Research Of Microscopic Pore Structure And Reservoir Evaluation Of Dingbian Chang 7 Tight Reservoir In Yanchang Oilfield

There are abundant tight oil resources and the main layers are Chang 6,Chang 7,Chang8 and Chang 9 of Triassic Yanchang Formation in Ordos Basin.The exploration and development of Chang 7 is relatively late,and the dynamic and static datas of the oilfield are few.The analysis and research on all aspects of Chang 7 tight oil reservoir is relatively weak,which hinders the exploration and development process of the oilfield and affects the development efficiency.Taking Chang 7 reservoir of Triassic Yanchang Formation in Dingbian area of Northwest Ordos Basin as an example,on the basis of predecessors'research,this paper carries out core observation,physical properties,grain size image,casting thin slice,scanning electron microscope,X-ray diffraction,CT scanning,high pressure mercury injection,constant velocity mercury injection,oil-water relative permeability,nuclear magnetic resonance and so on.The author makes the comprehensive study of sedimentary characteristics,petrological characteristics,physical properties,micro pore structure,oil-water seepage characteristics,occurrence characteristics of movable fluid of Chang 7 tight sandstone reservoir in Dingbian,and confirms the lower limit of reservoir physical properties and reservoir controlling factors and the reservoir comprehensive evaluation.Based on the research we get the following main achievements:?1?Delta and lacustrine facies deposits are developed in the study area of dense oil in Dingbian Chang 7.The main facies are subfacies subaqueous distributary channel and interdistributary bay microfacies in the front of the delta,and lacustrine mud microfacies in the lower part of Chang 7 are relatively developed.The lithology of the reservoir is mainly grey black,gray and gray white feldspar sandstone,followed by lithic feldspar sandstone.The main pore of reservoir is feldspar dissolved pore and intergranular pore,followed by lithic dissolved pore,and a few intercrystalline pore and micro-crack.At the same time,the type of throat is mainly lamellar or curved lamellar,followed by tube bundle and constricted larynx.?2?The capillary pressure curve is classified into four types according to the pressure of high pressure mercury injection drive less than 1 MPa,between 1¹.5 MPa,between 1.5².5 MPa,more than 2.5 MPa.Class?for low displacement pressure-micro-micro throat type,?type for low-middle displacement pressure-micro throat type,?type for middle displacement pressure-micro throat type,?type for middle-high displacement pressure-nano-suction throat type.The matching relationship between capillary pressure curve of constant velocity mercury injection and pore,throat and total mercury saturation,the curve pattern is divided into pore,pore-throat,throat zone.When mercury enters a porous throat with poor connectivity or disconnection,the capillary pressure rises,and the mercury intake is mainly restricted by the throat and micro pore.The amount of mercury intake into the throat determines mercury intake of the total pore throat and the study of the throat is very important for the tight reservoir.Empirical statistics,the relationship between saturation and porosity,and the minimum flowing pore throat radius method are used to determine the lower limit of physical utilization of tight reservoirs in the study area.The minimum porosity is 6.5%,and the minimum permeability is 0.04 mD.?3?According to the evaluation criteria of movable fluid saturation,class III and class V reservoirs are dominated.The range of cut-off value of T₂ is 1.70ms¹3.67ms,with the average of 5.78ms,and the low of mobile fluid saturation,with an average of 33.04%.Before and after centrifugation,the shape of T₂ spectrum curve was mainly double peak,and after centrifugation,the shape of T₂ spectrum curve was mainly distributed in the left side of the cut-off value of T₂.?4?Based on the theory of NMR,the corresponding relationship between T₂ value of NMR and pore throat radius is deduced,logarithmic linearization is taken,the constant term value of power function is solved by the least square method,and a new method of transforming T₂ spectrum of NMR into pore throat radius is established.The results are verified by extending the NMR data of tight reservoirs and the pore throat radius data of constant velocity mercury injection.The feasibility and practicability of the established method for studying the micro-pore structure of tight reservoirs are good.?5?According to the morphological characteristics of oil-water relative permeability curve,it can be divided into three types,class?for Kro uphill concave-Krw downhill type,class?for Kro uphill concave-Krw straight line type,and class?for Kro-Krw uphill concave type.The ultimate oil displacement efficiency of type I is 48.44%,and the development effect is good.The ultimate oil displacement efficiency of type II is 43.19%,and the development effect is slightly better.The ultimate oil displacement efficiency of type III is36.26%,and the development effect is poor.?6?The"eight-element comprehensive classification coefficient"method of tight oil in the study area is put forward.At the same time,F_eci>8 in type I reservoir,3<F_eci?8 in type II reservoir,-2<F_eci?3 in type III reservoir and F_eci?-2 in type IV reservoir are proposed.According to the actual data of the study area,the evaluation method of tight reservoir is validated.The result is reliable and the established evaluation method of tight reservoir has certain practicability.A comprehensive classification and evaluation map of tight oil reservoirs in block Y has been drawn by combining quantitative analysis of control area,distribution law of sedimentary facies and qualitative analysis of control characteristics.The main reservoir is the third type of tight oil reservoir.