ECBM schemes in relatively thin unminable seams typical of Northern Appalachian coal basin using a horizontal well configuration. Numerical simulations based upon public-domain coalbed reservoir properties indicated that injection of pure CO2 is likely to result in only limited incremental methane recovery if any over primary recovery, due to the low injection rates that can be achieved. On the other hand, the presence of the nitrogen component in the injected gas stream is capable of improving the efficiency of enhanced methane recovery significantly without compromising the net CO2 injection rates, as a result of improved injectivity over pure CO2 injection. There is, however, a trade off between incremental methane recovery and produced gas purity due to early nitrogen breakthrough.  相似文献   

4.
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   

5.
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   

6.
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   

7.
Studying gas transport mechanisms in coal seams is crucial in determining the suitability of coal formations for geosequestration and/or CO2-enhanced coal bed methane recovery (ECBM), estimating CO2 storage capacity and recoverable volume of methane, and predicting the long-term integrity of CO2 storage and possible leakages. Due to the dual porosity nature of coal, CO2 transport is a combination of viscous flow and Fickian diffusion. Moreover, CO2 is adsorbed by the coal which leads to coal swelling which can change the porous structure of coal and consequently affects the gas flow properties of coal, i.e. its permeability. In addition, during CO2 permeation, the coal seam undergoes a change in effective stress due to the pore pressure alteration and this can also change the permeability of the coal seam. In addition, depending on the in situ conditions of the coal seam and the plan of the injection scheme, carbon dioxide can be in a supercritical condition which increases the complexity of the problem. We provide an overview of the recent studies on porous structure of coal, CO2 adsorption onto coal, mechanisms of CO2 transport in coalbeds and their measurement, and hydro-mechanical response of coal to CO2 injection and identify opportunities for future research.  相似文献   

8.
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   

9.
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   

10.
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   

11.
The Late Miocene Muaraenim Formation in southern Sumatra contains thick coal sequences, mostly of low rank ranging from lignite to sub-bituminous, and it is believed that these thick low rank coals are the most prospective for the production of coal seam gas (CSG), otherwise known as coalbed methane (CBM), in Indonesia.As part of a major CSG exploration project, gas exploration drilling operations are being undertaken in Rambutan Gasfields in the Muaraenim Formation to characterize the CSG potential of the coals. The first stage of the project, which is described here, was designed to examine the gas reservoir properties with a focus on coal gas storage capacity and compositional properties. Some five CSG exploration boreholes were drilled in the Rambutan Gasfield, south of Palembang. The exploration boreholes were drilled to depths of ~ 1000 m into the Muaraenim Formation. Five major coal seams were intersected by these holes between the depths of 450 and 1000 m. The petrography of coal samples collected from these seams showed that they are vitrinite rich, with vitrinite contents of more than 75% (on a mineral and moisture free basis). Gas contents of up to 5.8 m3/t were measured for the coal samples. The gas desorbed from coal samples contain mainly methane (CH4) ranging from 80 to 93% and carbon dioxide (CO2) ranging from 6 to 19%. The composition of the gas released into the production borehole/well is, however, much richer in CH4 with about 94 to 98% CH4 and less than 5% CO2.The initial results of drilling and reservoir characterization studies indicate suitable gas recovery parameters for three of the five coal seams with a total thickness of more than 30 m.  相似文献   

