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.     

Determination of in-situ stress direction from cleat orientation mapping for coal bed methane exploration in south-eastern part of Jharia coalfield, India

Accurate prediction of in-situ stress directions plays a key role in any Coal Bed Methane (CBM) exploration and exploitation project in order to estimate the production potential of the CBM reservoirs. Permeability is one of the most important factors for determination of CBM productivity. The coal seams in Jharia coalfield generally show low permeability in the range of 0.5 md to 3 md. To estimate the in-situ stress direction in the study area, an attempt has been made to undertake the cleat orientation mapping of four regional coal seams of two underground coal mines located at south-eastern part of Jharia coalfield, India. Cleat orientation mapping is critical to determine the maximum principal compressive horizontal stress (S_H) direction for CBM exploration and exploitation, which in turn controls the direction of maximum gas or water flow though coal beds. From the field study it is found that the average face and butt cleat azimuths are towards N15°W and N75°E respectively. Average permeability of the four above-mentioned major coal seams has been calculated from well logs of nine CBM wells distributing over an area of 7.5 km², adjacent to the underground mines. The cleat orientations are congruous with the regional lineament pattern and fits well with the average permeability contour map of the study area to infer the orientation of in-situ maximum horizontal stress. Goodness of fit for the exponential regressions between vertical stress and permeability for individual coal seams varies between 0.6 and 0.84. The cleat orientation is further validated from the previous fracture analysis using FMI well log in Parbatpur area located southern part of the Jharia coalfield. The major coal seams under the study area exhibit directional permeability, with the maximum permeability, oriented parallel to the direction of face cleat orientation.     

Controlling Bottom Hole Flowing Pressure Within a Specific Range for Efficient Coalbed Methane Drainage

The stress state of coal surrounding a coalbed methane (CBM) production well is affected by the bottom hole flowing pressure (BHFP). The permeability of coal shows a marked change under compression. The BHFP must be restricted to a specific range to favor higher permeability in the surrounding coal and thus higher productivity of the well. A new method to determine this specific range is proposed in this paper. Coal has a rather low tensile strength, which induces tensile failure and rock disintegration. The deformation of coal samples under compression has four main stages: compaction, elastic deformation, strain hardening, and strain softening. Permeability is optimal when the coal samples are in the strain softening stage. The three critical values of BHFP, namely, p _wmin, p _wmid, and p _wupper, which correspond to the occurrence of tensile failure, the start of strain softening, and the beginning of plastic deformation, respectively, are derived from theoretical principles. The permeability of coal is in an optimal state when the BHFP is between p _wmin and p _wmid. The BHFP should be confined to this specific range for the efficient drainage of CBM wells. This method was applied to field operations in three wells in the Hancheng CBM field in China. A comprehensive analysis of drainage data and of the BHFP indicates that the new method is effective and offers significant improvement to current practices.     

基于不同方向模量损失率的含瓦斯煤各向异性渗透模型

煤是自然界中一种常见的沉积岩,它具有显著的各向异性特征。然而目前针对煤体渗透特性研究,多数学者为了简化问题,多假设煤体为各向同性材料,提出了相应的各向同性渗透模型。这类模型并不能完全反映含瓦斯煤气-固耦合真实工程和室内试验的实际情况。假设煤体为横观各向同性,推导出以不同方向模量损失率为关键参数的煤体各向异性渗透模型,在此基础上,推导出含瓦斯煤的气-固耦合控制方程,并植入Comsol计算平台,系统研究煤体各向异性对气体扩散和渗透的影响。理论和数值研究结果表明：不同方向的模量损失率 反映出煤体结构各向异性变化程度,若 不同,其煤体各方向渗透特性也不相同;煤体渗透率的改变主要受解吸附效应和有效应力作用双重影响, 反映了这两种效应对于渗透率的影响程度;单轴应变或位移控制边界条件下,水平方向的模量损失率 对于垂直方向的煤体渗透率改变量 的影响程度大于对水平方向的煤体渗透率改变量 的影响程度,垂直方向模量损失率 对 的影响则弱于对于 的影响。     

