swelling mechanism investigation of microgel with double-cross-linking structures

Swelling Mechanism Investigation of Microgel with Double-Cross-Linking StructuresHu Jia,* Qiang Ren, Wan-Fen Pu,* and Jinzhou Zhao

State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum University, Chengdu, Sichuan610500, People’s Republic of China

ABSTRACT: Microgel is a novel conformance control technology that can be injected into high permeability zones andtransport into deep area through pore throats. It can efficiently plug large pores to force injection water diversion to achieveconformance control. In this paper, we synthesized the microgel with a double-cross-linking structure of non-labile and labilecharacteristics. Then, a polarizing microscope, laser particle analyzer, Brookfield DV-III, and atomic force microscopy (AFM)were used to investigate the swelling mechanism of the microgel in high-saline water (total dissolved solids is from 30 to 100 g/L) at 65 °C. Results show that the swelling ability of the double-cross-linking structure microgel is not sensitive to salinity and thesolution viscosity can increase to a certain degree with the maximum value of 18 mPa s. The decomposition of labile cross-linkerresults in a loose and explosive structure, which seems like a “mushroom cloud”. Because of the high-density cross-linking sitesprovided by the non-labile cross-linker, the integrity of the spherical feature of the microgel was reserved after aging for 70 days.On the basis of the viscosity increase of microgel solution during the swelling process, a concept to develop a novel water shutoffagent is proposed. That is, the conventional cross-linkers, such as formaldehyde, methenamine, or phenolic or chromic salt, canbe added to microgel solution during the injecting process. Thus, the cross-linker can have the potential to cross-link with thegroups that released during the swelling process to form a bulk gel system, which can further improve the water shutoffperformance.

1. INTRODUCTIONAs a chemical method of enhanced oil recovery (EOR),injecting a polymer solution together with a cross-linkerproposed for conformance control application has been widelyused in mature oilfield development.1,2 Conformance control isa technique to block the already well-swept layers of reservoirfor mobilizing pockets of unswept oil/gas.3 Cross-linkers, suchas chromium(III) salt,4−6 phenol formaldehyde,7−9 polyethyle-nimine (PEI),10−12 etc., cross-linking with acrylamide-basedcopolymer or hydrolyzed polyacryamide (HPAM) can form apolymer gel in subterranean formation during a few hours toseveral days.In this process, gelation performance has been found to

depend upon many parameters; therefore, the gelation timeand, thus, the depth of the gel penetration are quite difficult topredict. These difficulties result from the uncertaintiesconcerning different factors: shear stresses, in both surfacefacilities and near-wellbore areas, and also the physical−chemical environment around the well (salinity, temperature,and pH).3 Among all of the parameters, shear degradation ofthe polymer/cross-linker in porous media seems to be moreimportant.13,14 Furthermore, both polymer and/or cross-linkeradsorption in the near-wellbore region and dilution bydispersion during placement can also greatly affect thetreatment effectiveness. On the basis of the problemsencountered, size-controlled gel,15,16 “bright water”,17,18 pre-formed particle gel (PPG),19−21 etc. were developed in recentyears. Especially for “bright water”, a kind of microgel, in fact, isprepared by emulsion polymerization. “Bright water” of sub-micrometer/micro size matching with pore throat and in anunswelled state can be easily injected and placed deep into thereservoir; when microgels encounter a high temperature in the

reservoir, they swell, to efficiently block high-permeability zonesin the heterogeneous reservoir to achieve the goal of fluiddiverting. The swelling characteristic of this microgel wascontrolled by two cross-linking structures. One is stable enoughto make the microgel have a stronger space network structurethat is not easy to degrade. However, the other one is labile andcan decompose easily. At the reservoir temperature, the labilecross-linker tends to slowly decompose after injected in theformation for more than 10 or dozens of days. At this moment,the microsphere size begins to gradually swell 10−100 times.Moreover, the microgel does not dissolve and still has anintegrated and independent structure.The synthesis method of the double-cross-linking structure

microgel was reported in refs 22−25, but a systematic researchon the swelling mechanism is not conducted. Swellingproperties of microgel can affect its field design as well asproduction performance; therefore, it is necessary to investigatethe swelling mechanism of the microgel with a double-cross-linking structure. In this paper, we first synthesize the microgelwith a double-cross-linking structure according to the syntheticmethods in refs 22−25. Then, four kinds of experimentalapparatuses, including polarizing microscope, laser particleanalyzer, Brookfield DV-III, and atomic force microscopy(AFM), are used to investigate the swelling mechanism. Thepolarizing microscope and laser particle analyzer are used toobserve the swelling characteristics at different times. Brook-field DV-III is used to periodically measure the viscosity of the

