experimental investigation of the novel phenol−formaldehyde cross-linking hpam gel system: based...

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727 r2011 American Chemical Society pubs.acs.org/EF Energy Fuels 2011, 25, 727736 : DOI:10.1021/ef101334y Published on Web 01/27/2011 Experimental Investigation of the Novel Phenol-Formaldehyde Cross-Linking HPAM Gel System: Based on the Secondary Cross-Linking Method of Organic Cross-Linkers and Its Gelation Performance Study after Flowing through Porous Media Hu Jia,* ,† Wan-Fen Pu, Jin-Zhou Zhao, and Ran Liao* ,†,‡ State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum University, Chengdu 610500, People’s Republic of China, and School of Chemistry and Chemical Engineering, Southwest Petroleum University, Chengdu 610500, People’s Republic of China Received September 30, 2010. Revised Manuscript Received December 28, 2010 The use of a polymer gel is an effective method for water shutoff in mature oilfield development. As for fractured reservoirs, in order to mitigate the filtration of gelant (fluid solution of cross-linker and polymer that exists before gelation) to matrix and increase the enduring erosion ability of mature gel, chromium(III) acetate, and phenol-formaldehyde cross-linking, the HPAM gel system of a secondary cross-linking method is used more often. Chromium(III) salt is often used as the first cross-linker. However, the cross- linking mechanism is achieved by an ion bond, which is less stable than a covalent bond when used as an organic cross-linker. Resorcinol and phenol-formaldehyde used as the first and secondary cross-linker, respectively, are discussed in this paper. Results showed that resorcinol can quickly cross-link with HPAM at room temperature. Gelant formulated with a combination of 0.3 wt % HPAM added to 10-30 mg/L resorcinol can increase its viscosity from 10.2 to 150 mPa 3 s within 2 h. SEM results show that the microstructure of the first cross-linking gel appears in typical dendritic shape, with branched chains diffused in arbitrary directions. The high shearing tolerant ability of the first cross-linking gel can be achieved by these branched chains. However, a tight 3-D network structure is formed in the microstructure of the secondary cross-linking gel. This is the benefit of the stability of the skeleton structure of gel enhancing. The main factors, including temperature and total dissolved solids (TDS), to affect the gelation performance of this secondary cross-linking gel are also discussed. Results show that gelation time decreased and gel strength increased with increasing temperature and TDS. Especially for TDS, the adverse law of the gelation performance with PEI/PAtBA or PEI/HPAM gel systems is shown. The gelation performance of a resorcinol/phenol-formaldehyde/HPAM gel system of a first cross-linking state after flowing through porous media is studied. Atomic force microscopy (AFM) scanning results show that in comparison to the original gel, the structure of the weak cross-linking (code B) gels has a certain degree of damage after flowing through porous media. However, the final gel strength of both gels do not show an apparent difference. This demonstrated that the first cross-linking achieved by resorcinol can guarantee the effectiveness of secondary cross-linking. This study suggests that a resorcinol/phenol- formaldehyde/HPAM secondary cross-linking gel system can be used for water shutoff in fractured reservoirs. 1. Introduction Most oil fields in the worldwide have already stepped into the stages of high water-cut and production decline. This will lead to more and more difficult oil-field development. Espe- cially for natural fractured reservoirs, water injection can break through quickly along the fractures, and production will face serious challenges with a high water-cut and a decline in oil/gas production. 1 Polymer gels treatment is effective for well water shutoff and profile control. However, the crucial problem for water shutoff in fractured reservoirs is how to control the filtration loss of water shutoff agents into porous media. 2-4 Chromium(III) acetate/partially hydrolyzed poly- acrylamide(HPAM)gel systems are commonly used, 5,6 and the influent fluid can exist in two states, respectively, of gelants (the fluid solution of cross-linker and polymer that exists before gelation) and preformed gel (any gel state that does not flow into or through porous rock, whether it is a rigid gel, a “weak” elastic gel, or a dispersion of gel aggregates). How- ever, gelants often experience problems with gravity segrega- tion in fractures. Also, when gelants contact reservoir fluids or rock minerals, compositional changes can occur that interfere with gelation. Most importantly, the low viscosity gelants can *To whom correspondence should be addressed. Telephone: 86-28- 15902825270. E-mail: [email protected] or [email protected]. (1) Bailey, B.; Crabtree, M.; Tyrie, J.; Elphick, J.; Kuchuk, F.; Romano, C.; Roodhart, L Water control. Oilfield Rev. 2001, 6, 31. (2) Seright, R. S. Use of preformed gels for conformance control in fractured systems. SPE Prod. Facil. 1997, 12, 5965. (3) Seright, R. S. An alternative view of filter-cake formation in fractures inspired by Cr(III)-acetate-HPAM gel extrusion. SPE Prod. Facil. 2003, 18, 6572. (4) Seright, R. S. Washout of Cr (III)-Acetate-HPAM Gels from Fractures. Presented at the SPE International Symposium on Oilfield Chemistry, Houston, TX, February 5-7, 2003; Paper 80200. (5) Cordova, M.; Cheng, M.; Trejo, J.; Johnson, S. J.; Willhite, G. P.; Liang, J. T.; Berkland, C. Delayed HPAM gelation via transient sequestration of chromium in polyelectrolyte complex nanoparticles. Macromolecules. 2008, 41, 43984404. (6) Simjoo, M.; Dadvand, K. A.; Vafaie, S. M.; Zitha, P. L. J. Water Shut-Off in a Fractured System Using a Robust Polymer Gel. Presented at the SPE European Formation Damage Conference, Scheveningen, The Netherlands, May 27-29, 2009; Paper 122280.

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727r 2011 American Chemical Society pubs.acs.org/EF

Energy Fuels 2011, 25, 727–736 : DOI:10.1021/ef101334yPublished on Web 01/27/2011

Experimental Investigation of the Novel Phenol-Formaldehyde Cross-Linking HPAM

Gel System: Based on the Secondary Cross-Linking Method of Organic Cross-Linkers

and Its Gelation Performance Study after Flowing through Porous Media

Hu Jia,*,† Wan-Fen Pu,† Jin-Zhou Zhao,† and Ran Liao*,†,‡

†State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum University, Chengdu 610500,People’s Republic of China, and ‡School of Chemistry and Chemical Engineering, Southwest Petroleum University, Chengdu

