Pore-scale analysis of coal structure and mechanical properties evolution through liquid nitrogen thermal shock

Liquid nitrogen fracturing is one of the potential feasible technologies for improving the stimulation efficiency of coalbed methane (CBM) reservoirs. At present, the visualization of pore-throat connectivity and microscopic seepage characteristics in coal rocks under liquid nitrogen thermal shock is still lack of studying. Hence, the influence of liquid nitrogen thermal shock on the micro-nano pore structure and mechanical property of coal rocks are not understood clearly. In order to provide theoretical basis for the stimulation behavior of liquid nitrogen fracturing in coal beds, this paper investigates the change of micro-nano pore structure and mechanical property of coal rocks before and after liquid nitrogen treatment means of CT scanning and atomic force microscope (AFM). In addition, the influence of liquid nitrogen thermal shock on the seepage routes of coal rock are revealed. The following research results can be obtained. First, the number and scales of pores in the coal increase after liquid nitrogen thermal shock. In this experiment, porosity is increased by 200%, micro-fracture is dominant and its volume proportion is increased to 90.0% from 7.7% before liquid nitrogen treatment. Second, the three-dimensional pore structure reconstruction model obtained by CT shows that after the liquid nitrogen treatment, the number, total length and total volume of throats in the coal rock are increased by 170%, 140% and 130% and the pore connectivity is improved greatly. Third, after liquid nitrogen treatment, the sample's absolute permeability is improved significantly. In this experiment, the absolute permeability of coal after liquid nitrogen treatment is 77 times higher than that before liquid nitrogen cooling. The micro-fractures induced by thermal stress are the main percolation routes in coal after liquid nitrogen cooling. Fourth, pores and fractures are newly formed on both matrix and mineral domains, and the surface roughness is increased. In the meantime, the elastic modulus in matrix and mineral domains of coal drops, and the average elastic modulus drops by 81% and 91%, respectively. In conclusion, liquid nitrogen thermal shock leads to the increase of microscopic defects in coal and the deterioration of mechanical property. Liquid nitrogen fracturing is expected to be a new kind of efficient and green CBM reservoir stimulation technology.