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An analytical transient coal permeability model:Sorption non-equilibrium index-based swelling switch

Journal of Rock Mechanics and Geotechnical Engineering 2024年第4期16卷 1175-1191页

作者： Xingxing Liu Jishan Liu Liang Chen Jinchang Sheng Huimin Wang College of Civil and Transportation Engineering Hohai UniversityNanjing210098China School of Engineering The University of Western Australia35 Stirling HighwayCrawleyWA6009Australia College of Water Conservancy and Hydropower Engineering Hohai UniversityNanjing210098China

Long-term permeability experiments have indicated that sorption-induced swelling can switch from internal to bulk depending on the evolutive sorption status.However,this sorption swelling switch mechanism has not been considered in current analytical permeability models.This study introduces a normalized sorption non-equilibrium index(SNEI)to characterize the sorption status,quantify the dynamical variations of matrix swelling accumulation and internal swelling partition,and formulate the sorption swelling switch model.The incorporation of this index into the extended total effective stress concept leads to an analytical transient coal permeability model.Model results show that the sorption swelling switch itself results in the permeability switch under stress-constrained conditions,while the confined bulk swelling suppresses the permeability recovery to the continuous reduction under displacement-constrained conditions.Model verifications show that current experimental observations correspond to the early stages of the transient process,and they could be extended to the whole process with these models.This study demonstrates the importance of the sorption swelling switch in determining permeability evolution using simple boundary conditions.It provides new insights into experimentally revealing the sorption swelling switch in the future,and underscores the requirement of a rigorous model for complex coupled processes in large-scale coal seams.

关键词： coal permeability Sorption swelling switch Sorption non-equilibrium Boundary conditions

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Revaluating coal permeability-gas pressure relation under various gas pressure differential conditions

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International Journal of coal Science & Technology 2023年第4期10卷 203-216页

作者： Chunguang Wang Hongxu Wang Derek Elsworth Guanglei Cui Bingqian Li Meng Zhou Wenxin Li Jiyuan Zhang College of Energy and Mining Engineering Shandong University of Science and TechnologyQingdao 266590China Energy and Mineral Engineering GeosciencesG3 Centerand EMS Energy InstitutePennsylvania State UniversityUniversity ParkPAUSA Key Laboratory of Ministry of Education On Safe Mining of Deep Metal Mines Northeastern UniversityShenyang 110004China Pingdingshan Tianan coal Mining Co. LTDPingdingshanHenan 467003China School of Petroleum Engineering China University of Petroleum(East China)Qingdao 266580China

Identifying changes in coal permeability with gas pressure and accurately codifying mean efective stresses in laboratory samples are crucial in predicting gas-fow behavior in coal reservoirs. Traditionally, coal permeability to gas is assessed using the steady-state method, where the equivalent gas pressure in the coal is indexed to the average of upstream and downstream pressures of the coal, while ignoring the nonlinear gas pressure gradient along the gas fow path. For the fow of a compressible gas, the traditional method consistently underestimates the length/volume-averaged pressure and overestimates mean efective stress. The higher the pressure diferential within the sample, the greater the error between the true mean pressure for a compressible fuid and that assumed as the average between upstream and downstream pressures under typical reservoir conditions. A correction coefcient for the compressible fuid pressure asymptotes to approximately 1.3%, representing that the error in mean pressure and efective stress can be on the order of approximately 30%, particularly for highly pressure-sensitive permeabilities and compressibilities, further amplifying errors in evaluated reservoir properties. We utilized this volume-averaged pressure and efective stress to correct permeability and compressibility data reported in the literature. Both the corrected initial permeability and the corrected pore compressibility were found to be smaller than the uncorrected values, due to the underestimation of the true mean fuid pressure, resulting in an overestimation of reservoir permeability if not corrected. The correction coefcient for the initial permeability ranges from 0.6 to 0.1 (reservoir values are only approximately 40% to 90% of laboratory values), while the correction coefcient for pore compressibility remains at approximately 0.75 (reservoir values are only approximately 25% of laboratory value). Errors between the uncorrected and corrected parameters are quantifed under various factors, such as confning pressure,

关键词： Gas compressibility coal permeability Pressure diferential Mean gas pressure

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Controlling effects of differential swelling index on evolution of coal permeability

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Journal of Rock Mechanics and Geotechnical Engineering 2020年第3期12卷 461-472页

作者： Chuanzhong Jiang Zhenfeng Zhao Xiwei Zhang Jishan Liu Derek Elsworth Guanglei Cui Key Laboratory of Ministry of Education on Safe Mining of Deep Metal Mines Northeastern UniversityShenyang110819China Beijing Key Laboratory for Precise Mining of Intergrown Energy and Resources China University of Mining and TechnologyBeijing100083China Oil&Gas Technology Research Institute Changqing Oilfield CompanyXi’an710018China Department of Chemical Engineering School of EngineeringThe University of Western Australia35 Stirling HighwayCrawleyWA6009Australia Department of Energy and Mineral Engineering G3 Centre and Energy InstituteThe Pennsylvania State UniversityUniversity ParkPA16802USA

