Mechanical anisotropy and depth effects of deep coal seams

Authors

• MEI Xiaofan, School of Energy Resource, China University of Geosciences (Beijing), Beijing 100083, ChinaFollow
• WANG Cong, School of Energy Resource, China University of Geosciences (Beijing), Beijing 100083, China; Shanxi Center of Technology Innovation for Coal Measure Mineral Resources Comprehensive Exploration, Taiyuan 030000, China
• WANG Kaifeng, CNOOC Research Institute Ltd., Beijing 100028, China
• TANG Shuheng, School of Energy Resource, China University of Geosciences (Beijing), Beijing 100083, China; Shanxi Center of Technology Innovation for Coal Measure Mineral Resources Comprehensive Exploration, Taiyuan 030000, ChinaFollow
• ZHANG Songhang, School of Energy Resource, China University of Geosciences (Beijing), Beijing 100083, China; Shanxi Center of Technology Innovation for Coal Measure Mineral Resources Comprehensive Exploration, Taiyuan 030000, China
• XI Zhaodong, School of Energy Resource, China University of Geosciences (Beijing), Beijing 100083, China; Shanxi Center of Technology Innovation for Coal Measure Mineral Resources Comprehensive Exploration, Taiyuan 030000, China

Abstract

Objective The mechanical properties of coal seams determine the stability, fracability, and fracturing performance of deep coal reservoirs. Since the in situ stress applied to coal increases with burial depth, deep coal differs from its shallow counterpart in damage behavior, key mechanical parameters, and failure mechanism. Furthermore, deep coal exhibits pronounced mechanical anisotropy.Methods Focusing on the No. 9 coal seam in the Linxian area along the eastern margin of the Ordos Basin, this study carried out triaxial compression tests at 60℃ under different confining pressures to determine the mechanical anisotropy of deep coal. By comparing the full stress-strain curves, key mechanical parameters, and failure modes of coal specimens with sampling directions parallel to face cleats, parallel to butt cleats, and perpendicular to beddings, this study revealed the confining-pressure effects and directional differences in the mechanical responses of deep coal. In addition, the anisotropic failure criterion for deep coal was established.Results and Conclusions The results indicate that, under confining pressures of 0 MPa and 20 MPa, the coal specimens showed distinct linear stress-strain relationships in the elastic deformation stage and then underwent rapidly growing strain in the plastic deformation stage, with coal failure dominated by tensile splitting. Under confining pressures ranging from 30 MPa to 60 MPa, the slopes of the stress-strain curves increased, with the failure mode shifting from tensile-dominated to shear-dominated failure. Comparison reveals that the deformability of the coal specimens with three sampling directions decreased in the order of bedding-perpendicular, butt-cleat-parallel, and face-cleat-parallel directions, while their strength and stiffness decreased in the order of face-cleat-parallel, butt-cleat-parallel, and bedding-perpendicular directions. The compressive strength, residual strength, and elastic modulus of the coal specimens generally increased as the confining pressure rose, with amplitudes increasing at lower rates under high confining pressures. The Poisson's ratios of the coal specimens showed an overall downward trend but dispersed to some extent. Correlation analysis indicates that the elastic modulus and residual strength of the coal specimens were positively correlated with their compressive strength, while the relationship between the Poisson's ratio and compressive strength of the samples exhibited pronounced directional differences. Based on triaxial compression data in the three sampling directions and the Mohr-Coulomb shear failure criterion, this study determined the internal friction angles and cohesion in different directions and established an anisotropic shear failure criterion for deep coal. The results indicate that increasing deep in situ stress would enhance the load-bearing capacity of the coal seam and change its failure mode, while directional mechanical differences remained significant. The results of this study will provide a mechanical basis for the stability analysis, fracability evaluation, fracturing parameter design, and shear failure determination of deep coal reservoirs.