610500, People’s Republic of China

Received September 30, 2010. Revised Manuscript Received December 28, 2010

The use of a polymer gel is an effective method for water shutoff in mature oilfield development. As forfractured reservoirs, in order to mitigate the filtration of gelant (fluid solution of cross-linker and polymerthat exists before gelation) tomatrix and increase the enduring erosion ability ofmature gel, chromium(III)acetate, and phenol-formaldehyde cross-linking, the HPAM gel system of a secondary cross-linkingmethod is used more often. Chromium(III) salt is often used as the first cross-linker. However, the cross-linking mechanism is achieved by an ion bond, which is less stable than a covalent bond when used as anorganic cross-linker. Resorcinol and phenol-formaldehyde used as the first and secondary cross-linker,respectively, are discussed in this paper. Results showed that resorcinol can quickly cross-link withHPAMat room temperature. Gelant formulated with a combination of 0.3 wt % HPAM added to 10-30 mg/Lresorcinol can increase its viscosity from 10.2 to 150 mPa 3 s within 2 h. SEM results show that themicrostructure of the first cross-linking gel appears in typical dendritic shape, with branched chainsdiffused in arbitrary directions. The high shearing tolerant ability of the first cross-linking gel can beachieved by these branched chains. However, a tight 3-D network structure is formed in themicrostructureof the secondary cross-linking gel. This is the benefit of the stability of the skeleton structure of gelenhancing. Themain factors, including temperature and total dissolved solids (TDS), to affect the gelationperformance of this secondary cross-linking gel are also discussed. Results show that gelation timedecreased and gel strength increased with increasing temperature and TDS. Especially for TDS, theadverse law of the gelation performance with PEI/PAtBA or PEI/HPAM gel systems is shown. Thegelation performance of a resorcinol/phenol-formaldehyde/HPAM gel system of a first cross-linkingstate after flowing through porous media is studied. Atomic force microscopy (AFM) scanning resultsshow that in comparison to the original gel, the structure of the weak cross-linking (code B) gels has acertain degree of damage after flowing through porous media. However, the final gel strength of both gelsdo not show an apparent difference. This demonstrated that the first cross-linking achieved by resorcinolcan guarantee the effectiveness of secondary cross-linking. This study suggests that a resorcinol/phenol-formaldehyde/HPAM secondary cross-linking gel system can be used for water shutoff in fractured reservoirs.

1. Introduction

Most oil fields in the worldwide have already stepped intothe stages of high water-cut and production decline. This willlead to more and more difficult oil-field development. Espe-cially for natural fractured reservoirs, water injection canbreak through quickly along the fractures, and productionwill face serious challenges with a highwater-cut and a declinein oil/gas production.1 Polymer gels treatment is effective forwell water shutoff and profile control. However, the crucial

problem for water shutoff in fractured reservoirs is how tocontrol the filtration loss of water shutoff agents into porousmedia.2-4 Chromium(III) acetate/partially hydrolyzed poly-acrylamide(HPAM)gel systems are commonly used,5,6 andthe influent fluid can exist in two states, respectively, of gelants(the fluid solution of cross-linker and polymer that existsbefore gelation) and preformed gel (any gel state that does notflow into or through porous rock, whether it is a rigid gel, a“weak” elastic gel, or a dispersion of gel aggregates). How-ever, gelants often experience problems with gravity segrega-tion in fractures.Also, when gelants contact reservoir fluids orrockminerals, compositional changes can occur that interferewith gelation.Most importantly, the low viscosity gelants can

*To whom correspondence should be addressed. Telephone: 86-28-15902825270. E-mail: [email protected] or [email protected].(1) Bailey, B.; Crabtree, M.; Tyrie, J.; Elphick, J.; Kuchuk, F.;

Romano, C.; Roodhart, L Water control. Oilfield Rev. 2001, 6, 31.(2) Seright, R. S. Use of preformed gels for conformance control in

fractured systems. SPE Prod. Facil. 1997, 12, 59–65.(3) Seright, R. S. An alternative view of filter-cake formation in

fractures inspired by Cr(III)-acetate-HPAM gel extrusion. SPE Prod.Facil. 2003, 18, 65–72.(4) Seright, R. S. Washout of Cr (III)-Acetate-HPAM Gels from

Fractures. Presented at the SPE International Symposium on OilfieldChemistry, Houston, TX, February 5-7, 2003; Paper 80200.

(5) Cordova,M.; Cheng, M.; Trejo, J.; Johnson, S. J.; Willhite, G. P.;Liang, J. T.; Berkland, C. Delayed HPAM gelation via transientsequestration of chromium in polyelectrolyte complex nanoparticles.Macromolecules. 2008, 41, 4398–4404.

(6) Simjoo, M.; Dadvand, K. A.; Vafaie, S. M.; Zitha, P. L. J. WaterShut-Off in a Fractured SystemUsing a Robust Polymer Gel. Presentedat the SPE European Formation Damage Conference, Scheveningen,The Netherlands, May 27-29, 2009; Paper 122280.

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Energy Fuels 2011, 25, 727–736 : DOI:10.1021/ef101334y Jia et al.

invade into porous media. Therefore, damage to the porousrock is maximized. Alternatively, formed gels have significantlyfewer problems with gravity segregation and chemical interfer-ence than gelants. However, formed gels exhibit higher pressuregradients during placement that may affect their distance ofpropagation especially along small aperture fractures. Also,water loss can be caused by the higher placement pressure.4

Using partially formed (partially mature state) gel is aneffective method to control extruding through fractures. Thisgel system used in Sydansk et al.’s experiments exhibited aneffective viscosity of approximately 500 cP during placement,with formulation of a 1.5 wt % high-MW and 2.0 wt % low-MW polymer gel, when gels becoming matured exhibit excep-tionally good fracture-plugging characteristics.7 Therefore,prior weak cross-linking gel systems are more welcome. Onthe basis of this concept, a secondary cross-linking gel systemwas developed. Gel washout in fractures can be reduced usingsecondary cross-linking reactions. The concept of secondarygelation reaction is to inject a gel that undergoes two separatecross-linking rections. The first (primary) reaction is timed totake place before entry into the fracture. The second cross-linking reaction forms a cross-linked polymer thatwill not enterthe porous rock but will be sufficiently fluid to exhibit relativelylow pressure gradients during extrusion. The second reactionstrengthens the gel and significantly increases the gel’s resistanceto washout. The cross-linker for the second reaction should notgel with any component that leaks into the porous rock. In thisway, damage to the porous rock is minimized. Many studies onsecondary cross-linking gel systems have been completed bySeright in2003.4 Inhis experiments, chromium(III) acetateas thesecondary cross-linker was mixed with the formed resorcinol-formaldehyde-HPAM gel and injected into the fracture beforechromium(III) acetate had time to react.

However, the gelation mechanism of chromium(III) isachieved by an ionic bonding, which is less stable than covalentbonding through organic cross-linkers.8 Research on the sec-ondary cross-linking gel system of two organic cross-linkingreactions is scarcely.Therefore, researchon thegel systemof twocross-linking reactions by covalent bonding is necessary.

For high-temperature (>90 �C) reservoirs, acrylamide-basedcopolymerswithanorganic cross-linkingagent, suchasphenol-formaldehyde, can be used to form a gel with thermal stabi-lity.9,10 Phenol-formaldehyde cross-linking acrylamide-basedcopolymers gel systems were used more in the past decades.However, phenol-formaldehyde is toxic, and the wide applica-tion of this cross-linker is restrained. We have found that someoilfields in recent years are still using phenol-formaldehydecross-linking gel systems for water shutoff in onshore oil field

complex reservoirs.11-13 One symposium paper states: “There-fore, according to the comprehensive surveyof the effects ofH2Son gel-typed plugging agents, the phenol formaldehyde resin/HPAM gel is recommended as the plugging agents suitable foroil and gas reservoir containing H2S.”

