Smart Profile Control by Salt-Reversible Flocculation of Cationic

Oct 22, 2013 - A flocculation system is detailed to deal with the size matching problem between pore throats and microgels for profile modification in...
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Smart Profile Control by Salt-Reversible Flocculation of Cationic Microgels and Polyacrylamide Guanghui Li,†,* Guicai Zhang,† and Lei Wang‡ †

School of Petroleum Engineering, China University of Petroleum, Qingdao, Shandong 266555, People’s Republic of China Department of Petroleum Engineering, Colorado School of Mines, Golden, Colorado 80401, United States

‡

ABSTRACT: A flocculation system is detailed to deal with the size matching problem between pore throats and microgels for profile modification in high permeable formation. This system consists of partially hydrolyzed polyacrylamide (HPAM) and cationic microgels synthesized by inverse suspension polymerization with acrylamide and methylacryloxylethyl trimethyl ammonium chloride as monomers. By adding HPAM into cationic microgel suspensions, flocculation occurs because of electrostatic attraction. Then it reversibly restructures from bulk to an insulated microgel particle at a critical salt concentration (CSC). In the profile modification test, this flocculation system diverts nearly 100% chase water into the low permeable tube compared to 20% by using microgels only. The performance improvement testifies the mechanism that HPAM flocculates around the cationic microgels to make up the interstitials between microgels and irregular pore throats. This flocculation plug can be removed by water with salinity above CSC due to the desorption of HPAM on cationic microgels.

1. INTRODUCTION Due to the washing out of excessive injection, most matured water-flooding reserviors are sufferring large permeability variance, yielding water channels of high permeability, and leaving significant oil unswept. Therefore, facile profile modification treatment for high permeable formation is more urgently needed than ever before. Considerable attention has been paid to gel technology with varied formulations for different reservoir conditions, usually including bulk gel,1 colloidal dispersion gel,2,3 and preperformed gel.4,5 However, most of these methods have been proved only marginally effective when dealing with water channeling in high permeable zone.6−8 In 1996, the Industry Consortium (BP, Chevron, Texaco, and Nalco Company) developed a novel “Bright Water” system, which was a microgel emulsion with particle diameters around 0.1−3 μm.9 And this system has been practiced for more than 85 treatments since the first application in Minas Field, Indonesia. But all of the successful applications published are restricted to the formation with permeability lower than 1500 × 10−3 μm2 (mD).10−13 For more permeable formation with large permeability range, size mismatching between microgels and pore throats probably discounts its performance. To deal with this mismatching problem, Yao et al.14 defined a matching factor, which is the ratio of particle diameter to average pore-throat diameter. They found that great profile-control performance could be achieved at the narrowly optimal matching factor 1.35−1.55; while beyond this range, the mismatching occurred and resulted into poor effect in profile control. This mismatching problem can be schematized by the large permeability of interstitial between irregular pore throats and trapped microgels, which is illustrated in Figure 1(a). To solve the above-mentioned problem, partially hydrolyzed polyacrylamide (HPAM) is carefully considered due to its flocculation effect with cationic particle. The flocculation phenomenon between microgels and opposite polyelectrolyte © 2013 American Chemical Society

Figure 1. The schematic diagram of plugging effect by microgels in water channel.

has been extensively studied in water treatment, papermaking, and biomedical engineering.15−18 And the flocs can restructure and change phase behaviors with component properties17,19 and environmental conditions.18,20 This restructuring feature gives an inspiration for solving the problem in an environmentally-sensitive way. In this work, microgels with a series of cationic degrees (CD, defined by mole fraction of cationic monomer in all monomers) were synthesized by inverse suspension polymerization with acrylamide (AM) and methylacryloxylethyl trimethyl ammonium chloride (DMC) as monomers. These particles can first narrow pore throats by retention on the pore walls. And then HPAM injected subsequently flocculates around these particles, making up the interstitials to solve the mismatching problem, as shown in Figure 1(b). The properties of these particles and the plugging mechanisms are validated in the following sections. Received: September 18, 2013 Revised: October 22, 2013 Published: October 22, 2013 6632

