Pore-scale analysis of coal cleat network evolution through liquid nitrogen treatment: A Micro-Computed Tomography investigation

Coalbed methane extraction suffers from low permeability, and liquid nitrogen treatment has been suggested as one of the methods to address this issue. This study thus investigates cryogenic liquid N₂ fracturing of a bituminous coal at pore scale through 3D X-ray micro-computed tomography. The μ-CT results clearly demonstrate that freezing the coal with liquid nitrogen increases the porosity by over 11% and creates fracture planes with large apertures originating from the pre-existing cleats in the rock. The images also suggest connection establishment of the cleat network with originally isolated pores and micro-cleats following the freezing, thereby increasing pore network connectivity. Furthermore, SEM images of the frozen sample highlights the appearance of continuous wide conductive fractures with the maximum aperture size of 9 μm. The analysis of mechanical properties through Nano-indentation technique shows a decrease of up to 25% in the indentation modulus due to increase in the compressibility of the cracked rock. Moreover, as the main purpose of this fracturing treatment, the permeability evolution of the coal is examined computationally and experimentally. Lattice Boltzmann simulations on the μ-CT images highlight two-fold enhancement in permeability measure of the treated rock. Additionally, the outcome of core flooding tests shows 2.5 times increase in the permeability measure after liquid nitrogen exposure. This study thus provides a visual understanding of fracturing mechanism associated with liquid nitrogen treatment of a coal, and quantifies the pore structure and permeability evolution of the rock.