Effects of oil viscosity on the plugging performance of oil-in-water

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General Research

Effects of oil viscosity on the plugging performance of oil-in-water emulsion in porous media Zan Chen, Mingzhe Dong, Maen M. Husein, and Steven L. Bryant Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b00889 • Publication Date (Web): 08 May 2018 Downloaded from http://pubs.acs.org on May 9, 2018

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Industrial & Engineering Chemistry Research

Effects of oil viscosity on the plugging performance of oil-in-water emulsion in porous media Zan Chen, Mingzhe Dong*, Maen Husein, Steven Bryant Department of Chemical and Petroleum Engineering, University of Calgary, Calgary, AB Canada T2N 1N4

Abstract Oil-in-water (O/W) emulsion is an effective plugging agent for improving fluid mobility conformance. Most studies to date have attributed the plugging mechanism solely to the “Jamin effect”. In this work, we investigate the influence of oil phase viscosity on the plugging performance of O/W emulsions in porous media. Sandpack flow tests were conducted to quantitatively evaluate such effects. Systematic data of pressure drop versus oil viscosity under varied sandpack permeabilities and flow rates were reported. The oil viscosities of emulsions used in this study ranged from 9.4 cP to 496.0 cP. During flow tests, potential interference from interfacial tension (IFT) and droplet size was eliminated. The results show that oil viscosity of O/W emulsion considerably affects pressure drop across the sandpack. Frictional resistance increases as oil viscosity increases, but the rate of increment tapers down significantly. The plugging effect due to oil viscosity is dependent on sandpack permeability and emulsion injection rate. The effective viscosity of the emulsions increases as oil viscosity increases and decreases as the injection rate increases. The droplet-wall friction at pore throat, the slipping behavior and the change of flow path are considered as essential mechanisms for such results. Keywords: oil-in-water emulsion; emulsion flow in porous media, viscosity, sandpack model.

1. Introduction Emulsion flow in porous media is of great interest to the petroleum industry. It has been reported that approximately two-thirds of the world’s crude oil is produced in some form of emulsions.1 Rapid water channeling during waterflooding in a heterogeneous reservoir may lead to low sweeping efficiency. This problem can be mitigated by injecting O/W emulsions as blocking agents to plug off the highly permeable channels and thus improve the uniformity of the water front.

2-5

Such emulsions can also be used as displacing fluid for an enhanced oil recovery

process. 5-8 Corresponding author at: University of Calgary, Calgary, Alberta, T2N 1N4, Canada. Email: [email protected] (Mingzhe Dong)

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O/W emulsion is a system where oil droplets are dispersed in the aqueous continuous phase. Surfactants are widely used for stabilizing emulsion systems, as they not only aggregate at the interface to reduce the interfacial tension, but they also provide a repulsive energetic barrier between droplets.9 Alkali aqueous solutions display a positive synergistic effect with the surfactants at low concentrations. However, this effect is reversed when the alkali concentration is high.10,11 Viscosities of O/W emulsions are generally slightly higher than those of the continuous aqueous phase and can be easily injected into wellbores. Conversely, water-in-oil (W/O) emulsions may display much higher viscosities than the continuous oil phase and are generally difficult to inject into reservoirs.12 Comparing with the gel system for conformance control, O/W emulsions show better injectivity, easier blockage removal and less formation damage.13 O/W emulsions also allows a wider range of channel plugging by controlling the size of droplets.14 Additional pressure is required to push oil droplets through pore throats when O/W emulsions travel through porous media. Most studies have attributed the mechanisms of such rheological change to the “Jamin effect”, which is caused by the difference of capillary pressure between the upstream and downstream ends of the droplet.2-8, 15-18 McAuliffe pointed out that for an emulsion to be most effective as a blocking agent, the size of the oil droplet should be slightly larger than the pore throat.2 Soo and Radke observed that drops were not only captured at the pore throat, they are also absorbed and captured at the rock surface.19 Wang further concluded that three types of plugging mechanisms can occur during emulsion flow in porous media: single large droplet trapped at the pore throat, multiple small droplets simultaneously trapped at the pore throat, and plugging caused by droplet adsorption.20 Some correlations were developed to describe the capillary trapping.21-23 Alvarado and Marsden proposed a simple viscosity model where they treated the emulsion as a single-phase fluid. This model, however, does not consider permeability reduction.21 The restriction model by Devereux accounted for permeability reduction, nevertheless, treated the reduction as a temporary phenomenon.22 Soo and Radke developed a filtration model, which incorporates the capture of droplets by the sand grain surface. This model is, however, complex and involves some parameters that are not applicable to real life.23 Despite the abundance of literature, both experimental and mathematical modeling, no study on the effects of oil phase viscosity on plugging performance has been reported.


