Research On Microcosmic Characteristics And Quality Evolution In Low Permeability Sandstone Reservoir

With the technique of High pressure Hg injection, of rate-controlled Hg penetration, of NMR, of X-CT scanned image, of ESEM, of blue epoxy resin-impregnated thin sections, of water driving experiment, of geoanalysis, the aim is focused on studying the properties of reservoir's micropore structure, the characteristics of micro-fractures, the characteristics of microscopic water displacement, and the evolution of low-permeability sandstone reservoir's quality in several representative areas in Yanchang Formation of Ordos Basin. The conclusions are in the following:(1)The factors that affect physical properties are complicated. The variety of pores and throats are the main reasons that the permeability is low. The sorting. coefficient of pore throat, whether it is high or low, is detrimental to the fluid flowing in the reservoir. The reservoir quality is controlled by the throats.(2)The parameters of mobile fluid saturation, of mobile fluid's porosity, of initial oil saturation, of mobile oil saturation, of mobile oil's percent, of oil-displacement efficiency are low and strongly heterogeneous. The lower the permeability is, the larger the attenuation rate of those parameters is. They are mainly affected by the reservoir's micropore structure. Some neglected factors in medium-high permeability reservoir research must be emphasized in low permeability one.(3)The distribution of micro-fractures is flaser. The main direction of micro-fractures is the same as that of tectonic fractures. The quantitative research of micro-fracture's parameters indicates that the micro-fracture is the main factor which improves low permeable reservoir's permeability. The existence of relatively high permeability reservoir lies in the difference and heterogeneity of the distribution in low permeability reservoir.(4)Jamin effect, stress sensitivity, reservoir sensitivity damage, and spontaneous imbibition are strong, the proportion of thin pore-throat is higher in the development of low permeability oil fields. Lowing Jamin effect is an effective way to develop low permeability reservoir highly. If the velocity of water driving is near to that of spontaneous imbibition, the development efficiency will be high.(5)Controlled by sedimentation and diagenesis, the proportion of intergranular pore and of secondary dissolved pores are great in relatively good reservoirs. While the proportion of secondary dissolved pores and of micropores are high in relatively bad reservoirs. Statistical analysis in rock thin-section and the porosity evolution show that the cementation and dissolution are intensive in Yan 25 area, the compaction, dissolution and metasomatism are intensive in Zhuang 40 area. Sorting coefficient is the main factor affecting error, and the overlapping of the diagenetic stages is also one factor.(6)The cementation, in a sense, improves reservoir's resistance to compacting, also provides the material foundation for secondary solution effect. However, the cement occupies some sandstone pores. When the cement is calculated, the dissolved cement should be included.(7)Diagenesis is the essential controlling factors of reservoir quality evolution, and the type is secondary low permeability reservoir(or low permeability reservoir of diagenesis) in Yan 25 and Zhuang 40 areas. There is the type of the compaction and cementation in Yan 25 area, the type of compaction and metasomatism in Zhuang 40 area. Based on the tectonics framework and sedimentary environments, the diagenesis and its differences are the essential geological agencies that control the reservoir quality. Because different reservoir rocks are in different sedimentary microfacies, their primary texture, mineral composition and rock pore structure are also different. Although different primary texture and mineral composition had experienced the same diagenetic histories, the same diagenesises function differently on pore evolution in different areas. Therefore, there are different porosity evolution histories, which are embodied the discrepancies of pore structures and reservoir properties.