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.     

Chemical structures of coal lithotypes before and after CO2 adsorption as investigated by advanced solid-state C nuclear magnetic resonance spectroscopy

Four lithotypes (vitrain, bright clarain, clarain, and fusain) of a high volatile bituminous Springfield Coal from the Illinois Basin were characterized using advanced solid-state ¹³C nuclear magnetic resonance (NMR) spectroscopy. The NMR techniques included quantitative direct polarization/magic angle spinning (DP/MAS), cross polarization/total sideband suppression (CP/TOSS), dipolar dephasing, CH_n selection, and recoupled C-H long-range dipolar dephasing techniques. The lithotypes that experienced high-pressure CO₂ adsorption isotherm analysis were also analyzed to determine possible changes in coal structure as a result of CO₂ saturation at high pressure and subsequent evacuation. The main carbon functionalities present in original vitrain, bright clarain, clarain and fusain were aromatic carbons (65.9%-86.1%), nonpolar alkyl groups (9.0%-28.9%), and aromatic C-O carbons (4.1%-9.5%). Among these lithotypes, aromaticity increased in the order of clarain, bright clarain, vitrain, and fusain, whereas the fraction of alkyl carbons decreased in the same order. Fusain was distinct from other three lithotypes in respect to its highest aromatic composition (86.1%) and remarkably small fraction of alkyl carbons (11.0%). The aromatic cluster size in fusain was larger than that in bright clarain. The lithotypes studied responded differently to high pressure CO₂ saturation. After exposure to high pressure CO₂, vitrain and fusain showed a decrease in aromaticity but an increase in the fraction of alkyl carbons, whereas bright clarain and clarain displayed an increase in aromaticity but a decrease in the fraction of alkyl carbons. Aromatic fused-rings were larger for bright clarain but smaller for fusain in the post-CO₂ adsorption samples compared to the original lithotypes. These observations suggested chemical CO₂-coal interactions at high pressure and the selectivity of lithotypes in response to CO₂ adsorption.     

Interpretation of carbon dioxide diffusion behavior in coals

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.     

CO2地质封存研究与中国CO2地质封存潜力评述

二氧化碳（CO₂）捕获、利用和封存（CCUS）技术是减少人类活动排放CO₂的关键技术,也是我国实现“碳中和、碳达峰”目标的重要手段。面对我国复杂的地质条件,构建适用于我国的CO₂地质封存评价体系,有针对性的选取数学模型评价我国主要沉积盆地CO₂封存潜力,是我国发展CCUS技术的前提。本文根据国内外学者研究成果,阐述了CO₂深部咸水层封存、驱油封存和驱气封存的机制,在我国尚未建立统一CO₂地质封存评价体系的情况下,从区域地质条件、储存适宜性、经济性和社会影响4方面提出了适用于我国的CO₂地质封存评价体系;结合国际CCUS项目案例评价了CO₂地质封存的安全性,并通过碳收集领导人论坛（CSLF）和美国能源部（DOE）等机构提出的数学模型,分别计算了我国难开采煤层、深部咸水层和枯竭油气藏的CO₂储量,科学评价了我国主要沉积盆地CO₂地质封存潜力,结果表明中国主要沉积盆地CO₂理论封存总量高达41 244.87×10～8t。     

Quick-look assessments to identify optimal CO2 EOR storage sites

A newly developed, multistage quick-look methodology allows for the efficient screening of an unmanageably large number of reservoirs to generate a workable set of sites that closely match the requirements for optimal CO₂ enhanced oil recovery (EOR) storage. The objective of the study is to quickly identify miscible CO₂ EOR candidates in areas that contain thousands of reservoirs and to estimate additional oil recovery and sequestration capacities of selected top options through dimensionless modeling and reservoir characterization. Quick-look assessments indicate that the CO₂ EOR resource potential along the US Gulf Coast is 4.7 billion barrels, and CO₂ sequestration capacity is 2.6 billion metric tons. In the first stage, oil reservoirs are screened and ranked in terms of technical and practical feasibility for miscible CO₂ EOR. The second stage provides quick estimates of CO₂ EOR potential and sequestration capacities. In the third stage, a dimensionless group model is applied to a selected set of sites to improve the estimates of oil recovery and storage potential using appropriate inputs for rock and fluid properties, disregarding reservoir architecture and sweep design. The fourth stage validates and refines the results by simulating flow in a model that describes the internal architecture and fluid distribution in the reservoir. The stated approach both saves time and allows more resources to be applied to the best candidate sites.     