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Some insights on the role of oil viscosity can, nevertheless, be inferred from microfluidic studies on droplet flow in capillaries. A capillary tube with a large radius

can

be assimilated as a pore body, and a constricted capillary tube that is comparable or smaller than the droplet diameter can be assimilated as a pore throat. The scenario where droplet flows in the large radius tube is not particularly of interest since plugging generally does not occur in the pore body. The microfluidic research in this area mainly focuses on droplet deformation, break up, coalescence and the corresponding flow behavior caused by such droplet dynamics.24-27 Loewenberg and Hinch showed that higher deformations are obtained for lower viscosity ratios.24 Bentley et.al claimed that a critical value of the viscosity ratio exists beyond which droplet breakup does not occur.25-26 Komrakova et al. modeled droplet deformation and breakup with three dimensionless numbers; including viscosity ratio, Reynolds number and capillary number.27 Despite these observations, it has been well accepted in the petroleum industry that O/W emulsion can be treated as a Newtonian fluid when containing 50% oil or less.2,21 Studies on droplet flow in a capillary tube with comparable or smaller radius provide more worthwhile information.28-33 Under this scenario, a significant additional pressure drop is required to push the droplet through the capillary, due to frictional resistance. The droplet viscosity plays a governing role. Martinez and Udell concluded that when the ratio of droplet radius to capillary radius is less that 0.7, the additional pressure is not affected by a change in the viscosity ratio of the two fluids and the capillary number.28 Ho and Leal reported that, for a given dispersed fluid viscosity, the pressure drop increases solely with an increase in length of the contact region with the tube wall.29 Tsai and Miksis numerically analyzed forces acting on a droplet travelling through a constriction.30 They reported that pressure drop decreases sharply as the front of droplet passes through the constriction, and snap-off does not occur for high droplet viscosity. Arriola et al. suggested that the wall effect must be considered when estimating the pressure drop.31 Dong et al. studied the mobilization of an oil slug in a capillary tube and showed that pressure drop in the order of 476 times higher than values calculated by Poiseuille’s law is encountered.32 Pang et al. derived a model for calculating total pressure drop and pressure distribution for foam flow in porous media.33 Their model can be modified to simulate emulsion flow in porous media by incorporating the viscous effects induced by oil viscosity. By analogy, oil phase viscosity would slightly affect the frictional resistance of droplets flowing through the pore body and significantly affect the frictional resistance of droplets



flowing through the pore throat, which consequently influences the total pressure drop associated with O/W emulsion flow in porous media. To the best of the author’s knowledge, no previous research has addressed this effect on the plugging performance of O/W emulsions.

