Occurrence mechanism, environment and dynamic evolution of gas and water in deep coal seams

Authors

LI Yong, College of Geoscience and Surveying Engineering, China University of Mining and Technology (Beijing), Beijing 100083, ChinaFollow
XU Lifu, College of Geoscience and Surveying Engineering, China University of Mining and Technology (Beijing), Beijing 100083, China
LIU Yu, College of Geoscience and Surveying Engineering, China University of Mining and Technology (Beijing), Beijing 100083, China
WANG Ziwei, College of Geoscience and Surveying Engineering, China University of Mining and Technology (Beijing), Beijing 100083, China
GAO Shuang, College of Geoscience and Surveying Engineering, China University of Mining and Technology (Beijing), Beijing 100083, China
REN Ci, College of Geoscience and Surveying Engineering, China University of Mining and Technology (Beijing), Beijing 100083, China

Abstract

Accurately understanding of the occurrence states, relative content, and distribution characteristics of gas - water under deep conditions has important guiding significance for efficient exploration and development of coalbed methane (CBM). Based on the theoretical model, molecular simulation and systematic analysis of gas-water, the occurrence states of gas-water in coal seam is clarified, and the boundary and dynamic evolution process of gas-water dynamic migration and accumulation are revealed. Considering the coal - water interface interaction, the mobility and occurrence states of water, the coalbed water can be divided into movable water (gravity water and capillary water), bound water (adsorbed water, zeolite water, crystallization water and interlayer water) and structural water. The adsorbed water, capillary water and gravity water are dominated by pores, and zeolite water, structural water, crystallization water and interlayer water are dominated by minerals. The molecular simulation results show that water molecules are saturated and filled in 0.7 nm pores, with consistent adsorption and desorption processes, while weak adsorption layers and free states appear in larger pores. The adsorption process of water molecules is manifested in stages: single molecule oxygen-containing group adsorption, monolayer strong adsorption, multi-layer weak adsorption, water clusters formation, and pore filling. Methane molecules stably fill pores (1.5 nm) with 3 layers of adsorption, and coexist in monolayer adsorption and free state in the larger pores(>1.5 nm), resulting in the widespread presence of free state in pores above mesopores. According to the boundary between adsorption and free gas mentioned above, the theoretical calculation formulas for free gas and adsorption gas have been improved to provide new ideas for gas content calculation. Deep thermogenic CBM is the residual gas generated after large-scale hydrocarbon generation and expulsion from coal. During the hydrocarbon expulsion process, the water is driven by methane and the evaporation diffusion process results in a small amount of residual water (bound water and structural water) remaining in the pores, which cannot be changed in the later stage. Assuming a static water pressure of 20 MPa, under reservoir pressures of 0, 5, 10, 15, and 20 MPa, the maximum pore size that external water can invade is 7, 9, 13, 27 nm and non-invasive. Controlled by differential preservation conditions, coal-derived gas not only forms overpressure and under pressure differential gas bearing systems, but also forms multiple types of gas bearing modes in coal measures. The above work has clarified the micro-occurrence mechanism and evolution mode of CBM and water, which has guiding significance for the enrichment characteristics and efficient development design of deep CBM.

Keywords

deep coalbed methane, coalbed water, molecular simulation, free gas, reservoir pressure

DOI

10.12363/issn.1001-1986.23.10.0617

Recommended Citation

LI Yong, XU Lifu, LIU Yu, et al. (2024) "Occurrence mechanism, environment and dynamic evolution of gas and water in deep coal seams," Coal Geology & Exploration: Vol. 52: Iss. 2, Article 6.
DOI: 10.12363/issn.1001-1986.23.10.0617
Available at: https://cge.researchcommons.org/journal/vol52/iss2/6

Reference

[1] 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.

[2] 秦勇，申建，史锐. 中国煤系气大产业建设战略价值与战略选择[J]. 煤炭学报，2022，47(1)：371−387.

QIN Yong，SHEN Jian，SHI Rui. Strategic value and choice on construction of large CMG industry in China[J]. Journal of China Coal Society，2022，47(1)：371−387.

