The Mechanism Research of Imbibition in Tight Oil Reservoir

Author:Shen An Zuo

Supervisor:liu yi kun


Degree Year:2017





The conventional oil field has been put into development for many years in our country,remaining recoverable reserves is limited,with the further development of oil and gas fields,the unconventional oil and gas resources have been paid more attentions.Under the inspiration of the successful development of tight oil and gas in the United States,tight oil has been became a typical unconventional replacement resource of oil and gas.According to the China’s petroleum industry standard "Tight Oil Geological Evaluation Method" of 2013,tight oil refers to the oil in the tight sandstone,tight carbonate and other reservoir which matrix permeability under overburden pressure is less than or equal to 0.2×10-3μm2,the air permeability is less than or equal to 2×10-3μm2.The natural productive capacity of tight oil is lower than the industrial oil flow.However,the tight oil can be obtained with certain economic conditions and technical measures such as horizontal wells,fracturing.At present horizontal well and volume fracturing are the main development mode.Resource evaluation,seismic interpretation,well logging evaluation,fast drilling and volume fracturing have been taken into use.However,under the restriction of development approach and displacement efficiency,imbibition becomes an important mechanism in the development process of tight oil after fracturing.Aiming at the phenomenon of imbibition in porous media,a study of imbibition kinetics and the behavior mechanism of surfactant on imbibition process has been carried out.First of all,the kinetic model of single phase fluid spontaneous imbibition under the capillary pressure in vertical capillary has been established.The effect of inertia force on suction fluid and the influence of displacement and ambient pressure on imbibition have been analyzed through dimensionless analysis.The oil-water phase capillary imbibition considering the differentiation of gravity had been built based on single tube model;the suction front height and recovery ratio were calculated and the effect of oil/water viscosity ratio on the suction was analyzed under the given conditions.On the basis of the conventional scale imbibition kinetic model,imbibition model with the micro scale flow effect,which considering the influence of the roughness of the capillary wall and the boundary layer flow had been established;the characteristics of micro scale imbibition flow had been analyzed.Secondly,convection and diffusion processes of imbibition in porous media are described by the nonlinear diffusion equation;countercurrent imbibition in water-wet media was analyzed emphatically.The approximate analytical solution of the water saturation in one-dimensional had been solved by simplifying the diffusion coefficient.Based on the critical detchment velocity of the oil droplets on a solid surface,relationship between the fracture velocity and media imbibition had been established.The numerical simulation of the two-dimensional countercurrent imbibition had been carried the addition operator splitting algorithm and semi-implicit method.Finally,a comprehensive analysis of the improvement behavior mechanism of surfactant imbibition considering wettability altering and lower interfacial tension had been taken out.The relaxation effect,Marangoni effect and interfacial tension driven flow caused by the surfactant in microscale were discussed.Combined with the modified relative permeability curve and capillary pressure curve,the numerical model of surfactant imbibition had been established,and the influences of wettability altering and lower interfacial tension on imbibition were analyzed.Research shows that: the smaller radius capillary increased the imbibition power of wetting phase,but at the same time it increase the viscous resistance of imbibition in the form of second power,so at the same time,a larger radius of capillary obtained deeper imbibition depth,however,smaller radius capillary got greater balance height when the time can be infinitely increased.Based on the analysis of dimensionless number Re,Ca and Bo,the porous medium of the oil reservoir imbibition is considered as laminar flow,interfacial tension plays the major role in the fluid imbibition,and gravity is less important,no other hydrodynamic effect on the suction surface.Lower the oil water viscosity ratio would contribute to increase the depth of imbibition.Rough wall equivalent solid layer was the main influence factor of microscale imbibition and it reduced the effective space of flow in nanometer scale capillary imbibition.Boundary layer increased the nonlinear feature,but had a little effect in the low velocity flow of microscale imbibition.The imbibition height considering microscale effects was slightly lower than that calculated by the conventional dynamic equation.Through the approximate analytical expression of water saturation and oil recovery of one-dimensional imbibition,the imbibition effect could be evaluated.Based on the status of pore developed,fracture fluid velocity could be optimization design to increase the oil recovery rate of imbibition.Two dimensional numerical solution of nonlinear diffusion equation can be more intuitive and accurate describe the countercurrent imbibition of wetting phase in the porous media.A coupling relationship between interfacial tension and fluid suction height existed in the surfactant-solution imbibition,the hight can’t be simply described by the conventional imbibition kinetics model.Interface relaxation of the surfactant solution caused a peak value of imbibition height and then tended to stable height as the inertia force was ignored.Surfactant could lead to Marangoni convection and interfacial tension driven flow which could improve the recovery of crude oil in nanometer micro-scale.A two dimensional surfactant solution imbibition model was established considering the wettability-alteration and interfacial tension,which could directly reflect the countercurrent imbibition of surfactant solution.