2. Experimental 2.1 Materials Dioctyl sodium sulfosuccinate (AOT, 70% pure, Stepan Co., Ltd, USA) was used as the surfactant. Sodium hydroxide (96% pure, Science Co., Ltd, USA) was dissolved in deionized water to prepare aqueous solutions with the desired alkaline concentration. Crude oil was obtained from an Alberta heavy oil reservoir. The viscosity of the oil was measured as 15200 cP at 40 oC on a Brookfield Viscometer (Model DV-II+, Brookfield Engineering, USA). The crude oil was diluted with kerosene to achieve desired viscosities. Viscosities of the diluted oil samples were 496.0 cP, 201.7 cP, 47.4 cP, 23.2 cP and 9.4 cP at room temperature (22 oC). Sandpack flow tests were conducted to simulate emulsion flow in the porous media. The sandpack holder is 11.15 cm in length and 2.84 cm in diameter. Clean sands (water wet) were purchased from Bell & Mackenzie Co., Ltd (Canada). The size distribution was from 140 to 270 meshes. 2.2 Emulsion preparation and characterization In order to investigate the effect of oil viscosity on the plugging performance, potential interference from interfacial tension and droplet size needs to be eliminated. The surfactant concentration, alkaline concentration, emulsifying time and emulsifying speed are adjustable parameters between the different mixes for attaining nearly the same interfacial tension and droplet size. After numerous attempts, specific formulas for preparing each emulsion sample were obtained, as presented in Section 3.2. It was observed that the stability of emulsion was very sensitive to the concentration of alkaline, thus the alkaline concentration was fixed at 0.012 %. For each flow test, 100 grams of emulsion was prepared, which guaranteed at least four pore volumes (PV) of injection. The oil content was maintained at 10 grams and the mass of other constituents were calculated from the corresponding emulsion formulas. The homogenizer (Model AHS 250, VWR International, USA), with a stirring speed of up to 50,000 rpm was used for mixing the constituents. In addition, the desired emulsion stability must be achieved for each mix. The stability was determined by visual inspection and by comparing the droplet distribution at the top and bottom of the emulsion. Since the time for all sandpack flow tests were within 3


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hours, emulsion samples that maintained their properties for more than four hours were considered sufficiently stable for the experiments. 2.2.1 Interfacial tension measurement IFT was measured by a spinning drop interfacial tensiometer (Model M6500, Grace Instrument, USA). A capillary tube was filled with mixtures of surfactant solution and alkaline solution. The oil drop was injected into the tube and the tube was then rotated at a speed of 10,000 to 11,000 rotations per min. Circulation of cold water was applied to maintain the operating temperature at 25 oC. Equilibrium was reached when there was no change in drop shape for 10 min. Details on how the instrument calculates IFT from the drop shape can be found elsewhere.34 2.2.2 Droplet size distribution The droplet distribution of the O/W emulsions was collected on a microscope (Model BM1000, Jiangnanyongxin, China) connected to a digital camera (Model Moticam 5, Motic Asia, China) with 400 times magnification power. The collected images were analyzed using an image processing software (Motic Images Plus 2.0) to obtain droplet size distribution. 2.3 Sandpack flow test 2.3.1 Experimental setup The experimental setup for the sandpack flow tests consisted of a displacement pump (Model 500D, Teledyne Isco, USA) for fluid injection, two cylinders for holding emulsion and deionized water, a sandpack holder filled with clean sands and a pressure transducer (Model Heise PM, Ashcroft Inc, USA) for measuring the injecting pressure at the inlet of the sandpack. A schematic diagram of the experimental setup is shown in Figure 1.

Figure.1 Diagram of the experimental setup for emulsion flow in sandpacks 2.3.2 Procedures Dry sands were mixed with a small amount of water and then tightly packed into the sandpack holder. Three sandpack models were prepared in this study, with permeability of 1.5 darcy, 3.0



darcy and 4.5 darcy, respectively. The sandpacks were first saturated with water by injecting DI water at 0.5 ml/min in order to measure permeability and porosity. The permeability was determined from Darcy’s equation and the porosity was determined from the volume of water in the sandpack. The sandpacks qualified for flow tests if the measured permeability was in the range of ± 10% from the desired values. Water was first injected into the sandpack and was then switched to emulsion when the pressure drop along the sandpack no longer changed. The emulsion cylinder was placed horizontally and was continuously rotated during the injection of emulsion, in order to eliminate the effect of gravity, i.e., droplet creaming. The inlet pressure was recorded at a specified time interval once the emulsion injection started, while the outlet was open to the atmosphere. When the inlet pressure became stable, emulsion samples were collected from the inlet and outlet of the sandpack for quality tests. If the difference in emulsion droplet size distributions was negligible between the inlet and outlet, the corresponding pressure was recorded and used to calculate the effective emulsion viscosity.