Received: May 14, 2014Revised: September 11, 2014Published: September 12, 2014

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© 2014 American Chemical Society 6735 dx.doi.org/10.1021/ef5012325 | Energy Fuels 2014, 28, 6735−6744

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microgel solution. AFM is used to study the microstructure ofthe swelled microgel, including verifying whether the microgelhas two types of cross-linking structures and analyzing themicrostructure of the microgel after the decomposition of thelabile cross-linker. Finally, the application prospect of thedouble-cross-linked microgel used for water shutoff isdiscussed.

2. MATERIALS AND METHODS2.1. Materials. The materials used in our experiments without

further purification included acrylamide (AM), 2-acrylamide-2-methylpropanesulfonic acid (AMPS), NaOH, Tween-80, Span-60,N,N′-methylenebis(acrylamide) (MBA) (non-labile cross-linkingmonomer), poly(ethylene glycol(200) diacrylate) (PEG 200 DA)(labile cross-linking monomer), initiator 2,2′-azobis(2-methylpropio-namide) dihydrochloride (V-50), aviation kerosene, ethylenediamine-

tetraacetic acid (EDTA) (chelating agent), and deionized water. Theabove-mentioned materials were used throughout this work forpolymerization. The rest reagent NaCl used to prepare the saline waterwas analytical reagent (AR) grade.

Among the materials, the labile cross-linking agent is PEG 200 DA

( ) and the non-

labile cross-linking agent is MBA ( ).

2.2. Methods. Microgels are prepared by inverse emulsionpolymerization. The representative steps are as follows: (1) Blending8.38 g of AMPS into 14 g of deionized water, and then solution pHwas adjusted to neutral by the addition of NaOH. (2) Blending 20 g ofAM and 1.2 g of Tween-80 into 15.6 g of deionized water for fully

Figure 1. Swelling prosperities of sample 1 microgel.

Energy & Fuels Article

dx.doi.org/10.1021/ef5012325 | Energy Fuels 2014, 28, 6735−67446736

stirring. (3) Blending 30 g of aviation kerosene and 6 g of Span-60 forfully stirring. Whereafter, the mixed solution was added to the three-necked flask used for polymerization. (4) Blending the mixed solutionprepared in steps 1−3, and then 0.2 g of EDTA, 0.03 g of PEG 200DA, and 0.01 g of MBA were successively added to the mixed solution,which will be loaded in the three-necked flask. Next, the mixedsolution is fully stirred accompanied by the aeration of N2 forthorough emulsification and oxygen removal. (5) The three-neckedflask is put in the water bath at 50 °C, and then the stirring speed isadjusted to 300 rpm, accompanied by the addition of 0.03 g of V-50 tothe solution. The reaction time is controlled at 6−8 h. (6) Cleaningthe synthesized emulsified microgel with acetone over and over againto prepare microgel samples for observation study.After that, the above-mentioned experimental apparatus was

employed to investigate the expansion mechanism of the microgel.The mean particle size (d50) shows the central tendency of the particlesize distribution. In this case, d50 is the particle diameter correspondingto the cumulative weight of 50% on the cumulative particle sizedistribution curve, which will be used for discussion in subsequentsections.

3. RESULTS AND DISCUSSION

3.1. Dynamic Swelling Ability of the Microgel. Themicrogel with different particle sizes can be obtained bychanging the concentration of the emulsifying agent, rotatingspeed, or other affecting factors. To investigate the swellingmechanism of the double-cross-linking microgel at 65 °C, threerepresentative microgel samples 1, 2, and 3 with differentsalinities are chosen. The test samples were prepared by theaddition of 2 wt % microgel in different saline water with 30,50, and 100 g/L NaCl.

