Effects of LN2 Freeze–Thaw Preconditioning on Damage Characteristics and Hydraulic Fracturing Breakdown Pressure of Dry and Water-Saturated Coals

Abstract

Deeply buried coal seams generally exhibit an extremely low permeability. Liquid nitrogen (LN2) freeze–thaw (F–T) preconditioning followed by hydraulic fracturing is an effective approach to enhancing coal seam permeability. To evaluate the effects of LN2 F–T cycles on the hydraulic fracturing behavior of dry and water-saturated coals, the P-wave velocity, pore structure, and mechanical properties of coal samples subjected to different F–T cycles were first investigated, and then hydraulic fracturing tests were conducted on the treated samples. The results show that F–T cycles cause significant damage to the coal. A complex fracture network is formed on the surface of dry coals, whereas dominant fractures develop on the surfaces of saturated coals. As the number of cycles increases, the P-wave velocities of both dry and saturated coals decrease, with the rate of decline gradually diminishing. Under the same number of cycles, the P-wave velocity of saturated coals is lower than that of the dry coals. The numbers of micropores, mesopores, and macropores in both dry and saturated coals increase with increasing F–T cycles, with macropores showing the most pronounced increase. After six LN2 F–T cycles, the number of macropores in saturated coals increases by 366.86%, whereas the corresponding increase for dry coals is 138.41%. As the number of F–T cycles increases, the peak strength and elastic modulus of both dry and saturated coals gradually decrease. Saturated coals undergo more severe damage than dry coals after the same number of F–T cycles. After LN2 F–T treatment, both dry and saturated coals exhibit reduced breakdown pressure during the subsequent hydraulic fracturing process, and the reduction extent becomes more pronounced with the increasing number of cycles. After three F–T cycles, the breakdown pressures of dry and saturated coals decreased by 19.5% and 23.61%, respectively. These results indicate that LN2 F–T preconditioning prior to hydraulic fracturing is a promising technique for reducing the breakdown pressure and enhancing the fracturing efficiency of low-permeability coal seams.