Effects of depth on gas-bearing properties of coal reservoirs and their coupling relationships with coalbed methane accumulation

Authors

CHEN Shida, School of Energy Resources, China University of Geosciences (Beijing), Beijing 100083, China; Coal Reservoir Laboratory of National Engineering Research Center of Coalbed Methane Development and Utilization, Beijing 100083, ChinaFollow
HOU Wei, National Engineering Research Center of China United Coalbed Methane Co., Ltd., Beijing 100095, China; PetroChina Coalbed Methane Company Limited, Beijing 100028, China
TANG Dazhen, School of Energy Resources, China University of Geosciences (Beijing), Beijing 100083, China; Coal Reservoir Laboratory of National Engineering Research Center of Coalbed Methane Development and Utilization, Beijing 100083, China
LI Xiang, National Engineering Research Center of China United Coalbed Methane Co., Ltd., Beijing 100095, China; PetroChina Coalbed Methane Company Limited, Beijing 100028, China
XU Hao, School of Energy Resources, China University of Geosciences (Beijing), Beijing 100083, China; Coal Reservoir Laboratory of National Engineering Research Center of Coalbed Methane Development and Utilization, Beijing 100083, China
TAO Shu, School of Energy Resources, China University of Geosciences (Beijing), Beijing 100083, China; Coal Reservoir Laboratory of National Engineering Research Center of Coalbed Methane Development and Utilization, Beijing 100083, China
LI Song, School of Energy Resources, China University of Geosciences (Beijing), Beijing 100083, China; Coal Reservoir Laboratory of National Engineering Research Center of Coalbed Methane Development and Utilization, Beijing 100083, China
TANG Shuling, School of Energy Resources, China University of Geosciences (Beijing), Beijing 100083, China; Coal Reservoir Laboratory of National Engineering Research Center of Coalbed Methane Development and Utilization, Beijing 100083, China

Abstract

Burial depth serves as a comprehensive factor influencing coalbed methane (CBM) accumulation. Understanding the effects of depth on gas-bearing properties is critical for ascertaining the occurrence states and accumulation mechanisms of deep/shallow CBM. Based on the present situation of CBM exploration and the dissection of data from CBM exploration wells on the eastern margin of the Ordos Basin, this study explored the effects of burial depth on the gas content, saturated adsorbed gas capacity, and gas saturation of coal seams, as well as their coupling relationship with accumulation processes, using conventional and unconventional petroleum accumulation. The results indicate that CBM accumulation results from the coupling of hydrocarbon generation and gas supply in the tectonic subsidence stage with the phase transformation and dissipation of gas during the rollback and uplift of strata. This is manifested as a deep coupling of self-sealing- and buoyancy-driving gas accumulation. The variations in gas-bearing properties involve two critical depth thresholds: one for the turning of saturated adsorbed gas capacity and one for free gas retention. Notably, the two thresholds do not exhibit absolute synchronicity: (1) The saturated adsorbed gas capacity is an intrinsic property of coals under specific temperature and pressure conditions, not strictly constrained by preservation conditions. Its depth-varying evolutionary process governs the phase transformation of methane. The compensating effects of pressure gradients and metamorphic grades lead to a significantly deeper turning point (zone) of current regional saturated adsorbed gas capacity. (2) The migration and accumulation, along with transformation and shaping, of free gas are dictated by the cap rock conditions during the rollback and uplift of strata, involving the comprehensive influence from burial depth - structure - hydrodynamic field coupling effects, as well as buoyancy and reservoir/cap rock capillary pressures. Minor uplift amplitude and low transformation intensity are necessary for free gas retention and preservation. Free gas will dissipate extensively in strata shallower than the retention depths due to reduced sealing properties. In the Liulin-southern Yanchuan area on the eastern margin of the Ordos Basin, the total gas content of coal seams increases almost linearly with the burial depth, exhibiting a subtle convergence trend in deep layers. Coals with different metamorphic grades in this area show depth thresholds ranging from 1600 to 2200 m for the turning of theoretical saturated adsorbed gas capacity. However, regional differentiation of coal ranks causes a continuous increase in the in-situ saturated adsorbed gas capacity with the burial depth. The Daning-Jixian block has a critical depth of approximately 2000 m for free gas retention, with an average gas saturation of 120% at a depth of 2500 m and an estimated gas saturation of 136% at a depth of 3000 m. Different areas exhibit differential CBM accumulation settings and geological conditions, necessitating a specific analysis of the effects of depth on gas-bearing properties. The analysis should focus on the comprehensive influence of the spatio-temporal evolution of both the phase transformation of methane and the sealing conditions of strata on current gas and water distributions. This is crucial for achieving the zone-orientated evaluation and efficient production design of deep CBM.