12.
For the degassing of coal seams, either prior to mining or in un-minable seams to obtain coalbed methane, it is the combination of cleat frequency, aperture, connectivity, stress, and mineral occlusions that control permeability. Unfortunately, many potential coalbeds have limited permeability and are thus marginal for economic methane extraction. Enhanced coalbed methane production, with concurrent CO2 sequestration is also challenging due to limited CO2 injectivity. Microwave energy can, in the absence of confining stress, induce fractures in coal. Here, creation of new fractures and increasing existing cleat apertures via short burst, high-energy microwave energy was evaluated for an isotropically stressed and an unstressed bituminous coal core. A microwave-transparent argon gas pressurized (1000 psi) polycarbonate vessel was constructed to apply isotropic stress simulating ~ 1800 foot depth. Cleat frequency and distribution was determined for the two cores via micro-focused X-ray computed tomography. Evaluation occurred before and after microwave exposure with and without the application of isotropic stress during exposure. Optical microscopy was performed for tomography cleat aperture calibration and also to examine lithotypes influences on fracture: initiation, propagation, frequency, and orientation. It was confirmed that new fractures are induced via high-energy microwave exposure in an unconfined bituminous core and that the aperture increased in existing cleats. Cleat/fracture volume, following microwave exposure increased from 1.8% to 16.1% of the unconfined core volume. For the first time, similar observations of fracture generation and aperture enhancement in coal were also determined for microwave exposure under isotropic stress conditions. An existing cleat aperture, determined from calibrated X-ray computed tomography increased from 0.17 mm to 0.32 mm. The cleat/fracture volume increased from 0.5% to 5.5%. Optical microscopy indicated that fracture initiated likely occurred in at least some cases at fusain microlithotypes. Presumably this was due to the open pore volumes and potential for bulk water presence or steam pressure buildup in these locations. For the major induced fractures, they were mostly horizontal (parallel to the bedding plane) and often contained within lithotype bands. Thus it appears likely that microwaves have the potential to enhance the communication between horizontal wellbore and existing cleat network, in coal seams at depth, for improved gas recovery or CO2 injection.  相似文献   

13.
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   

14.
This paper presents reviews of studies on properties of coal pertinent to carbon dioxide (CO2) sequestration in coal with specific reference to Victorian brown coals. The coal basins in Victoria, Australia have been identified as one of the largest brown coal resources in the world and so far few studies have been conducted on CO2 sequestration in this particular type of coals. The feasibility of CO2 sequestration depends on three main factors: (1) coal mass properties (chemical, physical and microscopic properties), (2) seam permeability, and (3) gas sorption properties of the coal. Firstly, the coal mass properties of Victorian brown coal are presented, and then the general variations of the coal mass properties with rank, for all types of coal, are discussed. Subsequently, coal gas permeability and gas sorption are considered, and the physical factors which affect them are examined. In addition, existing models for coal gas permeability and gas sorption in coal are reviewed and the possibilities of further development of these models are discussed. According to the previous studies, coal mass properties and permeability and gas sorption characteristics of coals are different for different ranks: lignite to medium volatile bituminous coals and medium volatile bituminous to anthracite coals. This is important for the development of mathematical models for gas permeability and sorption behavior. Furthermore, the models have to take into account volume effect which can be significant under high pressure and temperature conditions. Also, the viscosity and density of supercritical CO2 close to the critical point can undergo large and rapid changes. To date, few studies have been conducted on CO2 sequestration in Victorian brown coal, and for all types of coal, very few studies have been conducted on CO2 sequestration under high pressure and temperature conditions.  相似文献   

15.
从吐哈盆地和保德河盆地的沉积、地层、成煤、构造等背景出发,结合两盆地煤层气勘探所取得的煤储层资料,对比分析了低煤阶盆地煤层气成藏及资源开发潜力。分析表明,吐哈盆地和保德河盆地虽然分属于不同的大地构造单元,盆地演化不同,但其含煤性相似,特别是同属低变质的褐煤—亚烟煤类,含煤性较好,沉积、水文地质条件近似,因而盆地煤层气成藏具有一定的可比性。美国保德河等低变质煤盆地煤层气商业性开发成功的事实说明,对于低变质煤盆地,即使气含量较低,但只要成藏条件合适,其仍然具有煤层气资源开发潜力   相似文献   

