Effect of liquid nitrogen cooling on mechanical characteristics and fracture morphology of layer coal under Brazilian splitting test

Liquid nitrogen (LN2) fracturing, as an environmentally friendly waterless fracturing technology, is given more and more attention in coalbed methane (CBM) exploitation. In this paper, a series of microscopic tests and a thermal damage simulation were combined to evaluate the changes in the internal structure of the coal caused by the LN2 cooling, firstly. Then, Brazilian splitting tests were carried out on the bedding coal, and influences of cryogenic cooling fracturing and bedding orientation on mechanical properties and fracture morphology of the bedding coal were analyzed. The results show that the micro-structure of the coal is obviously damaged under the LN2 cooling due to the thermal tensile stress. The P-wave velocity, tensile strength, Brazilian splitting modulus, and brittleness index of the coal sample have significant anisotropic characteristics and their values are reduced after the LN2 cooling. The weakening degree in mechanical characteristics of the coal induced by the LN2 cooling is also closely related to the bedding direction of the coal. After the LN2 cooling treatment, the maximum and minimum reduction of the tensile strength is at the bedding angle 0 & DEG; and 45 & DEG;, respectively, with the reduction of 55.81% and 6.98%, respectively. The maximum and minimum changes of the Brazilian splitting modulus and brittleness index before and after the LN2 cooling locate at 0 & DEG; and 60 & DEG;, respectively. The LN2 cooling treatment can increase the ductility of the coal. The length and surface complexity of the induced fracture increase after the LN2 cooling. The increase in the fracture length is particularly evident at the high bedding angle, while the significant increase of the crack surface complexity is more likely to occur at the low bedding angle. The results contribute to the design of LN2 fracturing for coal beds.

Automated summary

In this study, the effects of liquid nitrogen cooling on the internal structure of coal and the mechanical properties and fracture morphology of bedding coal were investigated. The results showed that LN2 cooling damaged the micro-structure of coal and reduced its mechanical properties, with the degree of weakening dependent on the bedding direction. Additionally, LN2 cooling increased the ductility of coal and induced longer and more complex fractures.