13 Though the phenol-formaldehyde cross-linkinggel systemhas somedefects, it showsseveral advantages in many aspects. In addition, phenol-formaldehyde is economical compared toother lowtoxicorganiccross-linkers. Thus, there is still muchwork to do perfect this gelsystem.

We know that phenol and its derivatives together with for-maldehyde are often used as the compositions for a phenolic-aldehyde cross-linker system.However, the activation tempera-ture of phenol-formaldehyde cross-linked with HPAM or anacrylamide-based copolymer is only above 70-80 �C, andphenol-formaldehyde-based gel systems are often used in hightemperature reservoirs. At the low reservoir temperature, con-densation reaction for cross-linking will become slow andcannot guarantee better results in the formation. Resorcinolas one of the phenol derivatives was only reported as theaccelerator to accelerate gelation, and whether it can be usedas the first cross-linker in a secondary cross-linking gel system isworth discussing. Therefore, research on the gelation perfor-mance of a resorcinol-added phenol-formaldehyde cross-link-ing HPAM gel system is needed.

The goal of this paper includes the following: (1) investigat-ing the gelation performance of resorcinol cross-linkingHPAM under room temperature and studying the gel micro-structure achieved by the two cross-linking reactions, respec-tively, (2) determining the effects of the following parameterson gelation performance of secondary cross-linking gel sys-tems: concentrations of two cross-linkers, total dissolvedsolids (TDS), and temperatures, and (3) studying the gelationperformance of secondary cross-linking gel systems of organiccross-linkers after flowing through porous media.

2. Measurement of the Gelation Rate and Strength

Gelation time is defined as the time needed to reach theinflection point on the viscosity versus time curve. This methodhas been widely used by several authors.8,14 In this study, aBrookfield viscometer DV-III was used to measure viscosity.Samples of the gelling solutions were prepared at room tem-perature and loaded regularly into theviscometer tomeasure theviscosity at room temperature. This aims to understand the firstcross-linking reaction rate to optimize the concentration re-quirements of the first cross-linker.

Before gel formation, the viscosity of the gelling solution isrelatively low; therefore, it can be measured accurately. Aftergel formation, it is hard to obtain the accurate viscosity value.

The bottle testing method, as an experimental technique,provides a semiquantitative measurement of gelation rate andgel strength. Also, it is an convenient and inexpensive methodto study thegelationkinetic.15Gel strengthduringdevelopment

(7) Sydansk,R.D.;Al-Dhafeeri,A.M.;Xiong,Y.; Seright,R. S. Polymergels formulated with a combination of high- and low-molecular-weightpolymers provide improved performance for water-shutoff treatments offractured production wells. SPE Prod. Facil. 2004, 19, 229–236.(8) Al-Muntasheri, G. A.; Nasr-El-Din, H. A.; Peters, J. A.; Zitha,

P. L. J. Investigation of a high-temperature organic water-shutoff gel:Reaction mechanisms. SPE. J. 2006, 11, 497–504.(9) Albonico, P.; Bartosek,M;Malandrino, A.; Bryant, S.; Lockhart,

T. P. Studies on Phenol-Formaldehyde Crosslinked Polymer Gels inBulk and in Porous Media .Presented at the SPE International Sympo-sium on Oilfield Chemistry, San Antonio, TX, February 14-17, 1995;Paper 28983.(10) Moradi-Araghi, A. A review of thermally stable gels for fluid

diversion in petroleum production. J. Pet. Sci. Eng. 2000, 26, 1–10.(11) Banerjee, R.; Ghosh, B.; Khilar, K. C.; Boukadi, F.; Bemani, A.

Field application of phenol-formaldehyde gel in oil reservoirmatrix forwater shut-off purposes. Energy Sources, Part A. 2008, 30, 1779–1787.(12) Banerjee, R.; Patil, K.; Khilar, K. C. Studies on phenol-

formaldehyde gel formation at a high temperature and at different pH.Can. J. Chem. Eng. 2006, 84, 328–336.

(13) You, Q.; Wang, Y. F.; Zhou, W.; Zhao, F. L.; Zhang, J.; Yang,G. Effects of Hydrogen Sulfide on Gel Typed Plugging Agents. Pre-sented at the SPE International Symposium on Oilfield Chemistry,Woodlands, TX, April 20-22, 2009; Paper 121470.

(14) Reddy, B. R.; Eoff, L.; Dalrymple, E. D.; Black, K.; Brown, D.;Rietjens, M. A Natural polymer-based cross-linker system for confor-mance gel systems. SPE. J. 2003, 8, 99–106.

(15) Simjoo, M.; Vafaie, S. M.; Dadvand, K. A.; Vafaie, S. M.;Hasheminasab,R. Polyacrylamide gel polymer as water shut-off system:Preparation and investigation of physical and chemical properties in oneof the Iranian oil reservoirs conditions. Iran. J. Chem. Chem. Eng. 2007,26, 99–107.

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of gelation kinetic was expressed as an alphabetic code of Athrough I, which is shown in Table 1.16 For no detectable gelformation that has the same viscosity as the initial solution, it iscoded as “A”. Likewise, code I indicates that there is nodeformation on the gel surface upon inversion.

The gelation time of water shutoff gel is usually consideredas theperiodof timewhengelling solutions ina codeAstate turntoa flowinggel codeCstate andcanalsobe called initial gelationstrength (gel point). Strength code and apparent viscositymeasurement methods were all used in this paper. The strengthcode method was used to measure the gel strength and stabilityafter gelation, and the viscometermethodwas used toobtain theaccurate viscosity during the first cross-linking reaction.

3. Experimental Studies

3.1. Materials. The commercial polymer employed in thesestudies has a highMw (18000 kDa) and high degree of hydrolysis(25-35%) HPAM. Standard reagent-grade formaldehyde, resor-cinol, andphenol, the former furnishedas a 37%aqueous solution,the latter (resorcinol and phenol) furnished as A.R. grade with netweight more than 99.5%, were employed. Deionized water andfresh water were obtained in own laboratory. All salts, such asNaCl,KCl, andCaCl2,whichwereused to investigate the influenceon gelation performance, were A.R. grade and used as received.