Keywords

deep coal seam, mechanical anisotropy, depth effect, high-temperature triaxial compression test, Mohr-Coulomb shear failure criterion

DOI

10.12363/issn.1001-1986.26.03.0120

Recommended Citation

MEI Xiaofan, WANG Cong, WANG Kaifeng, et al. (2026) "Mechanical anisotropy and depth effects of deep coal seams," Coal Geology & Exploration: Vol. 54: Iss. 9, Article 11.
DOI: 10.12363/issn.1001-1986.26.03.0120
Available at: https://cge.researchcommons.org/journal/vol54/iss9/11

Reference

[1] 秦勇,汤达祯,刘大锰,等. 煤储层开发动态地质评价理论与技术进展[J]. 煤炭科学技术,2014,42(1):80−88. QIN Yong,TANG Dazhen,LIU Dameng,et al. Geological evaluation theory and technology progress of coal reservoir dynamics during coalbed methane drainage[J]. Coal Science and Technology,2014,42(1):80−88.

[2] 鞠玮,姜波,秦勇,等. 滇东恩洪区块地应力分布及深部煤层气临界深度预测[J]. 煤炭科学技术,2020,48(2):194−200. JU Wei,JIANG Bo,QIN Yong,et al. Distribution of in–situ stress and prediction of critical depth for deep coalbed methane in Enhong Block of eastern Yunnan region[J]. Coal Science and Technology,2020,48(2):194−200.

[3] 孙钦平,赵群,姜馨淳,等. 新形势下中国煤层气勘探开发前景与对策思考[J]. 煤炭学报,2021,46(1):65−76. SUN Qinping,ZHAO Qun,JIANG Xinchun,et al. Prospects and strategies of CBM exploration and development in China under the new situation[J]. Journal of China Coal Society,2021,46(1):65−76.

[4] 叶建平,侯淞译,张守仁. "十三五"期间我国煤层气勘探开发进展及下一步勘探方向[J]. 煤田地质与勘探,2022,50(3):15−22. YE Jianping,HOU Songyi,ZHANG Shouren. Progress of coalbed methane exploration and development in China during the 13th Five–Year Plan period and the next exploration direction[J]. Coal Geology & Exploration,2022,50(3):15−22.

[5] 徐凤银,王成旺,熊先钺,等. 鄂尔多斯盆地东缘深部煤层气成藏演化规律与勘探开发实践[J]. 石油学报,2023,44(11):1764−1780. XU Fengyin,WANG Chengwang,XIONG Xianyue,et al. Evolution law of deep coalbed methane reservoir formation and exploration and development practice in the eastern margin of Ordos Basin[J]. Acta Petrolei Sinica,2023,44(11):1764−1780.

[6] 申建,秦勇,傅雪海,等. 深部煤层气成藏条件特殊性及其临界深度探讨[J]. 天然气地球科学,2014,25(9):1470−1476. SHEN Jian,QIN Yong,FU Xuehai,et al. Properties of deep coalbed methane reservoir–forming conditions and critical depth discussion[J]. Natural Gas Geoscience,2014,25(9):1470−1476.

[7] 李辛子,王运海,姜昭琛,等. 深部煤层气勘探开发进展与研究[J]. 煤炭学报,2016,41(1):24−31. LI Xinzi,WANG Yunhai,JIANG Zhaochen,et al. Progress and study on exploration and production for deep coalbed methane[J]. Journal of China Coal Society,2016,41(1):24−31.

[8] 闫霞,徐凤银,聂志宏,等. 深部微构造特征及其对煤层气高产"甜点区"的控制:以鄂尔多斯盆地东缘大吉地区为例[J]. 煤炭学报,2021,46(8):2426−2439. YAN Xia,XU Fengyin,NIE Zhihong,et al. Microstructure characteristics of Daji area in east Ordos Basin and its control over the high yield dessert of CBM[J]. Journal of China Coal Society,2021,46(8):2426−2439.

[9] 何发岐,董昭雄. 深部煤层气资源开发潜力:以鄂尔多斯盆地大牛地气田为例[J]. 石油与天然气地质,2022,43(2):277−285. HE Faqi,DONG Zhaoxiong. Development potential of deep coalbed methane: A case study in the Daniudi gas field,Ordos Basin[J]. Oil & Gas Geology,2022,43(2):277−285.

