Preparation and Properties of Polymer/Vermiculite Hybrid Superabsorbent Reinforced by Fiber for Enhanced Oil Recovery

A series of polymer/clay hybrid superabsorbent composites (SACFs) comprising acrylamide, acrylic acid, sodium 2-acrylamidotetradecyl sulfonate, fiber, and vermiculite by in situ intercalation and exfoliatedmethodwas successfully synthesized.The structure of SACFs was characterized by IR, SXRD, and SEMmeasurements. Much notable absorbency for SACF-2 was observed compared to that for SACF-1 in the absence of hydrophobic group in the high cationic solution due to the alkyl carbon chain and sulfonic acid group of hydrophobicmoistures protecting the cations from attacking the carboxylate groups.What is more, high temperature fiber which acts as bridge connection for the polymeric network structure enhanced both toughness and strength for SACF-4 in the harsh conditions. At the total dissolved substance of 212000mg/L for Tarim Basin injected water and the temperature of 120C, desired absorbency as well as water retaining property for SACF-4 was observed during the long period of thermal ageing. Core flooding experiments demonstrated that SACFs could migrate as amoeba in the porous medium and accumulated in the narrow channel to adjust injection profile, promoting the subsequent water diverting into the unswept zones. Finally, characteristic parameters for SACFs calculated from flooding experiment further confirmed these polymer/clay hybrid composites reinforced by fiber would have robust application in the mature oilfield for profile control.


Introduction
As for the mature oilfield, oil production rapidly decreased with water cut from oil well increasing because of years of water flooding making reservoirs much more heterogeneous [1].Polymeric matrix hybrid clay superabsorbent attracted considerably industrial interests due to their swelling and water retention in the porous medium.Moreover, such granular superabsorbent can migrate like amoeba in the reservoir, adjust injection profile, and prompt the subsequent water flowing into the fresh formation, implementing deep profile control [2,3].
However, conventional polymeric networks such as polyacrylamide/montmorillonite (MMT), epoxy resin/MMT, and poly(acrylamide/acrylate)/kaoline composites suffer great deal in the harsh conditions with reservoir temperature higher than 100 ∘ C and total salt concentration higher than 100000 mg/L being rich in divalent oins [4].Such superabsorbent containing flexible groups, for instance, carboxyl group and amide pendent, displays uncontrolled swelling, poor toughness and quick water release.Such drawbacks limit particle gels' wide application in mature oilfields [5].It is noted that hydrophobic groups modified polyacrylamide hybrid clay exhibits reasonable thermal endurance and salt resistance [6,7].What is more, interpenetrating polymeric network enhanced by nature materials, such as polysaccharide [8], fiber [9,10], and rubber [6], shows more stability in mature formation, guaranteeing particle gels as good candidates for water shutoff and profile-control operations.
In this paper, alkyl sulfonate hydrophobic monomer modified polymeric vermiculite superabsorbent gel reinforced by fiber is preferentially synthesized.The structure of gels was characterized by Fourier transform infrared (FTIR), X-ray diffraction (XRD), and scanning electron microcopy (SEM).The effect of temperature and salt concentration on the water swelling and aged stable properties for particle gels has been investigated.Finally, three groups of core displacement experiments have been conducted to measure their shut-off capacity and profile control ability.To our knowledge, it is the first to report superabsorbent filled by fiber used as profile control agent for enhanced oil recovery.A series of superabsorbent composites composed of AM, AA, and high temperature tolerance fiber (HTF) was synthesized by varying the amount of NaAMC 14 S proportion.8.25 g AM, 1.75 g AA, and appropriate amount of NaAMC 14 S were dissolved in desired volume of deionized water.The pH value of the mixture was tuned to 7.0-7.2using sodium hydroxide solution.Then approximate 20 g CVMT and 0.5 g HTF was added into the mixture solution, and added some volume of deionized water to produce the 20 wt% for total polymerizable monomer (shown in Table 1).The resulting mixture was conducted with a small N 2 stream for half an hour and then kept still for 2 hours to provide monomer intercalation into CVMT.Subsequently, the solution was heated to 45 ∘ C. Polymerization in situ is initiated by a small amount of V-50 [12] and CVMT exfoliation by polymeric gel is produced by addition of a small proportion of NMBA.The resulting bulk gel was washed twice using acetone and extracted with ethanol to remove any impurities.The product was dried under vacuum at 90 ∘ C for 6 h to constant weight and mashed bulk particles into 40-mesh powders.