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silicon sand. The diameter of glass bead lies within 170−430 μm with an average value of 340 μm. The corresponding mean diameter of the pore throat is calculated to be 52.63 μm on the premise of hexagonal close packing, in which the pore-throat diameter is 1/6.46 times of the average bead diameter.23 The tube packing method was described by Ding, et al.24 The test procedure is as follows: (1) inject water; (2) inject 0.5 pore volume (PV) microgel suspension with mass concentration of 0.4%; (3) inject 0.5 PV HPAM with concentration of 2000 mg/L; and (4) switch the flow direction of the low permeable tube to avoid blocking of the sand filter net at the end of tube after the previous two slugs, then continue waterflood until flow pressure is steady. A control experiment was done with an additional procedure after the four slugs. This procedure was that the highly permeable tube was flooded with 2%NaCl solution for 5PV. The tube permeabilities in the control experiment were 5323 and 382 mD, respectively. In all processes, pressure was recorded by pressure transducer and outflow was weighted to determine the flow fraction of these two tubes.

2. MATERIALS AND METHODS 2.1. Microgel Synthesis. Acrylamide (analytically pure) was recrystallized by chloroform and 78 wt % water solution of methylacryloxylethyl trimethyl ammonium chloride (DMC) was extracted by benzene. Methylene bisacrylamide (MBA) was used as the cross-linker. Span80 (Sorbitan monooleate, chemically pure) and 2,2′-Azobis [2-(2-imidazolin-2-yl)propane] dihydrochloride(Va-044) were used as dispersant and initiator, respectively; Industrial white oil, with a boiling range between 150 and 300 °C, was chosen as continuous phase. Partially hydrolyzed polyacrylamide (HPAM) was determined with the intrinsic viscosity of 2.087 L/g in 1 mol/L NaCl solution at 30 °C and hydrolysis degree about 25.2%. The inverse suspension polymerization was carried out as the following procedures. The oil phase was prepared by 0.60 g Span80 and 59.40 g white oil. The aqueous phase consisted of 50% comonomer (AM/DMC), 1% MBA, 0.05% Va-044, and 48.95% water (based on the total weight of water phase). Then, oil phase was put in four neck round-bottomed flask and stirred at 450 rpm in 50 °C water bath. Nitrogen purging was carried out for 15 min before 30 g aqueous phase was added by dropwise using a syringe pump at a rate of 1.0 mL/min. The reaction mixture was maintained for another 2 h after the addition process. Samples were withdrawn at the end of the reaction using a hypodermic syringe to measure the particle size distribution. In order to avoid irreproducibility, the sample point was labeled on the reactor and fixed at 3 cm away from the impeller shaft. At the end of the reaction, stirring was stopped and the two phases, white oil and copolymer particles, were obtained from top and bottom, respectively. Then these particles were rinsed by acetone and dried under a primary vacuum at 50 °C for 24 h. The final products were white powder and used for other experiments. 2.2. Morphology and Particle Size. Morphology and particle size were analyzed by observing under microscope. The microscope accuracy is 0.4 μm, and amplification factors are 40, 100, 400, and 1000. At least 400 particles were numerated to calculate the mean diameter and particle size distribution. 2.3. Zeta Potential. The products were diluted by freshly deionized water to 0.1% in mass concentration. The particle mobility was recorded by visual microscope of electrophoresis meter at 20 °C. The microsphere products with cationic degree of 5%, 10%, 15%, 20%, 25%, 30%, and 40% were all determined. For each product 10 runs were taken. Then the zeta potential was further calculated by Smoluchowski formula which was built in the attached software of the meter. 2.4. Flocculation and Adsorption. The HPAM solution with concentration of 10 000 mg/L was added by dropwise into 50 mL 0.1 wt % microgels suspension, and flocculation formed immediately due to the electrostatic interaction. This flocculation can disappear due to the addition of sufficient salt and violent oscillation, so the critical salt concentration (CSC) for this phase change was evaluated for microgels with varied CDs. Additionally, the adsorption amount of HPAM on the particles with 25% CD was determined as a function of HPAM concentration at 20 °C by a semiquantitative method. The principle is that the viscosity of HPAM is proportional to its concentration in dilute concentration regime.21,22 This method was performed as the following procedures. The above flocculation system was oscillated for 2 h in thermostat, which allowed the sufficient reaction. Then it was centrifugated at 4000 rpm for 30 min to get supernatant. The viscosity of supernatant was measured by Brookfield Viscometer LVDVII plus pro with ULA spindle at 20 °C. According to the viscosity-concentration standard curve of HPAM, the adsorption capacity was obtained regardless of possible preferential adsorption for HPAM with different molecular weight. 2.5. Profile-Control Test. The objective of the combined HPAM and cationic microgels is to selectively block highly permeable channels and divert subsequent water to sweep low permeability zone. So this test was implemented by paralleled sand-pack tubes of 30 cm in length and 2.5 cm in diameter at 20 °C. These two tubes were packed to get different permeabilities, the higher one of 5092 mD with 40−60 mesh glass beads and the lower one of 301.3mD with 80−120 mesh

