The evolution process of fractures and their modification effects on the liquid–solid interface during liquid nitrogen cyclic freeze–thaw of coal and shale

Abstract

Liquid nitrogen cyclic freezing-thawing fracturing (LNCFT) is an innovative method for the waterless fracturing of coal and rock, harnessing the effects of low temperature, phase change, and expansion. However, the mechanisms behind micro-fracture propagation, evolution, and surface modification during LNCFT remain elusive. This study concentrates on anthracite coal and roof shale in the Jiaozuo mining area. It conducts LNCFT experiments under various saturated water conditions and employs a stereoscopic fluorescence microscope and an optical contact angle measuring instrument for observations, analyzing the surface modification effects resulting from the propagation and evolution of fractures on solid surfaces. The study unveils the following findings: (1) LNCFT effectively creates a network of micro-fractures. The quantity and length of fractures significantly increase with the number of fracturing cycles, and the effect is more pronounced with higher water content, especially in coal compared to shale. Fracture evolution falls into three categories: extension, intersecting, and newly formed independent fractures, with the scale of extension and intersecting of the original fractures determining the modification effect. (2) The LNCFT process exhibits distinct stages and selectivity. The strength of early-stage modifications (before eight cycles) directly influences the overall modification effect. (3) Throughout the LNCFT process, the solid–liquid surface contact angle first increases and then decreases with the number of fracturing cycles. This pattern can be analyzed using the Wenzel model. Liquid nitrogen fracturing increases the surface roughness of coal and shale, leading to a transition from hydrophobicity to hydrophilicity on their surfaces. The research findings of this paper provide theoretical guidance for the modification of low-permeability reservoirs.