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Normally, when analyzing Driving Forces to determine the Displacement Efficiency (ED) during flooding, only Viscous Forces (vμ), which are proportional to the Macro Pressure Gradient (dp/dl), are considered. Micro Forces in pores are not considered. The micro forces considered in this paper are: First, the Normal Force caused by the change in shape of the flow lines in pores; second, the kinetic force caused by the change in momentum, which is due to the change in flow lines in pores also. Both these two micro forces are caused by the change in flow lines, the larger the change in flow lines, the larger the two micro forces. Analysis, calculations and experiments show that the Flow Lines in pores are different between Visco-Elastic Fluids and Newtonian Fluids. Flow lines in pores of visco-elastic fluids, compared to Newtonian fluids, look more like and “expanding” and “contracting” Piston Flow. The change in flow lines changes the micro Forces, these Micro forces act on locations that caused the flow lines to change, mainly the Protruding Portions of residual “oil blobs”. This enhanced Micro force causes the protruding portions of the “oil blobs’ to change shape and mobilize. The results of the calculated magnitude of the enhanced micro forces are shown in this paper. For visco-elastic fluids, micro forces in pores are much larger that that for Newtonian fluids and must be considered when analyzing the “Displacement Efficiency”. Results on “Visualization Core Models” confirm the above calculations and analysis. The affect of ED by core flooding using visco-elastic fluids is shown. Due to this mechanism, “Very High Concentration Polymer Flooding” is being implemented in the field, the incremental recovery is nearly twice that of conventional polymer flooding. Tests show that only the “First Normal Force Differential” (N1) and its corresponding “Weisenberg Number” (We) affect the shape of the flow lines, other elastic properties and the viscosity of the driving fluid do not affect flow lines, therefore they do not affect the ED either. The above analysis of the affect of visco-elasticity on flow lines, micro forces and ED is new in the study of flow of fluids in porous media and in Reservoir engineering; It is of significant value to further understand the mechanism of increasing ED in porous media; Useful in designing, screening, developing and evaluating better chemical products, chemical floods and methods to further increase oil recovery; “High Concentration Polymer Flooding” that utilizes the high visco-elasticity of the driving fluid to increase ED is field trailed in a large scale (4 large field tests) and shows a very much higher incremental recovery over water flood and conventional polymer flood, this is a “New Flooding Method” that has high potentials. The above also shows that this is a new mechanism for displacement by flooding in porous media with multiple phases in its pores.

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractResidual oil after water flooding appears in three types: oil drops, films and clusters. Work performed using Newtonian fluids shows that to mobilize the residual oil remaining after water flooding, the driving forces, on condition of oil-water IFT (interfacial tension) being constant, need to be increased by one thousand to ten thousand times over that of waterflooding before the capillary forces retaining the residual oil can be overcome [1] . Viscous forces cannot be increased by such a large magnitude in the field.Polyacrylamide (PAM) solutions have little effect on the oilwater IFT. However, results in the laboratory and field show that after flooding by polymer fluids, all types of microscopic scale residual oil in porous media are lowered [2] . The amount that is lowered is related to the elasticity of the fluids. Other conditions being the same, the higher the elasticity, the lower the residual oil saturation. The results of lab tests (including microscopic visualization cores) show that viscoelastic fluid flooding can lower the residual oil saturation in cores of different wettabilities. The main forces to mobilize the residual oil by viscoelastic fluids are not entirely the same as that of Newtonian fluids. It is not only a force perpendicular to the oil-water interface overcoming the restraining capillary force, but also a viscous dragging force parallel to the oil-water interface that mobilizes the residual oil.

The displacement efficiency (De) in porous media is usually analyzed by the ratio of the macro pressure gradient driving force and interfacial tension (IFT) between the driving fluid and residual oil. However, when the pressure gradient is constant, macro forces cannot explain the increase in De by driving fluids with elastic properties. Therefore, the change of micro forces acting on residual oil between driving fluids with and without elastic properties is analyzed.This paper shows the influence of viscoelasticity on De; the difference in the stress of non elastic fluids and fluids with elastic properties when flowing is analyzed; the effect of this stress difference on the micro flow lines in pores is mathematically simulated; the affect of the changes in micro forces caused by the change in micro flow lines on residual oil is shown; this enhanced micro force (without changing the macro pressure gradient) mainly acts on the protruding portion of different types of residual oil in pore(s), causing the protruding portion to change shape and move (mobilize); results on visualization core models confirm the above calculation and analysis; the displacement results on cores by fluids with different elastic properties in the lab are shown; the results of considering the phenomena that elasticity increases the De in numerical simulation are shown and compared with field results; large scale field results on polymer flooding and pressure coring are also shown.The above mathematical simulation, analysis, lab tests and field results all show that the micro forces acting on residual oil is different when the elastic properties of the displacing fluid varies, resulting in an increase in De for viscoelastic displacing fluids at constant pressure gradient conditions. This method of micro flow line and force analysis and its conclusions should be useful to further understand the mechanism of De in porous media; should be useful in designing, screening and developing better chemical flooding products, methods and projects to further increase oil recovery; should be useful in the analysis of phenomena associated with injecting non Newtonian fluids and gels.