[3] 徐凤银，侯伟，熊先钺，等. 中国煤层气产业现状与发展战略[J]. 石油勘探与开发，2023，50(4)：669−682.

XU Fengyin，HOU Wei，XIONG Xianyue，et al. The status and development strategy of coalbed methane industry in China[J]. Petroleum Exploration and Development，2023，50(4)：669−682.

[4] 李勇，王延斌，倪小明，等. 煤层气低效井成因判识及治理体系构建研究[J]. 煤炭科学技术，2020，48(2)：185−193.

LI Yong，WANG Yanbin，NI Xiaoming，et al. Study on identification and control system construction of low efficiency coalbed methane wells[J]. Coal Science and Technology，2020，48(2)：185−193.

[5] 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.

[6] 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.

[7] ZHU Hengzhong，LIU Ping，CHEN Ping，et al. Analysis of coalbed methane occurrence in Shuicheng Coalfield，southwestern China[J]. Journal of Natural Gas Science and Engineering，2017，47：140−153.

[8] LI Yong，YANG Jianghao，PAN Zhejun，et al. Unconventional natural gas accumulations in stacked deposits：A discussion of upper Paleozoic coal–bearing strata in the east margin of the Ordos Basin，China[J]. Acta Geologica Sinica(English Edition)，2019，93(1)：111−129.

[9] 陈贞龙. 延川南深部煤层气田地质单元划分及开发对策[J]. 煤田地质与勘探，2021，49(2)：13−20.

CHEN Zhenlong. Geological unit division and development countermeasures of deep coalbed methane in southern Yanchuan Block[J]. Coal Geology & Exploration，2021，49(2)：13−20.

[10] 刘程瑞. 煤储层纳米级微孔超压环境形成机制研究[D]. 焦作：河南理工大学，2021.

LIU Chengrui. Study on the formation mechanism of nanoscale microporous overpressure environment in coal reservoirs[D]. Jiaozuo：Henan Polytechnic University，2021.

[11] 张开仲，程远平，王亮，等. 基于煤中瓦斯赋存和运移方式的孔隙网络结构特征表征[J]. 煤炭学报，2022，47(10)：3680−3694.

ZHANG Kaizhong，CHENG Yuanping，WANG Liang，et al. Pore network structure characterization based on gas occurrence and migration in coal[J]. Journal of China Coal Society，2022，47(10)：3680−3694.

[12] 桑树勋，朱炎铭，张井，等. 煤吸附气体的固气作用机理(Ⅱ)：煤吸附气体的物理过程与理论模型[J]. 天然气工业，2005，25(1)：16−18.

SANG Shuxun，ZHU Yanming，ZHANG Jing，et al. Solid–gas interaction mechanism of coal–adsorbed gas(II)：Physical process and theoretical model of coal–adsorbed gas[J]. Natural Gas Industry，2005，25(1)：16−18.

[13] 陈昌国，鲜晓红，张代钧，等. 微孔填充理论研究无烟煤和炭对甲烷的吸附特性[J]. 重庆大学学报(自然科学版)，1998，21(2)：75−79.

CHEN Changguo，XIAN Xiaohong，ZHANG Daijun，et al. The volume filling theory of the adsorption of methane on anthracite and its char[J]. Journal of Chongqing University(Natural Science Edition)，1998，21(2)：75−79.

[14] 程远平，胡彪. 微孔填充：煤中甲烷的主要赋存形式[J]. 煤炭学报，2021，46(9)：2933−2948.

CHENG Yuanping，HU Biao. Main occurrence form of methane in coal：Micropore filling[J]. Journal of China Coal Society，2021，46(9)：2933−2948.

[15] MOSHER K，HE Jiajun，LIU Yangyang，et al. Molecular simulation of methane adsorption in micro– and mesoporous carbons with applications to coal and gas shale systems[J]. International Journal of Coal Geology，2013，109：36−44.

[16] 薛禹群. 地下水动力学[M]. 北京：地质出版社，1997.