3. Results and discussion 3.1 Emulsion stability Emulsion stability was studied by visual inspection and quality tests of emulsion samples. Figure 2 shows a photograph of emulsions with different oil phase viscosities after standing for 4 hours. Emulsions with higher oil phase viscosities appeared to be darker than those with lower oil viscosity. No obvious separated layers were observed. The quality tests show that there was a slight creaming of droplets in the upper part due to gravitational separation, which can affect the consistency of plugging in the sandpack flow test. Nevertheless, this creaming is expected to be minimum during the experimental run, since the cylinder holding the emulsions was continuously rotated.


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9.4 cP

23.2 cP

47.4 cP

201.7 cP

496.0 cP

Figure. 2 Stability scan of emulsion samples with varied oil viccosities at 22 oC after 4 hours 3.2 Emulsion characterization The interfacial tension was influenced by the concentration of surfactant and sodium hydroxide, whereas the droplet size was greatly affected by the energy provided by the homogenizer. Table 1 shows the values of the parameters required to obtain the same IFT and droplet size distribution among the emulsion samples, with different oil phase viscosities. Less surfactant was added for the oil sample with a higher viscosity to obtain the same IFT value. For example, when oil viscosity increases from 9.4 cP to 496.0 cP, the required surfactant concentration decreases from 0.65% to 0.25%. This is attributed to naturally occurring surface active agents, which are activated in presence of NaOH. The higher viscosity oil phase contained more of these agents. For emulsions with higher oil viscosity, higher emulsifying speed and longer emulsifying times are required. This correlates to more energy needed to breakup the higher viscosity oil into droplets. Table 1 also shows the measured bulk viscosity of the emulsion. Rotation speed of the viscometer is 100 rpm for all measurements. The bulk emulsion viscosities range from 1.45 cP to 1.86 cP, which slightly increase with oil viscosity as a result of increased internal friction caused by droplet collision and sliding.35 The bulk elasticity of emulsion was not measurable due to low oil volume fraction.36 Table 1. Composition and emulsifying speed required to attain same IFT and droplet size distribution of the emulsion used in this study. The oil mass% were maintained constant at 10 wt%



3.2.1 Interfacial tension measurement For a typical field application of O/W emulsion, the interfacial tension between oil and water needs to be optimized.18 Low IFT cannot provide sufficient capillary force for plugging, while high IFT induces emulsion instability and may cause significant droplet breakup at the pore throat. In this work, a moderate IFT was selected; slightly less than 2 mN/m. Figure 3 shows the IFT for the oil samples with different viscosities. IFT values were controlled to be in the range of 1.82 -1.95 mN/m by applying the previous obtained formulas. Thus, interference from interfacial tension on pressure drop can be eliminated.

Figure. 3 IFT values for the different oil phases used in this study at 25 oC. 3.2.2 Droplet distribution Emulsions generally display better blocking performance when the droplet diameter is slightly larger than the diameter of the pore throat. During the flow tests with a sandpack having 1.5 darcy permeability, it was found that emulsions with an average droplet size of 3 to 4 microns


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outperformed the others. As stated earlier, similar droplet distribution was obtained for all the emulsion samples by adjusting the mixing speed and time. Figures 4 and 5 show images of multi-layer droplets and their size distribution for the five emulsion samples used in this study. The droplets are evenly distributed, with no signs of flocculation and coalescence. A normal distribution with an average diameter of around 3.6 microns was achieved for all emulsions. Potential interference from droplet size was thus neglected in the sandpack flow tests.