3.2. Methods and Procedures. Low Mw acrylamide-basedcopolymers, including HPAM, PAtBA, and AM/AMPS(Na),used as water shutoff agents for fractured reservoirs, have beenreported by several authors.8,17,18 These polymers or copolymersusually have a low degree of hydrolysis (less than 10%) and areoften used at a high concentration ranging from 1.5 to 7.0 wt %.Therefore, the well treatment cost is higher if large amounts areneeded. As for the HPAM series polymers, the higher the polymerMw, the larger the molecular coil size, and the hydrodynamicvolume of polymer chains is larger than that of low Mw HPAM.In addition, the degree of hydrolysis is a key factor in deciding theviscosity of the polymer solution. HPAM with a high degree ofhydrolysis often contains more carbonyl carbon attached to theamide group, and the polymer chains are more spread out in water.This was obvious for highMwHPAM.As for the highMwHPAMwith high degree of hydrolysis, a HPAM solution of a certain TDScan show a relatively high viscosity even at a low polymer concen-tration. Using large dosages of polymer gel of low polymer con-centration forprofile control ismorewelcome.Therefore,wechoosethehighMwHPAM(Mw=18000kDa) inour study.Experimentalmethods and procedures are summarized as follows.

1.Optimization of the First Cross-LinkerConcentration. In thefirst section, all experiments were conducted at room tempera-ture. Polymer and resorcinol were dissolved in fresh water toprepare a gelling solution. Other aqueous-phase salinities usedin the experimental tests will be specified in this paper. After thegelling solution was prepared, the viscosity was measuredregularly at room temperature to investigate the first cross-linking reaction rate. Therefore, a suitable resorcinol concen-tration requirement was obtained for other studies.

2. Phenol-Formaldehyde Cross-Linker Preparation and Con-centrationOptimizing. Series formulas of phenol-formaldehydecross-linker were given by several authors. The prior literatureindicates that phenol and formaldehyde are conveniently employedin a 1:1 weight ratio, though Falk preferred a ratio of about 1.3:1.19

Later studies show that gel properties donot depend strongly on theweight ratio of phenol:formaldehyde, and a 1:1 weight ratio isemployed in Bryant et al.’s study.20 Formaldehyde solution addedto maintain a formaldehyde-phenol mole ratio of 3.0 and 3.5 wasgiven by Banerjee et al.12 In our study, a phenol-formaldehydecross-linker was prepared by a simple method: one gram of phenolwas dissolved in 10 mL formaldehyde to prepare this second cross-linker. This is similar to Bryant et al.’s method. A suitable phenol-formaldehyde concentration requirement was also obtained in thissection through many screening procedures.

3. Optimization of Two Types of Cross-Linkers of a SecondaryCross-Linking Gel System. Two cross-linkers were dissolved inHPAM solutions. Therefore, the gelation performance of thesecondary cross-linking gel system was studied by adjusting theconcentrationof cross-linkers andHPAM.However, in this section,the first cross-linker concentrationwasonly investigatedbecause thefirst cross-linking reaction was only studied at room temperature,and the influence of the optimized first cross-linker’s concentrationon the gelation performance of the secondary cross-linking gelsystem was needed for this study. In addition, low HPAM concen-trationmeans low cost. Therefore, in our study,HPAMwas used ata given low concentration of 0.3%.

4. SEM Analysis. The gel microstructure, respectively, achievedby the first and the secondarycross-linking reactionwas investigatedthrough SEMmethods.

5. Gel Performance Evaluation. TDS and temperature are themost important factors affecting gel treatment efficiency in reservoirenvironments. Therefore, the effect of TDS and temperature on thegelationperformance of secondary cross-linking gel systemwas alsostudied.

6. Gelation Performance Study after Flowing through PorousMedia. Injecting a polymer solution together with a cross-linkerhas been widely used. In this process, gelling properties have beenfound to depend on many factors, so the gelling time and, thus, thedepth of the gel penetration is quite difficult to predict. Thesedifficulties result from the uncertainties concerningdifferent factors:shear stresses, both in surface facilities and in near-wellbore areas,

Table 1. Gel Strength Code

gel strength code gel description

A No detectable gel formed: The gel appears to have the same viscosity as the original polymer solution.B Highly flowing gel: The gel appears to be only slightly more viscous than the initial polymer solution.C Flowing gel: Most of the gel flows to the bottle cap by gravity upon inversion.D Moderately flowing gel: Only a small portion (5-10%) of the gel does not readily flow to the bottle cap

by gravity upon inversion (usually characterized as a tonguing gel).E Barely flowing gel: The gel can barely flow to the bottle cap and/or a significant portion (>15%)

of the gel does not flow by gravity upon inversion.F Highly deformable nonflowing gel: The gel does not flow to the bottle cap by gravity upon inversion.G Moderately deformable non flowing gel: The gel deforms about half way down the bottle by gravity upon inversion.H Slightly deformable nonflowing gel: only the gel surface slightly deforms by gravity upon inversion.I Rigid gel: There is no gel surface deformation by gravity upon inversion.

(16) Sydansk,R.D.; Argabright, P. A. Conformance Improvement ina SubterraneanHydrocarbon-Bearing FormationUsing a PolymerGel.U.S. Patent 4,683,949, 1987.(17) Reddy, B. R.; Eoff, L.; Dalrymple, E. D.; Black, K.; Brown, D.;

Rietjens, M. A. Natural polymer-based cross-linker system for confor-mance gel systems. SPE. J. 2003, 8, 99–106.(18) Vasquez, J.; Dalrymple, E. D.; Eoff, L.; Reddy, B. R.; Civan, F.

Development and Evaluation ofHigh-TemperatureConformance Poly-mer Systems. Presented at the SPE International Symposium onOilfieldChemistry, Houston, TX, February 2-4, 2005; Paper 93156.

(19) Falk,D.O. Process for Selectively PluggingPermeableZones in aSubterranean Formantion. U.S. Patent 4,485,875, 1984.

(20) Bryant, S. L.; Bartosek, M.; Lockhart, T. P. Laboratory evalua-tion of phenol-formaldehyde/polymer gelants for high-temperatureapplications. J. Pet. Sci. Eng. 1997, 17, 197–209.

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and also the physical-chemical environment around the well (pH,salinity, and temperature). Among all factors, shear degradation ofthe polymer/cross-linker in porousmedia ismore important.More-over, both polymer and/or cross-linker adsorption in the nearwellbore region and dilution by dispersion during placement canalso affect the effectiveness of the treatment. For example, deepdiverting gels (DDG) have been proposed as a means of achievingplacement of a cross-linked polymer slug deep within the reservoir.An acrylamide-acrylate polymerwasmixedwith aluminum citrateunder controlled conditions. A field trial, the process showed thatthe injection profile wasmodified as expected, but the improvementwas premature and did not last long. Loss of the aluminums to theporous medium was believed to be the major limiting factor.21

On the basis of these problems, preformed particle gel(PPG),22,23 Bright Water,21,24-27 and size-controlled gel28-31

etc. were developed in recent years and have achieved muchsuccess in the aspect of deep profile control.However, a polymersolution together with a cross-linker composed bulk gel system

also has unique superiority, and economy is a prominentadvantage. Therefore, research on the gelation performance ofsecondary cross-linking gel systems after flowing through por-ous media is needed and aims at better understanding the use ofthis gel system. In order to accelerate secondary cross-linking forphenol-formaldehyde, we chose the test temperature of 90 �C.Figure 1 shows the setup diagram.

