Microscopic Pore-throat Structure And Seepage Characteristics Of Chang 3 Ultra-low Permeability Reservoir In Triassic Yanchang Formation In W Oilfield

The Chang 3 reservoir in Weibei Oilfield,Ordos Basin,is a low-porosity,ultra-low-permeability-ultra-low permeability reservoir.Compared with conventional reservoirs,the reservoirs in the study area have lower porosity and permeability,fine pores,tight pore throat structure,and better seepage characteristics.complex.In the development process,there are problems such as unclear understanding of seepage mechanism,unclear relationship between microscopic pore throat structure and fluid production degree,and insufficient understanding of the increase in oil recovery by imbibition and the law of starting pressure gradient with permeability.In response to the above problems,various experimental tests were carried out to study the petrological characteristics,microscopic pore-throat structure characteristics and seepage characteristics of the reservoir,and to clarify the seepage characteristics and influencing factors of the reservoir in the study area.The following understandings are mainly obtained through research:(1)The reservoirs in the study area are developed with feldspar lithic quartz sandstone and feldspar sandstone.The sandstone has the highest content of quartz in the clastic components,followed by feldspar,and the content of cuttings and interstitials is less;calcite and clay minerals are the main fillings.Types of pores,chlorite and kaolinite are more developed in clay minerals.The average porosity of the reservoir is 10.4%,and the average permeability is 0.67×10^-3?m².It is a low-porosity,ultra-low-permeability reservoir.(2)The reservoirs in the study area mainly developed intergranular dissolved pores and flaky and curved throats.The pore throat size is mainly small pores and fine throats.The pore throat structure in the study area can be divided into three types,? to ? pore throats.The structure gradually deteriorated;(3)The main types of water drive in the pore network of Chang 3 reservoir in W Oilfield are uniform,network-uniform,network and finger-network,etc.Piston-type water drive is the main channel in the pores;the pore throat types are different The microscopic water drive seepage characteristics are different,and the final oil displacement efficiency is quite different;the type ? to type ? displacement type gradually changes from uniform to finger-network type,and the oil displacement efficiency gradually decreases;physical properties,pore throat structure,and displacement Displacement pressure and injection multiple have certain effects on oil displacement efficiency;(4)The irreducible water saturation of the reservoir in the study area is 26.4-55.0%,the isotonic point saturation is between 46.3% and 67.3%,the main body is greater than 50%,and the reservoir wettability is weak water wetness;the residual oil saturation is 24.2%?38.4%;the relative permeability curve is divided into three categories,from the curve ? to ? as a whole shift to the right,the irreducible water saturation,isotonic point saturation and residual oil saturation gradually increase,and The seepage capacity of the reservoir gradually weakens;(5)With the change of imbibition time,the overall imbibition rate shows a decreasing trend.The smaller the physical property,the higher the oil recovery efficiency of reservoir imbibition.The existence of cracks improves the oil imbibition and displacement efficiency to a certain extent;the same porosity Different media and pore structure have different imbibition efficiency.Type ? reservoirs have the highest imbibition oil displacement efficiency,with an average of 33.1%;Type ? reservoirs have the lowest imbibition oil displacement efficiency,with an average of 19.3%.(6)The starting pressure gradient of the reservoirs in the study area is mainly distributed in0.04-0.46MPa/m,and the average starting pressure is 0.087MPa/m.The starting pressure gradient changes exponentially as the permeability decreases.The relationship between the permeability and the starting pressure gradient is: y=0.0964 x-0.973;changes in reservoir physical properties,pore structure and effective stress all have a certain impact on the threshold pressure gradient.