3.1.1. Polarizing Microscope Analysis. 3.1.1.1. Sample 1 at30 g/L Saline Water. The experimental observations are shownin Figure 1, which indicates that the initial diameter of sample 1microgel generally distributed within 8−15 μm and the shapeof the particles was of uniform sphere. After 1 day, the microgelbegins to swell obviously. It can be seen from the micrographthat the diameter of the microgel mainly distributed within 17−50 μm, microgels swelled notably but the spheres were stillneat, and the interface between the spheres was very clear. The

Figure 2. Swelling prosperities of the sample 2 microgel.



micrograph became a bit blurry after 7 days under the samelight intensity, indicating that part of the microgel dissolved,while most of the microsphere particles remained full ofspherical conformation. At this time, the diameter of themicrogel generally distributed within 25−80 μm, which hadswelled to a certain degree compared to the swellingperformance after 1 day. No obvious changes occurred after15 and 20 days, in comparison to the swelling performanceafter 7 days. The diameter of the microgel generally distributedwithin 30−80 μm, and the spheres were still intact. Theswelling ratio is still at a higher level and can efficiently plug thehigh permeability zone.In addition, the viscosity of microgel solution is increased

with time. The initial viscosity of the microgel solution with 2wt % microgel was 1.2 mPa s, which is close to the water-phaseviscosity. Furthermore, the initial diameter was within 8−15 μm

that can be easily injected to the high-permeability reservoirswith a permeability of 500−4000 mD. The solution viscosityslightly increased to 3 mPa s after 1 day and reached the highestvalue of 10 mPa s after 7 days. This also further proved thereason why the polarizing micrographs became blurry. After 7days, the viscosity of the microgel solution began to reduce andwas then kept at around 6.2 mPa s. Because of the swelling, themechanism of the micrographs mainly depends upon theloosening efficacy of the labile cross-linker and the free watermolecules can enter into the inner structure of the microgel,which exhibits a loose and decomposition state to stimulateswelling. The cross-linked microsphere particles also have acertain water absorption swelling ability, but under such a highsalinity, the cross-linked microgel simply relying on waterabsorption swelling performance will be greatly reduced; hence,

Figure 3. Swelling prosperities of the sample 3 microgel.



the main swelling mechanism is the loosening efficacy of thelabile cross-linker.At the same time, the amide groups cross-linked with the

labile cross-linker will be released and the partial hydrolysis ofthe amide groups will give rise to the solution viscosity increase.When the decomposition of the labile cross-linker reaches thegreatest degree, the non-labile cross-linker can still make themicrogel keep an ideal cross-linking structure. The hydrolysis ofthe amide groups will be limited at high salinity. In addition, thecarboxyl groups hydrolyzed will be curbed and lead to the

decrease of the viscosity. However, there is no continuousincrease of the viscosity caused by the loosening efficacy of thelabile cross-linker; therefore, the viscosity of the microgelsolution tends to be smooth at the end.For the general hydrogels, the swelling ratio usually reduced

with the increase of water salinity.3,26−28 The experimentalresults show that the swelling mechanism of the double-cross-linking structure microgel is mainly due to the looseningefficacy of the labile cross-liker, which is different from generalhydrogels. The viscosity of the microgel with a high

Figure 4. Particle size dynamic changing versus time for the sample 1 microgel.



concentration of 2 wt % only increased to the highest value ofaround 10 mPa s, indicating that the high-strength cross-linkingsites formed by the non-labile cross-linker can make the

spherical particle remain in the configuration integrated and notcompletely decompose. It can also be seen from the polarizingmicrographs that the swelled microgel is still in a dense state




and does not appear with an obvious sparse phenomenon.Therefore, the viscosity increase property of the double-cross-linked microgel will play an important role in mobility controlas well as water diversion.

3.1.1.2. Sample 2 at 50 g/L Saline Water. The experimentalobservations are as shown in Figure 2. Similar to sample 1, theinitial diameter of sample 2 generally distributed within 6−12μm and the shape of the microgel is in a uniform sphere state.




