The Flow Characteristics Influenced By Wettability In Micro-Scale Structures

As an indispensable energy and the economic artery of a country, Petroleum has been received widespread attention. The resource of high or middle permeability reservoirs is becoming exhausting. The oil production rate has reduced. The development of low and ultra-low permeability reservoirs is becoming an important resources in recent years. At the same time, with the development of Micro-Electro Mechanical Systems, the micro fluidics shows a huge potential in some applications, such as medicine, aerospace and micro-mechanical device manufacturing. The research of flow in micro-scale attracts a increasing attention. So it is useful to do the theoretical and experimental studies of the low permeability porous media in micro-scale and nano-scale.First, based on molecular dynamics and fluid mechanics, the wall slippage and Threshold Pressure Gradient (TPG) in the hydrophilic micro-tubes and the porous media flow were studied by Molecular Dynamics Simulation (MDS) and experiments.The slippage characteristics of fluid flow in the different hydrophilic channels were studied by MDS method on Poiseuille flow between two parallel planes. The simulation results indicate that there exists negative slippage in the small shear rate for the hydrophilic boundary, a adhesive force exists between the fluid and the wall. Under the condition of the same wettability, the slippage is only relative to shear rate rather than channel width. The absolute slip length decreases with the augment of shear rate and keeps constant when the shear rate increases to a certain extent.The slippage characteristics of fluid flow in the different hydrophilic channels were also studied by experiments, Deionized water flow in micro-tubes of15μm and10μm diameter were carried out and be contrasted with results of MDS.The experiment results show that there is a linear correlation between the flow rate and the shear rate, and there exists the TPG on condition of low flow velocity. The existence of TPG is induced by the negative slippage near the wall. The slip length has a nonlinear relationship with the shear rate, and nonlinearity gradually weakens with the increase of the shear rate until the slippage tends to stable value. The experiment results are in good agreement with pre-simulation results, which illustrate there are negative slippage and adhesive fluid layer near the solid wall with hydrophilic boundary. The TPG is proved when liquid flow in low permeability porous media. In addition, the relationship of slippage and the shear rate is fitted with exponential function:y=y0+AeBx, which shows the slip length decreases exponentially with the increase of shear rate.Then the1.2cm*20cm column was packed with glass beads of320mesh as a porous media, and some experiments of water flow were done. The results further proved the existence of TPG in low permeability porous media, whose size are about under the1micron scale. So the mechanism of TPG in low permeability porous media is the negative slippage with the hydrophilic boundary.Finally, the method of static and steady method were combined to survey TPG of a micro-tube, this method was called the global pressure method. The experimental results not only demonstrate the existence of TPG, and minimum TPG can be directly measured by this method. The value of dynamic measurement is14.5times greater than one by the static method. The former TPG values in petroleum laboratories are too large and can be used in developments of low permeability reservoirs. The results show a good method to measure TPG of micro-tubes and low permeability porous media.