Visual Description Of The Formation Process Of Viscous Fingering And Its Control Method

Water injection is an economical and effective secondary recovery technology in oil fields.However,the injected fluid will be channeling during the injection due to the influence of the longitudinal heterogeneity,formation thickness,and the properties of the reservoir fluid,which greatly reduces the final oil recovery.The fingering phenomenon,the areal channeling of injected fluid,caused by the unstable fluctuation of the force at the interface of the displacement front is common in the process of oil and gas field development.Determining the influencing factors,occurrence and development laws,and control methods of fingering is of great significance to improve the oil recovery and the swept volume of the injected fluid.In this paper,a large-scale glass etching microscopic model(5cm×3cm)is designed based on the real core pore structure.Combined with the displacement experiment of the sand filling model and plate core model,the influence of multiple factors on the occurrence and development of fingering was clarified,and the control method of fingering was explored.The displacement experiments of 5 kinds of displaced phase viscosity and 5 kinds of injection velocity were carried out by using the large size micromodel.The fingering coefficient is established by using the dimensionless length and sweep area of the displacement process,which can be used to characterize the whole fingering process.The suppression effects of pressure pulse frequency,pulse amplitude,and median injection velocity on viscous fingering were explored and optimized.Finally,the parameters obtained from the microfluidic experiment were upgraded to the mesoscopic sand filling model and the large plate model based on the injection velocity,then,the effects of three injection methods(viscosity increasing,velocity decreasing,and pressure pulse)in improving the finger progression and expanding the scope were verified.The results show that the two-phase viscosity ratio and injection velocity directly affect the morphology of the displacement front.Increasing the viscosity of the displacement phase and decreasing injection velocity can delay the breakthrough time of the injected fluid,extend the swept volume,and make the viscous fingering change to the capillary fingering.Although increasing injection velocity will aggravate the phenomenon of viscous fingering,the final spread area will be enlarged.The displacement front can be quantitatively divided into four categories according to the morphology and fractal dimension of displacement front: viscous fingering,crossover zone,capillary fingering,and stable displacement.The fingering coefficient curve is,in which the higher value of a indicates the lower final recovery,the smaller value of b shows the swept volume will be more significantly extended after the injected fluid breakthrough,and the larger initial endpoint value of the curve reveals the more severe fingering.Pressure pulse injection can improve viscous fingering,and the best improvement can be achieved by increasing the pulse frequency,median velocity,and moderate amplitude.Increasing displacement phase viscosity,decreasing injection velocity and pressure pulse injection can change the displacement front from viscous fingering into capillary fingering at mesoscopic and macroscopic scales,improving the swept volume.Compared with continuous water flooding,polymer flooding and pressure pulse injection can improve oil recovery by 19.39% and 7.23%,respectively.