On computation of strain‐dependent permeability of rocks and rock‐like porous media

Determination of transport properties of geomaterials is an important issue in many fields of engineering analysis and design. For example, in petroleum engineering, in situ permeability of an oil reservoir may be crucial in establishing its viability for exploitation, whilst prevention of leakage from underground storage facilities for oil and gas, nuclear waste as well as viability of CO₂ sequestration projects crucially depends on its long‐term values. Permeability is indirectly related to the porosity, pore‐size distribution and pore architecture of the porous media. These parameters evolve when a strain field is imposed. Physical measurement of permeability under a strain field in laboratory conditions is difficult, expensive and prone to a number of uncertainties. In the past, pore network models have been used to compute permeability of materials under stress/strain‐free conditions. In this paper, we propose an enhanced pore network model to compute permeability of rocks and rock‐like porous media under a stress/strain field. Data of pore‐size distribution obtained from mercury intrusion porosimetry are used to compute permeability of rock samples from various unspecified oilfields in the world. It is shown that the two permeabilities can be predicted from the model with sufficient accuracy. A hypothesis for change in porosity, pore‐size distribution and pore architecture as a result of imposed mechanical strains is then proposed. Based on this, permeability is computed again for one of the rock samples under uniaxial and triaxial compressive and tensile strain fields. It is shown that depending on the state of strain field imposed, permeability evolves in an anisotropic manner. Permeability under tensile strain field increases dramatically compared with the reduction that takes place under compressive strain field of the same magnitude. Copyright © 2014 John Wiley & Sons, Ltd.     

功率声波影响煤层甲烷储运的初步探讨

分析了功率声波对煤岩介质孔隙率和渗透率的影响规律及作用机理。结果表明,功率声波能够增加煤的孔隙体积,提高甲烷在煤层中的渗透率;功率声波对煤岩等介质的主要作用机理有机械作用、激波作用、定向作用、热效应、空化作用,使煤岩层产生微裂缝,改变煤岩的孔隙结构,降低甲烷气体的粘度,从而为煤层甲烷开发提供了一种新的思路。      

深部煤储层应力敏感性特征及其对煤层气产能的影响

深部煤储层处于高地应力环境中,其渗透率变化特征与浅部存在较大差异,为研究有效应力对深部煤储层渗透率的差异性影响,以及应力敏感性各向异性特征,以沁水盆地横岭区块15号煤层为研究对象,采样深度1 200~1 700 m,采用覆压孔渗实验,开展平行层理和垂直层理样品在不同有效应力下的渗透率变化规律研究,探究其应力敏感性特征及其对煤层气产能的影响。结果表明:渗透率随有效应力的增加呈幂指函数降低,平行层理面渗透率总体高于垂直层理面,且在2个方向上渗透率变化规律呈正相关性。选取储层孔裂隙压缩系数、渗透率损害率和渗透率曲率3个参数作为煤储层应力敏感性评价指标,其中,孔裂隙压缩系数随有效应力增加,以5 MPa为界限先后呈现正相关性和负相关性,渗透率损害率和渗透率曲率分别与有效应力呈指数上升和下降的规律。基于应力敏感性参数,推导出煤层气井产能模型,模型显示,不考虑应力敏感性的气井产量高于考虑应力敏感性,揭示了应力敏感性对煤层气产量的影响程度,即在5 MPa生产压差下,气井的产量降低幅度随应力敏感性系数的增大整体呈增高趋势。针对应力敏感性的阶段划分,研究区目标煤层在煤层气排采过程中应采用小–中–大的排采方案来控制生产流量。      

Laboratory Investigations of the Hydro-Mechanical–Chemical Coupling Behaviour of Sandstone in CO2 Storage in Aquifers