Geologic factors controlling CO2 storage capacity and permanence: case studies based on experience with heterogeneity in oil and gas reservoirs applied to CO2 storage

A variety of structural and stratigraphic factors control geological heterogeneity, inferred to influence both sequestration capacity and effectiveness, as well as seal capacity. Structural heterogeneity factors include faults, folds, and fracture intensity. Stratigraphic heterogeneity is primarily controlled by the geometry of depositional facies and sandbody continuity, which controls permeability structure. The permeability structure, in turn, has implications for CO₂ injectivity and near-term migration pathways, whereas the long-term sequestration capacity can be inferred from the production history. Examples of Gulf Coast oil and gas reservoirs with differing styles of stratigraphic heterogeneity demonstrate the impact of facies variability on fluid flow and CO₂ sequestration potential. Beach and barrier-island deposits in West Ranch field in southeast Texas are homogeneous and continuous. In contrast, Seeligson and Stratton fields in south Texas, examples of major heterogeneity in fluvial systems, are composed of discontinuous, channel-fill sandstones confined to narrow, sinuous belts. These heterogeneous deposits contain limited compartments for potential CO₂ storage, although CO₂ sequestration effectiveness may be enhanced by the high number of intraformational shale beds. These field examples demonstrate that areas for CO₂ storage can be optimized by assessing sites for enhanced oil and gas recovery in mature hydrocarbon provinces.     

Supercritical gas sorption on moist coals

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.     

Inducing fractures and increasing cleat apertures in a bituminous coal under isotropic stress via application of microwave energy

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.     

Impact of experimental parameters for manometric equipment on CO2 isotherms measured: Comment on “Inter-laboratory comparison II: CO2 isotherms measured on moisture-equilibrated Argonne premium coals at 55°C and up to 15 MPa” by Goodman et al. (2007)

Modelling the sorption properties of coals for carbon dioxide under supercritical conditions is necessary for accurate prediction of the sequestering ability of coals in seams. We present recent data for sorption curves of three dry Argonne Premium coals, for carbon dioxide, methane and nitrogen at two different temperatures at pressures up to 15 MPa. The sorption capacity of coals tends to decrease with increasing temperature. An investigation into literature values for sorption of nitrogen and methane by charcoal also show sorption capacities that decrease dramatically with increasing temperature. This is inconsistent with expectations from Langmuir models of coal sorption, which predict a sorption capacity that is independent of temperature. We have successfully fitted the isotherms using a modified Dubinin–Radushkevich equation that uses gas density rather than pressure. A simple pore-filling model that assumes there is a maximum pore width that can be filled in supercritical conditions and that this maximum pore width decreases with increasing temperature, can explain this temperature dependence of sorption capacity. It can also explain why different supercritical gases give apparently different surface sorption capacities on the same material. The calculated heat of sorption for these gases on these coals is similar to those found for these gases on activated carbon.     

Laboratory study of self-potential signals during releasing of CO2 and N2 plumes

Carbon Capture Sequestration (CCS) projects require, for safety reasons, monitoring programmes focused on surveying gas leakage on the surface. Generally, these programmes include detection of chemical tracers that, once on the surface, could be associated with CO₂ degassing. We take a different approach by analysing feasibility of applying electrical surface techniques, specifically Self-Potential. A laboratory-scale model, using water-sand, was built for simulating a leakage scenario being monitored with non-polarisable electrodes. Electrical potentials were measured before, during and after gas injection (CO₂ and N₂) to determine if gas leakage is detectable. Variations of settings were done for assessing how the electrical potentials changed according to size of electrodes, distance from electrodes to the gas source, and type of gas. Results indicated that a degassing event is indeed detectable on electrodes located above injection source. Although the amount of gas could not be quantified from signals, injection timespan and increasing of injection rate were identified. Even though conditions of experiments were highly controlled contrasting to those usually found at field scale, we project that Self-Potential is a promising tool for detecting CO₂ leakage if electrodes are properly placed.     