[10] 刘建忠,朱光辉,刘彦成,等. 鄂尔多斯盆地东缘深部煤层气勘探突破及未来面临的挑战与对策:以临兴–神府区块为例[J]. 石油学报,2023,44(11):1827−1839. LIU Jianzhong,ZHU Guanghui,LIU Yancheng,et al. Breakthrough,future challenges and countermeasures of deep coalbed methane in the eastern margin of Ordos Basin: A case study of Linxing–Shenfu Block[J]. Acta Petrolei Sinica,2023,44(11):1827−1839.

[11] 徐凤银,闫霞,李曙光,等. 鄂尔多斯盆地东缘深部(层)煤层气勘探开发理论技术难点与对策[J]. 煤田地质与勘探,2023,51(1):115−130. XU Fengyin,YAN Xia,LI Shuguang,et al. Theoretical and technological difficulties and countermeasures of deep CBM exploration and development in the eastern edge of Ordos Basin[J]. Coal Geology & Exploration,2023,51(1):115−130.

[12] 秦勇,申建,李小刚. 中国煤层气资源控制程度及可靠性分析[J]. 天然气工业,2022,42(6):19−32. QIN Yong,SHEN Jian,LI Xiaogang. Control degree and reliability of CBM resources in China[J]. Natural Gas Industry,2022,42(6):19−32.

[13] 郭广山,徐凤银,刘丽芳,等. 鄂尔多斯盆地府谷地区深部煤层气富集成藏规律及有利区评价[J]. 煤田地质与勘探,2024,52(2):81−91. GUO Guangshan,XU Fengyin,LIU Lifang,et al. Enrichment and accumulation patterns and favorable area evaluation of deep coalbed methane in the Fugu area,Ordos Basin[J]. Coal Geology & Exploration,2024,52(2):81−91.

[14] 闫涛滔,邓志宇,吴鹏,等. 鄂尔多斯盆地东缘临兴东区杨家坡区块煤层气井产能特征及主控因素[J]. 现代地质,2024,38(6):1545−1556. YAN Taotao,DENG Zhiyu,WU Peng,et al. Characteristics and key control factors of coalbed methane well productivity in the Yangjiapo Block,eastern Linxing district,Ordos Basin[J]. Geoscience,2024,38(6):1545−1556.

[15] 侯晓伟. 沁水盆地深部煤系气储层控气机理及共生成藏效应[D]. 徐州: 中国矿业大学, 2020. HOU Xiaowei. Study on gas controlling mechanism and coupled accumulation of deep coal measure gases in Qinshui Basin[D]. Xuzhou: China University of Mining and Technology, 2020.

[16] 秦勇. 中国深部煤层气地质研究进展[J]. 石油学报,2023,44(11):1791−1811. QIN Yong. Progress on geological research of deep coalbed methane in China[J]. Acta Petrolei Sinica,2023,44(11):1791−1811.

[17] LI Guoqing,YAN Detian,ZHUANG Xinguo,et al. Implications of the pore pressure and in situ stress for the coalbed methane exploration in the southern Junggar Basin,China[J]. Engineering Geology,2019,262:105305.

[18] 高向东. 临兴深部煤储层孔渗成因演化机制及压裂可改造性研究[D]. 北京: 中国矿业大学(北京), 2019. GAO Xiangdong. Study on porosity and permeability evolution mechanism and fracturing reconstruction feasibility of deep coal reservoirs in Linxing area[D]. Beijing: China University of Mining and Technology (Beijing), 2019.

[19] 陈世达,汤达祯,侯伟,等. 深部煤层气地质条件特殊性与储层工程响应[J]. 石油学报,2023,44(11):1993−2006. CHEN Shida,TANG Dazhen,HOU Wei,et al. Geological particularity and reservoir engineering response of deep coalbed methane[J]. Acta Petrolei Sinica,2023,44(11):1993−2006.

[20] 熊先钺,甄怀宾,李曙光,等. 大宁–吉县区块深部煤层气多轮次转向压裂技术及应用[J]. 煤田地质与勘探,2024,52(2):147−160. XIONG Xianyue,ZHEN Huaibin,LI Shuguang,et al. Multi–round diverting fracturing technology and its application in deep coalbed methane in the Daning–Jixian Block[J]. Coal Geology & Exploration,2024,52(2):147−160.

[21] COLMENARES L B,ZOBACK M D. Hydraulic fracturing and wellbore completion of coalbed methane wells in the Powder River Basin,Wyoming: Implications for water and gas production[J]. AAPG Bulletin,2007,91(1):51−67.

