The Characteristics Of Pore Structure And Influencing Factors Of Fluid Mobility In Tight Sandstone Reservoirs

As one of the largest oil-gas accumulation basins in China,the ordos basin has received extensive attention in its strategic position.The Chang 6 and Chang 8 member of Yanchang formation in Huaqing area of southwest basin which is an important field for exploration and development have been paid special attention by researchers.The study on pore structure and pore fluid is of great theoretical significance in improving the efficiency and economic of development of reservoirs.However,there are few studies on pore throat structure and characteristics of pore fluid distribution,seepage and displacement front in this area.In this study,typical diagenetic minerals were identified from the aspects of basic sedimentary features,lithologic composition and physical properties of the basin,and on this basis,diagenetic stages were divided and pore evolution was calculated.Meanwhile,qualitative methods such as casting thin section and scanning electron microscope were used to identify the main pore-throat types,and methods such as high-pressure mercury injection,constant speed mercury injection and isothermal adsorption were used to comprehensively characterize the micro-pore structure of the reservoir and explore the coupling relationship between structure and physical properties.On the basis of microscopic pore structure characteristics,oil-water phase permeability,nuclear magnetic resonance and permeability experiments was conducted to study the characteristics of seepage and movable fluid occurrence of the reservoir,and the coupling relationship between permeability efficiency and pore throat structure was discussed.Finally,the characteristics and laws of water flooding were analyzed and the comprehensive evaluation of each layer was carried out.The understandings obtained from this study are as follows:?1?The average porosity and permeability of Chang 6 layer in Huaqing area is 8.77%and0.271 10^-3?m² respectively.The average porosity of Chang 8 layer is 9.02%and the average permeability is 0.664 10^-3?m².The storage and seepage capacity of Chang 8 layer are better than Chang 6 layer.?2?The anti-compaction ability of Chang 6 layer in Huaqing area is poor,and the characteristics of pore reduction caused by compaction are obvious.The porosity reduction caused by cement in Chang 8 layer is obvious in early stage of diagenesis and in the late stage of diagenesis in Chang 6 layer.The dissolved pores in both layers play an active role in improving the physical properties of tight sandstone reservoirs.?3?The radius and the heterogeneity characteristic of throat are closely related to the seepage ability of the reservoir.The stronger the heterogeneity of throat is,the better the seepage ability of the reservoir is.The seepage ability Chang 8 layer is better than Chang 6layer because of a better pore throat configuration in Chang 8 layer.?4?The reservoir with concentrated micro-capillary developed is weak in percolation ability.The seepage capacity increases with the increase of pore throat radius when reservoirs with uniform micro-capillary development,The development of intergranular pores and a better pore-throat configuration result in a better storage and seepage capacity in Chang 8 layer,and the fluid mobility of reservoir is better than Chang 6 layer.?5?The efficiency of early stage was higher than that of late stage.A higher content of hydrophilic clay mineral in Chang 6 layer improved the permeability of the reservoir.The development of larger pores in Chang 8 layer can produce higher displacement efficiency in the early stage,and the better connection between the micro throat and larger radius pores can improve the final recovery rate of infiltration displacement in Chang 8 layer.?6?The displacement efficiency of Chang 8 layer is generally higher than that in Chang 6layer.Continuous water injection after water breakthrough can effectively increase the oil displacement efficiency of reservoir in Chang 8 layer.The poor pore-throat configuration in Chang 6 layer makes it easier for water phase to form an advantageous channel and the improvement of oil displacement efficiency is limited after water breakthrough.?7?The reservoir property of Chang 6 and Chang 8 layer in Huaqing area gradually becomes poor from type?to?type reservoir.However,the larger pore-throat radius and a better pore-throat configuration make the reservoir of the same type in Chang 8 layer have a better reservoir quality.Besides type?,the rest type of reservoir has a strong micro heterogeneity and continuous injection after water injection has limited effect on improving oil displacement efficiency and sweep area.Except for?type reservoir,other types can still maintain certain oil displacement efficiency after water breakthrough.The displacement efficiency and stable production cycle are obviously controlled by the type of reservoir.