[17] 傅雪海，秦勇，韦重韬. 煤层气地质学[M]. 徐州：中国矿业大学出版社，2007.

[18] YAO Yanbin，LIU Dameng，LIU Jungang，et al. Assessing the water migration and permeability of large intact bituminous and anthracite coals using NMR relaxation spectrometry[J]. Transport in Porous Media，2015，107(2)：527−542.

[19] CLARENCE K J. Analytical methods for coal and coal products[M]. Academic Press，1978.

[20] STRIOLO A，CHIALVO A A，CUMMINGS P T，et al. Water adsorption in carbon–slit nanopores[J]. Langmuir，2003，19(20)：8583−8591.

[21] 郑超，马东民，陈跃，等. 水分对煤层气吸附/解吸微观作用研究进展[J]. 煤炭科学技术，2023，51(2)：256−268.

ZHENG Chao，MA Dongmin，CHEN Yue，et al. Research progress micro effect of water on coalbed methane adsorption/desorption[J]. Coal Science and Technology，2023，51(2)：256−268.

[22] 周龙政，蔡进功，李旭，等. 泥页岩中水的赋存态研究进展及其意义[J]. 地球科学进展，2022，37(7)：709−725.

ZHOU Longzheng，CAI Jingong，LI Xu，et al. Research progress and significance of the occurrence of water in shale[J]. Advances in Earth Science，2022，37(7)：709−725.

[23] 汪灵. 矿物材料学原理[M]. 北京：地质出版社，2021.

[24] CHARRIÈRE D，BEHRA P. Water sorption on coals[J]. Journal of Colloid and Interface Science，2010，344(2)：460−467.

[25] LIU Lumeng，TAN Shiliang，HORIKAWA T，et al. Water adsorption on carbon：A review[J]. Advances in Colloid and Interface Science，2017，250：64−78.

[26] 张凯，汤达祯，陶树，等. 不同变质程度煤吸附能力影响因素研究[J]. 煤炭科学技术，2017，45(5)：192−197.

ZHANG Kai，TANG Dazhen，TAO Shu，et al. Study on influence factors of adsorption capacity of different metamorphic degree coals[J]. Coal Science and Technology，2017，45(5)：192−197.

[27] 杨亚璞，陈明义，田富超，等. 基于水蒸气吸附模型的低阶煤与高阶煤吸附水特性差异研究[J]. 煤矿安全，2021，52(4)：7−12.

YANG Yapu，CHEN Mingyi，TIAN Fuchao，et al. Research on differences of adsorbed water characteristics of low rank and high rank coals based on water vapor adsorption model[J]. Safety in Coal Mines，2021，52(4)：7−12.

[28] 赵梦尧，杨雪平，杨晓宁. 石墨烯狭缝受限孔道中水分子的分子动力学模拟[J]. 物理化学学报，2015，31(8)：1489−1498.

ZHAO Mengyao，YANG Xueping，YANG Xiaoning. Molecular dynamics simulation of water molecules in confined slit pores of graphene[J]. Acta Physico-Chimica Sinica，2015，31(8)：1489−1498.

[29] LI Yong，XU Lifu，CHEN Jianqi，et al. Hydration of transitional shale and evolution of physical properties constrained by concurrent NMR and acoustic observations[J]. Energy & Fuels，2023，37(11)：7809−7822.

[30] 倪冠华，李钊，解宏超. 基于核磁共振测试的煤层水锁效应解除方法[J]. 煤炭学报，2018，43(8)：2280−2287.

NI Guanhua，LI Zhao，XIE Hongchao. Experimental research on the methods to remove water blocking effect based on nuclear magnetic resonance testing[J]. Journal of China Coal Society，2018，43(8)：2280−2287.

[31] ZHANG Junjian，HU Qinhong，CHANG Xiangchun，et al. Water saturation and distribution variation in coal reservoirs：Intrusion and drainage experiments using one– and two–dimensional NMR techniques[J]. Energy & Fuels，2022，36(12)：6130−6143.

[32] 王浩明. 煤储层微观孔隙内气水动态流动规律研究[D]. 重庆：重庆大学，2021.

