Research On Microscopic Pore Structure And Imbibition Mechanism Of Tight Sandstone Reservoir

Recently,the majority of tight sandstone reservoirs are exoploited by water flood,which is a traditional method in oil field development.Due to the distinct difference between the permeability of fractures and matrix,quantity of oil is left in rock marix after the water displacement.It is testified that capillary force plays an important role in tight sandstone reservoirs development.Making good use of spontaneous imbibition to enhance the residual oil recovery of rock matrix is of great importance for the oil recovery enhancement in tight sandstone reservoir.Therefore,research on the law of spontaneous imbibition that take the micro pore structure of tight sandstone formation into consideration,is significant for the study of mechanism of oil displacement in tight sandstone reservoir.In this paper,tight sandstone reservoir in Wuqi area of ordos basin is taken as the research target.Testing methods of SEM,casting thinsections,low-temperature nitrogen adsorption,mercury injection with high pressure and mercury injection with constant rate were used to deeplystudy the microstructure as well as mechanism of imbibition in tight sandstone reservoir.Based on the above knowledge,the distribution of wetting phase and non-wetting phase in different sized pores is analyzed by NMR method.The effect of the microscopic pore structure of tight sandstone on imbibition is mainly studied.Based on fractal characteristics of tight sandstone reservoir microstructure,considering the change of effective stress,fractal criterion of imbibition mechanism model is established.The main results of the paper are summarized as follows:(1)The reservoiris mainly medium displacement pressure-micro-fine throat in wuqi area,the total pore volume is relatively large.Pore types are mainly intergranular pores and feldspar dissolved pores,the open flat shape slit pore developed around reservoir,middle pore and megalospore contained at the same time.The main throat type is the neck shape and flake or curved flake throat,both throat radius distribution range and the total number of throats are relatively large.Microscopic pore structure connectivity is better,contributing to oil and gas migration and imbibition oil recovery.(2)Imbibition process is affected by the core length and boundary condition,wettability,interfacial tension,initial water saturation,oil-water viscosity ratio,and many other factors,The optimal displacement speed of dynamic imbibition is 0.06 m L/min,capillary force and viscous force play a role maximum in imbibition recovery efficiency under the speed;(3)The effect that microscopic pore structure on imbibition cannot ignore,not the larger proportion of small pore is,the imbibition effect is better.Water drive gas spontaneous imbibition recovery is mainly controlled by the small pore,and for water flooding spontaneous imbibition and dynamic imbibition,mesoporous proportion plays a decisive role in imbibition recovery efficiency.The experimental results show that the recovery rate of medium pores is much bigger than that of other pores,medium pores play a main role in the process of spontaneous imbibition.Because of the complexity of porous media,the correlation between porosity and recovery efficiency is not very good,while permeability is good correlation with recovery efficiency: a better reservoir quality leads to a larger recovery efficiency.Average pore throat ratio and specific surface area of tight sandstone show a negative correlation with the recovery efficiency: the porous media with a larger surface area,also has a greater distribution of small and medium throat distribution,the larger pore throat ratio does harm to the imbibition of wetting phase as well as the discharge of the non-wetting phase.A larger medium pore proportion leads to a higher recovery efficiency,while a larger mesoporous proportion is unfavorable,because of the viscous force which has a negative influence on spontaneous imbibition.(4)The three parameters including maximum throat radius,fractal dimension,fractal dimension for tortuosity are most important in fractal criterion of imbibition mechanism model.With maximum throat radius and fractal dimension increasing,imbibition mechanism has changed to the gravitational differentiation dominant.