Pore structure characteristics of low-rank coal reservoirs with different ash yields and their implications for recoverability of coalbed methane—a case study from the Erlian Basin, northeastern China

doi:10.1007/s11707-022-1015-0

Front. Earth Sci.

2023, Vol. 17

Issue (1) : 18-29 https://doi.org/10.1007/s11707-022-1015-0

RESEARCH ARTICLE

Pore structure characteristics of low-rank coal reservoirs with different ash yields and their implications for recoverability of coalbed methane—a case study from the Erlian Basin, northeastern China

Dawei DONG¹, Jiaosheng YANG²(

), Qiujia HU³, Shitao CUI⁴, Fenjin SUN², Jidong ZHANG², Xinrui CUI³

¹. Shandong Institute of Petroleum and Chemical Technology, Dongying 257061, China
². PetroChina Research institute of petroleum exploration and development, Beijing 100083, China
³. Shanxi CBM Exploration and Development Branch, PetroChina Huabei Oilfield Company, Changzhi 046000, China
⁴. China National Logging Corporation, Xi’an 710077, China

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Abstract

Pores are the main accumulation sites and migration pathways for coalbed methane (also referred to as CBM). Pore structure restricts the content and recoverability of CBM from coal reservoirs. In this study, 12 representative coal samples with different ash yields that have similar tectonic characteristics and burial depths were collected from different mining areas in the Jiergalangtu and Huolinhe depressions in the Erlian Basin. These samples were used to study the restrictions of ash yield on the characteristics of coal pore structures and the recoverability of CBM through macroscopic and microscopic structure observation, scanning electron microscope observations, vitrinite reflectance tests, low-temperature N₂ adsorption, nuclear magnetic resonance (NMR), and micro-computed tomography. The results show that coal reservoirs in the study area vary greatly in ash yield, based on which they can be divided into three types, i.e., low-ash-content, ash-bearing, and high-ash-content coal reservoirs. In addition, the ash yield has a certain impact on the development of coal pores; coal samples with lower ash yields indicate the presence of well-developed medium-large pores and better connectivity. Ash yield also has a certain impact on the brittleness of coal wherein a lower ash yield implies the development of brittle coal that is more liable to fracture as compared to less brittle samples at the same pressure. Absorbed gas content also varies significantly with ash yield; a low ash yield impacts the gas saturation of coal. Overall, for coal reservoirs in the study area, their porosity, pore diameter, movable fluid porosity, adsorbed gas amount, and recoverability decrease as the ash yield increases.

Keywords coal reservoir ash pore structure recoverability Erlian Basin

Corresponding Author(s): Jiaosheng YANG

About author:

^* These authors contributed equally to this work.

Online First Date: 09 May 2023 Issue Date: 03 July 2023

Cite this article:

Dawei DONG,Jiaosheng YANG,Qiujia HU, et al. Pore structure characteristics of low-rank coal reservoirs with different ash yields and their implications for recoverability of coalbed methane—a case study from the Erlian Basin, northeastern China[J]. Front. Earth Sci., 2023, 17(1): 18-29.

URL:

https://academic.hep.com.cn/fesci/EN/10.1007/s11707-022-1015-0
https://academic.hep.com.cn/fesci/EN/Y2023/V17/I1/18

Fig.1 Division of tectonic units and location of study area in the Erlian Basin (modified after Zhao et al., 2015).

Fig.2 Relationship between R_o and burial depth of coal in the Huolinhe and Jiergalangtu depressions, Erlian Basin.

Tab.1 Experimental samples and experiments

Fig.3 Typical scanning electron microscope images of JM3 and JM2. (a) Scanning electron microscope image of JM3; (b) scanning electron microscope image of JM2.

Fig.4 CT Scanning of JM2 in the Jiergalangtu Depression. (a) Development characteristics of micro-fissures parallel to strata (section in the XY direction); (b) development characteristics of micro-fissures parallel to strata (section in the XZ direction); (c) development characteristics of micro-fissures parallel to strata (section in the YZ direction); (d) 3D view of rock skeleton and micro-fissures.

Fig.5 Relation between relaxation time and aperture conversion.

Fig.6 NMR T2 distribution of coal samples from Huolinhe and Jiergalangtu Depressions, Erlian basin.

Fig.7 Liquid nitrogen adsorption curves of coal samples from Houlinhe and Jiergalangtu Depressions,

Fig.8 Distribution characteristics of micro-pink pores in nitrogen adsorption at low temperature.

Fig.9 Effect of ash content on porosity. (a) Effect of ash content on porosity; (b) effect of yield on pores of different sizes.

Fig.10 Effects of ash content on fissures and movable porosity.

Fig.11 Effect of ash content on adsorption amount and desorption amount.

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