Abstract
Pores and fractures and their connectivity play a significant role in coalbed methane production. To investigate the growth characteristics and connectivity of pores and fractures in coal parallel and perpendicular to the bedding plane, the pores and fractures of high-rank coal samples collected from the southern Qinshui Basin were measured by low-field nuclear magnetic resonance, X-ray-computed tomography and field emission scanning electron microscopy. Then, the determinants of their connectivity were further discussed. The results show that the high-rank coal samples have similar pore size distributions both parallel and perpendicular to the bedding plane. They primarily contain mesopores (2–50 nm in width), followed by macrospores (> 50 nm in width). The research indicated that the high-rank coal connectivity parallel to the bedding plane is significantly better than that perpendicular to the bedding plane. The connectivity of high-rank coal is mainly determined by throats, and the orientation of the pores and fractures. The two connectivity modes in high-rank coal are “pore connectivity,” in which the throats are mainly pores with a low coordination number, and “microfissure connectivity”, in which the throats are mainly microfissures with a high coordination number.
Highlights
Pores in coal are the main spaces for coalbed methane (CBM) storage, while fractures are the main channel for gas and water flow (Liu et al 2015, 2018a)
This study aims to provide a better understanding of fluid migration and output during horizontal CBM well production
ΦN, porosity based on Nuclear magnetic resonance (NMR), %; Vs, volume of coal sample used for X-ray computed tomography (CT), μm3; Vp, pore volume based on X-ray CT, μm3; φp, porosity based on X-ray CT, %
Summary
Pores in coal are the main spaces for coalbed methane (CBM) storage, while fractures are the main channel for gas and water flow (Liu et al 2015, 2018a). For vertical CBM wells, the fluid flow in coal can be regarded as radial fluid flow; fluid migration perpendicular to the bedding plane can be ignored, and the coal structure, connectivity and permeability in this orientation can be disregarded (Clarkson and Bustin 1999; Wei et al 2007; Aminian and Ameri 2009). During horizontal CBM well production, fluid flow is mainly sphere-shaped flow in the horizontal section; fluid migration perpendicular to the bedding plane cannot be ignored, and the coal structure, connectivity and permeability in this orientation are important to CBM production (Keim et al 2011; Wen et al 2011; Fang et al 2019). The investigation of the growth characteristics and connectivity of pores and fractures in coal parallel and perpendicular to the bedding plane is significant to horizontal CBM well production and can provide a better understanding of fluid migration in coal
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