Experiments Procedures(1) Each component, HPAM, resorcinol, and phenol-formal-

dehyde, was dissolved in fresh water to prepare the gellingsolution.

(2)A high permeability synthetic core was used as the porousmedia. The core was vacuumized before use, and then Kwwill be measured by simulated water.

(3)After the first cross-linking reaction occurred, the weakcross-linked gelwas injected at a constant flow rateof 1mL/min flowing through the synthetic core. The effluent wascollected and rejected for full shearing three times.

(4) The original first cross-linking gel and the same gel afterflowing through the porous media were removed at thesame amount and then sealed inbottles andplaced inaovenat 90 �C. The gel strength code was recorded regularly.

(5)When both gelling solutions turned to flowing gel code B,the gel strength in this state was not too strong so that theycould be placed on a wet mica plate for scanning with anatomic force microscope (NANPSCOPE-3a, Digital In-struments Ltd.). In fact, the secondary cross-linking forphenol-formaldehyde had occurred. Then, the two sam-ples were scanned for analysis to compare the microcosmicstructures.

(6) In the meantime, the rest of the gel was rapidly put backinto the oven, and a bottle test was used to study thegelation performance of the original gel and the gel afterflowing through porous media.

4. Results and Discussion

The gelation performance can be affected by various pa-rameters. More detailed information is discussed in the fol-lowing sections.

4.1. Study on the Gelation Performance of Resorcinol

Cross-Linking with HPAM. In this experiment, HPAMsolutions were prepared with fresh water, and polymers wereused at a given low concentration of 0.3 wt %, as mentionedin the previous section. The viscosity of the original polymersolution is about 210 mPa 3 s at room temperature. Then, thegelation performance of mixed solutions formulated witha combination of 0.3 wt % HPAM added to 15-30 mg/Lresorcinol was studied.

As iswell-known,polymers, cross-linkers, andotheradditivesshould be dissolved inwater as quickly as possible on awell site.In considerationof thedissolving time andpumping timebeforethe first cross-linking gel was entered into the formation, wechoose 2has themaximal timeof the first cross-linking reaction.

Figure 1. Set-up diagram for core flooding experiments.

(21) Pritchett, J.; Frampton, H.; Brinkman, J.; Cheung, S.; Morgan,J. C.; Chang, K. T.; Williams, D.; Goodgame, J. Field Application of aNew In-DepthWaterflood Conformance Improvement Tool. Presentedat the SPE International Improved Oil Recovery Conference, KualaLumpur, Malaysia, October 20-21, 2003; Paper 84897.(22) Bai, B. J.; Li, L. X.; Liu, Y. Z.; Liu, H.; Wang, Z. G.; You, C.M.

Preformed particle gel conformance control: Factors affecting its prop-erties and applications. SPE Reservoir Eval. Eng. 2007, 10, 415–422.(23) Bai, B. J.; Liu, Y. Z.; Coste, J. P.; Li, L. X. Preformed particle gel

for conformance control: Transport mechanism through porous media.SPE Reservoir Eval. Eng. 2007, 10, 176–182.(24) Frampton, H.; Morgan, J. C.; Cheung, S. K.; Munson, L.;

Chang, K. T.; Williams, D. Development of a Novel WaterfloodConformance Control System. Presented at the SPE/DOE FourteenthSymposiumon ImprovedOil Recovery. Tulsa, Oklahoma,April 17-21,2004; Paper 89391.(25) Chang, K. T.; Frampton, H.; Morgan, J. C.Method of Recover-

ing Hydrocarbon Fluids from a Subterranean Reservoir. U.S. Patent2003/0,155,122, 2003.(26) Chang, K. T.; Frampton, H.; Morgan, J. C. Composition for

Recovering Hydrocarbon Fluids from a Subterranean Reservoir. U.S.Patent 7,300,973, 2007.(27) Ohms, D.; McLeod, J.; Graff, C. J.; Frampton, H.; Morgan,

J. C.; Cheung, S. K.; Yancey, K.; Chang, K. T. Incremental Oil Successfrom Waterflood Sweep Improvement in Alaska. Presented at the SPEInternational SymposiumonOilfield Chemistry,Woodlands, TX, April20-22, 2009; Paper 121761.(28) Chauveteau, G.; Tabary, R.; Renard, M.; Feng, Y. J.; Omari, A.

In-Depth Permeability Control by Adsorption of Soft Size-ControlledMicrogels. Presented at the SPE European Formation Damage Con-ference, Hague, The Netherlands, May 13-14, 2003; Paper 82228.(29) Chauveteau,G.; Tabary,R.; Blin,N.;Renard,M.;Rousseau,D.;

Faber, R. Disproportionate Permeability Reduction by Soft PreformedMicrogels. Presented at the SPE/DOE Fourteenth Symposium onImproved Oil Recovery, Tulsa, Oklahoma, April 17-21, 2004; Paper89390.(30) Rousseau, D.; Chauveteau, G.; Renard, M.; Tabary, R.; Zaitoun,

A.; Mallo, P.; Braun, O.; Omari, A. Rheology and Transport in PorousMedia ofNewWater Shutoff/ConformanceControlMicrogels. Presentedat the SPE International SymposiumonOilfield Chemistry,Houston, TX,February 16-19, 2005; Paper 93254.(31) Omari, A.; Tabary, R.; Rousseau, D.; Calderon, F. L.; Monteil,

J.; Chauveteau,G. Soft water-solublemicrogel dispersions: Struture andrheology. J. Colloid Interface Sci. 2006, 302, 537–546.

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After aging themixed solutions at room temperature (25 �C) for2 h, the viscosity measurements were carried out. The effect ofthe resorcinol concentration on the increasing viscosity of themixed solutions was significant. The relationship is shown inFigure 2.

The viscosity of the mixed solution has increased to acertain degree in 2 h. Most studies have demonstrated thatresorcinol has a high reactivity that reactswith formaldehydeto generate a derivant, even at room temperature.32-34 Thesederivants, together with formaldehyde, can cause a conden-sation reaction with HPAM at room temperature for gelformation. However, few studies have been completed forresorcinol cross-linking with HPAM or acrylamide-basedcopolymers. Our study demonstrated that using resorcinolalone as the cross-linker is feasible. It can cross-link withHPAM in a short time, even at low concentrations at roomtemperature. The value added for viscosity of the mixedsolutions is in the range from 10.2 to 150 mPa 3 s. Figure 2shows that the viscosity smoothly increased initially withresorcinol concentrations, increasing from 0 to 15 mg/L,then sharply increased at high resorcinol concentrationsfrom 15 to 30 mg/L. Viscosity increasing means that thecross-linking network structure formed by adding resorci-nol, and this is a benefit for the mixed solution injected intofractured reservoirs. Because the first cross-linking reactionoccurred in the surface, this not only determines the gelationperformance of the first cross-linking gel, it also relates to thegel performance for later secondary cross-linking. Therefore,negative effects on gelation, such as gravity segregation,dispersion of cross-linker etc., will be minimized due to theweak cross-linking.