Experimental Section
2.5.Characterization.IR was carried out using Shimadzu-1800S spectrometer on KBr pellets in the range of 4400-400 cm −1 .The peak intensities are characterized as follows: vs = very strong, s = strong, w = weak, and v corresponds to a stretching elongation.Small-angle X-ray diffraction (SXRD) measurement was recorded on a Rigaku Geiger Flex D/max-RB diffraction meter using Ka radiation (40 kV, 30 mA).The distance between two layers of crystal plate was calculated through Bragg equation: 2 sin  = , where  is the layer distance, nm,  is the diffraction angle,  is the length of the diffraction light, 0.154 nm, and  is the diffraction series, in the experiment  = 1.The morphology of samples was observed by Environment Scanning Electron Microscope (SEM XL 30).The sample solutions were maintained at −3.5 ∘ C and the pressure was controlled below 5 Torr to keep samples solution state during the whole observation.

Measurement of Water Absorbency of SACF.
A certain weight ( 0 ) of SACF was immersed into excess volume of salt solution and allowed to swell at the set temperature.After a period of time, the swollen SACF was filtrated through an 80mesh sieve to remove free water and weighed (  ).The water absorbency was calculated using the following equation: where   is the water absorbency per gram of dried SACF powder.
2.7.Core Flooding Experiment.Six Berea sandstone cores were preferentially prepared and the basic parameter of cores is given in Table 1.Flooding instrument and procedure of the experiment are depicted in Figure 1.The flooding rate for both water flooding and swelled particle gel flooding (40mesh SACF particle was swelling in the salt solution for 10 days) was set as 0.5 mL/min and kept unchanged.Injected pressure versus particle gel solution slug was automatically scaled by the flooding instrument.The residual resistance factor ( rr ) and the shut-off efficiency () of particle gel can be calculated through the inference of Darcy Formula [1]: Herein,   is aqueous phase permeability, mD;   is the core permeability after injection of some designed pore volume (PV) of SACF particle gel, mD.corresponds to a basal spacing of 1.77 nm compared to 1.43 nm pure UVMT [7], indicating that the basal spacing of original UVMT was expanded 0.34 nm after procedure of CTAC treatment.This peak was not observed in the SXRD pattern of the SACF, which indicated the fact that the crystalline plate had been further exfoliated by polymeric gel after the in situ copolymerized and crosslink reaction in the crystalline plates of polymeric monomer.The absence of such characteristic diffraction peak in the SXRD pattern of SACF was 2 = 4.17 with 2.35 nm of basal spacing.We could speculate that nanocomposite containing exfoliated UVMT and polymeric matrix was successfully synthesized by successive intercalation of CTAC and polymerizable monomers and in situ reaction of monomers.

SEM Morphology.
In order to investigate polymer/ silicate nanocomposite structure, SXRD analysis should be always accompanied with microstructure observation, such as TEM and SEM.This is because the coexistence of stacked interacted, disordered or exfoliated structure that a SXRD pattern may hide large part of exfoliated layers [6].Typical SEM morphologies of the nanocomposite of original UVMT and of various modified UVMT are depicted in Figure 4.
The unmodified UVMT nanosheets disorderly stacked to form multilayer aggregation due to the Van der Waals' force between adjacent layers (Figure 4(a)).Thus, neither monolayer nor exfoliated layer was existence.The original silicates were loose and porous enough for monomer intercalation and exfoliation.Poly(AM/AA/NaAMC14S)/CVMT morphology is depicted in Figure 4(b).Loose and multihole nanostructure with exfoliated silicate layer dispersed in the polymeric matrix was observed.The smooth network for polymeric bulk gel was disrupted when introducing proper silicate, producing the wrinkled and multihole microstructure which facilitated the external water into the network structure and prevented moisture loss.This confirmed that polymer exfoliated the multilayer plate pretreated by CTAC and the dispersion of nanocomposite into the polymeric matrix is well prepared.The microstructure of polymer/silicate composite filled by high temperature resistance fiber (HTF) is shown in Figure 4(c).The white strips in the Figure 4(c) represent the HTF, which randomly disperse in the polymeric matrix.The morphology of nanocomposite was displayed as scales, suggesting that fiber acted as linkage of the network structure in composite, reinforcing the toughness and strength of nanocomposite.

