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. •Pore-scale analysis of porosity and permeability evolution of a bituminous coal through liquid nitrogen fracturing.•In-situ micro-scale morphological alteration of the coal rock treated with liquid nitrogen through micro computed tomography.•Inspecting the mechanical properties alteration of the rock due to the fracturing damage using Nano-indentation technique.•Examining the surface morphological alteration of the coal frozen with liquid nitrogen via SEM.•Experimentally and numerically measuring the permeability enhancement through the treatment.