Experimental study on cracking of high temperature granite using liquid nitrogen

Key words:  liquid nitrogen cooling  granite  low temperature damage  thermal stress

Abstract:

High temperature rocks can be cracked or damaged when they contact with liquid nitrogen, and thus this method can be used to improve the efficiency of drilling and fracturing in hot dry rock formations. To study the effect of liquid nitrogen cooling on the physical and mechanical properties of hot rocks, air and liquid nitrogen were used to cool dry granite rock samples at different temperature of 25-600 ℃, and the sound velocity, permeability, tensile strength and uniaxial compressive strength of the rock samples were measured and compared. The experimental results show that liquid nitrogen cooling can effectively crack and damage the high temperature granite. The damage to the granite samples under 150-600 ℃ caused by liquid nitrogen cooling can lead the wave velocity decreasing 4.13％-10.04％, the permeability increasing 0.21-182.80 times, the tensile strength decreasing 4.95％-34.54％, the compressive strength decreasing 13.95％-29.30％, and the elastic modulus decreasing 7.33％-45.74％. It is notable that the higher of the temperature of rocks before cooling, the greater thermal stress can be resulted from the cooling process, and the greater degree of cooling damage can be caused.