However, there is no significant difference in the viscosityincreasing when resorcinol concentrations are 25 mg/L and30mg/L. In view of the property of gel pumping, gel viscosityshould not be too high and less than 300 mPa 3 s is better.Results show that viscosity of polymer solutions can reach

237.4-298.5 mPa 3 s by adding 15-20 mg/L resorcinol.Therefore, 15-20 mg/L of resorcinol can act as the appro-priate concentration in this gel system. In addition, theoverall cost also can be decreased by using resorcinol oflow concentrations.

4.2. Concentration Optimizing of the Secondary Cross-

Linker Phenol-Formaldehyde. In this study, fresh waterwas also used for preparing gelling solutions. All polymers werealso at a constant concentration of 0.3 wt %, and cross-linkerphenol-formaldehyde concentration varies from 0.4 to 1.2 wt%. We chose 65 �C as the relatively extreme low temperature.This way, the gelation performance improves at higher tem-peratures. For all of various gelling solutions, the gelation time,ranging from 9 to 14 days, decreased with increasing concentra-tion of phenol-formaldehyde (Table. 2). However, this isdifferent from previous study, showing that the condensationreaction for polymers cross-linkingoccurs only above 70-80 �Cand requires an alkaline pH. We, also, did not adjust the pH,and it still stayed in the neutral original state.

In general, the gelation performance is better when cross-linkers are at higher concentrations from 0.8 to 1.2 wt%.Wealso found that when cross-linkers at these higher concen-trations, there is no apparently discrepancy of the final gelstrength of all samples. The final strength code of testsamples with higher cross-linker concentrations can reachat the level of code E. This is consistent with Albonico et al.’sresults,9 showing that polymer gel formulated with a combi-nation of 0.5 wt % PAAm-AMPS copolymer addedto 0.6-1.0 wt % phenol-formaldehyde provided the finalgel strength around code E. The final gel strength will notchange by continuing to increase the phenol-formaldehydeat its relative high concentrations. This result can help toobtain suitable phenol-formaldehyde concentration require-ments for the application of these gel systems.

However, the gelling solution with lower concentrationcross-linkers showed poor gelation performance. The final gelstrength of samples for cross-linker concentrations of 0.4 wt %and 0.6 wt % were, respectively, at codes B and D. This meansthat for the gelling solution with the cross-linker concentrationof 0.4 wt% gelation did not occur. There are twomain reasonsthat explaining this phenomenon: (1) The polymer used has arelatively high degree of hydrolysis (25-35%), meaning morecarboxyl groups will be generated that can freely stretch to ahigh extent in freshwater.Thiswill lead to the shieldingof amidegroups for cross-linking. In addition, the polymer was at a lowconcentration of 0.3%, meaning that more surplus amidegroups cannot be easily obtained for cross-linking. Thus, poorgel strength canoftenbeobserved. (2) The lower the cross-linkerconcentration, the worse the degrees of cross-linking. Thesolubility of the polymer gel will increase with low cross-linkingdensity; therefore, the degree of water swelling of the gel wasreduced. So, gels with poor strength are often obtained.

4.3. Study onSecondaryCross-LinkingGel Systems. 4.3.1.Cross-Linker Concentration Optimization. In the previousstudy, resorcinol at concentrations from 10 to 30 mg/L canoccurwith first cross-linkingwithHPAMat room temperature,and the increasing viscosity is desired. However, the final gelstrength is too weak. As for the secondary cross-linker, with allother conditions being equal, the final gel strength only dependson the concentration of the phenol-formaldehyde. The pre-vious experiment’s data showed that there was little discrepancyin the final gel strength with phenol-formaldehyde at concen-trations ranging from 0.8 to 1.2 wt %.

Figure 2.Effect of resorcinol concentration on the viscosity increas-ing for the first cross-linking reaction at room temperature.

(32) Pekala, R. W. Low Density Resorcinol-Formaldehyde Aerogels.U.S. Patent 4,873,218, 1989.(33) Al-Muhtaseb, S. A.; Ritter, J. A. Preparation and properties of

resorcinol-formaldehyde organic and carbon gels. Adv. Mater. 2003,15, 101–114.(34) Calvo, E. G.; Ania, C. O.; Zubizarreta, L.; Men�endez, J. A.;

Arenillas,A.Exploring new routes in the synthesis of carbonxerogels fortheir application in electric double-layer capacitors. Energy Fuels. 2010,24, 3334–3339.

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Therefore, studies on the effect of resorcinol concentrationon gelation performance of the secondary cross-linking gelsystem are needed. In this experiment, cross-linkers formu-lated with a combination of 1.0 wt% phenol-formaldehydeadded to 10-30 mg/L resorcinol provide the organic cross-linkers for this secondary cross-linking gel system. Then,two types of cross-linkers were simultaneously dissolved inHPAM solutions with concentrations of 0.3 wt %. There-fore, the gelation performance of a secondary cross-linkinggel system was studied by changing the concentration ofresorcinol to obtain the appropriate resorcinol concentra-tions at 65 �C.

Table 3 shows that the initial gelation time decreased andthe final gel stength of the samples increased with increasingconcentration of resorcinol. For example, the final gel strengthcan reach codeDwhen resorcinolwas at a low concentrationof10mg/L.However, the gelation time is long, up to 10 days, andit seems that resorcinol did not better perform in the role ofaccelerator. Too long of a gelation time is detrimental to betterresults of gelation in the formation.

However, gelation acceleration was better achieved withhigh concentrations of resorcinol added. The gelation time isabout 2 days, and the final gel strength can reach code Ewhen resorcinol concentration is 30 mg/L. The higher theconcentration of resorcinol, the shorter the gelation time ofsecondary cross-linking gel systems, and also the gelationstrength slightly increased. However, in view of the well siterequirements, the initial gelation time should not be too shortfor large dose gel treatment. However, for fractured reser-voirs and these secondary cross-linking gel systems of lowHPAM concentration, the relatively high final gel strength ismore welcome. The gelation time is about 6 days, and thefinal gel strength is code Ewhen concentrations of resorcinolare 15 and 20 mg/L. In addition, the viscosity for the firstcross-linking gel in the surface is not too high. The previousresults show that the viscosity of weak cross-linking gel is lessthan 300mPa 3 s at room temperaturewhen concentrations ofresorcinol were 15 and 20 mg/L. Thus, it is beneficial for gelpumping.

