Study On The Flow Ability Of Remaining Oil In Extra-high Water Cut Stage

At present,the vast majority of onshore oilfields in China have entered an extra-high water cut stage.At this time,the flow of injected water in reservoir occupies the main position of fluid flow in reservoir.Remaining oil participates in reservoir fluid migration in the form of highly dispersed phase.Therefore,it is of great practical significance to study the flow ability of residual oil in ultra-high water cut period.Based on the results of physical simulation experiments and theoretical analysis,this paper proposes that the flow characteristics of underground oil migration can be characterized by a threshold pressure when the reservoir is in an extra-high water cut stage.The concept of threshold pressure for oil phase flow is proposed.For the reservoirs of ultra-high water cut stage,there is a certain corresponding relationship between oil saturation at each point in the reservoir and threshold pressure gradient of oil phase flow.The threshold pressure gradient of oil phase flow was measured accurately by "differential pressure-flow" method,and the non-linear percolation characteristics of reservoir were analyzed in depth.Through the design of a number of control experiments,it is proved that the threshold pressure gradient of oil phase flow in the oil saturation-threshold pressure gradient curve is mainly affected by the permeability of artificial cores and the pore structure inside cores.In addition,the influence of crude oil viscosity and salinity of injected water on the threshold pressure gradient is small.Both of them mainly affect the initial oil saturation and remaining oil saturation of reservoirs.The relationship between the threshold pressure gradient of oil phase flow and core permeability and crude oil viscosity is deduced.Finally,two two-dimensional oil-water two-phase flow reservoir numerical models considering the threshold pressure gradient term of oil-phase flow are established for the first time.The model is compared with the original model.When the reservoir is in the initial stage of waterflooding development,the difference caused by the threshold pressure gradient of oil phase flow can not be considered.But in the ultra-high water cut stage.The smaller the residual oil saturation is,the more significant the effect of threshold pressure gradient on oil phase flow is.The residual oil saturation of the threshold pressure gradient model considering the nonlinear oil phase flow is obviously higher than that of the first two models.It plays an important guiding role in further improving the utilization of residual oil in ultra-high water cut stage.