Study On The Characteristics And Influence Factors Of Spontaneous Imbibition In The Fractured Reservoirs

Fractured reservoirs are widely distributed around the world.Spontaneous imbibition is the significant mechanism for oil recovery from matrix system in the fractured reservoirs.Understanding of the mechanism of imbibition and the effect of diverse influence factors on imbibition are of essential importance in designing the development projects and predicting the reservoirs production performances.The conventional study on the imbibition is mainly focused on the boundary condition when fracture system is completely filled with water.However,the boundary conditions of matrix blocks are complex and diverse in reservoir condition.In this paper,based on the characteristics of water-flooding in fracture system under reservoir condition,the boundary conditions of the matrix blocks are classified.On the basis of the petrophysics and percolation theory,the characteristics of imbibition and the effect of diverse factors on the imbibition are studied by use of numerical,experimental and analytical methods.The numerical models in core scale are developed for different boundary conditions.The pressure profiles and saturation distribution are studied.In addition,the characteristics of imbibition recovery curves are studied as well.The effect of core length and fluid viscosity on the oil recovery curves is discussed.Then,the viscosity term of the scaling equation is modified by combination of numerical and analytical methods.It is verified that the correlation of oil recovery curves is improved by use of the modified scaling equation.On the basis of hypothesis of piston-like displacement,the mathematical models are developed for different boundary conditions.The reliability of the piston-like models is verified by numerical and experimental methods.The mechanism of the asymmetry of oil production from each end face of the core with TEO boundary condition is discussed by use of the mathematical model.In addition,the effect of gravity on the spontaneous imbibition is discussed as well.The experimental apparatus of spontaneous imbibition for different boundary conditions are designed.The effect of fluid viscosity on final oil recovery and oil recovery curves is discussed for porous media with different pore structure.In addition,the mechanism of entrapment of non-wetting phase in porous media with different pore structure is discussed as well.By matching the theoretical results calculated by the piston-like models with experimental results,the effect of fluid viscosity on the effective relative permeability is discussed.The 2D numerical models in pore scale are developed and the boundary conditions were classified based on the open face of the core in contact with water completely or not.The characteristics of saturation distribution and oil recovery curves are discussed for various boundary conditions.For the open surface completely covered by water boundary condition,the characteristic length of the scaling equation is modified by combination of the piston-like model and characteristics of this shape of the oil recovery curves.For the open surface partly covered by water boundary condition,the main influence factors of spontaneous imbibition are discussed and the calculation method of the characteristic length is developed as well.