Experimental study on fracture characteristics of coal due to liquid nitrogen fracturing

Abstract

Liquid nitrogen (LN2) fracturing is expected to become an effective means of increasing coalbed methane production due to good reservoir adaptability. Advantages of the technology are the ultra-low temperature and huge gasification pressure during the fracturing, but how to give full play to the advantages of liquid nitrogen fracturing needs further exploration. In this paper, the fracture pressure, fracture degree, roughness of fracture surface and microstructure change of coal after LN2 fracturing are analyzed under different under experimental conditions, and the main factors affecting LN2 fracturing are discussed in combination with the numerical simulation results. The results show that the temperature rise and freeze–thaw treatment can effectively reduce the initiation fracture pressure of coal, with a decrease of more than 40%; The fracture degree and roughness of coal increase, especially the volume fracture of the freeze–thaw coal sample is the largest, and the cross-sectional area expansion rate (roughness index Sdr) increases as high as 325%. In addition, the SEM scanning results showed that the heating and freezing thawing treatment promoted the intergranular and transgranular destruction of the coal. Numerical analysis shows that temperature rise and freeze–thaw treatment can expand initial damage range around the borehole of LN2 fracturing instant, thereby promoting gas permeability efficiency, increasing pore pressure and improving the fracturing effect of coal. The temperature rise enhances the expansion effect of LN2 gasification, and the freeze–thaw treatment can give the full play to the low-temperature damage effect of LN2. Based on this, an improved technology of LN2 fracturing is put forward: Technology of fracturing and permeability enhancement with liquid nitrogen coupled high temperature.