Pore Structure Characteristics of Coal and Their Geological Controlling Factors in Eastern Yunnan and Western Guizhou, China

Zhengguang Zhang; Yong Qin; Tongsheng Yi; Zhenjiang You; Zhaobiao Yang

doi:10.1021/acsomega.0c02041

Pore Structure Characteristics of Coal and Their Geological Controlling Factors in Eastern Yunnan and Western Guizhou, China

ACS Omega. 2020 Jul 28;5(31):19565-19578. doi: 10.1021/acsomega.0c02041. eCollection 2020 Aug 11.

Authors

Zhengguang Zhang^{1

2

3}, Yong Qin^{1

2}, Tongsheng Yi⁴, Zhenjiang You³, Zhaobiao Yang^{1

2}

Affiliations

¹ Key Laboratory of Coalbed Methane Resources and Reservoir Formation Process, Ministry of Education, China University of Mining and Technology, Xuzhou 221008, China.
² School of Resources and Geosciences, China University of Mining and Technology, Xuzhou 221116, China.
³ School of Chemical Engineering, The University of Queensland, St. Lucia, Brisbane, Queensland 4072, Australia.
⁴ Guizhou Research Center of Shale Gas and Coalbed Methane Engineering Technology, Guiyang 550009, China.

Abstract

Coalbed is the carrier for coalbed methane (CBM) enrichment and migration. The pore structure characteristics of coal and their main geological controlling factors are critical to the exploration and development of CBM. In this paper, 20 coal samples were collected from eastern Yunnan and western Guizhou, China. Based on vitrinite reflectance, proximate analysis, maceral analysis, and low-temperature N₂ adsorption/desorption (LT-N₂GA) experiments, the hysteresis coefficient of low-temperature N₂ desorption was proposed, the types of pore structure were identified, and the effects of coal facies and rank on the pore structure were revealed. The results show that the R _o,max values of the 20 coal samples are between 0.74 and 3.38%, which belong to medium- and high-rank coal. In the coal macerals, the vitrinite is mainly collodetrinite. The inertinite is dominated by semifusinite, and some coal samples contain exinite. The coal samples investigated can be divided into two types. Type A samples mainly contain open pores, while type B samples are rich in bottle-shaped pores. Compared with type A coal samples, type B samples have the characteristics of smaller total pore volume (TPV), smaller average pore diameter (APD), larger specific surface area (SSA), and larger hysteresis coefficient. The coal samples are located in three regions of different coal facies, including low-level swamp (reed) facies, wetland herbaceous swamp facies, and wet forest swamp facies. The tissue preservation index (TPI) values of most coal samples are less than unity, which indicates that herbaceous plants have absolute dominance in the coal-forming plants in eastern Yunnan and western Guizhou. The maximum vitrinite reflectance (R _o,max), gelification index (GI), TPI, vitrinite content (V), inertinite content (I), Barrett-Joyner-Halenda pore volume (V _BJH), Brunauer-Emmett-Teller SSA (S _BET), and low-temperature N₂ desorption total hysteresis coefficient (H _t) were clustered using the R-type cluster analysis method. It is found that TPI is the main controlling factor of the pore structure of type A coal samples, while the pore structure of type B coal samples are jointly controlled by TPI and coal rank. Type B coal samples are mainly located in Zhuzang and Laochang high-rank coal research areas, while the distribution of type A coal samples is mainly in other medium-high-rank coal research areas. These results will contribute to the exploration and development of CBM and also guide the study of pore structures of other unconventional gas reservoirs.