This paper is devoted to experimental investigations of the hydro-mechanical–chemical coupling behaviour of sandstone in the context of CO₂ storage in aquifers. We focused on the evolution of creep strain, the transport properties and the elastic modulus of sandstone under the effect of CO₂–brine or CO₂ alone. A summary of previous laboratory results is first presented, including mechanical, poromechanical and hydro-mechanical–chemical coupling properties. Tests were then performed to investigate the evolution of the creep strain and permeability during the injection of CO₂–brine or CO₂ alone. After the injection of CO₂–brine or CO₂ alone, an instantaneous volumetric dilatancy was observed due to the decrease in the effective confining stress. However, CO₂ alone had a significant influence on the creep strain and permeability compared to the small influence of CO₂–brine. This phenomenon can be attributed to the acceleration of the CO₂–brine–rock reaction by the generation of carbonic acid induced by the dissolution of CO₂ into the brine. The original indentation tests on samples after the CO₂–brine–rock reaction were also performed and indicated that the elastic modulus decreased with an increasing reaction time. The present laboratory results can advance our knowledge of the hydro-mechanical–chemical coupling behaviour of sandstone in CO₂ storage in aquifers.     

煤储层应力敏感性试验及其评价新方法

煤储层应力敏感性是影响煤层气井产能的关键地质因素,在煤层气井排采过程中如何降低或避免煤储层应力敏感性对渗透性的影响是值得考虑的问题。通过不同应力下煤储层渗透性试验,研究了有效应力作用下煤储层渗透率的变化规律;在对已有应力敏感性评价参数分析的基础上,提出了新的应力敏感性系数S₁与S₂,揭示了有效应力对煤储层渗透性的影响规律。研究结果表明:煤储层渗透率随有效应力的增加按负指数函数规律降低,在煤层气开发中煤储层表现出明显的应力敏感性;试验煤样应力敏感回归系数a为0.099~0.115 MPa-1,平均为0.108 MPa-1,应力敏感性系数S₁为0.383~0.436,平均为0.414,应力敏感性系数S₂为0.572~0.666,平均0.625;应力敏感性系数S₁与S₂具有整体性与唯一性,可以结合应力敏感回归系数a进行煤储层渗透率应力敏感性评价。      

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.     

Modelling of anisotropic coal swelling and its impact on permeability behaviour for primary and enhanced coalbed methane recovery

Coal swelling/shrinkage during gas adsorption/desorption is a well-known phenomenon. For some coals the swelling/shrinkage shows strong anisotropy, with more swelling in the direction perpendicular to the bedding than that parallel to the bedding. Experimental measurements performed in this work on an Australian coal found strong anisotropic swelling behaviour in gases including nitrogen, methane and carbon dioxide, with swelling in the direction perpendicular to the bedding almost double that parallel to the bedding. It is proposed here that this anisotropy is caused by anisotropy in the coal's mechanical properties and matrix structure. The Pan and Connell coal swelling model, which applies an energy balance approach where the surface energy change caused by adsorption is equal to the elastic energy change of the coal solid, is further developed to describe the anisotropic swelling behaviour incorporating coal property and structure anisotropy. The developed anisotropic swelling model is able to accurately describe the experimental data mentioned above, with one set of parameters to describe the coal's properties and matrix structure and three gas adsorption isotherms. This developed model is also applied to describe anisotropic swelling measurements from the literature where the model was found to provide excellent agreement with the measurement. The anisotropic coal swelling model is also applied to an anisotropic permeability model to describe permeability behaviour for primary and enhanced coalbed methane recovery. It was found that the permeability calculation applying anisotropic coal swelling differs significantly to the permeability calculated using isotropic volumetric coal swelling strain. This demonstrates that for coals with strong anisotropic swelling, anisotropic swelling and permeability models should be applied to more accurately describe coal permeability behaviour for both primary and enhanced coalbed methane recovery processes.     

