| Recently,most petroleum fields in China have considered gas injection or WAG?water-alternating-gas?as an important means to develop residual oil at the late stage of waterflooding.Injected gas or free gas shows easy flow and low viscous resistance,and is more likely to enter the areas where the reservoir space is not affected by injected water,or occupy the original oil space in the dead oil area.The injection gas or free gas in porous media in the form of discrete bubbles was not been clearly observed and investigated.The main reasons are as follows:Firstly,the reservoir space is like a"dark box"and cannot be directly observed from the outside.Secondly,discrete bubbles in porous media of tubular pores or granular pores are strongly influenced by external factors,and their deformation is complex.Thirdly,the relationship between discrete bubbles and the shape or size of the pore throat,and liquid properties in porous media is unclear,and the migration law of gas phase is affected by more factors.In response to the problems mentioned,the motion behavior of discrete bubbles in porous media based on mechanical analysis characterization and laboratory experiments in this paper was studied,and the rising migration conditions and flowing rules of discrete bubbles in different environments are summarized.The following results and conclusions in this paper were obtained:1)The conditions and expressions for the upward migration of the mobile gas phase in the fluid saturation model of tubular pores or granular pores were established.Through the comparison of buoyancy and capillary resistance,the effects of capillary radius,interfacial tension,interfacial contact angle and other factors on two different forces was studied,and the critical conditions of bubble migration and capture were determined.Meanwhile,based on the coupling of various forces,such as buoyancy,liquid resistance?drag force?,interfacial tension and additional mass force,mathematical expressions of acceleration and average velocity of rising bubbles with and without touch to solid particles in saturated-liquid porous medium were established,and the effect of driving differential pressure on rising bubbles was analyzed.Finally,the effects of bubble equivalent diameter,solid particle size,porosity,contact angle,interfacial tension and other parameters on different forces and bubble relative velocity were presented by a graphical method.2)The effects of different factors on the behavoir characteristics of the rising bubbles in saturated Newtonian fluids were studied experimentally.The variation of parameters such as deformation coefficient,transverse-longitudinal ratio of rising bubbles,horizontal velocity and vertical velocity of bubbles with the size of injection hole size,gas injection velocity,inclination angle of model and glycerol mass fraction in distilled water and glycerin-aqueous solution were analyzed.More importantly,the influence of temperature?5100°C?on the behavior characteristics of rising bubbles in distilled water,formation water,ethanol and glycerine-aqueous solution was studied.The results showed that the temperature had important influence on rising bubbles in glycerol-aqueous solution.Therefore,a drag coefficient model was established to describe the rising motion of bubbles with the high Reynolds number and low Reynolds number.The results of the modified model was consistent with the experimental results,of which the standard error was less than 5.32%.3)Based on visualized fluid saturation models filled with particles,the interaction relationship of bubble and bubble,bubble and liquid,solid particles and bubbles were observed,and the behavoirs were analyzed,including bubble migration,coalescence,fragment,trapping.And the volume fraction of trapped bubbles was measured in saturated-liquid porous media,of which the changes were explained by force analysis.Meanwhile,the influence of bubble lengths,pore-throat ratio,solid particle size,model inclination and glycerin mass fraction on the rising velocity of discrete bubbles in the saturated liquid porous medium filled with glass beads was performed.4)The microbubbles in the two dispersed systems including N2-water and CO2-water were produced by using the orifice plate injection method and ultrasonic oscillation method,and the microscopic seepage of the water-gas dispersion system was investigated in the etching glass model and sand packed model,and the effect of two dispersion systems in different core displacement experiments on oil displacement efficiency and sweeping volume was performed.So,based on mechanical model and experimental dynamic evaluation,the microscopic flowing characteristics of dispersed bubbles in porous media were obtained,and the influence of different factors on bubble motion behaviors was analyzed,and then the rising motion changes of gas in pore throat were described visually,so as to reveal the dynamics and microscopic seepage mechanisms of gas flooding.Meanwhile,referring to micro-bubble generation and flowing process,it can be determined that the water-gas dispersion system solve the problem of low recovery in low permeability reservoir.It provides a scientific basis for the migration rules of injected gas in the pore space,and has an important significance for the efficient development of remaining oil. |
PDF Full Text Request