(1. School of Petroleum Engineering，China University of Petroleum，Qingdao，Shandong 266580，China； 2. Huabei Oilfield of PetroChina，Renqiu，Hebei 062550，China)
Abstract：Liquid nitrogen can be used for the fracturing treatment of coal-bed-methane(CBM) wells. In this study，the effects of liquid nitrogen on coal-rock samples are studied via thermal shock experiments and ultrasonic testing. A stress-strain analysis model is established to analyze the thermal effect in terms of the shrinkage of coal matrix and the expansion of freezing water in the pores of coal rocks. Experimental results show that micro-fractures can be induced inside the coal-rock matrix，which might be due to the thermal stresses exceeded the tensile strength of the coal-rock. Water in the pores of the coal-rock can freeze instantly when it contacts with liquid nitrogen，in which additional stress can be produced and it may generate micro-fractures in the coal-rock. After the liquid nitrogen treatment，the micro-fractures generated are found to be orthogonal to the plane of the natural fractures，and the results of ultrasonic testing show that both the sound velocity and amplitude reduced dramatically，indicating that the liquid nitrogen treatment can have a significant effect on the internal structure of the coal-rock samples，which is beneficial for generating a fracture network or new fractures during the fracturing treatment of CBM wells.
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2013, Vol. 32 
