磁场-渗流场耦合作用下的铁磁流体多孔介质流动数值模拟

Numerical simulation of ferrofluid flow in porous media under coupled magnetic and seepage fields

铁磁流体作为一种新型功能材料,由于同时具备磁性及流体性质被广泛应用于各个技术领域.本文从数值模拟理论上研究了利用磁场控制的铁磁流体驱油问题:针对外磁场作用下的铁磁流体多孔介质流动物理过程,将磁力项引入铁磁流体相运动方程从而耦合磁场-渗流场给出其流动方程,并采用全隐式有限体积法形成相应数值离散格式;通过二维均质填砂平板流动实验验证了模型和算法的正确性,在此基础上,对比了平面及纵向非均质多孔介质中注水及注铁磁流体的驱油效果,计算结果表明通过控制铁磁流体的驱替路径可以提高驱替波及范围继而提高采收率,为提高原油采收率提供了新的思路和方法.

Water flooding is an efficient approach to maintain reservoir pressure and has been used widely to enhance oil recovery. However, preferential water pathways generated after long-term water flooding process and the strongly heterogeneous reservoirs can significantly decrease the sweep efficiency. Therefore, the utilization ratio of injected water is seriously affected. Most water-flooded reservoirs are in their late development stage, and their water cuts are more than 90%. How to develop new flooding technology to further improve the oil recovery is a pressing problem. Ferrofluids are colloidal suspensions of magnetic particles of diameter approximately 10 nm stabilized in various carrier liquids. In the presence of an external magnetic field, a ferrofluid becomes magnetized as the particles align with the magnetic field. In the presence of a gradient in the magnetic field strength, a magnetic body force is produced on the ferrofluid so that the attractive magnetic forces allow the ferrofluid to be manipulated to flow in any desired direction through control of the external magnetic field without any direct physical contact. In consideration of this properties, the potential of using the ferrofluid as a new kind of displacing fluid for flooding in oil reservoir is been studied in this paper at the first time. Considering the physical process that the mobilization of ferrofluid through porous media by application of strong external magnetic fields, we introduced the magnetic force into the Darcy equation to coupling the magnetic field and seepage flow field. The steps involved in calculating ferrofluid flow in porous media are（1） calculation of the external magnetic field strength produced by permanent magnet,（2） calculation of the magnetization using the magnetization curve of ferrofluid and initial ferrofluid saturation,（3） calculation of the magnetic force on the fluid,（4） addition of the magnetic force to the pressure gradient and gravitational body force terms, and（5） calculation of the liquid phase pressure and saturation,（6） update the magnetization based on the calculated ferrofluid saturation for the next step calculation. Using the fully implicit finite volume method to solve the mathematical model, and the validity and accuracy of the mathematical model and numerical algorithm are demonstrated through a experiment which ferrofluid flowing from left to right with an initial circular injection through oil-saturated sand in a 2-D horizontal cell. At last, based on the above the numerical simulation method, the water-flooding and ferrofluid-flooding in horizontal and vertical heterogeneous porous media have been studied： because there is a relatively higher permeability zone between injection well and production well, most of the injected water flow along the higher permeability zone and small amount of oil have been displaced, the utilization ratio of injected water is low; however, the ferrofluid can be manipulated to flow in desired direction through control of the external magnetic field, so that using ferrofluid for flooding can raise the scope of the whole displacement, as a consequence, the oil recovery has been improved more than 20% compared to water flooding. Thus, through the experiments and theoretical calculations show that： the ferrofluid flooding is a large potential method for enhanced oil recovery.