NMR Visualization Experiments And Numerical Simulations On Nanoparticle Stabilized Foam Flooding

In recent years,increasing domestic oil production and reducing the dependence of the oil supply on foreign countries have been a strategic task to ensure China's energy security with serious contradiction between supply and demand of oil in China emerging.As one of the main ways to enhance oil recovery,foam flooding technology has great potential and broad application prospects in oil exploitation.However,this technique encountered many problems in practical applications:?1?The traditional foam based on surfactant has poor stability,and fails to perform an effective plugging under some extreme reservoir conditions;?2?The core flooding experiment is widely used to study the seepage characteristics of foam flooding oil,but it cannot provide a visual way to observe the flow features of the foam inside the core.This is one of the reasons that the seepage characteristics of the foam flooding oil have not been well understood yet;?3?The present static evaluation methods for foam stability should be improved because the foam seepage is a dynamic process of continuous defoaming and re-generation in the porous medium;?4?The mathematical models of the conventional foam flooding cannot apply to new foam systems which contain nanoparticles.The above problems have been the bottleneck restricting the development of foam flooding technology.In view of the above problems,the following aspects of work are carried out:?1?The mechanical mechanism of nanoparticle stabilized foam was analyzed;?2?A nanoparticle-stabilized foam system was prepared;?3?The visualized experimental method based on the nuclear magnetic resonance?NMR?technology for foam flooding oil was developed;?4?A dynamic stability evaluation method for the foam in porous media was proposed and used;?5?Numerical simulations of nanoparticle-stabilized foam flooding oil were conducted.The achievements and understandings include the following aspects:?1?The mechanical mechanism of nanoparticle-stabilized foam was preliminary understood.The nanoparticles are subjected to an interfacial force in the liquid film between the bubbles,which makes the nanoparticles and the liquid film in the foam are interdependent leading to the formation of a stable structure.The gap between the nanoparticles on the surface of the foam liquid film produces a capillary pressure,which prevents the rupture of the foam liquid film to a certain extent,thereby enhancing the stability of the foam.In the wedge-shaped region between the foams,self-assembly of nanoparticles occurs and the oscillatory structural forces is produced,then a structural barrier is formed to prevent the bubbles from coalescing.?2?A nanoparticle-stabilized foam system S-N-2 was developed.This kind of foam system has the properties of temperature resistance,salt tolerance and oil resistance.?3?A new method for visual experiment of foam flooding was established based on NMR technology.The foam displacement characteristics under different permeability cores,different foam slug lengths and different oil saturations were studied by this method.The experimental results show that the water flooding efficiency increases with the permeability of the cores in the range of 800×10^-3?m²³000×10^-3?m².A long foam slug will induce a better oil displacement effect.The oil saturation has a greater impact on the displacement efficiency of the foam,and a low oil saturation corresponds to a high displacement efficiency in the foam flooding stage.?4?Based on the T₂ spectrum of NMR and the law of mass conservation,a new method to study the dynamic stability of foam was established and used.The results show that the foam system S-N-2 has better stability in the seepage process than the foam system S-2 under the same displacement mode.?5?The one-dimensional and two-dimensional numerical simulation programs for nanoparticle stabilized foam flooding in homogeneous cores were developed.The influences of key parameters,such as nanoparticle concentration,injection flow rate and gas-liquid ratio,on the displacement efficiency of foam were studied by one-dimensional numerical simulation program.The results show that the displacement efficiency increases with the nanoparticle concentration when the concentration of nanoparticles is between 0⁰.50%.When the concentration of the nanoparticles is more than 0.50%,the displacement efficiency decreased with the nanoparticle concentration due to the prominent damage effect of the nanoparticles on the pore throat of the porous medium.The injection flow rate has a great influence on the initial stage of displacement?between 0 and 1.5 PV?and does not significantly affect the oil displacement efficiency of the foam after the foam of 1.5 PV was injected.The gas-liquid ratio has a great influence on the oil displacement efficiency,and there is an optimal gas-liquid ratio that makes the displacement efficiency reach the highest.The two-dimensional numerical simulation program was used to study the distribution characteristics of oil saturation in homogeneous cores during nanoparticle stabilized foam flooding.The simulation results are similar to those of laboratory visualization experiments,indicating that the two-dimensional numerical simulation program developed in this paper can be used to study the characteristics of oil saturation distribution in foam flooding process for homogeneous cores.