Fractures and their macro-mesoscopic mechanical evolution pattern in coals in the Benxi Formation, Ordos Basin

Authors

ZHU Boxu, Petroleum Engineering School, Southwest Petroleum University, Chengdu 610500, China; Underbalanced/Drilling and Completion Engineering Laboratory, National Engineering Research Center for Oil and Gas Drilling and Completion, Chengdu 610500, ChinaFollow
YANG Xu, Petroleum Engineering School, Southwest Petroleum University, Chengdu 610500, China; Underbalanced/Drilling and Completion Engineering Laboratory, National Engineering Research Center for Oil and Gas Drilling and Completion, Chengdu 610500, ChinaFollow
LI Gaoren, China Eploration and Development Research Institute, Changqing Oilfield Branch, China National Petroleum Corporation, Xi’an 710021, China
LI Gao, Petroleum Engineering School, Southwest Petroleum University, Chengdu 610500, China; Underbalanced/Drilling and Completion Engineering Laboratory, National Engineering Research Center for Oil and Gas Drilling and Completion, Chengdu 610500, China
FENG Jiaxin, Petroleum Engineering School, Southwest Petroleum University, Chengdu 610500, China; Underbalanced/Drilling and Completion Engineering Laboratory, National Engineering Research Center for Oil and Gas Drilling and Completion, Chengdu 610500, China

Abstract

Objective This study aims to determine the relationship between multi-scale fracture characteristics and the mechanical behavior of deep coals, thereby helping gain further insights into the mechanisms behind the macro-mesoscopic damage and failure of the coals and providing a scientific basis for the stability assessment and mechanical modeling of deep coalbed methane (CBM) reservoirs. To this end, it is necessary to examine the differences in the coals’ mechanical properties under varying scales. Methods This study conducted uniaxial and triaxial mechanical tests, nanoindentation experiments, and CT scanning of the No. 8 coal seam in the Benxi Formation within the Ordos Basin. Based on the differences in fracture density and modulus of elasticity across varying scales, this study developed an upscaling model through fitting. Then, it investigated the influence of confining pressure on the mechanical parameters of fractures using this model. Results and Conclusions The structurally intact coal samples exhibited a fracture network formed by the dense interactions between cleats on the end faces and inside. The end faces had a face cleat density of 6–12 per 5 cm and a butt cleat density of 9–16 per 5 cm. CT scanning-based statistics reveal an internal cleat density of approximately 10 per 5 cm. At the microscale, fracture widths ranged from 0.52 μm to 13.41 μm. The presence of fractures significantly reduced the macroscopic modulus of elasticity of the coal samples, with values obtained from triaxial tests proving about 21% lower than those from nanoindentation experiments. As confining pressure increased, fractures gradually closed, leading to nonlinear increases in both the peak strength and modulus of elasticity of the coal samples. Correspondingly, the failure mode transitioned from tensile splitting to shear failure. The fitted model indicates that normal stiffness increased nonlinearly with confining pressure, with the increasing rate gradually slowing down. This model can be used to predict the law of changes in the macroscopic modulus of elasticity of coals under varying confining pressures. The results of this study provide an experimental basis for the multi-scale refined characterization and parameter prediction of the mechanical behavior of the coal reservoirs in the Benxi Formation within the Ordos Basin.

Keywords

coal in the Benxi Formation, fracture, confining pressure, CT, modulus of elasticity, nanoindentation

DOI

10.12363/issn.1001-1986.25.02.0126

Recommended Citation

ZHU Boxu, YANG Xu, LI Gaoren, et al. (2025) "Fractures and their macro-mesoscopic mechanical evolution pattern in coals in the Benxi Formation, Ordos Basin," Coal Geology & Exploration: Vol. 53: Iss. 8, Article 6.
DOI: 10.12363/issn.1001-1986.25.02.0126
Available at: https://cge.researchcommons.org/journal/vol53/iss8/6

Reference

[1] 程爱平，高永涛，梁兴旺，等. 基于未确知聚类法的底板采动破坏深度动态预测[J]. 采矿与安全工程学报，2014，31(5)：739−744.

CHENG Aiping，GAO Yongtao，LIANG Xingwang，et al. Dynamic forecasting of mining–induced failure depth of floor based on unascertained clustering method[J]. Journal of Mining & Safety Engineering，2014，31(5)：739−744.

[2] 叶建平. 中国煤层气勘探开发及其科技进步历程回顾与思考[J]. 煤田地质与勘探，2025，53(1)：114−127.

YE Jianping. China’s CBM exploration and production and associated technological advancements：A review and reflections[J]. Coal Geology & Exploration，2025，53(1)：114−127.

[3] 陈军海，周忠鸣，李守定，等. 超深储层白云岩力学行为及宏细观破裂特征试验研究[J]. 工程地质学报，2024，32(2)：503−512.

CHEN Junhai，ZHOU Zhongming，LI Shouding，et al. Experimental study on mechanical behavior and macro and micro fracture characteristics of dolomite in ultra–deep reservoir[J]. Journal of Engineering Geology，2024，32(2)：503−512.

