Formation Mechanism Of Remaining Oil For Fractured-Vuggy Carbonate Reservoirs Considering Filling Medium

Research of remaining oil formation mechanism in fractured-vuggy carbonate reservoirs is the important foundation for enhancing oil recovery.This thesis first analyzed connection form and filling characteristics by literature research and field outcrop observation.Based on similarity criterion,the filling vug model,blind-end model and fractured-vuggy network model of 2D and 3D were built.Then water injection experiment was conducted to study the two-phase flow regularity,remaining oil distribution and forming mechanism using those physical models.Using CHNSD model and BJS conditions based on Phase-field method and physical properties,we built numerical model and analyzed the fluid exchange and velocity field by numerical simulations.Combined the physical and numerical simulations,we derived four dimensionless numbers to analyze the oil-water flow mechanism and critical conditions in fractured-vuggy media.The results showed that CHNSD model could welly simulate the two-phase flow in fractured-vuggy media and has a good agreement with physical simulation results.For water injection in filling vugs,pressure difference between injection and production is very low due to free flow in unfilled area,so the gravity differentiation is the main effect for oil production.The residual oil and the shielded oil are main kinds of remaining oil in filling medium.Injection pressure also has low influence on oil-water substitution in filled vugs,so gravity and capillary force are the main acting force on the coupling interface.The oil-water substitution number(Cr)can describe the relationship between the gravity and capillary force,Cr is applicable in filled vugs and the critical number is 1.With stronger water-wettability,higher permeability,lower interfacial tension,higher oil recovery and better substitution can be reached.So the surfactant solution injection can increase the oil recovery in oil-wet filling medium.For oil-water substitution in pore or fracture for blind-end vug,gravity and capillary force are the main acting force.Blind-end residual oil is the main remaining oil in blind-end vugs,and there are critical conditions for substitution.The critical substitution number is Cr=1.With larger pore size(or fracture aperture),higher difference of density,lower oil-water interfacial tension,higher Cr and substitution velocity could be reached.Convective flow is the only way for oil-water substitution in pore connected with blind-end vug,and the wettability can only decide the fluid position rather than the substitution results.Fracture has more space for oil-water substitution and higher substitution velocity can be reached for water-wet surface.The critical size for pore and fracture is about1cm and 1mm,the capillary force can be neglected above this size and water-oil can substitute freely under gravity.For 2D fractured-vuggy network,inter-well connectivity decides the main flow direction and remaining oil between wells.Fractured-vuggy connectivity on micro scal determines the by-pass oil.Based on equivalent permeability,plane flooding balance number(C_h)can describe the dimensionless water breakthrough time considering well spacing difference.Water breakthrough time is similar when C_h is around 1,water logging can be prevented and low remaining oil saturation can be got.Reasonable range of C_h in this research is about0.75~1.13.For 3D fractured-vuggy network,the fractured-vuggy connectivity,formation structure and fluid properties co-decide the flow velocity in different directions.Condidering the formation size,vertical flooding balance number(C_v)can describe the dimensionless water breakthrough time in horizon and vetical directions.When production well only connect to top layer,the C_v should be controled above 1 to make full use of gravitational differentiation incase of water logging on top layer,then higher sweep efficiency and oil recovery can be reached.