离子浓度及表面结构对岩石孔隙内水流动特性的影响

酸性环境引发的岩石孔隙表面溶解增加了孔隙内水溶液的盐离子浓度,破坏了孔隙的表面结构.本文采用分子动力学模拟的方法研究了纳米级岩石孔隙内水溶液的流动特性,分析了盐离子浓度和孔隙表面结构对水流速度分布的影响及原因.研究结果表明:纳米级岩石孔隙内的水溶液流动符合泊肃叶流动特性,流速呈"抛物线"分布;随盐离子浓度增加,水溶液内部氢键网络变得更为致密,水黏度随其呈线性增长;水溶液中离子浓度越大,孔隙表面对水流动的阻力越大,最大流速越小,速度分布的"抛物线"曲率半径越大;岩石孔隙表面结构的破坏改变了流动表面的粗糙程度,增加了孔隙表面对H2O分子的吸引力.随表面结构破坏程度的增大,水溶液在近壁区域的密度增大,流速降低;当表面破坏程度达到50%时,水溶液在近壁区域出现了明显的负边界滑移现象.

Effects of salt concentrations and pore surface structure on the water flow through rock nanopores

The surface dissolution of rock nanopores, caused by the acidic environment, increases the salt concentration of water solution flowing in the nanopores, thereby destroying the surface structure of the rock,which can be found in CO2 geological sequestration and crude oil and shale gas exploration. In this paper, the molecular dynamics method is adopted to study the flow characteristics of water solution in the forsterite(Mg2SiO4) slit nanopores, by which the effects of salt concentration and structure destruction of pore surface on the velocity profiles of water solution confined in nanopores are systematically analyzed. The hydrogen bond density, radial distribution function(RDF) and water density distribution are calculated to explain the changes in viscosity, velocity profiles and interaction between water and nanopore surface. The results show that as the salt concentration increases, the water solution flow in the rock nanopore obeys the Hagen-Poiseuille equation,and the velocity profiles of water solution remain parabolic shape. However, the hydrogen bond network among water molecules becomes denser with salt concentration increasing, which can account for the linear increase in the viscosity of water solution. Besides, the higher salt concentration gives rise to the larger water flow resistance from the pore surface. As a result, with the salt concentration increasing, the maximum of water velocity decreases and the curvature radius of the parabolic velocity profile curve becomes bigger. Moreover, the surface structure destruction in rock nanopores changes the roughness of surface in the flow channel, which enhances the attraction of nanopore surface to H2O. As the structure destruction of nanopore surface deteriorates, the water density near the rough surface moves upward, whereas the velocity of water near the rough surface declines obviously. Interestingly, when the degree of surface structure destruction reaches 50%, a significant negative boundary slipping near the rough surface appears.