(a) 9.4 cP

(b) 23.2 cP

(d) 201.7 cP

(c) 47.4 cP

(e) 496.0 cP

Figure. 4 Image of droplet distribution for all emulsion samples

Figure. 5 Droplet size distribution for all emulsion samples



3.3 Pressure drop profile 3.3.1 Effect of oil viscosity Figure 6 shows the pressure drop across the sandpack at different stages of injection. The sandpack permeability in these experiments was 3.0 darcy, and the injection rate was kept constant at 0.5 ml/min. Generally, the injection pressure became stable after 2 PV of emulsions were injected. The pressure increment was relatively limited within the first 0.5 PV injection of the emulsion, followed by a rapid growth between 0.5 PV and 2 PVs. Subsequently, the pressure increase tapered down and eventually became constant. During the first PV injection, there was no droplet observed in the effluent. After that, small droplets started to present at the outlet. The average size of droplets increased as the injection further proceeded. Upon reaching steady state, no significant difference in emulsion droplet size distribution was observed between the inlet and outlet. Some studies have reported significant pressure fluctuations during the injection process; such fluctuations were barely observed in this experiment. This is possibly due to the fact that sandpack permeability was high and the injection rate was relatively low, so the plugging process was slowly and steadily established. In addition, Figure 6 indicates that oil phase viscosity significantly impacts plugging performance. Higher oil phase viscosity leads to a higher steady state pressure drop. For example, when oil viscosity increases from 9.4 cP to 496 cP, the steady state pressure drop increased from 1.7 kPa to 5.8 kPa. The small variation in IFT does not account for such increment since the pressure drop induced by capillary force is relatively small in all scenarios. The effect of surfactant adsorption is expected to be minimum since surfactant molecules at the oil-water interface are not easy to be captured by the matrix and excessive emulsion injection ensures similar degree of adsorption for all tests. The influence of the slightly change in sandpack permeability is also neglected.


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Figure. 6 Pressure drop across sandpacks during injection of emulsions with different oil phase viscosities (sandpack permeability = 3.0 darcy, injection rate = 0.5 ml/min) The increment in pressure drop can be explained with the following model. The porous media can be regarded as a group of interconnected pore units with varied sizes and shapes. Figure 7 depicts the process of an oil droplet flowing through a pore throat. The radius of the undeformed oil droplet was assumed to be larger than that of the pore throat, and the pore wall was water wet. The emulsion was injected at a constant flow rate. In this scenario, the pressure difference across the pore unit at steady state is impacted by two forces: the capillary force and the frictional force. The capillary force arises from the difference in capillary pressure between the front and back of the droplet, while the frictional resistance is due to the viscosity of the emulsion, in both the dispersed phase and the continuous phase.

Figure. 7 Schematic representation of an oil droplet flowing through a pore throat



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Frictional resistance occurs at the pore throat where the droplet contacts the pore wall (contact area), and this force is strongly affected by the viscosity of the oil phase. It is a function of flow velocity, oil viscosity, oil droplet size and pore throat geometry. Frictional resistance also occurs at the pore body due to viscosity of the continuous water phase and the suspended oil droplets inside it. In these sections, the droplets do not directly contact the wall, so the viscous effect can be approximated using the bulk viscosity of the emulsion. The total pressure drop across the control volume shown in Figure 7 can be formulated by combining capillary force, frictional force at a pore throat and frictional force in the pore body: !

!

%

"#

$

"#

(1)

where σ is the interfacial tension between oil and water, Rf and Rb are the droplet radius at the front and end, x1 and x2 are the boundaries of contact area between oil droplets and pore wall, and

&

are the boundaries of pore unit, dpo/dx and dpem/dx are the pressure gradients in the