16.
Characterization of coal reservoirs and determination of in-situ physical coal properties related to transport mechanism are complicated due to having lack of standard procedures in the literature. By considering these difficulties, a new approach has been developed proposing the usage of relationships between coal rank and physical coal properties. In this study, effects of shrinkage and swelling (SS) on total methane recovery at CO2 breakthrough (TMRB), which includes ten-year primary methane recovery and succeeding enhanced coalbed methane (ECBM) recovery up to CO2 breakthrough, and CO2 sequestration have been investigated by using rank-dependent coal properties. In addition to coal rank, different coal reservoir types, molar compositions of injected fluid, and parameters within the extended Palmer & Mansoori (P&M) permeability model were considered. As a result of this study, shrinkage and swelling lead to an increase in TMRB. Moreover, swelling increased CO2 breakthrough time and decreased displacement ratio and CO2 storage for all ranks of coal. Low-rank coals are affected more negatively than high-rank coals by swelling. Furthermore, it was realized that dry coal reservoirs are more influenced by swelling than others and saturated wet coals are more suitable for eliminating the negative effects of CO2 injection. In addition, it was understood that it is possible to reduce swelling effect of CO2 on cleat permeability by mixing it with N2 before injection. However, an economical optimization is required for the selection of proper gas mixture. Finally, it is concluded from sensitivity analysis that elastic modulus is the most important parameter, except the initial cleat porosity, controlling SS in the extended P&M model by highly affecting TMRB.  相似文献   

17.
Field experiments and laboratory studies have shown that swelling of coal takes place upon contact with carbon dioxide at underground pressure and temperature conditions. Understanding this swelling behavior is crucial for predicting the performance of future carbon dioxide sequestration operations in unminable coal seams conducted in association with methane production. Swelling is believed to be related to adsorption on the internal coal surface. Whereas it is well established that moisture influences the sorption capacity of coal, the influence of water on coal swelling is less well-defined. This paper presents the results of laboratory experiments to investigate the effect of moisture on coal swelling in the presence of carbon dioxide, methane and argon. Strain development of an unconfined sample of about 1.0–1.5 mm3 at 40 °C and 8 MPa (and at other pressures) was observed in an optical cell under a microscope as a function of time. Both air dried and moisturized samples were used. Results confirmed different swelling behaviors of coal with different substances: carbon dioxide leads to higher strain than methane, while exposure to argon leads to very little swelling. The experiments on moisturized samples seem to confirm the role of moisture as a competitor to gas molecules for adsorption sites. Adsorption of water could also explain the observed swelling due to water uptake at atmospheric pressure. A re-introduction of carbon dioxide, after intermediate gas release, results in higher strains which indicate a drying effect of the carbon dioxide on the coal. The results of this study show that the role of water cannot be ignored if one wants to understand the fundamental processes that are taking place in enhanced coalbed methane operations.  相似文献   

18.
结合CO_2地质利用与封存技术机理,在国际权威潜力评估公式的基础上,系统地提出了适合中国地质背景的次盆地尺度CO_2封存潜力评估方法及关键参数取值。同时,以四川盆地为例,依次开展了枯竭油田地质封存与CO_2强化石油开采、枯竭气田与CO_2强化采气、不可采煤层地质封存与CO_2驱替煤层气,以及咸水层地质封存技术的CO_2地质封存潜力。结果表明,四川盆地利用深部咸水层与枯竭天然气田CO_2地质封存潜力最大,期望值分别达154.20×10~8t和53.73×10~8t。其中,枯竭天然气田因成藏条件好、勘探程度高、基础建设完善,为四川盆地及其周边利用枯竭气田CO_2地质封存技术实现低碳减排提供了早期示范机会。CO_2地质利用与封存潜力评估方法,对进一步开展全国次盆地尺度理论封存潜力评估与工程规划具有重要意义。  相似文献   

