Experimental study of the effect of liquid nitrogen penetration on damage and fracture characteristics in dry and saturated coals

Abstract

Liquid nitrogen (LN2) fracturing is characterized with high rock breaking efficiency, no pollution and no water consumption compared with traditional hydraulic fracturing techniques. So far, LN2 fracturing remains as a new reservoir stimulation technique whose mechanism in breaking formation rocks is still not fully understood. To investigate the effect of LN2 penetration on damage and fracture characteristics in dry and saturated coals, some coal samples were wrapped with plastic film to prevent LN2 penetrating before LN2 cooling. Acoustic and permeability measurements were performed on samples before and after LN2 cooling. Then, scanning electron microscopy (SEM) analyses and mechanical tests were performed on samples with and without LN2 cooling. The results indicate that LN2 penetration has significant influence on the damage of dry coals, while little influence on the damage of saturated coals. The decreasing rate in P-wave velocity, the increasing rate in permeability and the size and number of micro-fractures of dry samples with LN2 penetration are significantly greater than that of samples without LN2 penetration. The saturated samples after LN2 cooling almost show the similar damage and fracture characteristics, and they sustained more serious damage than dry coals. Although the interconnected fracture networks were formed on the surfaces of dry samples, relatively short micro-fractures with poor connectivity and small aperture were formed inside the dry coal rocks, especially inside the dry coal rocks without the penetration of LN2. In contrast, crush areas and major cracks with branches were formed inside the water-saturated coal samples.