Cryogenic fracturing of coal with LN2 treatment: A sustainable approach for enhancing coalbed methane extraction in water-scarce regions

Abstract

Alteration of the pore space in coal fractured by liquid nitrogen (LN₂) significantly influences the coalbed methane (CBM) process to overall porosity, effective permeability, pore rugosity, and adsorption capacity. The impact of LN₂ treatment on the pore structures of coal samples from the Karaganda Coal Basin, Kazakhstan, was analyzed to enhance CBM extraction. This study employs Mercury Intrusion Porosimetry (MIP) and Low-Pressure Nitrogen Gas Adsorption (LN2GA) isotherm for analyzing the effects of varying freezing times and freezing-thawing cycles on the pore structure of coal. The maximum nitrogen adsorption capacity (25.02 cc/g) and the total injected mercury volume (0.226 cc/g) in the specimens were positively correlated (R² = 0.98) with the total freezing time and the number of freezing-thawing cycles. As LN₂ freezing time increases, fractal dimensions decrease, indicating a more uniform pore structure. The values drop from D1 = 2.43, D2 = 2.80 (0 min) to D1 = 2.17, D2 = 2.62 (180 min), suggesting reduced roughness and complexity. The peak intrusion volumes increased from 0.185 cm³/g (0 min) to 0.217 cm³/g (180 min), indicating that prolonged freezing expands the coal structure to create additional pores. Ejection efficiency improved from 68.32 % (0 min) to 77.22 % (180 min), reflecting better pore connectivity as the freezing duration increases. This study is paramount in the cryogenic fracturing of coal formation and may be opted against hydraulic fracturing for various industrial applications in water-scare regions.