3. RESULTS AND DISCUSSION 3.1. Morphology and Particle Size. A series of microgels with cationic degrees of 5%, 10%, 15%, 20%, 25%, 30%, and 40% was synthesized. The particle morphology of 25% CD microgels is shown as a typical example in Figure 2. It can be

Figure 2. The morphology photo of microgels with 25% cationic degree. The scale bar is 200 μm.

seen that most microgel particles are closely spherical and have a wide distribution. The 25% CD microgels were used for the following profile control test. As shown in Figure 3, the swollen size distribution ranges from several to more than 100 μm, and conforms to logarithm normal distribution. It can be observed

Figure 3. Size distribution of swollen microgels with 25% cationic degree. 6633

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restructuring of flocs, it opens a way using this flocculating system to control profile with formation salinity lower than CSC and remove the flocculation by injecting higher salinity water. Further, this structure evolution also leads to capacity change of HPAM adsorption on cationic microgels, as shown in Figure 6. The presented curve indicates that the adsorption process of

as well that the mean diameter is 63 μm with standard deviation about 0.518. 3.2. Flocculation and Adsorption. Due to the electrostatic attraction between cationic microgels and HPAM, microgel suspension flocculated rapidly when it was dripped by HPAM solution. These flocculation structures varied at different salt concentrations. Taking 25% CD microgels as an example, the floc structures transform from bulk [in Figure 4(inset b)] to insulated particles [in Figure 4(inset a)] at the

Figure 6. The relationship of HPAM adsorption capacity to its concentration at 20 °C.

Figure 4. The evolution of flocculation morphology for microgels with varied cationic degrees as a function of salt concentration. Insets (a) and (b) are photos of the flocs with salt concentration higher and lower than restructuring salt concentration, respectively. The scale bar is 200 μm in these photos.

HPAM on microgels can be represented by the adsorption curve of Langmuir type. In this process, the HPAM fast adsorbs on microgel surface, initially with a flat configuration. As the polymer concentration increases, the subsequently adsorbed polymer molecules may “crowd” the flat adsorbed polymer molecules into the spacial configuration. Considerable rearrangement of the polymer segments on the surface is likely to take place during this crowding stage and maximum adsorption may be reached slowly.26 According to the adsorption curve, the maximum adsorbing capacity is about 409 mg/g in 0 wt % NaCl solution and lowers to 237 mg/g in 2 wt % NaCl solution, which indicates the desorption feature of HPAM under high salinity. It is reasonable that the desorbed HPAM can be easily carried away within removing flocculation by higher salinity water. According to this adsorption curve, the HPAM usage for sufficiently flocculating 0.4 wt % microgels is designed to be 2000 mg/L in next profile control test. 3.3. The Profile-Control Test. Figure 7 shows the pressure response (bottom) and flow fractions (top) in the profile control test. From Figure 7, the initial injection water goes along the high permeability tube with fairly low pressure

critical salt concentration (CSC). When diluted by distilled water, the isolated floc reverses back to bulk state. A similar phenomenon is found for microgels with different CDs, and the CSC increases with the CD, as indicated by the line in Figure 4. This structure evolution is consistent with rearrangement process of adsorbed polyelectrolyte on the polystyrene particles investigated by Adachi, et al.15 And it can be explained that when salt concentration is lower than CSC, the polyelectrolyte coil has a spatially adsorbing conformation with tails and loops extending outside the electrostatic double layer of microgels, so a bridging effect occurs; and beyond this concentration, polyelectrolyte coils tend to be a flat adsorbing conformation, resulting in the insulated floc structure.17,19 Besides, in Figure 5, the zeta potential increases to the maximum at 30% CD, corresponding to the tendency of CSC.

