Study On The Formation And Distribution Characteristics Of Microscopic Remaining Oil In Ultra-high Water Cut Stage

The Yangsanmu Oilfield is located in the 2nd structural zone of the Yangsanmu Buried Hill in the Huanghua Depression.After 40 years of development by the Guantao Formation reservoir,it has entered an ultra-high water-cut period,with highly-dispersed distribution of macroscopic remaining oil,and obscure microscopic occurrence and distribution law.The understanding of the formation and distribution of microscopic remaining oil established in the middle and low water-cut stage can no longer effectively guide the efficient mining in the ultra-high water-cut stage.This is not only a problem faced by Yangsanmu Oilfield,but also a universal problem faced by old oilfields in Chinese eastern area.With the data of the newly-drilled closed coring well Yang11-16-1 in Yangsanmu Oilfield,and methods like core analysis and description,casting thin section analysis,and mercury intrusion test,reservoir characteristics research is conducted and reservoir pore structure types are classified in this paper.According to the porosity and permeability of core samples,the guantao formation in Yangsanmu Oilfield is a high-porosity and high-permeability reservoir,among which there are four typical pore structures:medium-pore coarse throat,small-pore fine throat,medium-pore coarse throat and middle-pore coarse throat.Based on this,different types of reservoir samples are selected for microscopic remaining oil simulation experiments.Experimental research found that:1)The formation of the remaining oil is mainly controlled by five effects-peeling,bypass and fingering,blocking,penetrating,and fluid adherence to the wall.2)There are three different types(five patterns)of microscopic remaining oil distribution in ultra-high water cut period.One is the discrete type-dispersed oil droplet-like oil,the other is semi-bound type-corner-like and pore-wall oil film-like oil,and the third is bound type-throat retention and patchy oil.3)Compared with water flooding,polymer flooding,binary displacement of polymer and surfactant and ASP flooding,the morphology of the microscopic residual oil changes:the discrete microscopic residual oil gradually develops towards small and circular direction.The micro-residual oil of semi-bound type varies from convex to concave and thin.The displacement efficiency is improved by changing from continuous and full distribution to non-full intermittent distribution.4)The displacement medium can effectively displace the remaining oil.The ASP flooding containing alkali,polymer and surfactant has the best effect,which can reach 23.79%;the second is the binary displacement of polymer and surfactant,with displacement effect of 19.04%;the last is polymer flooding,and its displacement effect is 3.31%.5)The displacing effect of the three kinds of displacing media on the bound type patchy microscopic remaining oil is good on the whole,while that on the semi-bound type microscopic remaining oil is poor on the whole.The relative saturation content of specular microscopic remaining oil in each displacement sample is above 55%,indicating that this type of microscopic remaining oil is still the main potential target in the future.6)By comparing the same displacement scheme of different models,it can be concluded that the better the pore structure and permeability of the guantao formation reservoir,the better the displacement effect.The above understanding provides a basis for the economic and efficient development of Yangsanmu Oilfield and similar oilfields in the ultra-high water-cut period.