Microscopic Pore Structure and Coalbed Methane Desorption Law in Low-Permeability Coal Seams

doi:10.3969/j.issn.1006-6535.2024.02.015

Abstract

Abstract: In order to study the relationship between microscopic pore structure and gas desorption law in low-permeability coal seams, the gas content, porosity, permeability, and pore size distribution of coal samples from No.8 coal seam of Yishang slope in the Ordos Basin were determined by using experimental methods such as direct observation method, nuclear magnetic resonance (NMR) method, and liquid nitrogen adsorption method. The results show that the coalbed methane desorption characteristics are related to the pore size distribution, pore morphology, pore connectivity, specific surface area of the coal, and coal rock composition, etc; the pore characteristics and gas desorption laws of the 2 sets of coal seams less than 10 m apart in the study area have large differences. Among them, the microporous morphology of the glance coal was dominated by cylindrical pores open at one end and ink-bottle pores, with the percentage of micropores and adsorbed pores being 39.2%, and the microcracks in the coal body were relatively developed, but the connectivity between the micropores and mesopores was poor, and the microporous morphology of the dull coal was dominated by cylindrical pores open at one end, with the percentage of micropores and adsorbed pores being 33.2%, the microcracks were undeveloped, and the connectivity between different pores was poor. The gas desorption of glance coal has a large output in the early stage but decays quickly in the late stage, and the gas output has obvious paroxysmal characteristics; the gas desorption of dull coal has a small output in the early stage, which is about 1/2 of that of the glance coal, and the decrease of the output in the late stage is relatively flat. The pore structure, pore type, permeability, stress sensitivity, and types of fracture fillers shall be fully considered in the development strategy for low-permeability coal seams. On the basis of fine characterization of the pore structure, a reasonable development plan should be formulated in accordance with the characteristics of different desorption periods of coalbed methane and in conjunction with geological and development conditions. This study has important guiding significance for understanding the mechanism of coalbed methane gas production and its controlling factors, and improving the mining efficiency of coalbed methane.

Key words: pore structure, gas desorption, pore size distribution, liquid nitrogen adsorption, coalbed methane, low permeability

CLC Number:

TE122

Hu Xiong, Wu Changwu, Yang Xiuchun, Cheng Qianhui, Zhu Wentao, Ma Liang, Zhu Xueguang, Xu Borui. Microscopic Pore Structure and Coalbed Methane Desorption Law in Low-Permeability Coal Seams[J]. Special Oil & Gas Reservoirs, 2024, 31(2): 129-135.