WANG Haoming. Research on law of dynamic flow of gas and water in micro–porosity of coal mine methane reservoir[D]. Chongqing：Chongqing University，2021.

[33] 周效志，桑树勋，易同生，等. 煤层气合层开发上部产层暴露的伤害机理[J]. 天然气工业，2016，36(6)：52−59.

ZHOU Xiaozhi，SANG Shuxun，YI Tongsheng，et al. Damage mechanism of upper exposed producing layers during CBM multi–coal seam development[J]. Natural Gas Industry，2016，36(6)：52−59.

[34] 苏现波，刘保民. 煤层气的赋存状态及其影响因素[J]. 焦作工学院学报，1999，18(3)：157−160.

SU Xianbo，LIU Baomin. Occurrence of coaled methane in coal–seam and it’s controls[J]. Journal of Jiaozuo Institute of Technology，1999，18(3)：157−160.

[35] ORTIZ L，KUCHTA B，FIRLEJ L，et al. Methane adsorption in nanoporous carbon：The numerical estimation of optimal storage conditions[J]. Materials Research Express，2016，3(5)：055011.

[36] 宋金星，苏现波，王乾，等. 考虑微孔超压环境的煤储层含气量计算方法[J]. 天然气工业，2017，37(2)：19−25.

SONG Jinxing，SU Xianbo，WANG Qian，et al. A new method for calculating gas content of coal reservoirs with consideration of a micro–pore overpressure environment[J]. Natural Gas Industry，2017，37(2)：19−25.

[37] 高鉴东. 新疆阜康矿区煤储层吸附气与游离气含量模拟[D]. 徐州：中国矿业大学，2015.

GAO Jiandong. Simulation on adsorbed gas and free gas content of coal reservoir in Fukang Mining Area，Xinjiang[D]. Xuzhou：China University of Mining and Technology，2015.

[38] 马东民. 煤层气吸附解吸机理研究[D]. 西安：西安科技大学，2008.

MA Dongmin. Research on the adsorption and desorption mechanism of coalbed methane[D]. Xi’an：Xi’an University of Science and Technology，2008.

[39] LI Yong，WANG Zhuangsen，TANG Shuheng，et al. Re–evaluating adsorbed and free methane content in coal and its ad–and desorption processes analysis[J]. Chemical Engineering Journal，2022，428：131946.

[40] 辛治国，侯加根，冯伟光. 密闭取心饱和度校正数学模型[J]. 吉林大学学报(地球科学版)，2012，42(3)：698−704.

XIN Zhiguo，HOU Jiagen，FENG Weiguang. Correction model of oil and water saturation in sealing core[J]. Journal of Jilin University(Earth Science Edition)，2012，42(3)：698−704.

[41] 苗雅楠. 煤层气藏气水赋存方式及产气机理研究[D]. 北京：中国石油大学(北京)，2019.

MIAO Yanan. Adsorption characteristics and production mechanisms of coalbed methane reservoirs[D]. Beijing：China University of Petroleum(Beijing)，2019.

[42] LI Yong，ZHANG Cheng，TANG Dazhen，et al. Coal pore size distributions controlled by the coalification process：An experimental study of coals from the Junggar，Ordos and Qinshui Basins in China[J]. Fuel，2017，206：352−363.

[43] 李勇，吴鹏，高计县，等. 煤成气多层系富集机制与全含气系统模式：以鄂尔多斯盆地东缘临兴区块为例[J]. 天然气工业，2022，42(6)：52−64.

LI Yong，WU Peng，GAO Jixian，et al. Multilayer coal–derived gas enrichment mechanism and whole gas bearing system model：A case study on the Linxing Block along the eastern margin of the Ordos Basin[J]. Natural Gas Industry，2022，42(6)：52−64.

[44] PANG Xiongqi，JIA Chengzao，CHEN Junqing，et al. A unified model for the formation and distribution of both conventional and unconventional hydrocarbon reservoirs[J]. Geoscience Frontiers，2021，12(2)：695−711.