Methane and carbon dioxide sorption on a set of coals from India

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.     

水浸干燥后煤的孔隙结构及瓦斯吸附特性变化规律

在富含水煤系或水力措施后的煤层中,受水溶液的浸泡,煤的孔隙结构及吸附特性发生改变,为了深入研究其变化规律,在实验室利用蒸馏水对2种不同变质程度煤样进行了长时间（60 d）浸泡,采用低温N₂吸附实验和CO₂吸附实验测试水浸前后煤样的孔隙结构变化规律,采用高压容量法测试水浸前后煤样的瓦斯吸附特性。结果表明,水浸干燥后煤体孔容和比表面积总体呈降低趋势。其中,低温N₂吸附实验结果表明,煤体中大中孔的比表面积最高可降低48.9%;CO₂吸附实验结果表明,水浸干燥后2种煤样的微孔孔容和比表面积也呈不同程度的降低趋势。将水浸煤样孔隙结构变化分为3个阶段,即矿物质溶出“增孔”阶段、煤基质局部膨胀变形“缩孔”阶段和煤基质整体溶胀变形“扩孔”阶段。此外,水浸干燥后煤对瓦斯的吸附能力下降,主要是由于水浸促使煤体产生膨胀变形,且导致微孔隙相互连通,从而降低了煤体微孔孔容和比表面积,降低瓦斯吸附能力。研究成果对进一步掌握富含水煤系或水力化措施后煤层的瓦斯抽采具有指导意义。      

Methane and CO2 sorption and desorption measurements on dry Argonne premium coals: pure components and mixtures

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 (VR_r=0.25%) and the highest ratios (2.7 and 3.16) were encountered for the low-rank coals (VR_r 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.     

http://www.sciencedirect.com/science/article/pii/S1674987112000308

The Panguan Syncline contains abundant coal resources,which may be a potential source of coalbed methane.In order to evaluate the coalbed methane production potential in this area,we investigated the pore-fracture system of coalbed methane reservoirs,and analyzed the gas sorption and seepage capacities by using various analytical methods,including scanning electron microscopy（SEM）,optical microscopy,mercury-injection test,low-temperature N₂ isotherm adsorption/desorption analyses,lowfield nuclear magnetic resonance and methane isothermal adsorption measurements.The results show that the samples of the coal reservoirs in the Panguan Syncline have moderate gas sorption capacity.However, the coals in the study area have favorable seepage capacities,and are conductive for the coalbed methane production.The physical properties of the coalbed methane reservoirs in the Panguan Syncline are generally controlled by coal metamorphism:the low rank coal usually has low methane sorption capacity and its pore and microfractures are poorly developed;while the medium rank coal has better methane sorption capacity,and its seepage pores and microfractures are well developed,which are sufficient for the coalbed methane’s gathering and exploration.Therefore,the medium rank coals in the Panguan Syncline are the most prospective targets for the coalbed methane exploration and production.     

Flue gas and pure CO2 sorption properties of coal: A comparative study

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.     

Coupled flow and geomechanical processes during enhanced coal seam methane recovery through CO2 sequestration

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.     

Selection of coals of different maturities for CO2 Storage by modelling of CH4 and CO2 adsorption isotherms

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.     

Assessing the potential for CO2 adsorption in a subbituminous coal,Huntly Coalfield,New Zealand,using small angle scattering techniques

Small angle scattering techniques (SAXS and SANS) have been used to investigate the microstructural properties of the subbituminous coals (R_max 0.42–0.45%) from the Huntly Coalfield, New Zealand. Samples were collected from the two thick (> 5 m) coal seams in the coalfield and have been analysed for methane and carbon dioxide sorption capacity, petrography, pore size distribution, specific surface area and porosity.Specific surface area (SSA) available for carbon dioxide adsorption, extrapolated to a probe size of 4 Å, ranged from 1.25 × 10⁶ cm^? 1 to 4.26 × 10⁶ cm^? 1 with total porosity varying from 16% to 25%. Porosity was found to be predominantly composed of microporosity, which contributed the majority of the available SSA. Although considerable variation was seen between samples, the results fit well with published rank trends.Gas holding capacity at the reservoir pressure (approximately 4 MPa) ranged from 2.63 to 4.18 m³/t for methane on a dry, ash-free basis (daf) and from 22.00 to 23.72 m³/t daf for carbon dioxide. The resulting ratio of CO₂:CH₄ ranged from 5.7 to 8.6, with an average of 6.7:1.Holding capacities for both methane and carbon dioxide on a dry ash free basis (daf) were found to be correlated with sample microporosity. However, holding capacities for the two gases on an as analysed (aa) basis (that is including mineral matter and moisture), showed no such correlation. Carbon dioxide (aa) does show a negative correlation with both specific surface area and microporosity. As the coals have low inorganic matter content, the reversal is thought to be related to moisture which is likely concentrated in the pore size range 12.5–125 Å. Methane holding capacity, both daf and aa, correlates with macroporosity, thus suggesting that the holding capacity of micropores is diminished by the presence of moisture in the pores.     