Wellbore cement fracture evolution at the cement–basalt caprock interface during geologic carbon sequestration

Composite Portland cement–basalt caprock cores with fractures, as well as neat Portland cement columns, were prepared to understand the geochemical and geomechanical effects on the integrity of wellbores with defects during geologic carbon sequestration. The samples were reacted with CO₂–saturated groundwater at 50 °C and 10 MPa for 3 months under static conditions, while one cement–basalt core was subjected to mechanical stress at 2.7 MPa before the CO₂ reaction. Micro-XRD and SEM–EDS data collected along the cement–basalt interface after 3-month reaction with CO₂–saturated groundwater indicate that carbonation of cement matrix was extensive with the precipitation of calcite, aragonite, and vaterite, whereas the alteration of basalt caprock was minor. X-ray microtomography (XMT) provided three-dimensional (3-D) visualization of the opening and interconnection of cement fractures due to mechanical stress. Computational fluid dynamics (CFD) modeling further revealed that this stress led to the increase in fluid flow and hence permeability. After the CO₂-reaction, XMT images displayed that calcium carbonate precipitation occurred extensively within the fractures in the cement matrix, but only partially along the fracture located at the cement–basalt interface. The 3-D visualization and CFD modeling also showed that the precipitation of calcium carbonate within the cement fractures after the CO₂-reaction resulted in the disconnection of cement fractures and permeability decrease. The permeability calculated based on CFD modeling was in agreement with the experimentally determined permeability. This study demonstrates that XMT imaging coupled with CFD modeling represent a powerful tool to visualize and quantify fracture evolution and permeability change in geologic materials and to predict their behavior during geologic carbon sequestration or hydraulic fracturing for shale gas production and enhanced geothermal systems.     

Accumulation models for coalbed methane in medium- to high-rank coals: examples from the southern Qinshui Basin and southeastern Ordos Basin

The majority of known coalbed methane (CBM) production worldwide comes primarily from high-abundance CBM-enrichment areas or ‘fairways.’ The high-abundance CBM-enrichment areas are primarily characterised by large CBM resources with high single-well productions. CBM accumulation areas from the medium- to high-rank coals in the southern Qinshui Basin and the Hancheng CBM fields in the Ordos Basin were investigated based on regional geological analyses and physical analogue experiments. The results show that gas contents in the study areas increase with depths over the range from approximately 300 to 800 m, while permeabilities generally decrease with depths. Intervals with optimal gas content and permeability exist at a moderate depth along an inclined coal seam under the coupled control of temperature and stress. Brittle–ductile transition deformation increases the permeability and the pore-specific surface areas of coals. The gas content and permeability of the CBM reservoirs are shown to be two key factors determining the formation of high-abundance CBM areas. The coupling of gas enrichment and high permeability provides a favourable combination for CBM accumulation and high production. Combining CBM exploration and development practices in the study areas with physical analogue experiments, two CBM-enrichment models for medium- to high-rank coal have been recognised for different geological conditions, including (1) the model controlled by the depth in the slope zone and (2) the model controlled by the coal brittle and ductile in the deformation zones.     

Modeling of ECBM recovery from Amasra coalbed in Zonguldak Basin,Turkey

Zonguldak coal basin is the only productive hard coal basin of Turkey. The eastern part of the basin is called as Bartin–Amasra District, which has deeper coal seams. The depth and difficulty of mining these coal seams make this district an important candidate for coalbed methane (CBM) recovery. However, there is not enough reservoir data for modeling purposes. In this study, the lithologic information collected for coal mining industry was used to determine the correlations and the continuity of the coal seams. The lithologic information was examined and the depths of the coal seams and the locations of the exploration boreholes were used to perform a reliable correlation using a new method. As a result of the correlation study, 63 continuous coal layers were found. A statistical reserve estimation of each coal layer for methane was made by using Monte Carlo simulation method. The initial methane in place found in the coal layers both in free and adsorbed states were estimated using probabilistic simulations resulted in possible reserve (P10) of 2.07 billion m³, probable reserve (P50) of 1.35 billion m³ and proven reserves (P90) of 0.86 billion m³.Among the determined continuous coal layers, coal layer #26 was selected for a preliminary investigation of the applicability of enhanced coalbed methane (ECBM) recovery and CO₂ storage. The scarcity of coal seam reservoir data required a sensitivity study for the effects of reservoir parameters on operational performance indicators. The effects of adsorption, coal density, permeability, cleat porosity and permeability anisotropy parameters were examined using the Computer Modeling Group's (CMG) GEM module.     