Swelling Behavior of SACF.
The effect of AM, AA, and clay content on the swelling properties of polymer/silicate composite has been extensively reported, and the understanding of these properties has been constructed.Here, we are focused on the hydrophobic group (NaAMC 14 S) and fiber content upon the composite absorbent properties in the salt solution.

Effect of NaAMC 14 S Content on Water Absorbency.
A typical profile of water absorbency for SACFs as a function of various ionic strengths in the salt (NaCl, MgCl 2 , and CaCl 2 ) solution is depicted in Figure 5.The water absorbency of SACFs was maximum value in the aqueous solution.Such value decreased as the ionic strength of all three cationic salt solutions increased.One observed that hydrophobic moistures play a significant role in controlling absorbency (SACF-2) in the salt solution compared to that of absorbency for polymer/silicate without hydrophobic group.For SACF-2, the curve of water absorbency decreased slowly with increasing ionic strength.This can be explained such that the increasing cationic concentration compressed the loose network by screening the negative charges of carboxylate group, resulting in the release of water in the salt solution.The hydrophobic microdomain formed by alkyl carbon chain of SACF-2 prevented the cations from attacking the carboxylate group.On the other hand, sulfonic acid groups brought into chain backbone can partly enhance composite slat resistance.Therefore, more notable absorbency for SACF-2 was observed in comparison to SACF-1.The water absorbency in the monovalent salt solution is higher than these for divalent (calcium and magnesium ions) salt solution.This is because divalent cationic would interact with carboxylate group to form complex.

Effect of HTF on Water
Absorbency.The curve of water absorbency for SACFs versus cationic concentration is shown in Figure 6.For SACF-2 in the absence of HTF, much more absorbent capability was observed in comparison to SACF-4 filled by small amount of HTF in the low salinity solution.With further increasing salt concentration, the absorbency for SACF-2 and SACF-4 was decreased.However, the decreased trend of absorbency for SACF-4 is much more flatted than that of SACF-2.This can be interpreted such that the hydrophobicity of fiber would partly hinder the water suction, resulting in the lower water absorbency [13].On the other hand, ionic strength with high salt concentration is the dominating factor affecting the sucking ability of composite.Elevated cationic concentration screened the carboxyl groups, producing the absorbent gel water release.Hence, similar absorbent values for SACF-2 and SACF-4 at the highest salt concentration are observed.

Salt and Thermal Stability for SACF in the Harsh
Condition.The water absorbency for SACFs depended on aged time in the various salt concentrations at 120 ∘ C as shown in Figure 7.At the superhigh temperature of 120 ∘ C, salt concentration significantly affected the absorbency and water retention of SACF, which controlled the absorbent profile against aged time.The absorbency of SACFs increased rapidly within 10 days ageing and then remained stable at the total dissolved concentration of 53000 mg/L of diluted Tarim Basin formation water.The dynamic swelling behavior of SACF was consistent with Kelvin-Viogt viscoelastic model reported by Wu and Liu [10].SACF-4 reinforced by fiber exhibited lower absorbency in comparison to SACF-2 in the absence of fiber.At the ultrahigh salt concentration, the dynamic swelling behavior of SACFs is more complicated.
The absorbency rapidly increased in the initial aging stages and the maximum time of SACFs expansion is displayed.Subsequently, the released-water behavior of SACFs shrinks such particle gels.We noted that SACF-4 exhibits slow waterreleased property, and absorbent index kept stable after 40 days of ageing.With further prolonging ageing period, few swelling gels broke into smashes, indicating that reasonable thermal stability as well as enough toughness for SACF-4 was given by introducing hydrophobic group and fiber into the superabsorbent composite.For SACF-2, notable released-water behavior shrinks swelling gels during 35 days of thermal ageing.With increasing ageing period, few swelling gels broken into smashes result in absorbent gels' further shrinkage.The different thermal stability for SACF-2 in the salt solution can be interpreted by the following reasons: (1) the partly degraded polymeric matrix defects network structure leading to the undesirable water-retaining capability for absorbent composite; (2) reinforced by fiber, bridge connection was constructed to facilitate network structure to be more tough and intensive.At the highest total dissolved substance of Tarim Basin formation water (212000 mg/L), notable absorbency for SACF-4 was exhibited compared to that of SACF-2 during the 60 days of thermal ageing.

