中文摘要：

液氮温度极低，约在-195．56～-180．44℃之间，当与岩石接触时会对岩石孔隙结构产生损伤。根据这一特点，低温液氮有望作为压裂流体对储层进行压裂改造。为了研究液氮冻结对岩石孔隙结构损伤的影响，选取两种不同砂岩岩样，分别在不同初始含水饱和度条件下进行液氮冻结处理。对冻结前、后的岩样进行孔隙度以及核磁共振测试，得到岩样在冻结前、后的孔隙度、横向弛豫时间疋分布以及疋谱面积变化情况。试验结果表明：液氮冻结会对岩石的孔隙结构产生损伤，损伤程度受到岩性、孔隙度和岩石含水饱和度等因素影响；岩石含水饱和度越大，损伤就越严重，当岩石含水饱和度达到100％时，岩石表面产生了明显裂纹；岩石在液氮冻结下损伤形式主要是微孔隙的发育和扩展，微孔隙的增加会使岩石孔隙结构的连通性增强，甚至会产生新的大尺寸孔隙，从而对孔隙结构造成严重损伤。

英文摘要：

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.