The microgel also began to swell after 1 day. It can be seenfrom the micrograph that the diameter of the microgelgenerally distributed within 25−44.72 μm. The micrographbecame a bit blurry after 7 days under the same light intensity,indicating that part of the microgel dissolved but most of themicrosphere particles remained full of spherical conformation.At this time, the diameter of the microgel generally distributedwithin 29.61−61.44 μm. d50 of the microgel reached the highestvalue of 70.09 μm, which is not obviously swelled at 15 and 25days, and the shape of the particles were of uniform sphere.In addition, d50 and viscosity curves show that the median

diameter and solution viscosity increased with time in the initialstage. The initial viscosity of 2 wt % microgel solution was 1.2mPa s, which is close to the water-phase viscosity. The viscosityreached up to the highest value of 18 mPa s at the seventh day,followed by a decrease in later stage, and was kept at 12 mPa sat the 25th day. The phenomenon of completely dissolved didnot occur after 4 months.3.1.1.3. Sample 3 at 100 g/L Saline Water. The

experimental observation results of sample 3 are shown inFigure 3, which indicates that the microgel still has a goodswelling ability and thermostability. This phenomenon seems tobe inconsistent with the mechanism that the polymer moleculechain will crimp because of the shielding effect under highsalinity. It can be explained that the main swelling mechanismof the double-cross-linking microgel is the decomposition ofthe labile cross-linker, not only limited to microgel swelling byabsorbing water. The decomposed labile cross-linker cangenerate many loose network structures around the micro-sphere particles. Because of the difference of osmotic pressure,a high-salinity solution can freely infiltrate into the internalstructure of the microgel to promote swell. After 25 days, itsstructure was still integrated; after 30 days, d50 increases toabout 70 μm. It was almost in the same swelling ratio as thesalinity of 30 and 50 g/L. However, high salinity can lead toserious crimping of the hydrolyzed carboxyl groups, which canact as the reason for the slow increase of solution viscosity.The viscosity of the microgel solution was only about 2 mPa

s at a high salinity of 100 g/L after aging for 25 days, which canfurther prove the swelling mechanism of the double-cross-linking microgel mainly because of the decomposition of thelabile cross-linker for volume swelling, not only for single waterabsorption.The above studies proved that the swelling ability of the

double-cross-linking structure microgel is not sensitive tosalinity. Even under the high salinity, the swelling ratio has not

been shown to be reduced. Of course, with the salinityincreases, the viscosity reduces. Furthermore, the structures ofthe three samples were still integrated after long-termobservation, indicating that the long-term stability is very good.

3.1.2. Particle Size Analysis. 3.1.2.1. Sample 1 at 30 g/LSaline Water. The laser particle analyzer was used to test theparticle size distribution to understand the swelling ratio of themicrogel. The experimental results of sample 1 are shown inFigure 4. It can be seen from Figure 4 that the particle sizedistribution curves of the original sample and the swelledsample are steep. Furthermore, the curve kurtosis has atendency of becoming steeper in the late swelling period,indicating that the grain size sorting of the microgel becomesbetter with time. In addition, the microgel swelled rapidly in thefirst 7 days, and d50 increased from 13.35 to 76 μm. However,no changes occurred in the next almost 20 days. d50 was about77 μm at 15 and 20 days. The particle size analysis shows goodagreement with the conclusions obtained by the polarizingmicroscope test. In the later observation, the completedissolving of the microgel is not seen.

3.1.2.2. Sample 2 at 50 g/L Saline Water. It can be inferredfrom Figure 5 that the grain size sorting of the microgel(sample 2) becomes better with time. The microgel swelledrapidly in the first 7 days, and d50 increased from 13.35 to 73.37μm. In the follow-up evaluation, d50 does not increase obviouslyand remains at about 77 μm. It is similar to the evaluation resultof the microgel at salinity of 30 g/L. The microgel was notcompletely dissolved in the later observation. High salinity cannot only curb the swelling ratio of the microgel but can also bebeneficial to keep the configuration integrity of the microgel;hence, it will not rapidly decompose.

3.1.2.3. Sample 3 at 100 g/L Saline Water. Figure 6 showsthe particle size distribution curve of sample 3 at differenttimes, indicating that the microgel swelled rapidly in the first1−5 days, d50 reached up to about 70 μm, and the diameter andsolution viscosity changed little in the later 2 days. The swellingratios of the three samples with different salinities from 30 to100 g/L were almost the same, indicating that the swellingratios of the double-cross-linking microgel is not sensitive tosalinity, while high salinity only has the negative effect on theviscosity.In the later observation of the above-mentioned microgel

simples, spherical particles are still visible after aging for half ayear; these microgels do not completely decompose along witha certain viscosity. The results show that the double-cross-

Figure 7. Schematic diagram of the loosening efficacy of the labile cross-linker.