In this paper, the rheological characteristic of HPAM solutions is studied and the oil displacement efficiencies of viscous-elastic HPAM solutions (concentration in series) and viscous glycerin solutions (no elasticity, viscosity in series) are studied by flooding at visual microscopic glass models and artificial cores. Also, the effect and mechanism of the viscous-elastic behavior of HPAM solutions and glycerin solution on the displacement of residual oil films under mixed wettability conditions is studied, the relationship between the elastic behavior of HPAM and oil displacement efficiencies is given. The results of rheological characteristic measurement indicate that both the viscosity and the first normal stress differential (N1) of HPAM solutions increase with the increase in concentration, HPAM solutions show strong viscous-elastic characteristics under high concentration conditions. The results of microscopic oil displacement experiments indicate the displacement efficiencies of HPAM solution flooding and glycerin solution flooding gradually increase with the increase in the viscous-elastic behavior of HPAM solutions and the viscosity of glycerin solutions. However, for HPAM solution flooding, the displacement efficiency is always higher and residual oil saturation lower than that of glycerin at the same viscosity and same capillary number. Last, the displacement experiment in artifical cores is conducted and the similar resultes are validated. Introduction In the field of petroleum engineering, it has long been believed that the mechanism of polymer flooding in Enhanced Oil Recovery (EOR) is mainly to improve the mobility ratio and macroscopically increase the sweep efficiency[1] and another main accepted mechanism of mobilizing residual oil after water flooding is that there must be a rather large viscous force perpendicular to the oil-wet interface to push the residual oil, this force must overcome the capillary forces retaining the residual oil, move it, mobilize it and recovery it[2].In the last few years, the conclusions of some papers[3–7] indicate the viscous-elastic behavior of HPAM (partially hydrolyzed polyacrylamide) solutions can enhance the oil displacement efficiency. Wang Demin[3–4] puts forward the major mechanism of polymer flooding increasing the micro-scale displacement efficiency is that the flow pattern and magnitude (value) of the viscous force parallel to the oil-water interface caused by the flow of viscoelastic fluids in porous media is different than for Newtonian fluids. This difference is the main cause of the increase in oil displacement efficiency for viscoelastic fluids flowing in porous media. Different polymer fluids had quite different elastic properties. The difference in incremental recovery can be more than 6%OOIP (original oil in place), which is substantial, it can make a polymer flood successful or not. Up to the present, papers about the separate effects of viscous and elastic behavior each on oil displacement efficiency have not been published, i.e. the viscous and elastic behavior of HPAM solutions are not taken into account separately in flooding, the effect of elastic behavior of the HPAM solutions on oil displacement efficiency has not been described quantitatively. Therefore, study on the effect of the elastic behavior of HPAM solutions on displacement efficiency is very necessary. Equipment, Material and Basic Step in Experiment In this study, oil displacement experiment is conducted in simplified microscopic pore model that is etched in glass and in artifical core. For the microscopic oil displacement experiment, the images recorded in the process of oil displacement are translated into digital signal by the image collection system, the behaviors of microscopic oil displacement of HPAM and glycerin solution are analyzed through the image analysis technology. Equipment Microscopic Oil displacement equipment consists of displacement part, image collection part and image analysis system. The experiment equipment and flow chart is as Fig.1. The rheometer is made in Germany HAAKE Company and is used for analyzing the rheological behavior of HPAM solution.

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractResidual oil after water flooding appears in three types: oil drops, films and clusters. Work performed using Newtonian fluids shows that to mobilize the residual oil remaining after water flooding, the driving forces, on condition of oil-water IFT (interfacial tension) being constant, need to be increased by one thousand to ten thousand times over that of waterflooding before the capillary forces retaining the residual oil can be overcome [1] . Viscous forces cannot be increased by such a large magnitude in the field.Polyacrylamide (PAM) solutions have little effect on the oilwater IFT. However, results in the laboratory and field show that after flooding by polymer fluids, all types of microscopic scale residual oil in porous media are lowered [2] . The amount that is lowered is related to the elasticity of the fluids. Other conditions being the same, the higher the elasticity, the lower the residual oil saturation. The results of lab tests (including microscopic visualization cores) show that viscoelastic fluid flooding can lower the residual oil saturation in cores of different wettabilities. The main forces to mobilize the residual oil by viscoelastic fluids are not entirely the same as that of Newtonian fluids. It is not only a force perpendicular to the oil-water interface overcoming the restraining capillary force, but also a viscous dragging force parallel to the oil-water interface that mobilizes the residual oil.

