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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T2分布以及T2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。
Liquid nitrogen temperature is very low, about between -195.56 ~ -180.44 ℃, when in contact with the rock will damage the rock pore structure. According to this feature, cryogenic liquid nitrogen is expected to be fractured fluid fracturing transformation. In order to study the effect of freezing of liquid nitrogen on the damage of rock pore structure, two different sandstone samples were selected and subjected to liquid nitrogen freezing treatment under different initial water saturation conditions. Porosity and nuclear magnetic resonance (NMR) tests were performed on the rock samples before and after freezing. The porosity and T2 distribution of T2 and T2 spectra before and after freezing were obtained. The experimental results show that the freezing of liquid nitrogen can damage the pore structure of rock, and the degree of damage is affected by lithology, porosity and water saturation of rock. The larger the water saturation of rock, the more serious the damage. When the rock water saturation At 100%, there are obvious cracks on the rock surface. The damage form of rock under liquid nitrogen freezing is mainly the development and expansion of micro-pores. The increase of micro-pores will increase connectivity of the rock pore structure and even produce new large-size Pores, which cause serious damage to the pore structure.