Understanding the capillary imbibition laws of brackish water in rocks is necessary to reveal the mechanism of fluid, salt, and ion transport. In this study, we investigated the capillary imbibition laws of a Na2SO4 solution of different concentrations in sandstone by measuring the parameters of water absorption mass, water migration front height, nuclear magnetic resonance (NMR) T2 spectra, and stratified moisture distribution. The results indicate the following: (1) With an increase in the salt solution concentration, the water absorption rate of samples increases, specifically manifested in an increase in the rate of absorption mass and a rising rate of the absorption front. (2) With an increase in the salt solution concentration, the total NMR signals in samples measured at the end of water absorption decreases; that is, the total amount of water absorption decreases. (3) When the solution concentration exceeds 0.50 g/L, variations in the NMR signal of samples and the absorbed water mass over time are not synchronic and are even opposite at some stages. Based on the capillary dynamic theories of liquid, the influence of salts on solution properties and the modification of the pore structure by crystallization are considered when discussing the underlying mechanism of capillary imbibition in sandstone. By calculating the physical properties such as the density, viscosity, surface tension, and contact angle of solutions with different concentrations, the imbibition process does not exhibit any significant variation with the difference in the properties of the liquid. The equivalent capillary radii of the samples at varying salt concentrations are obtained by fitting the capillary dynamics curves with the theoretically calculated values. The equivalent capillary radii of samples in higher salt concentrations are larger,i.e., the difference in capillary imbibition laws introduced by the salt concentration should be attributed to modifications to the pore structure caused by salt crystallization.

中文翻译：

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砂岩中淡水-咸水的毛细管自吸规律

了解岩石中微咸水的毛细管自吸规律对于揭示流体、盐和离子传输的机制是必要的。本研究通过测量吸水质量、水迁移前沿高度、核磁共振（NMR）T2谱和分层水分分布等参数，研究了不同浓度Na2SO4溶液在砂岩中的毛细管渗吸规律。结果表明：(1)随着盐溶液浓度的增加，样品的吸水率增加，具体表现为吸收质量率的增加和吸收前沿率的上升。 (2)随着盐溶液浓度的增加，吸水结束时测得的样品中总NMR信号减小；即，总吸水量减少。 (3)当溶液浓度超过0.50 g/L时，样品的NMR信号与吸收水质量随时间的变化不同步，甚至在某些阶段相反。基于液体毛细管动力学理论，在讨论砂岩毛细管渗吸的基本机制时，考虑了盐类对溶液性质的影响以及结晶对孔隙结构的改变。通过计算不同浓度溶液的密度、粘度、表面张力、接触角等物理性质，吸吸过程不会随液体性质的差异而表现出明显的变化。通过将毛细管动力学曲线与理论计算值进行拟合，获得不同盐浓度下样品的等效毛细管半径。盐浓度较高时样品的等效毛细管半径较大，即盐浓度引起的毛细管吸入规律的差异应归因于盐结晶引起的孔隙结构的改变。