contact area and the rest of pore body. From Newton’s inner friction law, as oil viscosity increases, the pressure gradient in the contact area dpo/dx would increase significantly, making it more difficult for the droplet to pass through the pore throat. Within the range of 496 cP (as in this study), it is considered that droplets do not show solid-like behavior. The pressure gradient in the pore body dpem/dx also increases due to the slight increment of bulk viscosity of the emulsion, leading to an even higher total pressure drop across the pore unit for maintaining constant flow rate. It is important to notice that as the pore wall is water wet, the “contact” between oil droplet and pore wall is separated by a very thin water layer where the water molecules are strongly bounded to the solid surface.37-39 This water layer will have an impact on the pressure gradient of dpo/dx (A detailed discussion is presented in Section 3.4). 3.3.2 Effect of sandpack permeability Flow tests were also conducted in sandpacks with different permeabilities. Figure 8 shows that the pressure drop is strongly affected by sandpack permeability, especially when permeability is low. The oil phase viscosity in this experiment was kept constant at 47.4 cP and the emulsion was injected at 0.5 ml/min. As sandpack permeability increased from 1.5 darcy to 3.0 darcy, the pressure drop to reach a steady state flow decreased sharply from 13.2 kPa to 3.0 kPa. The pressure drop across the sandpack decreased less significantly when permeability was further


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increased from 3.0 to 4.5 darcy and stabilized at 1.7 kPa. Such results indicate that more droplets are trapped in sandpacks with lower permeability. The flow tests were repeated for other oil viscosities. Figure 9 shows that the steady state pressure drop decreases as the sandpack permeability increases for all emulsion samples. Also, emulsions with higher oil viscosities lead to a larger pressure drop across the sandpack. For example, for the sandpack with a permeability of 1.5 darcy, the pressure drop was as high as 24 kPa for a selected emulsion sample (i.e., emulsion with oil viscosity of 496 cP), demonstrating great potential in the profile control of the formation. However, for the sandpack with a permeability of 4.5 darcy, pressure drop was below 4 kPa, no matter which emulsion sample was applied. The sandpack with the permeability of 3.0 darcy had a moderate pressure drop for the emulsions investigated in this study. Sandpack permeability is an indicator of the ratio of the droplet size to the pore size. Lower permeability means that there exists more contact area between the oil droplet and the pore wall. Viscous resistance is affected by both oil viscosity and total contact area between oil and pore wall. As Figure 9 shows, the relationship between pressure drop and permeability is represented by a curve with a decreasing slope. The degree of curvature is larger for higher oil viscosities, indicating that emulsions with higher oil viscosities are more sensitive to permeability changes.

Figure. 8 Pressure drop across sandpacks with different permeabilities during emulsion injection (oil viscosity = 47.4 cP, injection rate = 0.5 ml/min)



Figure. 9 Pressure drop profiles with different sandpack permeability for all emulsion samples 3.3.3 Effect of the injection rate The injection rate affects both viscous resistance and capillary trapping. Figure 10 shows the pressure drop under different injection rates in the sandpack with a permeability of 3.0 darcy. Higher injection rate leads to a higher steady state pressure drop. In addition, the steady state pressure drop was attained at slightly higher volume of injected emulsions. This is possibly because upon reaching steady state, the average size of droplets being trapped in the sandpack are larger for higher injection rates. As injection rate increases, the driving pressure increases. The small droplets that are previously trapped at low pressure will be pushed through the porous media, replaced by larger droplets. More emulsions are required for the supplement of large droplets. Figure 11 shows that the plugging performance of the different emulsion samples were influenced by the injection rate. Comparing Figure 9 with Figure 11, one can see that the influence of injection rate is less significant than that of sandpack permeability. Moreover, the relationship between pressure drop and flow rate is nearly linear, especially for emulsions with low oil viscosities.


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Figure. 10 Pressure drop across sandpacks during emulsion injection with different injection rates (emulsion oil phase viscosity = 47.4 cP, sandpack permeability = 3 darcy)

Figure. 11 Pressure drop profile with different injection rates for different emulsion samples (sandpack permeability = 3 darcy) 3.4 Effective viscosity of the emulsions The effective viscosity of emulsion is commonly used to represent the emulsion rheology in porous media, which is calculated using the following equation derived from Darcy’s law,

' where '+ is the viscosity of water, cP;

() *"

and

*)

(2)

