Characteristics of high-rank coal structure parallel and perpendicular to the bedding plane via NMR and X-ray CT

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.

Correlation Evaluation and Schematic Analysis of Influencing Factors Affecting Pore and Fracture Connectivity on the Microscale and Their Application Discussion in Coal Reservoir Based on X-ray CT Data

“…Therefore, the connectivity of the pore/fracture system has always been the key factor affecting CO 2 injection and CH 4 production efficiencies, whether directly mining CBM or by CO 2 injection (i.e., CO 2 -ECBM) . The pore/fracture system contains geometric and topological structures, and their core parameters include coordination number, Ferret diameter, Euler characteristic number, shape factor, and tortuosity. , It is a new research idea in the field of digital rock physics to discuss the influence of characteristic parameters of pore/fracture structures on self-connectivity from the perspective of pore/fracture structures.…”

“… 3 The pore/fracture system contains geometric and topological structures, and their core parameters include coordination number, Ferret diameter, Euler characteristic number, shape factor, and tortuosity. 4 , 5 It is a new research idea in the field of digital rock physics to discuss the influence of characteristic parameters of pore/fracture structures on self-connectivity from the perspective of pore/fracture structures.…”

Experimental Investigation on the Pore Structure Evolution of Coal in Underground Coal Gasification Process

“…Also, the process provides sites for subsequent gasification. Thus, the pore structure evolution during coal pyrolysis has been a research focus in the literature, − generally using thermogravimetry (TG)-mass spectrometry, − mercury intrusion porosimetry, , acoustic emission, X-ray computed tomography, , nuclear magnetic resonance, , scanning electron microscopy (SEM), − and liquid-nitrogen adsorption . Besides, the evolution characteristics of gases during pyrolysis which are mainly comprised of H 2 O, H 2 , CO, CH 4 , and CO 2 have been investigated in detail. , Nevertheless, most of the experiments are insufficient for the study of the pore structure evolution of coal in the UCG process that involves oxygen and high temperature (more than 600 °C).…”