Microscopic Characteristics Of Pore Structure And Seepage Of Yanchang Formation In Wangpanshan Area In Ordos Basin

The reservoirs with low permeability(extra-low permeability)account for a large proportion in the petroleum industry in China.Compared with conventional oil and gas reservoir,the degree of geological understanding and exploration of such reservoirs need to be improved.It's necessary to improve the technology of remaining oil recovery and find new methods for oil exploration and development.The research layer,Chang 4+5 and Chang 6 layer in Ordos Basin,which is also the main production zone of Changqing Oilfield have faced several problems during development including high rising in water cut,high degree of complexity of oil-water relationship and large number of wells with low production and so on The reasons for those problems are the weak knowledge of reservoir characteristics which includes the physical property,diagenesis,pore throat structure,distribution of movable fluid,seepage mechanism and oil displacement efficiency and so on.Therefore,several experiments such as core observation,physical property testing,casting thin sections,scanning electron microscopy,particle size analysis,X-ray diffraction have been conducted to make a comprehensive study of physical property and characteristics of diagenesis and its facies Constant pressure mercury intrusion and constant rate mercury intrusion have been used to reveal the characteristics of pore structure including types,radius and its distribution.Then,experiments including nuclear magnetic resonance,relative permeability and oil displacement in real sandstone have been carried out to quantify the characteristics of microscopic seepage Finally,the principles of reservoir classification have been established and the production performance of each type of reservoir have been analyzed.The understandings obtained from research are as follows(1)The structure of Chang 4+5 and Chang 6 layers in Wangpanshan Area is stable and simple,and the inheritance of each layer is good.The main type of rock is feldspathic sandstone and lithic feldspathic sandstone.The average porosity of Chang 4+5 is 10.65%and the average permeability is 0.80×10-3?m2.The average porosity and permeability of Chang 6 is 10.86%and 0.91×10-3?m respectively.The value of permeability indicates both layers belong to low to extra-low permeability reservoirs.The relationship between physical property and main detrital component is better in Chang 6 layer(2)The diagenesis types are similar in both layers with a minor difference in cementation degree.The porosity reduction caused by compaction and cementation is about 70%Diagenetic facies can be divided into 6 types which are kaolinite chlorite cementation intergranular pore facies,kaolinite cementation solution-intergranular pore facies,chlorite-kaolinite cementation dissolution pore facies,kaolinite cementation dissolution pore facies,illite cementation dissolved pore facies and carbonate cementation tight facies.The quality of reservoir gets worse as facies changes and the type of diagenesis facies can be distinguished through combination of difference of electric logging curve characteristics and logging intersection map(3)The constant rate mercury intrusion can be used to get the information of size,numbers and distribution of pore throat with radius>0.1 ?m while constant pressure mercury intrusion can gain information of pore throat with radius>3nm.The joint of data of two mercury intrusion methods can characterize the pore throat structure precisely.Average pore radius and mainstream throat radius have the most significant impact on reservoir quality(4)The reservoir in Chang 4+5 layer is mainly consist of type ?(50%)and the average movable fluid saturation is 39.37%.The type of reservoir in Chang 6 layer is mainly consist of type ?(50%)and the average movable fluid saturation is 51.37%.The value of T2 cutoff is in the range of 3.86ms-16.68ms.Reservoir quality and movable fluid saturation get worse as the area lies in the left of T2 cutoff gets larger.The impact of types and occurrence characteristics of clay minerals on fluid mobility in micropores is significantly different in Chang 4+5 and Chang 6 layers respectively and the impact of chlorite is stronger than other clay minerals in Chang 6 layers.Movable fluid saturation is affected by multi-factors including physical property,pore types,pore structure and interstitial content and is a key index for reservoir classification and evaluation(5)The reservoir with low saturation of bound water and residual oil and large area of two-phase co-seepage has a higher movable fluid saturation.The intergranular pore developed well and the facial porosity is higher.Pore connectivity and sorting in this reservoir is better Homogeneous and reticular displacement are common in the reservoir,and the oil displacement efficiency is 46%.The heterogeneity of pore throat structure is an important factor affecting the oil displacement efficiency(6)Eight sensitivity parameters including porosity,permeability,clay mineral content,average pore radius,mainstream throat radius,start-up pressure gradient,movable fluid saturation and oil displacement efficiency were coupled to establish reservoir classification and evaluation.The movable fluid saturation of typeIreservoir in Chang 4+5 and Chang 6 is high.The contribution capacity of this type is large and the distribution area is small.The distribution of ?,? reservoir is large and the recovery degree is low.The focus of development at high water-cut stage should be put on these types of reservoir.Reservoir with type IV has a poor physical property results in a high cost in development.