+

are the steady state pressure drops for the

emulsion and water, respectively, kPa. The effective viscosity of emulsion corresponds to the viscosity of single phase Newtonian fluid that gives the same shear stress at the same shear rate. It accounts for all the rheology behavior of emulsion when flowing in porous media such as droplet deformation and droplet-wall friction. Figure 12 shows that, although all emulsion samples share similar bulk viscosities (between 1.45 cP and 1.84 cP), their effective viscosities



are significantly different. Figure 12a indicates that the effective viscosity increases as oil viscosity increases, but with a significantly decreasing slope. For example, in the 1.5 darcy sandpack, an emulsion sample with 9.4 cP oil displayed an effective viscosity of 7.4 cP, whereas the emulsion sample with 496.0 cP oil displayed an effective viscosity of 24.8 cP, for the same injection rate of 0.5 ml/min. The trend between effective viscosity and oil viscosity appears to level off for all flow rates and permeabilities. Two possible explanations are proposed for such trends.

(a) Sandpack permeability = 1.5 darcy

(b) Sandpack permeability = 3.0 darcy

(c) Sandpack permeability = 4.5 darcy

Figure.12 Effective viscosity of emulsions with different oil phase viscosities The first explanation relates to the increase in the slipping effect with oil viscosity. Wall slipping occurs at the fluid-solid interface when the force of attraction between fluid molecules and solid molecules is weaker than that between fluid particles. Since the sand grains are water wet, oil droplet cannot displace all the water as it passes through the pore throat. A thin film of monolayer, or a few layers, of water molecules adheres to the pore. Such adhesion greatly reduces the attractive force between the pore wall and oil droplet and results in significant


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slipping behavior, as schematically depicted in Figure 13. The influence of wall slipping is negligible at macroscopic scale but is important at microscopic pore throats. Previous studies showed that wall slipping is affected by the fluid viscosity.40-43 Craig et al. reported that a low viscosity fluid does not induce slipping and the slip length increases with the fluid viscosity.40 This observation was confirmed by other researchers.41,42 The slip length measures the distance where the tangential velocity component vanishes relative to the wall, which is correlated to the degree of wall slipping. Increased wall slipping induces lower velocity gradient, hence, leads to less shear force on the droplet. The shear stress is, nevertheless, proportional to the viscosity of the fluid. These two opposing effects; namely lower velocity gradient and higher fluid viscosity, lead to higher overall shear force, however not proportional to the increase in viscosity of the oil droplet.

Figure. 13 Schematic representation of wall slipping as oil droplet enters the pore throat. Another explanation is that frictional resistance is not only affected by oil viscosity, but is also influenced by the flow path. Once a pore throat is plugged, the subsequent emulsion will change its original direction and diverges into other channels. Once steady state is reached, several fixed flow paths would be established in the porous medium. Figure 14 illustrates the flow paths at steady state for different oil phase viscosities. As oil viscosity increases, the flow through narrow pore throats will become more difficult. Such paths are either blocked or only allow very low flow velocity. Most droplets will have to travel through paths where the pore throats are larger in size. For higher oil viscosity, the difficulty in traveling through narrow pore throats increases significantly. However, the flow in broader paths are less affected since there is



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less contact between the droplets and the pore wall. As a result, the frictional resistance does not increase proportionally with the oil viscosity. This explanation is also confirmed by comparing the pressure drop profile at different flow rates (Figure 11). For each oil viscosity, a nearly linear relationship between pressure drop and flow rate is obtained, indicating a steady flow in the established flow paths. It should also be noted that higher injection rates also lead to a lower effective viscosity for the emulsions. This is because the effective viscosity is proportional to the ratio of the pressure drop for emulsion and water. As flow rate increases, the pressure drop for water increases linearly, while the pressure drop for emulsion does not increase at the same rate.