[4] ZHAO Zhe，XU Wanglin，ZHAO Zhenyu，et al. Geological characteristics and exploration breakthroughs of coal rock gas in Carboniferous Benxi Formation，Ordos Basin，NW China[J]. Petroleum Exploration and Development，2024，51(2)：262−278.

[5] 徐长贵，季洪泉，王存武，等. 鄂尔多斯盆地东缘临兴–神府区块深部煤层气富集规律与勘探对策[J]. 煤田地质与勘探，2024，52(8)：1−11.

XU Changgui，JI Hongquan，WANG Cunwu，et al. Enrichment patterns and exploration countermeasures of deep coalbed methane in the Linxing–Shenfu Block on the eastern margin of the Ordos Basin[J]. Coal Geology & Exploration，2024，52(8)：1−11.

[6] 王志壮，吴鹏，孙强，等. 临兴区块深部煤层气井生产特征及影响因素[J]. 煤田地质与勘探，2024，52(8)：69−78.

WANG Zhizhuang，WU Peng，SUN Qiang，et al. Production characteristics of deep coalbed methane wells in the Linxing Block and associated their influencing factors[J]. Coal Geology & Exploration，2024，52(8)：69−78.

[7] 王鹏，李斌，王昆剑，等. 神府区块深部煤层气钻完井关键技术及应用[J]. 煤田地质与勘探，2024，52(8)：44−56.

WANG Peng，LI Bin，WANG Kunjian，et al. Critical drilling and completion techniques for deep coalbed methane in the Shenfu Block and their applications[J]. Coal Geology & Exploration，2024，52(8)：44−56.

[8] LI Song，TANG Dazhen，PAN Zhejun，et al. Geological conditions of deep coalbed methane in the eastern margin of the Ordos Basin，China：Implications for coalbed methane development[J]. Journal of Natural Gas Science and Engineering，2018，53：394−402.

[9] 侯伟，赵天天，张雷，等. 基于低场核磁共振的煤储层束缚水饱和度应力响应研究与动态预测：以保德和韩城区块为例[J]. 吉林大学学报(地球科学版)，2020，50(2)：608−616.

HOU Wei，ZHAO Tiantian，ZHANG Lei，et al. Stress sensitivity and prediction of irreducible water saturation in coal reservoirs in Baode and Hancheng Blocks based on low–field nuclear magnetic resonance[J]. Journal of Jilin University (Earth Science Edition)，2020，50(2)：608−616.

[10] PAN Weihui，LI Yunpeng，ZHANG Chuang，et al. Coal burst prevention technology and engineering practice in Ordos deep mining area of China[J]. Sustainability，2023，15(1)：159.

[11] 李地元，罗平框，朱志根，等. 深部矿山不同埋藏深度岩石拉压力学特性试验研究[J]. 工程地质学报，2024，32(1)：86−95.

LI Diyuan，LUO Pingkuang，ZHU Zhigen，et al. Experimental study on tensile and compressive mechanical properties of rocks from different depths at a deep mine[J]. Journal of Engineering Geology，2024，32(1)：86−95.

[12] 李庆辉，李少轩. 超深层砂岩储层岩石力学特性实验研究[J]. 岩石力学与工程学报，2021，40(5)：948−957.

LI Qinghui，LI Shaoxuan. Experimental study on mechanical properties of ultra–deep sandstone[J]. Chinese Journal of Rock Mechanics and Engineering，2021，40(5)：948−957.

[13] 任非. 三轴压缩条件下煤岩力学特性及破坏模式[J]. 煤矿安全，2018，49(1)：37−39.

REN Fei. Mechanical properties and failure modes of coal and rock under triaxial compression[J]. Safety in Coal Mines，2018，49(1)：37−39.

[14] 付裕，陈新，冯中亮. 基于CT扫描的煤岩裂隙特征及其对破坏形态的影响[J]. 煤炭学报，2020，45(2)：568−578.

FU Yu，CHEN Xin，FENG Zhongliang. Characteristics of coal–rock fractures based on CT scanning and its influence on failure modes[J]. Journal of China Coal Society，2020，45(2)：568−578.

[15] WANG Zhaohui，SUN Wenchao，SHUI Yanting，et al. Computed tomography observation and image–based simulation of fracture propagation in compressed coal[J]. Energies，2023，16(1)：260.

[16] DU Feng，WANG Kai，ZHANG Guojun，et al. Damage characteristics of coal under different loading modes based on CT three–dimensional reconstruction[J]. Fuel，2022，310：122304.

[17] 孙长伦，李桂臣，GOMAH M E，等. 基于纳米压痕技术的破碎煤样力学特性实验研究[J]. 煤炭学报，2020，45(增刊2)：682−691.

SUN Changlun，LI Guichen，GOMAH M E，et al. Experimental investigation on the mechanical properties of crushed coal samples based on the nanoindentation technique[J]. Journal of China Coal Society，2020，45(Sup.2)：682−691.

[18] 陈立超，王生维. 煤岩断裂力学性质对储层压裂改造的影响[J]. 天然气地球科学，2020，31(1)：122−131.

