Study On The Law And Characteristics Of Imbibition In Low Permeability Sandstone Reservoir

Fracturing is one of the key technologies to improve oil recovery in low permeability reservoir.After fracturing,reservoir structure is changed from single pore medium system to fracture-pore dual medium system.Water enter into matrix from fracture by capillary pressure,and oil in matrix was displaced.Therefore,it is of great significance to exert the imbibition effect of retention fracturing fluid on the basis of reducing the retention damage.The distribution of oil and water in the process of imbibition and factors influencing oil recovery were studied through model derivation and laboratory experiment.Firstly,some simple and complex capillary models were built,and then imbibition mechanism and influence law was analyzed through calculation and simulation.Subsequently,the influence of interfacial tension,permeability,oil viscosity and salinity on imbibition were researched through spontaneous imbibition experiment.Finally,combined with advanced nuclear magnetic resonance core analysis technology,the distribution characteristics and variation law of fluid in the process of imbibition and displacement were studied.The differences and relations between static imbibition and dynamic displacement was also analyzed.Several results are obtained.Firstly,imbibition process is mainly water-oil displacement in small pore.Imbibition velocity increases with decreasing capillary radius and increasing interfacial tension.Secondly,there is a best interfacial tension which making oil recovery maximum.With the increase of permeability and salinity,the decrease of oil viscosity,oil recovery increases.Thirdly,T₂ relaxation time spectrum displayed bimodal distribution,and left peak was significantly higher than the right peak.Under the effect of imbibition,oil in small pores provided a major contribution to recovery,which accounted for 63%.The oil recovery increases with the increase of time,but imbibition velocity gradually decrease.Fourthly,small pores produce more oil than medium and large pores in static imbibition,however,dynamic displacement characteristic is related to the pore structure of rock.Displacement is a faster process compared with imbibition,and displacement effect is better.However,the contribution of imbibition to displacement recovery is so high as 40%that imbibition can't be ignored.