High Rank Coal Pore Fracture Structure and its Impact on Reservoir Characteristics in the Southern Qinshui Basin

Changjiang Ji,Dingding Guo,Guofu Li,Haijin Hao,Zhimin Song

doi:10.3389/feart.2022.842275

Abstract

Although the southern Qinshui Basin is the most successful area for coalbed methane (CBM) development in China, the production of CBM wells in different blocks in the area is significantly different. One of the key reasons is the difference in pore structure in various-ranked coal. In this study, No. 3 coal seam of Sihe and Zhaozhuang blocks in southern Qinshui Basin was selected as the research object to investigate the high rank coal pore fracture structure and its impact on reservoir characteristics. Mercury intrusion porosimetry (MIP), low-temperature liquid nitrogen adsorption (LTN2A), scanning electron microscopy (SEM), and isothermal adsorption tests were conducted. The results show that the Sihe No.3 coal seam was mainly composed of open cylindrical and flat pores with a high proportion of transition pores (10–100 nm), large specific surface area, good connectivity, strong adsorption capacity, high gas content, and reservoir energy. Zhaozhuang No.3 coal had high proportion of mesopores (100–1,000 nm), small specific surface area, poor pores connectivity, weak adsorption capacity, poor gas content, low reservoir energy, and critical desorption pressure. The proportion of cylindrical pores, parallel plate pores, and wedge-shaped pores closed at one end was high. The anomalies in pore morphology and pore structure characteristics of coal reservoir were the main factors that caused variation in gas production of No.3 coal seam in Sihe and Zhaozhuang blocks.

Highlights

Coalbed methane (CBM) is the typical natural gas stored in the pores of gas reservoirs
Two coal samples from the SH and ZZ blocks were selected for the Mercury intrusion porosimetry (MIP) test
The total mercury intake in the SH and ZZ blocks is not high, which reflects that micro/transition pores have developed in the blocks (Li CH et al, 2018)

Summary

Introduction

Coalbed methane (CBM) is the typical natural gas stored in the pores of gas reservoirs. The pore-fracture structure of coal has an important impact on the adsorption/desorption capacity of CBM, which directly determines the gas content and development potential of coal reservoir (Li et al, 2017; Mou et al, 2021). Many test methods are used to study the pore-fracture development characteristics, such as mercury intrusion porosimetry (MIP) (Debelak and Schrodt, 1979; Okolo et al, 2015; Sun et al, 2017; Ju et al, 2018; Gao et al, 2018), low temperature N2/CO2 adsorption (LTN2/CO2A) (Clarkson and Bustin, 1999; Wang et al, 2020; Yan et al, 2020; Mou et al, 2021; Pan et al, 2019), synchrotron small-angle X-ray scattering (SAXS) (Bale and Schmidt, 1984; Shi et al, 2018), small-angle neutron scattering (SANS) (Mastalerz and Oppel, 2012), low field nuclear magnetic resonance (NMR) (Yao. Pore Fracture Structure Reservoir Characteristics et al, 2010; Yao and Liu, 2012; Yao et al, 2014), atomic force microscope (AFM) (Pan et al, 2015), micro focus X-ray computed tomography (MFX-CT) (Mazumder et al, 2006), scanning electron microscope (SEM) (Klaver et al, 2012), transmission electron microscopy (TEM) (Song et al, 2019), and field emission-scanning electron microscopy (FE-SEM). The structural characteristics of mesoporous fractures in high rank coal in Qinshui Basin were analyzed by low field NMR, CT reconstruction analysis, and SEM (Song et al, 2018)

Results

Discussion

Conclusion