(a) low oil viscosity

(b) medium oil viscosity

(c) high oil viscosity

Figure. 14 Schematic representation of flow paths for different oil viscosity emulsions at steady state (arrows indicate the flow direction)

4. Conclusions Experimental tests were conducted to investigate the effects of oil phase viscosity on plugging performance of O/W emulsion in porous media. The following conclusions can be drawn from this study: (1) Oil phase viscosity considerably affects plugging performance of O/W emulsion in porous media. For example, the pressure drop increases 3.8 times when oil viscosity is increased from 9.4 to 496.0 cP in a sandpack with a permeability of 3.0 darcy. Increasing the oil viscosity results in a higher frictional resistance when droplets flow through the pore throats. (2) The pressure drop increases with oil viscosity, but levels off at high viscosities. Accordingly, continuous increase in oil viscosity does not improve the plugging performance. This is possibly resulted from the increased slipping behavior and the change of path where emulsions travel through. (3) The effect of oil viscosity on plugging performance is strongly affected by the sandpack permeability, especially for low permeabilities. As sandpack permeability increases from 1.5 to 3.0 darcy, the pressure drop reduction for emulsions with high oil viscosity can be 3.3 times larger than emulsions with low oil viscosity.


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(4) The effects of oil viscosity on plugging performance are also dependent on the injection rate of emulsion. Higher injection rates lead to higher pressure drops along the sandpacks. The relationship is nearly linear for all oil viscosities when the injection rate is in the range of 0.5 to 1.5 ml/min. (5) The effective viscosity of emulsion increases as the oil viscosity increases and decreases as the injection rate increases. This study implies that in field applications of O/W emulsion, on the condition of meeting stability requirement, it is favorable to choose oil with higher viscosity for achieving better plugging performance. Alternatively, it is unnecessary to pursue an extra-high oil viscosity as the improvement significantly tapers down. Droplets with extra-high viscosity may also show solidlike behavior when flowing through the pore throat, leading to demulsification and formation damage. The wettability and surface roughness of reservoir rock also affect the slipping behavior and the frictional resistance. Further investigation needs to be conducted on this topic.

Acknowledgement The authors acknowledge with thanks support from BitCan, PetroChina Canada and Natural Sciences and Engineering Council (NSERC) through a NSERC CRD grant.

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Graphics for manuscript

Figure.1 Diagram of the experimental setup for emulsion flow in sandpacks

9.4 cP

23.2 cP

47.4 cP

201.7 cP

496.0 cP

Figure. 2 Stability scan of emulsion samples with varied oil viccosities at 22 oC after 4 hours

1 ACS Paragon Plus Environment


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Figure. 3 IFT values for the different oil phases used in this study at 25 oC.

(a) 9.4 cP

(b) 23.2 cP

(d) 201.7 cP

(c) 47.4 cP

(e) 496.0 cP

Figure. 4 Image of droplet distribution for all emulsion samples


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Figure. 5 Droplet size distribution for all emulsion samples

Figure. 6 Pressure change across sandpacks during injection of emulsions with different oil phase viscosities (sandpack permeability = 3.0 darcy, injection rate = 0.5 ml/min)



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Figure. 7 Schematic representation of an oil droplet flowing through a pore throat

Figure. 8 Pressure change in sandpacks with different permeabilities during emulsion injection (oil viscosity = 47.4 cP, injection rate = 0.5 ml/min)


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Figure. 9 Pressure drop profiles with different sandpack permeability for all emulsion samples

Figure. 10 Pressure change in sandpacks during emulsion injection with different injection rates (emulsion oil phase viscosity = 47.4 cP, sandpack permeability = 3 darcy)



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Figure. 11 Pressure drop profile with different injection rates for different emulsion samples (sandpack permeability = 3 darcy)

(a) Sandpack permeability = 1.5 darcy

(b) Sandpack permeability = 3.0 darcy

(c) Sandpack permeability = 4.5 darcy

Figure.12 Effective viscosity of emulsions with different oil phase viscosities


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Figure. 13 Schematic representation of wall slipping as oil droplet enters the pore throat.

(a) low oil viscosity

(b) medium oil viscosity

(c) high oil viscosity

Figure. 14 Schematic representation of flow paths for different oil viscosity emulsions at steady state


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Effects of oil viscosity on the plugging performance of oil-in-water

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