19.
It is generally accepted that typical coalbed gases (methane and carbon dioxide) are sorbed (both adsorbed and absorbed) in the coal matrix causing it to swell and resulting in local stress and strain variations in a coalbed confined under overburden pressure. The swelling, interactions of gases within the coal matrix and the resultant changes in the permeability, sorption, gas flow mechanics in the reservoir, and stress state of the coal can impact a number of reservoir-related factors. These include effective production of coalbed methane, degasification of future mining areas by drilling horizontal and vertical degasification wells, injection of CO2 as an enhanced coalbed methane recovery technique, and concurrent CO2 sequestration. Such information can also provide an understanding of the mechanisms behind gas outbursts in underground coal mines.The spatio-temporal volumetric strains in a consolidated Pittsburgh seam coal sample were evaluated while both confining pressure and carbon dioxide (CO2) pore pressure were increased to keep a constant positive effective stress on the sample. The changes internal to the sample were evaluated by maps of density and atomic number determined by dual-energy X-ray computed tomography (X-ray CT). Early-time images, as soon as CO2 was introduced, were also used to calculate the macroporosity in the coal sample. Scanning electron microscopy (SEM) and photographic images of the polished section of the coal sample at X-ray CT image location were used to identify the microlithotypes and microstructures.The CO2 sorption-associated swelling and volumetric strains in consolidated coal under constant effective stress are heterogeneous processes depending on the lithotypes present. In the time scale of the experiment, vitrite showed the highest degree of swelling due to dissolution of CO2, while the clay (kaolinite) and inertite region was compressed in response. The volumetric strains associated with swelling and compression were between ± 15% depending on the location. Although the effective stress on the sample was constant, it varied within the sample as a result of the internal stresses created by gas sorption-related structural changes. SEM images and porosity calculations revealed that the kaolinite and inertite bearing layer was highly porous, which enabled the fastest CO2 uptake and the highest degree of compression.  相似文献   

20.
A pilot site for CO2 storage in coal seams was set-up in the Upper Silesian Coal Basin in Poland in the scope of the RECOPOL project, funded by the European Commission. About 760 tons CO2 were injected into the reservoir from August 2004 to June 2005. Breakthrough of the injected CO2 was established, which resulted in the production of about 10% of the injected CO2 in this period. This paper reports on activities performed under the European Commission project MOVECBM that aimed at the assessment of the storage performance of the reservoir in the follow-up period, i.e. whether the injected CO2 was adsorbed onto the coal or whether it was still present as free gas in the pore space. The injection well was used for this purpose, as the production well had to be abandoned for permitting reasons. Several operational periods can be defined between the last injection in June 2005 and the abandonment of the well in October 2007. In the first period the well was shut-in to observe the pressure fall-off, from about 15.0 MPa at the wellhead after the last injection until about 4.5 MPa at the end of 2005. This pressure fall-off curve showed that the reservoir permeability was very low. This seemed to confirm the observed swelling of the coal during the injection period. In the first months of 2006 the pressure at the wellhead was decreased by releasing gas in a controlled way. The amount and composition of the gas were measured. As a result of the pressure reduction, the well flooded with water. A production pump was placed on the former injection well, enabling active production from the coal from March to September 2007. Results of these operations showed that whereas the gas production rates were as expected based on the experience with the production well, the water production was remarkably low. This could be related to permeability issues or, alternatively, indicate a drying effect of the CO2 in the reservoir. Further, the gas composition showed a predominance of CO2 over CH4 during the gas release that changed gradually into a predominance of CH4 over CO2 during the production phase. Although stabilization was not reached within the given production period, the composition approached a 60% methane, 40% CO2 ratio. This indicates that the exchange of these gases is more complex than often envisaged. After removal of the pump the well was filled with water, which ceased the gas release. This indicates that the pressure in the reservoir was back to its original, hydrostatic, state. As the total volume of CO2 produced was only a fraction of the amount that was injected, it can be concluded that the CO2 was taken up by the coal and is currently adsorbed. This gives confidence in the long-term stability of the injected CO2.  相似文献   

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