[22] 李瑞,卢义玉,葛兆龙,等. 地面井卸压的煤层气开发新模式[J]. 天然气工业,2022,42(7):75−84. LI Rui,LU Yiyu,GE Zhaolong,et al. A new CBM development mode: Surface well pressure relief[J]. Natural Gas Industry,2022,42(7):75−84.

[23] 兰浩,杨兆彪,仇鹏,等. 新疆准噶尔盆地白家海凸起深部煤层气勘探开发进展及启示[J]. 煤田地质与勘探,2024,52(2):13−22. LAN Hao,YANG Zhaobiao,CHOU Peng,et al. Exploration and exploitation of deep coalbed methane in the Baijiahai uplift,Junggar Basin: Progress and its implications[J]. Coal Geology & Exploration,2024,52(2):13−22.

[24] 桑树勋,郑司建,王建国,等. 岩石力学地层新方法在深部煤层气勘探开发"甜点"预测中的应用[J]. 石油学报,2023,44(11):1840−1853. SANG Shuxun,ZHENG Sijian,WANG Jianguo,et al. Application of new rock mechanical stratigraphy in sweet spot prediction for deep coalbed methane exploration and development[J]. Acta Petrolei Sinica,2023,44(11):1840−1853.

[25] 申鹏磊,吕帅锋,李贵山,等. 深部煤层气水平井水力压裂技术:以沁水盆地长治北地区为例[J]. 煤炭学报,2021,46(8):2488−2500. SHEN Penglei,LYU Shuaifeng,LI Guishan,et al. Hydraulic fracturing technology for deep coalbed methane horizontal wells: A case study in north Changzhi area of Qinshui Basin[J]. Journal of China Coal Society,2021,46(8):2488−2500.

[26] 伍岳,夏东领,郭秀娟. 鄂尔多斯盆地南缘长8致密砂岩储层裂缝发育特征及主控因素[J]. 现代地质,2025,39(5):1282−1292. WU Yue,XIA Dongling,GUO Xiujuan. Fracture development characteristics and main controlling factors in the Chang 8 tight sandstone reservoir at the southern margin of the Ordos Basin[J]. Geoscience,2025,39(5):1282−1292.

[27] 陈立超,王生维. 煤岩弹性力学性质与煤层破裂压力关系[J]. 天然气地球科学,2019,30(4):503−511. CHEN Lichao,WANG Shengwei. Relationship between elastic mechanical properties and equivalent fracture pressure of coal reservoir near wellbore[J]. Natural Gas Geoscience,2019,30(4):503−511.

[28] 李勇,徐立富,张守仁,等. 深煤层含气系统差异及开发对策[J]. 煤炭学报,2023,48(2):900−917. LI Yong,XU Lifu,ZHANG Shouren,et al. Gas bearing system difference in deep coal seams and corresponded development strategy[J]. Journal of China Coal Society,2023,48(2):900−917.

[29] TAO Chuanqi,LI Yong,WANG Yanbin,et al. Characteristics of deep coal reservoir and key control factors of coalbed methane accumulation in Linxing area[J]. Energies,2023,16(16):6085.

[30] 陈杨,姚艳斌,崔金榜,等. 郑庄区块煤储层水力压裂裂缝扩展地质因素分析[J]. 煤炭科学技术,2014,42(7):98−102. CHEN Yang,YAO Yanbin,CUI Jinbang,et al. Analysis on geological control factors of hydraulic fracture extension of coal reservoirs in Zhengzhuang Block[J]. Coal Science and Technology,2014,42(7):98−102.

[31] 杨兆中,杨苏,张健,等. 800m以深直井煤储层压裂特征分析[J]. 煤炭学报,2016,41(1):100−104. YANG Zhaozhong,YANG Su,ZHANG Jian,et al. Fracturing characteristics analysis of 800 meters deeper coalbed methane vertical wells[J]. Journal of China Coal Society,2016,41(1):100−104.

