Research Advances on Characteristics of Pores and Fractures in Coal Seams

Abstract

The study of the characteristics and relationships of pores and fractures in coal seams is an important research in unconventional petroleum sedimentology， and is of great significance for understanding the micromechanisms of the existence， state， and mass transfer process of coal seam fluid， and for optimizing the exploitation of coalbed methane （CBM） where geologically appropriate. The pores and fractures in coal seams have complex genesis， with wide-ranging scale and strong heterogeneity. Their characteristics are the result of the combined actions of coalification， metamorphism types， tectonic evolution， coal components， and underground fluids. Coalification is the internal cause of pore and fracture characteristics， while tectonic stress is the major external cause. The combined actions of internal and external causes have formed the presently observed characteristics of pores and fractures in coal of different rank， in different coal-bearing basins， and in different tectonic locations. Moreover， the coal-forming materials， sedimentary environment， burial history and thermal history of coal-bearing basins each play important roles in determining the present pore and fracture characteristics. The study of pore and fracture characteristics is closely related to the efficient exploitation of CBM， which implies that considering pores and fractures in coal as an entire desorption–diffusion–seepage network in further research will be a sensible approach. The source and effectiveness of coal permeability is controlled by pore/fracture connected networks， nanoscale pore/fracture characteristics and their syntagmatic relations： interfacial properties of pores， fractures and coal seam fluids， together with multiscale pore/fracture characterization， all need to be further researched. The development and application of digital petrophysical characterization technology provides new methods and ideas for research into pore and fracture characteristics.