二氧化碳地质研究的意义及全球高含二氧化碳天然气的分布特点

二氧化碳地质研究具有重要的资源意义和地学意义。其资源意义表现在高纯CO₂天然气可视为珍贵的矿产资源,在工农业中有广阔的用途。CO₂地质研究的其它地学意义在于它是全球碳循环过程中的重要中枢,也是人类认识自然的极好指标。全球高含CO₂天然气的分布具有明显的区域性特点,主要分布在北美洛基山东麓、中国东部、南中国海、南澳大利亚和新西兰,等环太平洋国家和地区,还有欧洲喀尔巴阡地区和高加索地区。这些地区正好是中—新生代火山—岩浆活动激烈、构造十分发育、地热流值异常高的含烃或高含碳酸盐物质的沉积盆地。     

Acid-gas injection at West Stoddart, British Columbia: An analogue for the detailed hydrogeological characterization of a CO2 sequestration site

Geological sequestration of CO₂ is an option for significantly reducing emissions into the atmosphere. Various hydrocarbon companies in western Canada are currently injecting acid-gas (CO₂ and H₂S) into deep subsurface formations. At West Stoddart, in northeast British Columbia, acid-gas has been injected since 1998 at 1600 m depth into sandstones of the Triassic Halfway Formation, which forms a regional aquifer. A comprehensive subsurface characterization was conducted of the regional and local-scale geology, reservoir characteristics, mineralogy, in situ fluid properties, and hydrogeology. Preliminary results from geochemical and numerical multi-phase flow modelling suggest that the majority of the injected acid-gas will dissolve in the formation water and remain within a radius of a few kilometres of the injection well. The experience with the acid-gas injection at West Stoddart and other operations in the Alberta Basin has shown that the process of large-scale CO₂-injection into deep aquifers is technically feasible.     

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.     

CO<Subscript>2</Subscript> leakage through an abandoned well: problem-oriented benchmarks

The efficiency and sustainability of carbon dioxide (CO₂) storage in deep geological formations crucially depends on the integrity of the overlying cap-rocks. Existing oil and gas wells, which penetrate the formations, are potential leakage pathways. This problem has been discussed in the literature, and a number of investigations using semi-analytical mathematical approaches have been carried out by other authors to quantify leakage rates. The semi-analytical results are based on a number of simplifying assumptions. Thus, it is of great interest to assess the influence of these assumptions. We use a numerical model to compare the results with those of the semi-analytical model. Then we ease the simplifying restrictions and include more complex thermodynamic processes including sub- and supercritical fluid properties of CO₂ and non-isothermal as well as compositional effects. The aim is to set up problem-oriented benchmark examples that allow a comparison of different modeling approaches to the problem of CO₂ leakage.     

Gas–water–rock interactions in sedimentary basins: CO2 sequestration in the Frio Formation, Texas, USA

To investigate the potential for the geologic storage of CO₂ in saline sedimentary aquifers, 1600 ton of CO₂ were injected at 1500 m depth into a 24-m sandstone section of the Frio Formation — a regional reservoir in the US Gulf Coast. Fluid samples obtained from the injection and observation wells before, during and after CO₂ injection show a Na–Ca–Cl type brine with 93,000 mg/L TDS and near saturation of CH₄ at reservoir conditions. As injected CO₂ gas reached the observation well, results showed sharp drops in pH (6.5 to 5.7), pronounced increases in alkalinity (100 to 3000 mg/L as HCO₃) and Fe (30 to 1100 mg/L), and significant shifts in the isotopic compositions of H₂O and DIC. Geochemical modeling indicates that brine pH would have dropped lower, but for buffering by dissolution of calcite and Fe oxyhydroxides. Post-injection results show the brine gradually returning to its pre-injection composition.     