[32] 周延军,耿应春,王贵宾,等. 深部地层岩石力学性质测试与分析研究[J]. 岩土力学,2011,32(6):1625−1630. ZHOU Yanjun,GENG Yingchun,WANG Guibin,et al. Testing and analyzing rock mechanical characteristics for deep formation[J]. Rock and Soil Mechanics,2011,32(6):1625−1630.

[33] KONG Xiangguo,WANG Enyuan,HU Shaobin,et al. Fractal characteristics and acoustic emission of coal containing methane in triaxial compression failure[J]. Journal of Applied Geophysics,2016,124:139−147.

[34] 杨永杰,宋扬,陈绍杰. 三轴压缩煤岩强度及变形特征的试验研究[J]. 煤炭学报,2006,31(2):150−153. YANG Yongjie,SONG Yang,CHEN Shaojie. Test study of coal's strength and deformation characteristics under triaxial compression[J]. Journal of China Coal Society,2006,31(2):150−153.

[35] 赖勇. 围压对杨氏模量的影响分析[J]. 重庆交通大学学报(自然科学版),2009,28(2):246−249. LAI Yong. Effect analysis of confining pressure on Young's modulus[J]. Journal of Chongqing Jiaotong University (Natural Science),2009,28(2):246−249.

[36] 沈书豪,吴基文,翟晓荣,等. 深部地应力场下煤系岩石力学性质变化规律[J]. 采矿与安全工程学报,2017,34(6):1200−1206. SHEN Shuhao,WU Jiwen,ZHAI Xiaorong,et al. Variation of mechanical properties of coal and rocks in the deep mine in–situ stress field[J]. Journal of Mining & Safety Engineering,2017,34(6):1200−1206.

[37] RAN Qican,ZHAO Wenting,LIANG Yunpei,et al. Deciphering confining pressure effects on coal failure mechanisms using acoustic emission approaches[J]. Engineering Fracture Mechanics,2025,329:111592.

[38] 苏承东,宋常胜,苏发强. 高温作用后坚硬煤样单轴压缩过程中的变形强度与声发射特征[J]. 煤炭学报,2020,45(2):613−625. SU Chengdong,SONG Changsheng,SU Faqiang. Deformation intensity and acoustic emission characteristics of hard coal sample under uniaxial compression after high temperature[J]. Journal of China Coal Society,2020,45(2):613−625.

[39] CHENG Xianzhen,LUAN Hengjie,CHEN Lianjun,et al. Numerical investigation on mechanical properties of inhomogeneous coal under uniaxial compression and the role of cleat distribution[J]. Bulletin of Engineering Geology and the Environment,2021,80(9):7009−7027.

[40] 刘洪林,康永尚,王烽,等. 沁水盆地煤层割理的充填特征及形成过程[J]. 地质学报,2008,82(10):1376−1381. LIU Honglin,KANG Yongshang,WANG Feng,et al. Coal cleat system characteristics and formation mechanisms in the Qinshui Basin[J]. Acta Geologica Sinica,2008,82(10):1376−1381.

[41] RANJITH P G,PERERA M S A. Effects of cleat performance on strength reduction of coal in CO2 sequestration[J]. Energy,2012,45(1):1069−1075.

[42] ZHANG Zetian,ZHANG Ru,LI Guo,et al. The effect of bedding structure on mechanical property of coal[J]. Advances in Materials Science and Engineering,2014,2014(1):952703.

[43] HAZRA B,MOTRA H B,HOWER J C,et al. Elastic anisotropy and deformation characteristics of Pennsylvania anthracite[J]. International Journal of Coal Geology,2025,303:104740.

[44] 郝宪杰,魏英楠,杨科,等. 煤储集层起裂强度和损伤强度的各向异性特征[J]. 石油勘探与开发,2021,48(1):211−221. HAO Xianjie,WEI Yingnan,YANG Ke,et al. Anisotropy of crack initiation strength and damage strength of coal reservoirs[J]. Petroleum Exploration and Development,2021,48(1):211−221.

[45] ZHAO Yixin,ZHAO Gaofeng,JIANG Yaodong,et al. Effects of bedding on the dynamic indirect tensile strength of coal: Laboratory experiments and numerical simulation[J]. International Journal of Coal Geology,2014,132:81−93.

