Coal fracturing through liquid nitrogen treatment: a micro-computed tomography study

Abstract

Low permeability of coal has been a constant obstacle to economic production from coalbed methane reservoirs, and liquid nitrogen (LN2) treatment has been investigated as one approach to address this issue. This study examined LN2 fracturing of a bituminous coal at pore-scale through 3D X-ray micro-computed tomography. For this purpose, a cylindrical sample was immersed into LN2 for 60 min. The micro-CT results clearly showed that the rapid freezing of the coal with LN2 generated fracture planes with large apertures originating from the pre-existing cleats in the rock. This treatment also connected original cleats with originally isolated pores and micro-cleats, thereby increasing pore network connectivity. Moreover, scanning electron microscopy highlighted the appearance of continuous wide conductive fractures with a maximum opening size of 9 µm. Furthermore, a nano-indentation technique was used to test the effect of LN2 on coal mechanical properties. The indentation moduli decreased by up to 14%, which was attributed to the increase in the cracked rock compressibility, showing considerable fracturing efficiency of the LN2 treatment. Through in-situ microscopic visualisation and surface investigation, this study quantified the pore structure and connectivity evolution of the rock based on the morphological alteration, and demonstrated the promising effect of LN2 freezing on fracturing of bituminous coals, thus aiding coalbed methane production. The significance of this study was investigating the mechanisms associated with and the efficiency of LN2 treatment of a coal rock in a 3D analysis inside the rock.