砂岩吸水过程与吸水特性的核磁共振实验研究

岩石的吸水现象广泛存在于自然界与人为工程中，而毛细作用是非饱和状态下水分运移的主要驱动力，其规律研究具有重要科学意义与实际价值。针对水分运移难以监测的问题，将核磁共振技术用于人工砂岩吸水特性及其影响因素的研究，利用T₂谱分析样品孔隙水分变化，一维剖面成像获取其水分迁移信息。从实验结果可知，孔隙较大、连通性较好的砂岩吸水量大、吸水速率快。定量计算得到样品一维吸水率约为0.036 8 cm/min^1/2。此外，研究发现实验过程中的蒸发效应会对吸水过程产生不容忽视的影响。与称重法、CT法等相比，核磁共振实验研究具有快速直观、高精度、信息全面的优势。

Characterization of water imbibition in sandstones studied using nuclear magnetic resonance

Characterization of water imbibition in porous media is important in a variety of engineering and geological contexts.This work is aimed at investigating sorption characteristics of sandstones and how they are related to the pore structure of samples and boundary conditions.Over the past decades,NMR techniques have been proven to be reliable for visualizing the water distribution and quantifying the dynamic transport process of water in porous media.In this study,Carr-Purcell-Meiboom-Gill (CPMG) measurement was used to obtain the T₂ distribution which showed the change in water content in pores of different sizes with time.Moreover,one-dimensional magnetic resonance imaging (MRI) profile measurement was used to obtain the water content profiles during water imbibition into artificial sandstone samples in a quantitative and non-destructive manner.In order to understand the characteristics of water absorption and factors influencing the process,results of imbibition in samples of different permeability were compared by performing one-dimensional and three-dimensional NMR experiments.Sorptivity based on Fickian diffusion was adopted to make a quantitative analysis of sandstone samples. The results showed that sample of high permeability had a greater amount of water absorption with less time.In addition,the cumulative water absorption varied linearly with t^1/2 as expected.Therefore the sorptivity was derived directly to be 0.036 8 cm/min^1/2 with the correlation coefficient r=0.996 7.However,the increase of water content slowed down after the wetting front reached the top of the sample,which could be explained by the retarding effect of evaporation on the sample surface.