References

[1] 曹军涛,赵军龙,王轶平,等.煤层气含量影响因素及预测方法[J].西安石油大学学报,2013,28(4):665-666.
CAO Juntao,ZHAO Junlong,WANG Yiping,et al.Review of influencing factors and prediction methods of gas content in coal seams and prospect of prediction methods[J].Journal of Xi′an Shiyou University,2013,28(4):665-666.
[2] ZHANG Songhang,TANG Shuheng,TANG Dazhen,et al.Determining fractal dimensions of coal pores by FHH model:problems and effects[J].Journal of Natural Gas Science and Engineering,2014,21:929-939.
[3] 全国煤炭标准化技术委员会.煤层气含量测定方法:GB/T 19559—2008[S].北京:中国标准出版社,2008:11.
National Coal Standardization Technical Committee.Method for determining the content of coalbed methane:GB/T 19559-2008[S].Beijing:Standards Press of China,2008:11.
[3] ZHOU Sandong,LIU Dameng,CAI Yidong,et al.Fractal characterization of pore-fracture in low-rank coals using a low-field NMR relaxation method[J].Fuel,2016,181:218-226.
[4] 于艳梅,胡耀青,梁卫国,等.应用CT技术研究瘦煤在不同温度下孔隙变化特征[J].地球物理学报,2012,55(2):637-644.
YU Yanmei,HU Yaoqing,LIANG Weiguo,et al.Study on pore characteristics of lean coal at different temperature by CT technology[J].Chinese Journal of Geophysics,2012,55(2):637-644.
[5] MENDHE V A,BANNERjee M,VARMA A K,et al.Fractal and pore dispositions of coal seams with significance to coalbed methane plays of East Bokaro,Jharkhand,India[J].Journal of Natural Gas Science and Engineering,2017,38:412-433.
[6] COATES G R,XIAO L Z,PRAMMER M G.NMR logging principles and applications[M].Houston:Gulf Professional Publishing,1999:45-67.
[7] 王龙,张金川,唐玄.鄂尔多斯盆地下寺湾—云岩地区长7段页岩气测井评价与分布规律研究[J].中国石油勘探,2019,24(1):130-131.
WANG Long,ZHANG Jinchuan,TANG Xuan.Logging evaluation and distribution law of Chang 7 shale gas in Xiasiwan-Yunyan Area,Ordos Basin[J].China Petroleum Exploration,2019,24(1):130-131.
[8] 付金华,魏新善,任军峰,等.伊陕斜坡上古生界大面积岩性气藏分布与成因[J].石油勘探与开发,2008,35(6):665-667.
FU Jinhua,WEI Xinshan,REN Junfeng,et al.Distribution and genesis of large-scale Upper Palaeozoic lithologic gas reservoirs on Yi-Shaan Slope[J].Petroleum Exploration and Development,2008,35(6):665-667.
[9] 陈志勇,张启明.南海北部大陆架西区陆坡的形成与演化[J].石油勘探与开发,1992,19(1):1-6.
CHEN Zhiyong,ZHANG Qiming.The forming and evolution of the continental slope in western shelf area of northern South China Sea[J].Petroleum Exploration and Development,1992,19(1):1-6.
[10] 李绪宣,钟志洪,董伟良,等.琼东南盆地古近纪裂陷构造特征及其动力学机制[J].石油勘探与开发,2006,33(6):713-721.
LI Xuxuan,ZHONG Zhihong,DONG Weiliang,et al.Paleogene rift structure and its dynamics of Qiongdongnan Basin[J].Petroleum Exploration and Development,2006,33(6):713-721.
[11] 石油工业标准化技术委员会石油测井专业标准化委员会.岩样核磁共振参数实验室测量规范:SY/T 6490—2014[S].北京:石油工业出版社,2015:2.
Petroleum Logging Standardization Committee of Petroleum Industry.Standardization technical committee laboratory measurement specifications for rock sample NMR parameters:SY/T 6490-2014[S].Beijing:Petroleum Industry Press,2015:2.
[12] 全国有色金属标准化技术委员会.气体吸附BET法测定固态物质比表面积:GB/T19587—2004[S].北京:中国标准出版社,2005:1.
National Nonferrous Metals Standardization Technical Committee.Gas adsorption BET method for determining the specific surface area of solid substances:GB/T19587-2004[S].Beijing:Standards Press of China,2005:1.
[13] 全国筛网筛分和颗粒分检方法标准化技术委员会.压汞法和气体吸附法测定固体材料孔径分布和孔隙度第2部分:气体吸附法分析介孔和大孔:GB/T 21650.2—2008[S].北京:中国标准出版社,2008:7.
National Technical Committee for Standardization of Sieve Mesh Sieving and Particle Sorting Methods.Pore size distribution and porosity of solid materials by mercury porosimetry and gas adsorption - part 2:analysis of mesopores and macropores by gas adsorption:GB/T 21650.2-2008[S].Beijing:Standards Press of China,2008:7.
[14] 石油地质勘探专业标准化委员会.岩石比表面积和孔径分布测定静态吸附容量法:SY/T 6154—2019[S].北京:石油工业出版社,2015:2.
Standardization Committee for Petroleum Geological Exploration Determination of specific surface and pore size distribution of rocks - static adsorption capacity method:SY/T 6154-2019[S].Beijing:Petroleum Industry Press,2015:2.
[15] 许耀波,朱玉双.高阶煤的孔隙结构特征及其对煤层气解吸的影响[J].天然气地球科学,2020,31(1):86-87.
XU Yaobo,ZHU Yushuang.Pore structure characteristics of high rank coal and its effect on CBM desorption[J].Natural Gas Geoscience,2020,31(1):86-87.
[16] 喻廷旭,汤达祯,许浩,等.柳林矿区不同煤岩类型煤的孔隙特征[J].煤炭科学技术,2013,41(增刊2):363-364.
YU Tingxu,TANG Dazhen,XU Hao,et al.Pore characteristics of different coal types by lustre in Liulin Mining Area[J].Coal Science and Technology,2013,41(S2):363-364.
[17] 陈萍,唐修义.低温氮吸附法与煤中微孔隙特征的研究[J].煤炭学报,2001,26(5):552-556.
CHEN Ping,TANG Xiuyi.The research on the adsorption of nitrogen in low temperature and micro-pore properties in coal[J].Journal of China Coal Society,2001,26(5):552-556.
[18] 降文萍,张群,姜在炳,等.构造煤孔隙结构对煤层气产气特征的影响[J].天然气地球科学,2016,27(1):176-177.
JIANG Wenping,ZHANG Qun,JIANG Zaibing,et al.Effect on CBM drainage characteristics of pore structure of tectonic coal[J].Natural Gas Geoscience,2016,27(1):176-177.
[19] 刘爱华,傅雪海,梁文庆,等.不同煤阶煤孔隙分布特征及其对煤层气开发的影响[J].煤炭科学技术,2013,41(4):104-105.
LIU Aihua,FU Xuehai,LIANG Wenqing,et al.Pore distribution features of different rank coal and influences to coal bed methane development[J].Coal Science and Technology,2013,41(4):104-105.
[20] 康永尚,孙良忠,张兵,等.中国煤储层渗透率主控因素和煤层气开发对策[J].地质评论,2017,63(5):1410-1412.
KANG Yongshang,SUN Liangzhong,ZHANG Bing,et al.The controlling factors of coalbed reservoir permeability and CBM development strategy in China[J].Geological Review,2017,63(5):1410-1412.