Keywords

deep coalbed methane, gas-bearing property, conversion depth, compensating effect

DOI

10.12363/issn.1001-1986.23.10.0670

Recommended Citation

CHEN Shida, HOU Wei, TANG Dazhen, et al. (2024) "Effects of depth on gas-bearing properties of coal reservoirs and their coupling relationships with coalbed methane accumulation," Coal Geology & Exploration: Vol. 52: Iss. 2, Article 7.
DOI: 10.12363/issn.1001-1986.23.10.0670
Available at: https://cge.researchcommons.org/journal/vol52/iss2/7

Reference

[1] 徐凤银，聂志宏，孙伟，等. 大宁–吉县区块深部煤层气高效开发理论技术体系[J/OL]. 煤炭学报，2023：1–17 [2023-12-03]. https://doi.org/10.13225/j.cnki.jccs.YH23.1290.

XU Fengyin，NIE Zhihong，SUN Wei，et al. Theoretical and technological system for highly efficient development of deep coalbed methane in Daning–Jixian Block[J/OL]. Journal of China Coal Society，2023：1–17 [2023-12-03]. https://doi.org/10.13225/j.cnki.jccs.YH23.1290.

[2] 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.

[3] 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.

[4] 聂志宏，徐凤银，时小松，等. 鄂尔多斯盆地东缘深部煤层气开发先导试验效果与启示[J/OL]. 煤田地质与勘探，2023：1–11 [2023-12-03]. http://kns.cnki.net/kcms/detail/61.1155.P.20231113.1609.002.html.

NIE Zhihong，XU Fengyin，SHI Xiaosong，et al. Enlightenment and effect of deep coalbed methane development pilot test in the eastern margin of Ordos Basin[J/OL]. Coal Geology & Exploration，2023：1–11 [2023-12-03]. http://kns.cnki.net/kcms/detail/61.1155.P.20231113.1609.002.html.

[5] 郭绪杰，支东明，毛新军，等. 准噶尔盆地煤岩气的勘探发现及意义[J]. 中国石油勘探，2021，26(6)：38−49.

GUO Xujie，ZHI Dongming，MAO Xinjun，et al. Discovery and significance of coal measure gas in Junggar Basin[J]. China Petroleum Exploration，2021，26(6)：38−49.

[6] 康永尚，邓泽，皇甫玉慧，等. 中煤阶煤层气高饱和–超饱和带的成藏模式和勘探方向[J]. 石油学报，2020，41(12)：1555−1566.

KANG Yongshang，DENG Ze，HUANGFU Yuhui，et al. Accumulation model and exploration direction of high– to over–saturation zone of the medium–rank coalbed methane[J]. Acta Petrolei Sinica，2020，41(12)：1555−1566.

[7] 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.

[8] 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.

[9] 宋岩，赵孟军，柳少波，等. 构造演化对煤层气富集程度的影响[J]. 科学通报，2005，50(增刊1)：1−5.

SONG Yan，ZHAO Mengjun，LIU Shaobo，et al. The impact of structural evolution on the coalbed methane enrichment degree[J]. Chinese Science Bulletin，2005，50(Sup.1)：1−5.

[10] 秦勇，申建，王宝文，等. 深部煤层气成藏效应及其耦合关系[J]. 石油学报，2012，33(1)：48−54.

QIN Yong，SHEN Jian，WANG Baowen，et al. Accumulation effects and coupling relationship of deep coalbed methane[J]. Acta Petrolei Sinica，2012，33(1)：48−54.

[11] 秦勇，申建. 论深部煤层气基本地质问题[J]. 石油学报，2016，37(1)：125−136.

QIN Yong，SHEN Jian. On the fundamental issues of deep coalbed methane geology[J]. Acta Petrolei Sinica，2016，37(1)：125−136.

[12] 申建，杜磊，秦勇，等. 深部低阶煤三相态含气量建模及勘探启示：以准噶尔盆地侏罗纪煤层为例[J]. 天然气工业，2015，35(3)：30−35.

SHEN Jian，DU Lei，QIN Yong，et al. Three–phase gas content model of deep low–rank coals and its implication for CBM exploration：A case study from the Jurassic coal in the Junggar Basin[J]. Natural Gas Industry，2015，35(3)：30−35.

[13] 陈刚，李五忠. 鄂尔多斯盆地深部煤层气吸附能力的影响因素及规律[J]. 天然气工业，2011，31(10)：47−49.

CHEN Gang，LI Wuzhong. Influencing factors and patterns of CBM adsorption capacity in the deep Ordos Basin[J]. Natural Gas Industry，2011，31(10)：47−49.

[14] 郭广山，柳迎红，李林涛. 鄂尔多斯盆地东缘北段煤层含气量变化规律及控制因素[J]. 天然气地球科学，2021，32(3)：416−422.