Figure 3 shows that the viscosity of the gelling solutionslightly increased and reached about 240 mPa 3 s with theaddition of resorcinol within 2 h. It then gently increased in�100 h and later sharply increased and gelation occurred. The

gelation time is nearly 140 h according to the Hardy et al.’sgelation-determining method.35 Large amounts of polymergel can be injected into fractured reservoirs for this longgelation time. It seems that the viscosity increasing for thefirst cross-linking is not affected by the addition of phenol-formaldehyde. Our hypothesis that phenol-formaldehydecould be constrained in the weak cross-linking structure ofgel to guarantee the gelation performance in the formation.This is further investigated through gel shearing experimentsin porous media.

4.3.2. SEM Analysis. Polymer gel formulated with a com-bination of 0.3% HPAM added to 15 mg/L resorcinol and1.0% phenol-formaldehyde was also used to study the micro-structure of the gel. Scanning results (Figure 4) show that themicrostructure of the first cross-linking gel appeared in a typicaldendritic shape and branched chain diffused in an arbitrarydirection. No 3-D network structure was detected from this

Table 2. Gelation Performance of Phenol-Formaldehyde/HPAM (Mw 18000 kDa) Gel Systems

strength code of gel on different days (days)

HPAMConcn (wt %)

phenol-formaldehydeconcn (wt %)

1 2 3 4 5 6 7 8 9 10 14 23 30

0.3 0.4 A A A A A A A A A A B B B0.6 A A A A A A A A B B C D D0.8 A A A A A A A A B C E E E1 A A A A A A A B C E E E E1.2 A A A A A A A B C E E E E

Table 3. Gelation Performance of Secondary Crosss-Linking Gel Systems

strength code of gel on different days (days)

HPAM

concn (wt %)phenol-formaldehyde

Concn (wt %)resorcinol

(mg/L) 1 2 4 5 6 7 8 10 12 14

0.3 1.0 10 A A A A B B B C C D15 A A A B B C C D D E20 A A A B C C D D E E30 A C C D D D E E E E

Figure 3.Viscosity of the secondary cross-linking gel system at roomtemperature and set temperature (65 �C). Note: The gelling solutionwas formulated with a combination of 0.3%HPAMadded to 15mg/Lresorcinol and 1.0 wt % phenol-formaldehyde cross-linkers.

(35) Hardy, M. B.; Botermans, C. W.; Smith, P. New OrganicallyCross-Linked Polymer System Provides Competent Propagation atHigh Temperatures in Conformance Treatments. Presented at theSPE/DOE Symposium on Improved Oil Recovery, Tulsa, OK, April19-22, 1998; Paper 39690.

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picture. This suggested that the gelwasonly slightly cross-linkedby the first cross-linking reaction at room temperature.

Thus, the viscosity increase was not too high. This is bene-ficial for loss control of the gelling solution during injectioninto fractured reservoirs. The secondary cross-linker phenol-formaldehyde will be much constrained in the dendriticshape of the first cross-linking structure; and therefore,both polymer and/or phenol-formaldehyde adsorptionin the rock matrix and dilution by dispersion during theplacement can also be alleviated. Damage to the matrixwill be reduced to a maximal extent. Residual oil exist-ing in the rock matrix was recovered in a protectedenvironment after the fracture sealed by gel. On the otherhand, the dendritic shape of first cross-linking gel maybe helpful to the perfect rheological behavior of sheardilution.

In contrast, a tight 3-D network structure was formed inthe secondary cross-linking gel (Figure 5).

All the macromolecules in the structure of the secondarycross-linking gel are a water-soluble linear highMw polymerwith amounts of chain units. Hydrophilic groups, suchas ;;COONa, ;;CONH2, ;;COOH etc., existed inevery chain group. These hydrophilic groups were all insolvation state in water. Thus, the solvation layer canbe formed outside the macromolecule of the copolymerand can also increase the internal friction when relativemoving of the macromolecule of the copolymer occurred.Therefore, gel with high strength can often be obtainedby using the secondary cross-linking method, and also this3-D network structure may be helpful to gel’s thermo-stability and enduring erosion ability. This may be help-ful to explain the results obtained by Seright that gelwashout can be reduced using secondary cross-linkingreactions.4

4.4. Evaluation of the Secondary Cross-Linking Gel Sys-

tem. TDS and temperature are often the main factors toaffect gelation performance. Better understanding of the two

factors’ effects on the gelation performance of the secondary

cross-linking gel is important for gel treatment design. The

same Gelling solution related to SEM analysis was also used

in this section.4.4.1. Effect of TDS on Gelation Performance. Our tests

were conducted to investigate the effect of TDS on the

gelation time for secondary cross-linking gel systems at a

middle temperature of 65 �C. Divalent salts and their con-

centrations were 200 mg/L CaCl2 and 200 mg/L MgCl2,

respectively. The TDS of the gelling solution was increased

from10000 to 30000mg/L by increasing the concentration of

NaCl. This is to study the effect of TDS on the gelation time

and initial gelation strength (code C).Table 4 shows that the characteristic of the effect of

TDS on the gelation performance of the secondary cross-linking gel system is opposite to the effect of TDS on the

Figure 4. Scanned photo of the first cross-linking gel system.

Figure 5. Scanned photos of the secondary cross-linking gel system.

Table 4. Effect of TDS on Gelation Performance of Secondary Cross-Linking Gel Systems

strength code of gel on different days (days)

HPAMconcn (wt %)

phenol-formaldehyde(wt %)

resorcinol(mg/L) TDS (mg/L) 1 2 4 5 6 7 8 10 12 14 35 90

0.3 1.0 15 10000 A A A B B C C D D E E E15000 A A A B B C D D E E E E20000 A A B B C C D E E F F F25000 A A B C C D D E E F F F30000 A A B C D D E E F F F F

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PEI/PAtBA or PEI/HPAM gel system at high or lowtemperatures.36-38

The gelation time decreased, meanwhile the final gelstrength increased with increasing TDS. The gelation timeis about 7 days when TDS is 10000 and 15000 mg/L, and thefinal gel strength is only about code E. However, the gelationtime reduced to nearly 5 days with TDS of 30000 mg/L, andthe final gel strength can even reach codeF. This is because ofpolymer chains in the high TDS solution rolled due to thecharge repulsion. This will bringmore of a chance for a cross-linker hitting the acidamide groups attached in polymerchains; thus, the gelation rate increased. On the other hand,a double-electrode layer oppressed by monovalent and di-valent ions will lead to polymer linear chains shrunk in highdensity state; thus, gel strength increased.

All test samples do not have dehydration within 90 days,and they still keep a high strength state. This demonstratesthat the secondary cross-linking gel system can have a highsalinity tolerant ability. Thus, it is recommended to useinjected water to prepare this gel system.

4.4.2. Effect of Temperature on Gelation Performance.Weknow that temperature is a key factor affecting the gelationtime of the cross-linker, and fresh water was used to preparethe gelling solution with the same formulation in this experi-ment. The test temperatures are 65, 80, and 90 �C, aiming toenlarge the usage range in different temperatures.