我国煤岩动力灾害研究进展及面临的科技难题

为加快推进煤矿瓦斯抽采与利用技术的研发与应用,通过系统梳理我国煤矿瓦斯抽放向瓦斯抽采的转变的发展历程,总结归纳我国典型煤矿瓦斯精准高效抽采技术,全面介绍煤矿瓦斯安全集输与全浓度利用技术最新应用情况,以期为加速发展煤矿瓦斯抽采与利用技术、实现甲烷减排、助力“双碳”目标提供借鉴。

20世纪30—50年代是我国煤矿瓦斯抽放的起步阶段,主要在辽宁抚顺、山西阳泉、重庆天府等矿区采用井下钻孔法抽放本煤层瓦斯和上邻近层瓦斯保障采掘安全;60—70年代,黑龙江鸡西、贵州水城等矿区采取边掘边抽措施防治瓦斯超限与事故,取得了良好效果,开发了邻近层瓦斯抽放技术,针对低透气性煤层试验了煤层水力割缝、控制预裂爆破、水射流扩孔等方法;自20世纪80年代开始,随着工作面瓦斯涌出量大幅度增加,有针对性地应用本煤层瓦斯预抽、邻近层瓦斯抽放和采空区瓦斯抽放等多种方法对卸压瓦斯进行了有效抽放,但瓦斯利用进展缓慢。进入21世纪后,对煤矿瓦斯的认识已由单一灾害属性向灾害与资源共生属性转变,逐步实现了变瓦斯抽放为瓦斯抽采,结合不同矿区瓦斯地质条件差异性形成了以山西晋城四区联动井上下联合抽采模式、安徽两淮碎软低渗煤层保护层开采模式以及重庆松藻水力强化增透模式为典型代表的瓦斯抽采模式,研发形成了水封阻火泄爆、干式阻火器、细水雾和自动喷粉抑爆等安全输送技术与装置,迄今已获得广泛应用。在瓦斯利用方面,研发应用了瓦斯发电、直流氧化、多孔介质燃烧、蓄热氧化、乏风瓦斯掺混氧化等技术,为煤矿瓦斯“全浓度”利用提供了技术支撑。煤矿瓦斯抽采仍然面临普适性差、成本高等难题,利用技术主要面临经济性差的挑战。

煤矿瓦斯抽采与利用技术的发展方向,将聚焦至发展十年规划区域瓦斯超前预抽采技术体系、扩展煤与瓦斯共采模式、源头优化瓦斯利用各环节、提升瓦斯抽采管网智能化水平、产−学−研−政联合攻关助力瓦斯利用项目落地等方面,实现煤矿瓦斯的全浓度利用,助力煤炭行业双碳目标的实现。

     

Spatial characterization of the location of potentially leaky wells penetrating a deep saline aquifer in a mature sedimentary basin

This work was motivated by considerations of potential leakage pathways for CO₂ injected into deep geological formations for the purpose of carbon sequestration. Because existing wells represent a potentially important leakage pathway, a spatial analysis of wells that penetrate a deep aquifer in the Alberta Basin was performed and various statistical measures to quantify the spatial distribution of these wells were presented. The data indicate spatial clustering of wells, due to oil and gas production activities. The data also indicate that the number of wells that could be impacted by CO₂ injection, as defined by the spread of an injected CO₂ plume, varies from several hundred in high well-density areas to about 20 in low-density areas. These results may be applied to other mature continental sedimentary basins in North America and elsewhere, where detailed information on well location and status may not be available.     

温室气体CO2与钠长石相互作用实验研究

为寻求减缓全球变暖的途径,利用高压釜,开展不同温度下(75、100、125、150和175℃)CO₂与钠长石的水热实验研究,以探讨CO₂在长石砂岩中地质封存的可能性。结果表明,随着温度的升高,钠长石溶蚀强度逐渐增强,在150℃左右开始有菱镁矿、菱铁矿等碳酸盐矿物生成,175℃新矿物生成量增加,这表明CO₂能够以碳酸盐矿物的形式在含钠长石的长石砂岩中以矿物的形式被“固定”,其被“固定”的温度在150℃左右。