[46] LIU Jiajia,YANG Ming,WANG Dan,et al. Different bedding loaded coal mechanics properties and acoustic emission[J]. Environmental Earth Sciences,2018,77(8):322.

[47] BIENIAWSKI Z T,DENKHAUS H G,VOGLER U W. Failure of fractured rock[J]. International Journal of Rock Mechanics and Mining Sciences & Geomechanics Abstracts,1969,6(3):323−341.

[48] 许江,周婷,李波波,等. 三轴应力条件下煤层气储层渗流滞后效应试验研究[J]. 岩石力学与工程学报,2012,31(9):1854−1861. XU Jiang,ZHOU Ting,LI Bobo,et al. Experimental research on seepage lagging effects of coalbed methane reservoir under condition of triaxial stress[J]. Chinese Journal of Rock Mechanics and Engineering,2012,31(9):1854−1861.

[49] 孟召平,王保玉,谢晓彤,等. 煤岩变形力学特性及其对渗透性的控制[J]. 煤炭学报,2012,37(8):1342−1347. MENG Zhaoping,WANG Baoyu,XIE Xiaotong,et al. Mechanical properties of coal deformation and its influence on permeability[J]. Journal of China Coal Society,2012,37(8):1342−1347.

[50] 王向宇,周宏伟,钟江城,等. 三轴循环加卸载下深部煤体损伤的能量演化和渗透特性研究[J]. 岩石力学与工程学报,2018,37(12):2676−2684. WANG Xiangyu,ZHOU Hongwei,ZHONG Jiangcheng,et al. Study on energy evolution and permeability characteristics of deep coal damage under triaxial cyclic loading and unloading conditions[J]. Chinese Journal of Rock Mechanics and Engineering,2018,37(12):2676−2684.

[51] 张年学,盛祝平,李晓,等. 岩石泊松比与内摩擦角的关系研究[J]. 岩石力学与工程学报,2011,30(增刊1):2599−2609. ZHANG Nianxue,SHENG Zhuping,LI Xiao,et al. Study of relationship between Poisson's ratio and angle of internal friction for rocks[J]. Chinese Journal of Rock Mechanics and Engineering,2011,30(Sup.1):2599−2609.

[52] 经纬,薛维培,姚直书. 巷道围岩塑性软化区岩石内摩擦角与黏聚力变化规律[J]. 煤炭学报,2018,43(8):2203−2210. JING Wei,XUE Weipei,YAO Zhishu. Variation of the internal friction angle and cohesion of the plastic softening zone rock in roadway surrounding rock[J]. Journal of China Coal Society,2018,43(8):2203−2210.

[53] 章朋. 煤层气开发的地质力学条件及煤储层稳定性研究[D]. 北京: 中国矿业大学(北京), 2020. ZHANG Peng. Study on geomechanical condition and coal reservoir stability in coalbed methane development[D]. Beijing: China University of Mining and Technology (Beijing), 2020.

[54] 张国军,张勇. 基于摩尔–库伦准则的岩石材料加(卸)载分区破坏特征[J]. 煤炭学报,2019,44(4):1049−1058. ZHANG Guojun,ZHANG Yong. Partition failure characteristics of rock material loading and unloading based on Mohr–Coulomb criterion[J]. Journal of China Coal Society,2019,44(4):1049−1058.

[55] LI Hao,PEL L,YOU Zhenjiang,et al. Stress–dependent Mohr–Coulomb shear strength parameters for intact rock[J]. Scientific Reports,2024,14:17454.

[56] LI Hao,PEL L,YOU Zhenjiang,et al. Stress–dependent instantaneous cohesion and friction angle for the Mohr–Coulomb criterion[J]. International Journal of Mechanical Sciences,2024,283:109652.

[57] 李斌,刘艳章,林坤峰. 非线性Mohr–Coulomb准则适用范围及其改进研究[J]. 岩土力学,2016,37(3):637−646. LI Bin,LIU Yanzhang,LIN Kunfeng. Application scope of nonlinear Mohr–Coulomb criterion and its modification[J]. Rock and Soil Mechanics,2016,37(3):637−646.

[58] LABUZ J F,ZANG A. Mohr–Coulomb failure criterion[J]. Rock Mechanics and Rock Engineering,2012,45(6):975−979.