Adsorption Mechanism and Kinetic Characterization of Bituminous Coal under High Temperatures and Pressures in the Linxing‐Shenfu Area

The majority of coalbed methane(CBM) in coal reservoirs is in adsorption states in coal matrix pores. To reveal the adsorption behavior of bituminous coal under high-temperature and high-pressure conditions and to discuss the microscopic control mechanism affecting the adsorption characteristics, isothermal adsorption experiments under hightemperature and high-pressure conditions, low-temperature liquid nitrogen adsorption-desorption experiments and CO2 adsorption experiments were performed on coal samples. Results show that the adsorption capacity of coal is comprehensively controlled by the maximum vitrinite reflectance(Ro, max), as well as temperature and pressure conditions. As the vitrinite reflectance increases, the adsorption capacity of coal increases. At low pressures, the pressure has a significant effect on the positive effect of adsorption, but the effect of temperature is relatively weak. As the pressure increases, the effect of temperature on the negative effect of adsorption gradually becomes apparent, and the influence of pressure gradually decreases. Considering pore volumes of pores with diameters of 1.7-100 nm, the peak volume of pores with diameters 10-100 nm is higher than that from pores with diameters 1.7-10 nm, especially for pores with diameters of 40-60 nm, indicating that pores with diameters of 10-100 nm are the main contributors to the pore volume. The pore specific surface area shows multiple peaks, and the peak value appears for pore diameters of 2-3 nm, indicating that this pore diameter is the main contributor to the specific surface area. For pore diameters of 0.489-1.083 nm, the pore size distribution is bimodal, with peak values at 0.56-0.62 nm and 0.82-0.88 nm. The adsorption capability of the coal reservoir depends on the development degree of the supermicroporous specific surface area, because the supermicroporous pores are the main contributors to the specific pore area. Additionally, the adsorption space increases as the adsorption equilibrium pressure increases. Under the same pressure, as the maximum vitrinite reflectance increases, the adsorption space increases. In addition, the cumulative reduction in the surface free energy increases as the maximum vitrinite reflectance increases. Furthermore, as the pressure increases, the surface free energy of each pressure point gradually decreases, indicating that as the pressure increases, it is increasingly difficult to adsorb methane molecules.     

压裂液对煤岩储层解吸–扩散性能的影响

为预防压裂液自吸侵入煤岩基质孔隙,影响煤岩储层的解吸-扩散性能,以沁水盆地石炭统太原组15号煤为研究对象,开展了低伤害活性水压裂液对煤岩储层解吸-扩散性能的损害评价实验,并基于红外光谱、低温氮气吸附和扫描电镜分析了压裂液作用前后的煤岩表面性质和孔隙结构变化。实验结果表明:压裂液处理后,煤样的甲烷解吸率下降了10.23%,扩散系数损害率为16.67%;煤岩表面亲水性增强,液相滞留效应加剧,煤岩基质孔隙比表面增大、孔隙连通性变差、平均孔径减小,这些变化从根本上揭示了压裂液损害煤岩储层解吸-扩散性能的微观机理。最后,提出了基于纳米颗粒封堵技术和表面活性剂技术的煤岩储层解吸-扩散性能损害预防措施。      

颗粒煤甲烷吸附过程扩散特征

为了研究甲烷在颗粒煤中扩散、吸附至平衡过程的扩散特性,基于颗粒煤吸附甲烷幂函数扩散模型,利用磁悬浮天平高压等温吸附仪,测定不同压力下颗粒煤甲烷吸附过程中扩散量随时间变化值,研究颗粒煤甲烷吸附达到平衡前的扩散特征。实验结果表明:平衡压力对颗粒煤甲烷吸附和扩散特性影响显著;吸附量和平均扩散系数随着压力增大而增大;颗粒煤甲烷吸附过程扩散系数随时间呈幂函数衰减,前500 s衰减幅度较大,平均扩散系数与时间呈负相关关系。研究认为颗粒煤吸附甲烷幂函数扩散模型对于描述颗粒煤甲烷吸附扩散过程具有较高准确性,有助于分析煤层气排采过程煤层气吸附量的动态变化,提高煤层气采收率。      

CBM and CO2-ECBM related sorption processes in coal: A review

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.     

A theoretical model for gas adsorption-induced coal swelling

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.     

