Evolution of Pore Structure and Micromechanical Properties of Coal Subjected to Cyclic Liquid Nitrogen Treatment

Abstract

Coalbed methane (CBM) is one of the unconventional natural gas resources. Low and ultralow permeability significantly limit the efficiency of CBM production in surface or subsurface extraction systems. Liquid nitrogen fracturing is a novel technique recently proposed to enhance the permeability of coal seams. The cooling effect of liquid nitrogen may be beneficial for liquid nitrogen fracturing in coal seams. However, the effect of liquid nitrogen treatment and its cycling process on coal damage at the microscale is still unclear. In particular, the presence of water in porous and fractured coal systems can complicate the freezing and thawing processes. In this study, the evolution of the pore structure and mechanical properties at the microscale under cyclic liquid nitrogen treatment was studied in detail through multiple physical experiments before and after liquid nitrogen treatment. The main conclusions are as follows: (1) The mechanical parameters obtained through nano-indentation and the microporosity obtained through NMR spectroscopy barely changed after cyclic liquid nitrogen treatment. Liquid nitrogen treatment had limited effects on the coal matrix owing to the lack of water in the micropores. (2) Fracture propagation, significant increase in meso- and macroporosity, and permeability enhancement for 80 % water-saturated coal specimens all confirmed the significance of water in liquid nitrogen treatment. The tremendous frost heavying force during the water and ice phase transition can change the pore structure of the coal. (3) The variations in the microporosity and mechanical properties of the coal matrix were mainly attributed to the heterogeneous thermal expansion of coal minerals and organics. However, the thermal stress could not continuously induce a change in the microstructure of the coal matrix during cyclic liquid nitrogen treatment. In addition, accumulative plastic deformation of the fracture opening and closing may lead to reduction in the meso- and macroporosity. In contrast, (4) the frost heaving force of water was sufficient to cause microfracture propagation and enhance the meso- and macroporosity. Moreover, water migration caused by capillary force and refreezing of water as new crack tips might lead to continuous fracture propagation and an increase in coal permeability.

Keywords: Coalbed methane, liquid nitrogen treatment, nano-indentation, Permeability, pore structure variation