linked microgel synthesized by the method of inverse emulsionpolymerization has an excellent thermal stability.3.2. Microstructure Study on the Swelled Microgel

through AFM. Through some of the swelled microgelsdissolved, however, most of them have the characteristics of aninternal structure. The sample 1 microgel after 70 days of agingwas chosen to characterize the internal structure by the AFMstudy. The microstructures are shown in Figures 7 and 8.Figure 7 shows that a loose structure, such as a mushroom

cloud, is formed after the thermal decomposition of the labilecross-linker, while the structure of the microgel is still compactenough because of the non-labile cross-linker maintaining ahigh cross-linking density, which ensured the structuralintegrity. Figure 8 reflects the internal skeleton structure ofthe microgel. The skeleton chains are full of branches and infree stretching state. The stereo image reflects that the skeleton

of the swelled microgel still has a certain thickness of 5−10 nm.This special skeleton structure of the microgel is the nature thatcan ensure good thermal stability under high salinity.

4. FUTURE PROSPECTS FOR NOVEL WATER SHUTOFFAGENT

The novel water shutoff agent is based on the assumption thatthe water shutoff agent is a composite system prepared byinjection water, which contains a certain concentration of thedouble-cross-linking microgel and cross-linker.29 The microgelwill gradually release a part of carboxyl and amide groupsbecause of the thermal decomposition of the labile cross-linker,which can result in the increase of viscosity of the swelledmicrogel solution. When injecting microgel, it can be mixedwith the cross-linkers, such as formaldehyde, methenamine, orphenolic or chromic salt, and the cross-linker can cross-link

Figure 8. Internal skeleton structure of the polymer microgel.

Figure 9. Schematic diagram of the desired novel water shutoff agent.



with the groups released by labile cross-linker decomposition.Therefore, it is expected to form bulk gel to improve theplugging effect. Besides, the microgel will shrink in the oil phasethan cannot damage the oil-bearing zone. The desired functiondiagram of the novel water shutoff agent is shown in Figure 9.

5. CONCLUSION

(1) The swelling ability of the double-cross-linking structuremicrogel is not sensitive to salinity. The swelling mechanism isdifferent from the general hydrogels, and the main mechanismis the loosening efficacy of the labile cross-linker. The swellingratios of the three representative microgel samples are nearlyequal in the high salinity from 30 to 100 g/L. The medianparticle size (d50) increased with time in the first 7 days,followed by a little change in the later observation. d50 grows to72.96−77.54 μm after 25 days for the three samples. (2) Thesolution viscosity increased within a certain range during theswelling process. The solution viscosity with a salinity of 30 and50 g/L increased in the initial stage and decreased a little in thelater stage. The maximum value can reach up to 18 mPa s. Theviscosity of the microgel solution slowly increased at 100 g/Lsaline water and only reached up to 2 mPa s after 25 days. Thisis mainly because the carboxyl groups produced by theloosening efficacy of the labile cross-linker cannot efficientlystretch in high-salinity solution. It is expected to form bulk gelto improve the water shutoff performance by the addition ofcross-linkers, such as formaldehyde, methenamine, or phenolicor chromic salt, when injecting microgel. (3) The AFM analysisshows that the swelled microgel has a loose but integralstructure, which seems like a “mushroom cloud”, indicating thecharacteristic of the loosening efficacy of the labile cross-linker.The internal structure of the microgel exhibits skeletonstructures with chains that are in a state of free stretching.Because of the high-strength cross-linking sites formed by thenon-labile cross-linker, the entire structure of the microgelmaintains integrity. Furthermore, the special internal structureis the essence that can be used to explain why the double-cross-linking structure microgel has an excellent thermal stability athigh salinity.

■ AUTHOR INFORMATION

Corresponding Authors*E-mail: [email protected].*E-mail: [email protected].

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

This work is supported by the CNOOC Tianjin Branch (No.S10TJPXX049), the National Natural Science Foundation ofChina (NSFC) (51404202), the Scientific Research StartingProject of Southwest Petroleum University (SWPU)(2014QHZ001), and the Science and Technology Fund ofSWPU (2013XJZ007). The special fund of China’s CentralGovernment for the Development of Local Colleges andUniversities, the project of National First-Level Discipline inOil and Gas Engineering, is also greatly appreciated. Specialthanks to the assistance of Jun-Zhong Li (SWPU, now inCNOOC) in experimental work.

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swelling mechanism investigation of microgel with double-cross-linking structures

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