Table 5 shows that the gelation time of the secondarycross-linking gel system also increased with increasing tem-perature, and gel strength also increased. The strength codecan reach code G at 90 �C, but only code E at 65 �C. This isbecause high temperature can promote molecular motionaccelerated. On one hand, the condensation reaction forphenol-formaldehyde is accelerated, meanwhile the chancefor a cross-linker hitting amide groups increases, and thepolymer obtains more cross-linking units. Therefore, gelstrength becomes stronger at high temperatures. However,as for the HPAM series polymers, the temperature tolerantlimit is below 90 �C, above this temperature, gel stabilitycannot be guaranteed. Albonico et al.’s results show thatwith phenol-formaldehyde cross-linking, the heat and sali-nity tolerance copolymer PAAm-AMPS gel dehydrated in1 day at the same phenol-formaldehyde concentration of1.0 wt% at 120 �C.9However, no dehydrated gel was detectedinour experiments, and theorganic secondary cross-linkinggelsystem shows perfect thermostability at 90 �C after 30 days.

Therefore, selecting other high-temperature acrylamide-based polymers to compose this secondary cross-linking gelsystem is worth discussing in a later study.

4.4.3. Gelation Performance Study after Flowing throughPorous Media.After samples flowing through porous mediaand the original one have aged for 20 h (around code B), takeout a little sample and use the atomic force microscopymethod to scan the microstructure of the gel.

Figure 6 shows that the original sample has an obvious“skeleton structure”, and the cross-linking points (darkpoint) seem rather firm with an average height of 3.579 nm(Figure 7). It is demonstrated that the preliminary netstructure of gel was formed. The cross-linking point is ratherthick and solid, so that the branched chain can be tightlylocked. However, the network is still not too compact.Maybe this is only at the germinated time for gelation. Inthis period, the tight 3-D network did not obviously form. Incontrast, in the sample after flowing through porous media,the atomic force microscopy photo did not even show anobvious network structure. The cross-linking points seemrather sparse, and the branched chain was dim (Figure 8).The network structure is a dismemberment and showsfragmentary distribution. It seems that the first cross-linkingsolution had a certain extent of adsorption loss or the

Figure 6.Microstructure of original gel sample after aging for 20 h (around code B). (A): two-dimensional diagram; (B): three-dimensional diagram.

Table 5. Gelation Strength Code at Different Temperatures

strength code of gel at different times (days)

temp (�C) 1 2 3 4 5 6 8 12 30

65 A A A A A B C D E80 A B B C D E F F F90 A B C D E F G G G

(36) Al-Muntasheri, G. A.; Nasr-El-Din, H. A.; Hussein, I. A. ARheological Investigation of a high temperature organic gel used forwater shut-off treatments. J. Pet. Sci. Eng. 2007, 59, 73–83.(37) Eoff, L.; Dalrymple, D.; Everett, D.; Vasquez, J.Worldwide field

applications of a polymeric gel system for conformance applications.SPE Prod. Oper. 2007, 22, 231–235.(38) Jia, H.; Pu,W. F.; Zhao, J. Z.; Jin, F. Y. Research on the gelation

performance of low toxic PEI cross-linking PHPAM gel systems aswater shutoff agents in low temperature reservoirs. Ind. Eng. Chem. Res.2010, 49, 9618–9624.

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polymer chain is damaged to an extent after flowing throughporous media.

However, the effect of shearing of the first cross-linkingsolution in porous media on the follow-up gelation perfor-mance is still needs observed.

In this experiment, the gelation performance of thetwo samples will continually be observed through thebottle test and aims at understanding the differenceof the gelation performance between the original sampleand the sample after flowing through porous media.

Figure 8. Microstructure of the gel sample after flowing through porous media after aging for 20 h (around code B). (A): two-dimensionaldiagram; (B): three-dimensional diagram.

Figure 9. Comparison diagrams of the gelation performance of the two samples in different times Left: original sample; Right: sample afterflowing through porous media.

Figure 7. The thickness of microscopic cross-linking points of original gel.

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In this experiment, all samples were put in an oven at90 �C.

Comparison diagrams show that the gelation rate appar-ently reduced after flowing through porousmedia (Figure 9).However, the final gel strength of the two samples can reachthe same code H after 10 days. In observation at 60 days, thetwo test samples do not reach syneresis, and gel strength stillstays at codeH. Therefore, this further demonstrates that theweak first cross-linking gel can showperfect shear stability toprotect the subsequent gelation effectiveness, and also, thesecondary cross-linking gel system can provide wonderfulthermostability due to the tight 3-D cross-linking networkachieved by the covalent bond.

5. Conclusions

The gelation performance of the resorcinol/phenol-formal-dehyde/HPAM gel system is discussed systematically in thispaper. Our study demonstrates that the secondary cross-linkinggel system is an attractive system for water shutoff in fracturedreservoirs. The main conclusions drawn from this study arelisted below.

(1)The use of organic resorcinol as the first cross-linker isfeasible, and theweak cross-linking structure in dendriticshape is helpful for shear stability and filtration controlin fractured reservoirs.

(2)The secondary cross-linking mature gel system has atight 3-D cross-linking network due to covalent bondbinding that can achieve a wonderful thermostabilityand enduring erosion ability.

(3)TDS and temperature have positive effects on thestrength of this secondary cross-linking gel system.High-strength gel can be obtained by using high TDSsolutions or at a relative high temperature. However,how much of a higher temperature is appropriate relieson the property of polymer’s temperature resistance.

(4)The final gel strength and stability can be controlledby adjusting the resorcinol concentration, phenol-

formaldehyde concentration, HPAM, or other acryla-mide-based copolymer concentrations to prepare thissecondary cross-linking gel system to meet the well siterequirements.

(5)The final gel strength of the original sample and thesample after flowing through porousmedia do not showan apparent difference; however, the gelation rate de-creased after the sample flowed through porous media.This phenomenon can provide good application pros-pects for this secondary cross-linking gel system toensure a water shutoff success rate during the severeshearing in porous media.

However,whether a secondary cross-linkingmethod canbeapplied in the widely used PEI/PAtBA or other gel system isan interesting problem to be investigated in a later study.

Acknowledgment. This work was financially supported by theState Development Program of Large Gas Fields and CoalbedGas, China (2008ZX05049-05-03). The authors thank Dr. YulaTang of Chevron Global Upstream and Gas for his carefulediting of the English and his valuable suggestion on preparingthis paper. Special thanks to the following people of SouthwestPetroleumUniversity: YongGuo and Ji-Mao Li for their discus-sion of this study. The authors also thank the reviewers of thispaper for their many useful suggestions.

Nomenclature

PEI = polyethyleneimineHPAM= partially hydrolyzed polyacrylamidePAtBA= copolymer of acrylamide and t-butyl acrylatePAAm-AMPS = copolymer of acrylamide and 2-acryl-

amide-2-methylpropanesulfonateTDS = total dissolved solidsMw = molecular weightkDa = 1000 DaKw= water phase permeabilitySEM= scanning electron microscope3-D = three-dimensional