| The development and utilization of offshore oil and gas resources is important to ensure the oil supply in China.Air foam flooding based on gas flooding has the characteristics of water plugging without oil plugging,and effectively improves the recovery factor in the actual application of oilfield exploitation.However,the application of air foam flooding is restricted by the operating conditions of offshore platforms,and the offshore oil reservoirs are mostly heavy oil reservoirs.,Bubbles with stable size can be produced using the microfluidic technology for the preparation of foam and oil displacement.Besides,the microfluidic experimental platform can study the microscopic oil displacement mechanism.Therefore,the investigations of microbubble displacement process are significant for offshore oilfield exploitation.To study the process of microbubble flooding,the present work set up an experimental microfluidic platform to compare the difference between microbubble flooding and water flooding,and observed the flow law during microbubble flooding.Experimental measurements show that the effect of water flooding is less affected by the time factor,and the aqueous solution in the reservoir microfluidic chip can complete the displacement of paraffin oil in a very short time.The initial paraffin oil distribution conditions have a greater impact on oil displacement effect,and the channels with more residual water are more likely to be displaced.With the increase of the flow rate,the efficiency of water flooding has improved,but the shear force generated by the excessive flow prevents the paraffin oil from flowing out of the channel,which limits the further improvement of the water flooding efficiency.In the oil displacement process of microbubbles,two effects of microbubbles on the improvement of oil displacement were observed in the experiment:"oil washing" effect and plugging effect.The "washing oil" effect refers to the bubbles entering into the paraffin oil,reducing the viscosity of the paraffin oil,enhancing the fluidity of the paraffin oil,and occupying the gap between the paraffin oil and the channel,making it easier for the aqueous solution to displace the paraffin oil.The plugging effect is that the accumulation of bubbles to block the channels which have been displaced.The bubbles increase the flow resistance of the channels,and enhance the displacement of paraffin oil in the low permeability area.The experiment also compares the effects of microbubble flooding and water flooding,confirming that microbubbles have a better displacement effect than water flooding.The present work also investigated the oil displacement mechanism of bubbles and the flow behavior of bubbles using the open source computing platform OpenFOAM.A T-shaped geometric model was used to simulate the bubble generation process,and the results show that the bubble length decreases with increasing water phase flow rate and the bubble length can be scaled with flow rate as a power-law relationship.The bubble length varies linearly with gas pressure.The bubble generation period and distance decrease with the increasing of pressure,which indicates that the dryness of foam can be increased by the higher pressure of gas.To simulate the bubble flow at the bifurcation,a Y-shaped structure is used.Two flow behaviors of bubbles were obtained:breakup and non-breakup.The empirical correlation of the transition of two flow patterns is a power-law relationship.When the bubbles flow through the bifurcation structure,the pressure in the channel will increase because of the Jamin effect,which explains the plugging mechanism of bubble flooding.Using the geometric model of pore throat structure,bubble fusion phenomenon was observed at the structure that the dimensions are variable.When the bubbles flow through the pore-throat structure,eddy currents were generated which increase the scouring of the water solution to the wall surface,explaining the "oil washing" mechanism of the bubbles. |
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