Anisotropy of coal pore connectivity: experiments based on low-field nuclear magnetic resonance and low-temperature nitrogen adsorption

Coal, as an unconventional reservoir, is characterized by the development of micro/nano-meter pores and significant pore differences in different bedding directions. Pore heterogeneity and anisotropy are key factors contributing to the difference in reservoir properties. This not only affects the recovery of resources but also affects the interpretation of seismic data. Currently, research on coal anisotropy mainly focuses on the heterogeneity of pore distribution. Nonetheless, there are few studies and a lack of comprehensive knowledge regarding pore connectivity differences within the same macrolithotype of coal in different bedding directions. We present a novel method to study the anisotropy of pore connectivity in coal. This method used epoxy resin to seal different surfaces of cubic coal samples, combined with refined pore characterization techniques, to achieve anisotropy studies at different pore scales. These included differences in pore connectivity in different bedding directions and anisotropic characteristics of micron/nanoscale pores. The results indicated that the pores in various bedding directions differed considerably in type and distribution mode. There was a wide range of pore types in the bedding-parallel direction, and the pores showed regular variations. In contrast, the pore type was single in the bedding-normal direction, the pore distribution lacked an obvious pattern, and the pore complexity was higher. Moreover, the anisotropy of seepage pores was stronger than that of adsorption pores, which is the main reason for the anisotropy of reservoir properties. Based on the anisotropic study of coal pore connectivity, we can target the design of resource recovery processes to improve resource recovery. For example, in directions with low permeability, measures such as water injection can be taken to enhance permeability. Furthermore, one can also consider the pore size and connectivity in different bedding directions, introducing anisotropic parameters to optimize the permeability anisotropy model and provide better guidance for resource recovery.