GUO Guangshan，LIU Yinghong，LI Lintao. Study on variation law and controlling factors of coal gas content in north section of east margin of Ordos Basin[J]. Natural Gas Geoscience，2021，32(3)：416−422.

[15] 高丽军，谢英刚，潘新志，等. 临兴深部煤层气含气性及开发地质模式分析[J]. 煤炭学报，2018，43(6)：1634−1640.

GAO Lijun，XIE Yinggang，PAN Xinzhi，et al. Gas analysis of deep coalbed methane and its geological model for development in Linxing Block[J]. Journal of China Coal Society，2018，43(6)：1634−1640.

[16] 王延斌，陶传奇，倪小明，等. 基于吸附势理论的深部煤储层吸附气量研究[J]. 煤炭学报，2018，43(6)：1547−1552.

WANG Yanbin，TAO Chuanqi，NI Xiaoming，et al. Amount of adsorbed gas in deep coal reservoir based on adsorption potential theory[J]. Journal of China Coal Society，2018，43(6)：1547−1552.

[17] 苏现波，张丽萍，林晓英. 煤阶对煤的吸附能力的影响[J]. 天然气工业，2005，25(1)：19−21.

SU Xianbo，ZHANG Liping，LIN Xiaoying. Influence of coal rank on coal adsorption capacity[J]. Natural Gas Industry，2005，25(1)：19−21.

[18] 陈刚，秦勇，胡宗全，等. 不同煤阶深煤层含气量差异及其变化规律[J]. 高校地质学报，2015，21(2)：274−279.

CHEN Gang，QIN Yong，HU Zongquan，et al. Variations of gas content in deep coalbeds of different coal ranks[J]. Geological Journal of China Universities，2015，21(2)：274−279.

[19] 杨焦生，冯鹏，唐淑玲，等. 大宁–吉县区块深部煤层气相态控制因素及含量预测模型[J]. 石油学报，2023，44(11)：1879−1891.

YANG Jiaosheng，FENG Peng，TANG Shuling，et al. Phase control factors and content prediction model of deep coalbed methane in Daning–Jixian Block[J]. Acta Petrolei Sinica，2023，44(11)：1879−1891.

[20] 庞雄奇，贾承造，宋岩，等. 全油气系统定量评价：方法原理与实际应用[J]. 石油学报，2022，43(6)：727−759.

PANG Xiongqi，JIA Chengzao，SONG Yan，et al. Quantitative evaluation of whole petroleum system：Principle and application[J]. Acta Petrolei Sinica，2022，43(6)：727−759.

[21] 贾承造，庞雄奇，宋岩. 论非常规油气成藏机理：油气自封闭作用与分子间作用力[J]. 石油勘探与开发，2021，48(3)：437−452.

JIA Chengzao，PANG Xiongqi，SONG Yan. The mechanism of unconventional hydrocarbon formation：Hydrocarbon self–containment and intermolecular forces[J]. Petroleum Exploration and Development，2021，48(3)：437−452.

[22] 郭旭升，腾格尔，魏祥峰，等. 四川盆地深层海相页岩气赋存机理与勘探潜力[J]. 石油学报，2022，43(4)：453−468.

GUO Xusheng，BORJIGIN Tenger，WEI Xiangfeng，et al. Occurrence mechanism and exploration potential of deep marine shale gas in Sichuan Basin[J]. Acta Petrolei Sinica，2022，43(4)：453−468.

[23] 李曙光，王成旺，王红娜，等. 大宁–吉县区块深层煤层气成藏特征及有利区评价[J]. 煤田地质与勘探，2022，50(9)：59−67.

LI Shuguang，WANG Chengwang，WANG Hongna，et al. Reservoir forming characteristics and favorable area evaluation of deep coalbed methane in Daning–Jixian Block[J]. Coal Geology & Exploration，2022，50(9)：59−67.

[24] 任战利，祁凯，李进步，等. 鄂尔多斯盆地热动力演化史及其对油气成藏与富集的控制作用[J]. 石油与天然气地质，2021，42(5)：1030−1042.

REN Zhanli，QI Kai，LI Jinbu，et al. Thermodynamic evolution and hydrocarbon accumulation in the Ordos Basin[J]. Oil & Gas Geology，2021，42(5)：1030−1042.

[25] 孙斌，杨敏芳，杨青，等. 准噶尔盆地深部煤层气赋存状态分析[J]. 煤炭学报，2017，42(增刊1)：195−202.

SUN Bin，YANG Minfang，YANG Qing，et al. Analysis on occurrence state of deep coalbed methane in Junggar Basin[J]. Journal of China Coal Society，2017，42(Sup.1)：195−202.