Research On Imbibition Mechanism Of Tight Sandstone Reservoir

Different from the conventional sandstone reservoir,tight sandstone reservoir has poor reservoir property,small pore throat with complex structure,strong capillary force and imbibition phenomenon of fluid is very important in the development tight reservoir.The domestic and foreign research on the tight sandstone reservoirs are mostly concentrated in tight sandstone gas reservoir,the study on reservoir of tight sandstone reservoir is insufficient,but the same as gas reservoir,water blocking damage and how to improve the recovery of water flooding problems exsit in the development process of tight sandstone reservoir.Based on the previous study of the imbibition,with a tight sandstone oil reservoir in Ordos basin as the research object,combined porosity and permeability measurement,scanning electron microscopy,X-ray diffraction with other experimental methods,in this paper firstly the characteristics of tight sandstone reservoir are analyzed.On this basis,indoor experimental study of spontaneous imbibition in the tight sandstone is conducted,spontaneous imbibition model of tight sandstone is built,the influence factors of spontaneous imbibition of tight sandstone reservoir are analyzed and methods to raise the sandstone imbibition rate and recovery are advised.The main research contents and results are as follows:(1)Tight sandstone reservoir in study area has two characteristics: low porosity,low permeability,high clay content and high coefficient of heterogeneity.The seepage system has double pore structure,characterized by the matrix pores mainly with local characteristics of micro cracks.The main reservoir fluid are stored in the matrix pore complex structure,and the crack is the main transport channels for the fluid.According to the porosity and permeability diagram,we can see both poor correlation of two parameters and power-law relationship between permeability and reservoir quality index.(2)On the basis of theoretical research in the conventional sandstone spontaneous imbibition mechanism,the relationship between dimensionless imbibition time and permeability,fluid viscosity,interfacial tension,rock geometry and other factors are deduced.Through spontaneous imbibition experiments on tight sandstone,the spontaneous imbibition model of tight sandstone was established to describe the spontaneous imbibition law.Compared with conventional sandstone spontaneous imbibition law,relation curves of tight sandstone between imbibition recovery and dimensionless time are similar,but because of the complexity of the tight sandstone pore structure,for spontaneous imbibition model of tight sandstone in differentpermeability level,there are some differences in the parameters of the model.During the process of verifying the formula by experimental data,it is found that there is need to establish spontaneous imbibition curve model for tight sandstone cores of different permeability level,and the final curve has good normalization.By the dimensionless time,this curve can indirectly predict and evaluate imbibition oil production,oil recovery rate and recovery degree for any size of the actual reservoir,improving the application ability of imbibition experiment model and practical.(3)According to the particularity of the tight sandstone,gravimetric method is selected,static imbibition automatic measuring device is designed and built.The natural cores from tight oil reservoir is chosed and used in indoor static imbibition simulation experiment.The experimental results show that imbibition recovery of oil saturated cores is comprehensively influenced by factors such as permeability,interfacial tension,oil-water viscosity ratio,aging time,salinity and mineral content etc..Imbibition recovery of cores in research area is mostly more than 10%.(4)The tight sandstone imbibition process is controlled by capillary pressure,viscous force,inertia force,and gravity,and capillary pressure is the main power.Because of the complex pore structure and high tortuosity in tight sandstone,as spontaneous imbibition is a slow process,in order to improve the imbibition recovery and imbibition rate of tight sandstone reservoir,it is proposed to extend the imbibition time,properly decrease salinity,and add the surface active agent of suitable concentration.