coal permeability measurements are normally conducted under the assumption that gas pressure in the matrix is equalized with that in fracture and that gas sorption-induced swelling/shrinking strain is uniformly distributed within the coal.However,the validity of this assumption has long been questioned and differential strain between the fracture strain and the bulk strain has long been considered as the primary reason for the inconsistency between experimental data and poroelasticity solutions.Although efforts have been made to incorporate the impact into coal permeability models,the fundamental nature of those efforts to split the matrix strain between fracture and coal bulk remains questionable.In this study,a new concept of differential swelling index(DSI)was derived to theoretically define the relation among sorption-induced strains of the coal bulk,fracture,and coal matrix at the equilibrium state.DSI was a function of the equilibrium pressure and its magnitudes were regulated by the Langmuir constants of both the matrix and the coal bulk.Furthermore,a spectrum of DSI-based coal permeability models was developed to explicitly consider the effect of differential strains.These models were verified with the experimental data under the conditions of uniaxial strain,constant confining pressure,and constant effective stress.

关键词： coal permeability Differential swelling behavior Gas adsorption Equilibrium pressure

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Effect of adsorption-induced matrix swelling on coal permeability evolution of micro-fracture with the real geometry

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Petroleum Science 2021年第4期18卷 1143-1152页

作者： Ming-Yao Wei Jishan Liu Ying-Ke Liu Zhang-Hao Liu Derek Elsworth National and Local Joint Engineering Laboratory of Internet Application Technology on Mine IoT Perception Mine Research CenterChina University of Mining and TechnologyXuzhou 221116JiangsuChina State Key Laboratory of coal and CBM Co-Mining Jincheng 048012ShanxiChina School of Engineering The University of Western Australia35 Stirling HighwayPerthWA6009Australia Faculty of Safety Engineering China University of Mining and TechnologyXuzhou 221116JiangsuChina Institute of Rock and Soil Mechanics Chinese Academy of SciencesWuhan 430071HubeiChina Department of Energy and Mineral Engineering G3 Center and Energy InstituteThe Pennsylvania State UniversityUniversity ParkPA16802USA

Gas transport in coal induces effective stress variation,matrix swelling/shrinkage,and significantly affects matrix and fracture deformation,resulting in porosity and permeability evolution.However,the heterogeneity and anisotropy of coal are neglected in dual porosity models,which can lead to the deviation from the real physical mechanisms.To uncover the permeability evolution,especially the influence of dynamic matrix-fracture interaction for real fracture distribution,advanced virtual simulation is proposed.In this study,real fracture geometry is taken into account in the physical model based on the CT-scan image,while the mathematical models for coal deformation and gas flow are established.Our calculations are verified against a long-term experimental data under the same boundary condition.Accordingly,the real matrix-fracture interaction caused by adsorption-induced matrix deformation has been visually exhibited,and some new insight into the behavior of fracture permeability in real materials is offered.The results indicate the non-uniform distribution of fracture geometry is responsible for the nonmonotonic change of permeability.It also found that injection pressure,Langmuir strain constant and initial matrix permeability have important influences on the fracture permeability evolution.This research provides valuable insight into the understanding of the permeability change for the real fracture spatial distribution in coal.

关键词： Geometry distribution Adsorption swelling coal permeability Matrix-fracture interaction

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Direct Evidence of coal Swelling and Shrinkage with Injecting CO_(2) and N_(2) Using in-situ Synchrotron X-ray Microtomography

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Engineering 2022年第11期18卷 88-95页

作者： Guanglei Zhang P.G.Ranjith Herbert E.Huppert Department of Earth Science and Engineering Imperial College LondonLondon SW72BPUK Deep Earth Energy Laboratory Department of Civil EngineeringMonash UniversityMelbourneVIC3800Australia Institute of Theoretical Geophysics King’s CollegeCambridge CB21STUK

Deep coal seams are one of the world’s most widespread deposits for carbon dioxide(C02)disposal and are generally located near large point sources of CO_(2)emissions.The injection of CO_(2)into coal seams has great potential to sequester CO_(2)while simultaneously enhancing coalbed methane(CO_(2)-ECBM)recovery.Pilot tests of CO_(2)-ECBM have been conducted in coal seams worldwide with favorable early results.However,one of the main technical barriers in coal seams needs to be resolved:Injecting CO_(2)reduces coal permeability and well injectivity.Here,using in situ synchrotron X-ray microtomography,we provide the first observational evidence that injecting nitrogen(N_(2))can reverse much of this lost permeability by reopening fractures that have closed due to coal swelling induced by CO_(2)adsorption.Our findings support the notion that injecting minimally treated flue gas-a mixture of mainly N_(2) and CO_(2)-is an attractive alternative for ECBM recovery instead of pure CO_(2)injection in deep coal seams.Firstly,flue gas produced by power plants could be directly injected after particulate removal,thus avoiding high CO_(2)-separation costs.Secondly,the presence of N_(2)makes it possible to maintain a sufficiently high level of coal permeability.These results suggest that flue-gas ECBM for deep coal seams may provide a promising path toward net-zero emissions from coal mines.

关键词： CCS CO_(2)-ECBM Carbon neutrality X-ray imaging coal permeability

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