Visualization Experiments Of Nanoparticles Strengthened Foam Seepage Characteristics And Evaluation Of Foam Dynamic Stability

Improving and developing enhanced oil recovery technology is the key to ensuring steady oil production.Foam can prevent gas channeling and viscous fingering during the displacement process,as well as effectively increase the sweep coefficient.Foam flooding technology has been proved by field practice to be an effective way to improve the recovery of heterogeneous oil reservoirs,and foam stability is an important factor affecting the development of this technology.It was proved in practice that adding nanoparticles with appropriate parameters to the traditional surfactant foam system can significantly strengthen the stability of the foam,which make the foam system more suitable for harsh formation environment.However,there are three main problems for the current research on the application of nanoparticles strengthened foam flooding technology:（1）The mechanism of nanoparticles to improve foam stability involved the mechanical interaction between nanoparticles,foam liquid films,and foam gas.The effect of nanoparticles on the diffusion of gas in the liquid film,the mechanism by which nanoparticles slowed down the drainage of liquid film,and the effect of nanoparticles on the defoaming effect of the oil phase had not yet been fully explored.（2）Since the fluid flow state in general core displacement experiments was not visible,and the traditional visualization physical model was quite different from the actual formation environment,the research on the seepage characteristics and displacement mechanism of foam in heterogeneous reservoirs were not comprehensive.（3）The evaluation of the foam system for oil displacement generally used macro-static methods,while foam goes through a dynamic process of continuous destruction and regeneration in micro-pores,these macro-static methods could not fully reflected the micro-flow features of the foam migrating in the porous medium.In this situation,a dynamic stability evaluation method was urgently needed to evaluate and select foam systems suitable for reservoir characteristics.In view of the above problems,theoretical and experimental methods were used to analyze the mechanical mechanism of stabilizing foam with nanoparticles,and the nanoparticle strengthed foam system was prepared according to it.Then based on nuclear magnetic resonance technology,the seepage characteristics of nanoparticles strengthened foam in cores were studied,and the evaluation method of foam dynamic stability was established.The detailed research work includes:（1）The mechanism of inhibiting foam instability by nanoparticles was systematically analyzed from the three aspects:foam coarsening,liquid film drainage and defoaming in oil.The results showed that the nanoparticles prevented the foam gas from passing through the gas-liquid interfaces and the liquid films into the surrounding bubbles,thus reducing the coarsening rate of the foam.The micro-protruding structure formed by the bridge of nanoparticles in the liquid film of foam could hinder the liquid flow.The deformation of the liquid film leaded to the local capillary pressure which was in conflict with the drainage pressure,thus reducing the drainage rate of the foam.The adsorption of nanoparticles on the liquid-gas interface changed the composition of disjoining pressure and prevented the bubble rupture or coalescence caused by the instability of liquid film.The nanoparticles could effectively inhibit the spread of oil droplets on the surface of liquid film,which improve the stability of the foam in presence of oil phase.（2）According to the mechanical mechanism of stabilizing foam with nanoparticles,the nanoparticles（the SiO₂with an average particle size of 20 nm）strengthened foam system with good coordination was prepared.Subsequently,the visualization experiment method based on nuclear magnetic resonance was established,which provided a means to study the seepage characteristics and dynamic stability of nanoparticles strengthened foam system.（3）The displacement effects and blocking effects in artificial sandstone cores with different permeability have been tested for the SiO₂nanoparticles strengthened foam system and the SDS（Sodium dodecyl sulfonate）foam system.The results showed that compared with the SDS foam system,the nanoparticles strengthened foam had a higher resistance factor and a better displacement effect.After the subsequent water injection,the SiO₂nanoparticles increased the residual resistance factor of foam and enlarged the radius range of foam plugging pores.With the decrease of core permeability,the strengthening effect of nanoparticles on foam displacement and water plugging weakened.（4）The corresponding relationship between fluid signal and transverse relaxation time range in large,medium and small pores was established for double-layer heterogeneous core,the displacement characteristics and mechanism of foam in heterogeneous core were studied.The results showed that the water saturation decreased at different rates in the process of foam injection into heterogeneous core saturated with water.The foam injection parameters（injection rate,gas-liquid ratio,surfactant concentration）would significantly affect the sweep efficiency and the sweep rate of foam in different seepage layers and different pores.When the SiO₂nanoparticles were added to the SDS foam system,the sweep rate of foam increased,the displacement front moved significantly faster,and the displacement effect on large and medium pores was enhanced.（5）Based on the conservation of effective pore volume and nuclear magnetic resonance technology,an evaluation method for the dynamic stability of foam in the cores was established,and the oil displacement effect and dynamic stability factor were compared.The results showed that the dynamic stability factor of the foam had experienced three stages:sharp decreasing,progressive increasing and stabilization.Compared with surfactant foam,the nanoparticles strengthened foam improved the sweeping capacity and oil displacement efficiency in the low permeability layer,inhibited the unstable stage of foam development and improved the final equilibrium value of the dynamic stability factor.The high dynamic stability of the foam confirmed the mechanical mechanism that foam was strengthened by nanoparticles.The type of nanoparticles had a great influence on the stability of the foam,The SiO₂nanoparticles strengthened foam had stronger dynamic stability than the Al₂O₃and the CuO nanoparticles strengthened foam.With the increase of permeability difference between the two layers of heterogeneous cores,the minimum value of foam dynamic stability factor decreased and the equilibrium value of foam dynamic stability factor was lower.（6）Based on the experimental results,an expanded STARS foam seepage model suitable for nanoparticles strengthened foam was established,and the influence of key parameters was studied.The results showed that the model could well reflect the seepage characteristics and dynamic stability of nanoparticles strengthened foam in heterogeneous cores.（7）The nanoparticles strengthened foam was used to increase the oil recovery of fractured oil-wet cores.The characteristics of water flooding and foam flooding in cores before and after fractured（or aged by simulated oil）have been studied.The results showed that the nanoparticles strengthened foam could block the fractures and change the wettability of the pore surface,thus effectively displace the remaining oil in the matrix pores after water flooding,which resulted in higher oil displacement efficiency than that of the surfactant foam.