沁水盆地南部中高阶煤高压甲烷吸附行为

煤层气吸附作用是发生在煤基质内表面的物理过程,而煤岩复杂孔裂隙网络为高压甲烷吸附提供了丰富的空间。开展沁水盆地南部高阶煤30℃高压甲烷等温吸附实验,结合煤岩煤质参数与孔隙特征参数,通过改进的D-R模型分析了煤岩性质、孔隙特征与吸附参数的相关性。煤岩性质对最大吸附能力和吸附热参数的影响是多因素叠加的综合效应,而最大吸附能力与微孔体积,吸附体积校正参数与大中孔比表面积呈较好的正相关性,表明甲烷分子在煤基质内表面会根据孔径尺度大小呈现不同的吸附方式。据此提出高压甲烷在煤基质微孔中呈紧密堆积状态而在大中孔中呈多层分子堆叠状态的新认识,为进一步研究煤层气吸附机理提供了新的思路。      

Adsorption kinetics of CO2, CH4, and their equimolar mixture on coal from the Black Warrior Basin,West-Central Alabama

Laboratory experiments were conducted to investigate the adsorption kinetic behavior of pure and mixed gases (CO_2, 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.     

History matching of enhanced coal bed methane laboratory core flood tests

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.     

煤体性质对煤吸附容量的控制作用探讨

煤体性质是影响煤吸附容量的重要因素之一。通过对中国华北和西北两个重要煤层气富集区煤的煤岩学、煤化学和等温吸附实验分析,从煤级、显微组分、煤体变形三个方面对煤的吸附容量及其控制因素进行了, 分析探讨。结果表明,水分平衡条件下煤的吸附容量与煤阶的关系为倒U字型,吸附容量随煤阶的变化为跃变式, 基本与四次煤化作用跃变阶段相对应,主要受控于煤化作用过程中煤的亲甲烷能力和孔隙度的变化;煤体中惰质组含量较高时,其对煤体的吸附容量的影响较为明显,主要与惰质组中丝质体的高吸附能力有关;在构造应力作用下,煤体表面物化发生的变化使构造煤吸附容量比同一矿区同一煤层原生结构煤高。     

高压氮气置换甲烷对煤基质孔隙的影响

为探究高压气体吸附-解吸试验对煤基质中孔隙发育规模和结构的影响,选取安鹤矿区鹤壁六矿二1煤层煤样进行了高压氮气置换甲烷吸附-解吸试验,采用低温液氮吸附方法分别测定了高压氮气置换甲烷前后煤的低温液氮吸附解吸曲线,利用BET、BJH和QSDFT 3种分析模型,对煤基质中1.14~300 nm的孔隙规模、分布与结构特征进行了对比分析。分析结果显示煤样的孔容、比表面积和孔隙结构在高压气体置换过程中均发生了变化,孔隙BET比表面积从12.746 0 m2/g降低到7.227 0 m2/g,总孔容从0.009 0 cm3/g降低到0.006 6 cm3/g;孔隙发育规模与孔径分布均发生明显变化,但孔隙形态基本保持不变,孔径分布的变化主要表现为微孔孔容与比表面积的降低为主,而中孔和大孔基本保持不变。      

Swelling-induced volumetric strains internal to a stressed coal associated with CO2 sorption

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.     

Simulation study of CO2 sequestration potential of the Mary Lee coal zone, Black Warrior basin

Significant potential exists for CO₂ sequestration in coalbed methane reservoirs of the Black Warrior basin. Reservoir simulation is an appropriate approach to estimate both the storage capacity and methane recovery enhancement. However, prior to a reliable reservoir modeling and simulation, conducting an accurate and comprehensive reservoir characterization study is necessary. The purpose of the present study is twofold: (a) to provide a rigorous reservoir characterization study required for modeling Mary Lee coal group in the Blue Creek field of the Black Warrior basin; (b) to run fluid flow simulations to predict the performance of ECBM process applied to an under pressured zone of the Mary Lee coal group. According to the current well configuration of Blue Creek field, three applicable well patterns, namely a direct line drive, an inverted 5-spot and a normal 5-spot were separately (i.e., in three distinct cases) used for simulating ECBM. Simulations were run on an approximately 32 ha (80-acre) drainage area, and included coal matrix shrinkage/swelling effects. The injected gas was assumed to be pure CO₂. Using an inverted 5-spot pattern, simulations predicted that after 7.5 years of CO₂ injection, approximately 32,000 tonnes of CO₂ would be sequestered per 32 ha of this zone and that methane recovery would be enhanced by 36 %. Using a normal 5-spot pattern, CO₂ breakthrough would occur 2.4 years earlier, and about 40,000 tonnes CO₂ would be sequestered. However, methane production would be enhanced by 33 %. Considering methane recovery enhancement, direct line drive pattern delivered poor results in comparison with two other patterns. As expected, the results also showed that CO₂ injection would increase water production.