Abstract: Liquid nitrogen has an extremely cold temperature, which is between -195.56 ℃ and -180.44 ℃. When it contacts rock, it can make the rock pore structure damage. Based on this characteristic, cryogenic nitrogen can be used as fracturing fluid to fracture the formation. In order to research the effect of cryogenic nitrogen on pore structure damage, two kinds of sandstone samples are selected. These samples are frozen with cryogenic nitrogen at different original degrees of water saturations. Porosity and nuclear magnetic resonance characteristic of all samples are measured before and after experimental test. With the assistance of nuclear magnetic resonance (NMR) technique, porosities, transverse relaxation time T2 distributions and T2 spectrum areas of all samples are obtained. The results show that rock pore structure is damaged by cryogenic nitrogen freezing and the degree of damage is related to rock type, porosity and original degree of water saturation etc.. With the growth of water saturation, the degree of damage increases. Especially, when the degree of water saturation is 100%, obvious cracks appear in the samples. The form of pore structure damage is mainly the growth and development of micro-pore, which can cause the connectivity of the porous structure improving and promote the appearance of new pores with larger radius in specimens. Thereby, it cause serious damage on pore structure.

Key words: cryogenic nitrogen freezing, pore structure, rock damage, nuclear magnetic resonance (NMR), cryogenic nitrogen fracturing