Fang Chao, Sun Shuyu, Qiao Rui
Department of Mechanical Engineering , Virginia Tech , Blacksburg , Virginia 24061 , United States.
Division of Physical Science and Engineering , King Abdullah University of Science and Technology , Thuwal 23955-6900 , Saudi Arabia.
Langmuir. 2019 Aug 13;35(32):10341-10353. doi: 10.1021/acs.langmuir.9b01477. Epub 2019 Aug 2.
Thin brine films are ubiquitous in oil-brine-rock systems such as oil reservoirs and play a crucial role in applications such as enhanced oil recovery. We report the results of molecular simulations of brine films that are confined between model oil (decane) and rock (neutral or negatively charged quartz slabs), with a focus on their structure, electrical double layers (EDLs), disjoining pressure, and dynamics. As brine films are squeezed to ∼0.7 nm (∼3 water molecule layers), the structures of the water-rock and water-oil interfaces change only marginally, except that the oil surface above the brine film becomes less diffuse. As the film is thinned from ∼1.0 to ∼0.7 nm, ions are enriched (depleted) near the rock (oil) surface, especially at a bath ion concentration of 0.1 M. These changes are caused primarily by the reduced dielectric screening of water and the weakened ion hydration near water-oil interfaces and, to a smaller extent, by the increased confinement. When the brine film is ∼1.0 nm thick, hydration and EDL forces contribute to the disjoining pressure between the charged rock and the oil. The EDL forces are reduced substantially as the ion concentration increases from 0.1 to 1.0 M, and the magnitude of the reduction is close to that predicted by the Poisson-Boltzmann equation. When the brine film is thinned from ∼1.0 to ∼0.7 nm, the disjoining pressure increases by ∼10 MPa, which is mostly due to an increase in the hydration forces. The first layer of water on the rock surface is nearly stagnant, even in 0.74 nm-thick brine films, whereas the viscosity of water beyond the first layer is bulk-like, and the slip coefficient of oil-water interfaces is close to that under unconfined conditions. The insights that are obtained here help lay a foundation for the rational application of technologies such as low-salinity waterflooding.
薄盐水膜在油藏等油-盐水-岩石系统中普遍存在，在提高采收率等应用中起着关键作用。我们报告了限制在模型油（癸烷）和岩石（中性或带负电的石英板）之间的盐水膜的分子模拟结果，重点关注其结构、双电层（EDL）、分离压力和动力学。当盐水膜被挤压到约0.7纳米（约3个水分子层）时，水-岩石和水-油界面的结构仅略有变化，只是盐水膜上方的油表面变得不那么弥散。当膜从约1.0纳米变薄到约0.7纳米时，离子在岩石（油）表面附近富集（耗尽），特别是在浴离子浓度为0.1 M时。这些变化主要是由于水的介电屏蔽减弱以及水-油界面附近离子水化作用减弱，在较小程度上是由于限制增加。当盐水膜厚度约为1.0纳米时，水化和EDL力有助于带电岩石和油之间的分离压力。随着离子浓度从0.1 M增加到1.0 M，EDL力大幅降低，降低幅度接近泊松-玻尔兹曼方程预测的值。当盐水膜从约1.0纳米变薄到约0.7纳米时，分离压力增加约10 MPa，这主要是由于水化力增加。即使在0.74纳米厚的盐水膜中，岩石表面的第一层水几乎是静止的，而第一层以外的水的粘度类似本体水，油水界面的滑移系数接近无限制条件下的滑移系数。本文获得的见解有助于为低盐度水驱等技术的合理应用奠定基础。