Core Flooding Experiment.
The injected pressure profile as function of flooding pore volume (PV) for 0.25 wt.% SACFs solution and subsequent injecting water are shown in Figure 8.The climbing pressure for three types of polymer/clay composite was achieved and injected pressure increased accompanied by the increased injected pressure differential during injection of SACFs solution.The injected pressure was further raised at the onset of water flooding after 0.3 PV of SACFs flooding, which decreased rapidly to a platform value.Profile control ability for SACF can be recognized by illustrating the migration and swelling mechanism of particle gels in the porous medium.At the onset of migration, particle gels trend to larger porous channel due to their physical selection.Obviously, resistance of motion is caused with more amount of composite gels  entrance into porous medium, which is accompanied by the elevated injection pressure and increased injection pressure differential.The swelling particle gels further migrate in the porous medium and begin to accumulate in the narrow throat, leading to the injection pressure increased and to promote the subsequent flooding agent diversion.The accumulated particle gels are swept into scatter at the highest injection pressure.The scattered particle gels move as migrate amoeba in the narrow channel of the porous medium and the dropped injection pressure is measured by the flooding instrument.Finally, relative injection pressure is achieved when the composite stably immigrates in the porous medium.
In the experiment, we observe that SACF-4 containing hydrophobic group and fiber exhibits the most desirable profile control capability and it has the highest stable pressure region in comparison to SACF-1 and SACF-2.Characteristic parameters including initial injection pressure, breakthrough pressure, stable pressure, residual resistance factor, and shutoff efficiency for polymer/clay hybrid composites by core flooding experiment are illustrated in Table 2.

Conclusion
Alkyl sulfonate hydrophobic monomer called NaAMC 14 S was preferentially synthesized in the experiment and its structure was characterized by FTIR.Based on this hydrophobic monomer, novel superabsorbent composite filled with small amount of high temperature fiber (SACFs) through in situ intercalation and exfoliated technique was successfully synthesized.FTIR, SXRD, and SEM measurements confirmed that both the notion and the method for preparation of SACFs are scientific and practicable.The incorporation of hydrophobic segments into polymer/clay composite produces the controlled absorbent capacity for SACFs in the salt solution.More notable absorbency for SACF-2 was observed in the higher cationic solution resulting from the alkyl carbon chain and sulfonic acid group hindering the cations from attacking the carboxylate groups.Moreover, both toughness and strength for SACF-4 were reinforced by introducing amount of fiber which acted as bridge connection for the polymeric networks structure.At the total dissolved substance of 212000 mg/L with the temperature of 120 ∘ C, notable absorbency as well as water retaining property for SACF-4 was observed during 60 days of thermal ageing.Core flooding experiments demonstrated that SACFs could migrate as amoeba in the porous medium and accumulated in the narrow channel to adjust injection profile and promoted the subsequent water diverting into the unswept formation.Our investigation confirmed that polymer/clay hybrid superabsorbent composite containing hydrophobic group and fiber exhibited the reasonable profile control capability and desired injection pressure region which should have robust potential in the profile controlled application for enhanced oil recovery.

Figure 6 :
Figure 6: Water absorbency for SACFs as a function of cationic concentration at 120 ∘ C.

Figure 8 :
Figure 8: Injection pressure as function of particle gels' pore volume and of subsequent water displacement pore volume (Tarim Basin injected water, 120 ∘ C).

Table 2 :
Characteristic parameters for 0.3 SACF displacement of subsequent water flooding in the core flooding experiment.