| On the background of Surfactant-Polymer (SP) binary flooding major development tests of CNPC, the emulsion mechanism is still unknown for improving recovery efficiency. Emulsion phenomenon has been always found in oilfield tests and indoor experiments. A study on emulsion formation for SP and porous flow rules was carried on by indoor emulsion formation and numerical simulation methods.To research and simulate the emulsion features by indoor apparatus, high shear scattered emulsification machine was optimized to form the emulsion, which has the similar particle size distribution to the oil-in-water emulsion samples from oilfield tests, so the emulsion could meet the conditions of study on emulsion formation and porous flow rules. Analysis parameters such as average droplet diameter, sorting coefficient, droplet effective share, emulsification intensity and stability comprehensive coefficient can be used to assess these emulsion properties objectively, and the parameters emulsification intensity and droplet effective share are recommended to represent emulsion fliud characterization. Micro and macro property parameters are consistent. Formed with SP binary system and oil phase, the emulsion presents the non-Newtonian fluid properties and shear thinning ability, and its property is inline with power-law mode. Emulsion droplets can withstand the shear force under high shear rate, and its viscosity is greater than that of conventional polymer solution viscosity. Therefore, emulsion can reduce the loss of viscosity and improve the displacement efficiency in binary flooding.On the mechanism of emulsion formation for SP binary flooding, the shear force can change the conventional state of aggregation for oil and water phase, and the oil phase was dispersed into droplets. The stability of emulsion can be improved by polymer. Shear force and interfacial tension are the two main factors for emulsion formation. Both of reducing interfacial tension and increase the shear force are the effective methods of reducing particle size of emulsion droplets. By selecting the common surfactants of anionic type KPS, non-ionic DWS-3 and betaine SD-T, which are usually found in SP binary flooding tests, emulsion were formed by using the simulation oil from Qizhong block in Xinjiang oilfield respectively. The results have showed that the concentration of DWS-3, KPS and SD-T need to be greater than 0.1%,0.2% and 0.1% respectively when forming emulsion. Interfacial tension has a main impact on particle size in emulsification start-up, and the ultra-low interfacial tension (or low) is not the necessary condition for emulsion formation. The conditions of emulsion formation could be described as the oil and water two phases, droplet is dispersed by shear force and lower interfacial tension reducing droplet diameter or addition polymer to improve its stability. The emulsification ability for SP binary flooding is closely related to the surfactant concentration and agent types. Appropriate increase of surfactant concentration, and reducing the interfacial tension can promote the formation and stability of emulsion.In the process of emulsion formation, the droplet size distribution and its characteristics were conducted by biology microscope and sample analytic method. For medium permeability cores (K is about 200～500×10-3μm2), capillary number, viscosity ratio of oil and water phase have produced great effect on the intensity and seepage law for SP binary flooding. The emulsion began to form when capillary number threshold is about 3.0～8.0×10-4. As the viscosity ratio of oil and water phase gets larger, the capillary number gets bigger. When capillary number is larger than the threshold, emulsion begins to form and its intensity increases rapidly. With the growth of flow rate, two phase fluid flow dominated initially is transformed into emulsion flow mode gradually.The mechanism for SP binary flooding to improve oil recovery can be expressed as that start-up residual oil for emulsion formation, increase filtrational resistance and expanding the sweep volume, and the carrying force and viscous force also can drive out part of the residual oil. On the judgement of capillary number threshold for emulsion formation, the emulsion seepage process is divided into two steps of start-up residual oil for emulsion formation and emulsion system displacement. This method has simplified the study on seepage laws for binary composite emulsion system. In SP binary flooding, the capillary number and residual oil saturation are in the relationship of logarithm,and SP binary flooding can improve the oil displacement recovery about 15.0%-21.2%, while the emulsion flooding can improve the recovery efficiency from 8.4% to 12.9%. Emulsification intensity and oil recovery efficiency enhanced with the increase of capillary number, and the highest difference of recovery efficiency can reach up to 4.5%. Emulsification can improve the oil displacement efficiency.With capillary number threshold and oil-water viscosity ratio relation chart, emulsion seepage mathematical model was established for SP binary flooding system. By introducing emulsion coefficient and fitting correction, the relative permeability curve was amended in the process of emulsion phase seepage. The method could present the residual oil start-up to form emulsion, improving swept volume and mechanism of oil displacement efficiency. In this model, the emulsification coefficient was got by the method of test algorithm, its application value was increased by the comparison of theoretical calculation and indoor physical simulation. |
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