In this paper, the visco-elastic characteristics of polyacrylamide solution have been studied experimentally. The mechanisms of the effect of polymer solution with visco-elastic characteristic on each type of residual oil after water flooding are analyzed and the mechanisms of polymer solution with visco-elastic characteristic increasing microscopic oil displacement efficiency are studied. By the analysis of microscopic experiments of percolating flow, corresponding relationships between the characteristic parameter describing the visco-elastic behavior of polymer solution and displacement efficiency of residual oil in "dead ends" are given. It is shown that the larger the visco-elastic behavior of polymer solution, the higher the displacement efficiency of residual oil in "dead ends". A phenomenon that is not seen during water flooding is observed during polyacrylamide solution flooding in the microscopic experiment of percolating flow, that is, residual oil can be pulled into "oil threads" by the polyacrylamide solution. So a new type of oil flow channel, i.e. "oil thread" channel, can be formed and residual oil flows downstream through the "oil thread" channel. The probability of forming a steady "oil thread" flow channel in polymer flooding is also analyzed theoretically and proved. The research indicates that the mechanism of polymer solution microscopically increasing oil recovery is due to the visco-elastic characteristic of the polymer solution, the sweeping force acting on the residual oil for the visco-elastic polymer solution is larger than that of water. The residual oil is not pushed out by the polymer solution but pulled out by the polymer solution. It is also found that every type of residual oil after water flooding can be decreased by visco-elastic polymer solution, and the larger the visco-elastic property, the stronger the capability of the polymer solution to "sweep out" the residual oil. In brief, the results indicate that polymer flooding can not only macroscopically increase the sweep efficiency but also microscopically increase the oil displacement efficiency and a new type of flow channel, "oil thread", can also be formed. Besides new understanding on the mechanism of the flow of fluid through porous media and the displacement of residual oil, the above results should be beneficial to designing new EOR methods to further increase the recovery of oil fields. Introduction In the field of petroleum engineering, it has long been believed that the mechanism of polymer flooding in EOR is to improve the mobility ratio and macroscopically enhance sweep efficiency. However, the knowledge about the microscopic oil displacement mechanism of polymer solution is insufficient. Concerning the mechanism of polymer flooding, there are many different views. One is that injecting water with a certain viscosity has the same ultimate residual oil saturation as conventional water flooding, polymer flooding can not decrease the residual oil saturation within the sweeped area, and another is that the mechanism of polymer enhancing oil recovery is rather complex, and not yet understood completely so far[1]. In fact, the researches are far from perfect about the percolation flow behavior of polymer solutions in porous media, especially its microscopic physicochemical filtration flow rule. So it is necessary to further study the mechanism of polymer solution increasing the microscopic oil displacement efficiency.

The displacement efficiency (De) in porous media is usually analyzed by the ratio of the macro pressure gradient driving force and interfacial tension (IFT) between the driving fluid and residual oil. However, when the pressure gradient is constant, macro forces cannot explain the increase in De by driving fluids with elastic properties. Therefore, the change of micro forces acting on residual oil between driving fluids with and without elastic properties is analyzed.This paper shows the influence of viscoelasticity on De; the difference in the stress of non elastic fluids and fluids with elastic properties when flowing is analyzed; the effect of this stress difference on the micro flow lines in pores is mathematically simulated; the affect of the changes in micro forces caused by the change in micro flow lines on residual oil is shown; this enhanced micro force (without changing the macro pressure gradient) mainly acts on the protruding portion of different types of residual oil in pore(s), causing the protruding portion to change shape and move (mobilize); results on visualization core models confirm the above calculation and analysis; the displacement results on cores by fluids with different elastic properties in the lab are shown; the results of considering the phenomena that elasticity increases the De in numerical simulation are shown and compared with field results; large scale field results on polymer flooding and pressure coring are also shown.The above mathematical simulation, analysis, lab tests and field results all show that the micro forces acting on residual oil is different when the elastic properties of the displacing fluid varies, resulting in an increase in De for viscoelastic displacing fluids at constant pressure gradient conditions. This method of micro flow line and force analysis and its conclusions should be useful to further understand the mechanism of De in porous media; should be useful in designing, screening and developing better chemical flooding products, methods and projects to further increase oil recovery; should be useful in the analysis of phenomena associated with injecting non Newtonian fluids and gels.