CHEN Lichao，WANG Shengwei. Fracture properties of high–rank coal and its constraint on hydraulic fracturing stimulation of coal reservoir[J]. Natural Gas Geoscience，2020，31(1)：122−131.

[19] 郭晓娇，王雷，姚仙洲，等. 深部煤岩地质特征及煤层气富集主控地质因素：以鄂尔多斯盆地东部M区为例[J]. 石油实验地质，2025，47(1)：17−26.

GUO Xiaojiao，WANG Lei，YAO Xianzhou，et al. Geological characteristics of deep coal rock and main geological factors controlling coalbed methane enrichment：A case study of the M area in the eastern Ordos Basin[J]. Petroleum Geology & Experiment，2025，47(1)：17−26.

[20] 国家市场监督管理总局，国家标准化管理委员会. 煤和岩石物理力学性质测定方法 第1部分：采样一般规定：GB/T 23561. 1—2024[S]. 北京：中国标准出版社，2024.

[21] 王天明，司文，孙鹏飞，等. 基于试验分析的预制孔–裂隙型煤煤体破坏能量耗散规律研究[J]. 陕西煤炭，2025，44(5)：18−22.

WANG Tianming，SI Wen，SUN Pengfei，et al. Energy dissipation law of coal failure of precast pore–fracture coal based on experimental analysis[J]. Shaanxi Coal，2025，44(5)：18−22.

[22] 郝晨旭. 热–水–力作用下砂岩加、卸荷脆断力学特性试验研究[D]. 成都：成都理工大学，2021.

HAO Chenxu. Experimental study on mechanical properties of brittle fracture of sandstone under thermal–hydraulic–mechanical action under loading and unloading[D]. Chengdu：Chengdu University of Technology，2021.

[23] 郭敬杰，李伟，韩祥凯，等. 基于纳米压痕的构造煤与原生煤结构面力学特性[J]. 中国矿业大学学报，2024，53(3)：509−523.

GUO Jingjie，LI Wei，HAN Xiangkai，et al. Mechanical properties instructural planes of tectonic coal and primary coal through nano–indentation[J]. Journal of China University of Mining & Technology，2024，53(3)：509−523.

[24] SUN Changlun，LI Guichen，GOMAH M E，et al. Creep characteristics of coal and rock investigated by nanoindentation[J]. International Journal of Mining Science and Technology，2020，30(6)：769−776.

[25] 徐宝恒，郭大立. 大规模缝网压裂在深部煤层气中的应用[J]. 河南科技，2023，42(19)：81−84.

XU Baoheng，GUO Dali. Application of large–scale fracture network fracturing in deep coalbed methane[J]. Henan Science and Technology，2023，42(19)：81−84.

[26] SU Xianbo，FENG Yanli，CHEN Jiangfeng，et al. The characteristics and origins of cleat in coal from Western North China[J]. International Journal of Coal Geology，2001，47(1)：51−62.

[27] 李长海，赵伦，刘波，等. 微裂缝研究进展、意义及发展趋势[J]. 天然气地球科学，2020，31(3)：402−416.

LI Changhai，ZHAO Lun，LIU Bo，et al. Research status，significance and development trend of microfractures[J]. Natural Gas Geoscience，2020，31(3)：402−416.

[28] 傅雪海，秦勇，姜波，等. 煤割理压缩实验及渗透率数值模拟[J]. 煤炭学报，2001，26(6)：573−577.

FU Xuehai，QIN Yong，JIANG Bo，et al. Compress experiment of coal cleat and mathematical simulation of coal reservoir permeability[J]. Journal of China Coal Society，2001，26(6)：573−577.

[29] 何智海，倪雅倩，杜时贵，等. 纳米压痕技术在岩石材料中的应用与研究进展[J]. 岩石力学与工程学报，2022，41(10)：2045−2066.

HE Zhihai，NI Yaqian，DU Shigui，et al. Application and research progress of nanoindentation technology in rock materials[J]. Chinese Journal of Rock Mechanics and Engineering，2022，41(10)：2045−2066.

[30] 吴琼. 复杂节理岩体力学参数尺寸效应及工程应用研究[D]. 武汉：中国地质大学，2012.

WU Qiong. The mechanical parameters of jointed rock mass：Scale–effect research and its engineering application[D]. Wuhan：China University of Geosciences，2012.

[31] AI Chi，LI Yuwei. The model for calculating elastic modulus and Poisson’s ratio of coal body[J]. The Open Fuels & Energy Science Journal，2013，6(1)：36−43.

[32] 左建平，孙运江，刘海雁，等. 采矿岩石多尺度破坏力学[J]. 矿业科学学报，2021，6(5)：509−523.

ZUO Jianping，SUN Yunjiang，LIU Haiyan，et al. Multi–scale failure mechanics of rock in mining engineering[J]. Journal of Mining Science and Technology，2021，6(5)：509−523.

[33] CAI Yidong，LIU Dameng，MATHEWS J P，et al. Permeability evolution in fractured coal：Combining triaxial confinement with X–ray computed tomography，acoustic emission and ultrasonic techniques[J]. International Journal of Coal Geology，2014，122：91−104.