Fracture evolution in coalbed methane reservoirs subjected to liquid nitrogen thermal shocking

Abstract

Thermal shocking effect occurs when the coalbed methane (CBM) reservoirs meet liquid nitrogen (LN2) of extremely low temperature. In this study, 3D via X-ray microcomputer tomography (µCT) and scanning electron microscope (SEM) are employed to visualize and quantify morphological evolution characteristics of fractures in coal after LN2 thermal shocking treatments. LN2 thermal shocking leads to a denser fracture network than its original state with coal porosity growth rate increasing up to 183.3%. The surface porosity of the μCT scanned layers inside the coal specimen is influenced by LN2 thermal shocking which rises from 18.76% to 215.11%, illustrating the deformation heterogeneity of coal after LN2 thermal shocking. The cracking effect of LN2 thermal shocking on the surface of low porosity is generally more effective than that of high surface porosity, indicating the applicability of LN2 thermal shocking on low-permeability CBM reservoir stimulation. The characteristics of SEM scanned coal matrix in the coal powder and the coal block after the LN2 thermal shocking presented a large amount of deep and shallow progressive scratch layers, fracture variation diversity (i.e. extension, propagation, connectivity, irregularity) on the surface of the coal block and these were the main reasons leading to the decrease of the uniaxial compressive strength of the coal specimen.