Figure 5. The zeta potential changes as a function of cationic degree.

The reason is that the higher Zeta potential, the denser binding sites between microgel surface and polyelectrolyte, rendering that HPAM prefers being adsorbed in the patch-like conformation,25 so higher salinity is required to compress polymer coils into a flat geometry. In view of reversible

Figure 7. The pressure response (bottom) and flow fraction (top) change with injection volume. 6634

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permeable tube smoothly during injection. While injecting 2% NaCl solution, a wave-type pressure is observed before the injection volume reaches 1 PV, and afterward it gradually decreases to 0.074 MPa, which is very close to the stable injection pressure of 0.065 MPa of microgels. This wave-type pressure indicates the advance of flocs deep into the tube. Similar pressure curves are found on preperformed gel studied by Bai et al27 and microgels with optimal matching factor studied by Yao et al.14 The continuous decrease in pressure indicates that the flocculation becomes movable because of HPAM desorption in water of high salinity. As is stated above, the flocculation produced by HPAM and cationic microgels has potential to modify profile for the high permeability formation, and avoid the mismatching problem between particles and pore throats in using microgels only.

response of 0.004 MPa; then, microgels are injected and get ten times of initial pressure response, but the flow fractions are stable at 80% for high permeability tube and 20% for low permeability tube. This poor performance indicates an insufficient plugging due to the mismatching as shown in Figure 1(a), even though the mean diameter of microgels is 1.2 times larger than the mean pore throat of glass beads. In the HPAM slug, the pressure gradually rises up to 0.45 MPa, possibly attributing to the flocculation reaction, but the flow fraction of low permeable tube reversely goes down due to the entrance block of the low permeable tube by this reaction. In the final slug, water breaks through these flocculation block formed in the previous two slugs when the pressure goes up to 0.3 MPa, and then the flow is nearly completely diverted to the low permeable tube, demonstrating great performance of profile modification. After the test, three samples were taken from the location 1, 2, 3 of the highly permeable tube in Figure 8(a). The

4. CONCLUSIONS To deal with the mismatching problem between microgels and pore throats in highly permeable formation, a smart flocculation system sensitive to salinity was successfully designed. This system consisted of HPAM and cationic microgels synthesized by inverse suspension polymerization. The floc structures formed by electrostatic attraction between these microgels and HPAM reversibly change from bulk to an insulated microgel at critical salt concentration (CSC). The adsorption curve of HPAM on cationic microgels conforms to the Langmuir-type adsorption law and its adsorption capacity on 25% CD microgels decreases from 409 mg/g at 0% NaCl to 237 mg/g at 2% NaCl. This flocculation system shows far better performance in a paralleled tube test than that of microgels only, which proves the assumed mechanism that HPAM flocculates around cationic microgels to plug the interstitials between microgels and pore throats. This flocculation plug can be removed by water with salinity above the CSC.

Figure 8. The schematic setup of paralleled sand pack and adsorption morphology corresponding to the sample points. Part (a) is the schematic of paralleled sand pack; parts (b), (c), and (d) are photos of samples taken in high permeable tube from points 1, 2, and 3, respectively. The scale bar is 200 μm in the photos.

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morphologies of these samples are presented in Figure 8(b−d). It can be seen in the Figure 8(b),(c) that few cationic microgels are detained, which rules out possible performance contribution from the redistribution of microgels in depth; while a mount of flocculation is observed, as shown in the red circle of Figure 8(d), consolidating that the injected HPAM flocculates around the cationic microgels and the interstitials are plugged as the assumed mechanism in Figure 1(b). Figure 9 shows the pressure contrast of two steps: microgel injection and 2% NaCl injection in the control experiment. As shown in Figure 9, the microgels transport in the highly

AUTHOR INFORMATION

Corresponding Author

*Tel: +86 532 86981178; e-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors appreciate the great help from the Enhanced Oil Recovery Center in Oilfield Chemistry Department, China University of Petroleum.

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Figure 9. The pressure contrast of two steps: microgel injection and 2% NaCl injection in the control experiment. 6635

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