深部煤层气储层地质研究进展

深部煤层气资源潜力巨大,将是非常规天然气勘探开发的一个新领域。美国和加拿大部分地区已经成功实现了深部煤层气开发的商业化水平,而中国由于受当前开发技术和经济条件的限制,至今尚未形成规模性的开采。基于对近年来有代表性的学术论著的研究分析,从煤岩孔裂隙结构、吸附解吸性质、气体在煤层中的扩散渗流过程、煤储层的可改造特征等4个方面总结了深部煤层气储层物性的理论研究进展。研究指出深部煤储层处在高温、高压和高地应力的复杂地质环境中,煤储层储渗演化、煤层气吸附解吸扩散渗流平衡关系、煤岩应力应变行为等趋于复杂,开展特殊地质条件下的深部煤储层物性演化机理的研究,对我国深部煤层气资源的勘探开发具有重要的理论和现实意义。     

采动煤岩渗透率演化模型及数值模拟

为描述采动煤岩渗透率演化过程,引入强度退化指数,基于Hoek-Brown强度准则,建立了考虑围压影响的煤岩应变软化力学本构模型。给出了体积应变和渗透率的关系方程,结合应变软化模型建立了采动煤岩渗透率演化模型,并在FLAC下予以实现。通过数值模拟研究了不同围压下圆柱岩样的峰后应变软化力学行为和某煤矿工作面开采过程中煤岩的渗透率演化过程,结果表明：（1）该模型能较好地反映围压对煤岩峰后应变软化行为的影响;（2）随着工作面推进,越来越多的煤岩单元破坏,渗透率也不断增长,逐渐成为瓦斯等流体运移的主要通道。（3）模型能再现采动煤岩渗透率演化的动态过程,从而为煤与瓦斯共采、煤层瓦斯抽放和瓦斯灾害防治提供指导。     

Permeability evolution in fractured coal: The roles of fracture geometry and water-content

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.     

页岩储层裂隙渗透率模型和试验研究

页岩储层的裂隙渗透率是评价页岩气开采的重要参数,基于裂隙法向刚度的概念,考虑页岩储层变形过程中裂隙系统和基质系统之间的相互作用以及页岩气解吸引起的体应变,提出了与有效应力相关的页岩储层的渗透率模型。然后分别分析了页岩气藏在单轴应变和常体积条件下的渗透率模型,分析表明,单轴应变和常体积条件下（3个方向的总应变都为0）的裂隙渗透率模型完全一致。采用脉冲衰减渗透率仪测试了煤系页岩的裂隙渗透率,当有效应力从0.7 MPa增加到14.5 MPa时,渗透率从41.81×10-17 m2降到5.43×10-17 m2。为了阐述渗透率模型的有效性,利用煤系页岩的渗透率数据对有效应力-渗透率模型进行拟合。结果表明,当裂隙的法向刚度、张开度和煤系页岩的初始渗透率分别为57 922.5 MPa/m、0.000 17 m和50.15×10-17 m2时,有效应力-渗透率模型和煤系页岩的渗透率拟合程度较好。然后利用现场渗透率数据对该模型进行拟合,结果表明,当裂隙的法向刚度和张开度的关系符合反比例函数时,拟合程度非常好。该渗透率模型适合于单轴应变、常体积和常围压条件,可用于描述页岩气开采过程中页岩储层裂隙渗透率随孔隙压力的变化规律。同时,该渗透率模型和P&M模型以及S&D模型进行了比较,结果表明,该渗透率模型的拟合结果与S&D模型基本一致,但与P&M模型存在一定的差别。     

Assessment of shrinkage–swelling influences in coal seams using rank-dependent physical coal properties

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.     

Mechanical and flow behaviours and their interactions in coalbed geosequestration of CO2

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.