**2. Gelling polymer systems for conformance control of oil reservoirs**

Hydrogels used to control the anisotropic permeability profile of oil reservoirs are crosslinked polymers, swellable in water that retain the solvent within their three-dimensional structures without dissolving them [7–9].

Polyacrylamide homopolymer (PAM) and acrylamide copolymers such as: partially hydrolyzed polyacrylamide (PHPA), copoly(acrylamide-t-butyl acrylate) (PAtBA), copoly(acrylamide-2-acrylamido-2-methyl-propanesulfonic acid) (PAM-AMPS), and copoly(acrylamide-N-vinyl-2-pyrrolidone) (PAM-NVP)<sup>4</sup> are the most widely used polymers for conformance-improvement

<sup>4</sup> The incorporation of AMPS and/or NVP groups in acrylamide-copolymer chains prevents the acrylamide groups from autohydrolyzing at high temperatures, reducing the polymer susceptibility to precipitate out of the solution in the presence of hardness divalent ions (i.e. Ca2+ or Mg2+). For this reason, AMPS-NVP-acrylamide copolymers are mainly applied for the conformance control of reservoirs with harsh conditions (i.e. temperature > 90°C and salinity >100,000 ppm TDS) [10, 11].

**Figure 2.** Acrylamide-based polymers used for conformance control of oil reservoirs.

treatments of oil reservoirs **Figure 2**. Biopolymers such as xanthan gum, guar gum, chitosan, starch, cellulose, and scleroglucan have also been studied [7, 11, 12].

Furthermore, the porous matrix should present: (i) zones with high-permeability contrasts (e.g. 10:1), (ii) high thickness ratios (e.g. less-permeable zones being 10 times thicker than

Moreover, hydrogels that suffer severe dehydration and syneresis, greatly reducing their volume over time under reservoir conditions, are not suitable for reservoir permeability profile

Several gelling systems have been developed (**Table 1**) for the treatment of high-permeability

• adsorption/swelling of pre–cross-linked hydrogel within the reservoir rock (Section 2.2).

In *in situ* cross-linked polymer systems, a polymer solution and a cross-linking agent are injected together into the porous matrix forming an *in situ* hydrogel within the high-permeability zones

Gel dehydration can occur in fractures subjected to high pressure gradients. Gel syneresis can result from the increase in the cross-linking density due to excessive cross-linker agent addition and/or excessive cross-linking sites developed

Typical conditions found in oil reservoirs are: temperature ranging from 50 to 150°C, pressure between 10 and 50 MPa, salinity from 2000 up to 300,000 mg/L of Total Dissolved Solids (TDS) (with 1 to 35 mg/L of divalent ions), pH value

on the polymer chain overtime (e.g. autohydrolysis of acrylamide polymers at high temperatures) [17].

by means of the:

**temperature**

Colloidal dispersion gels [61–63] 40–90°C up to 15,000 mg/L TDS

BrightWater™ systems [69–73] 35–140°C up to 120,000 mg/L TDS Preformed particle gels [74–76] up to 120°C up to 300,000 mg/L TDS pH-sensitive gel systems [60, 77–79] up to 90°C up to 30,000 mg/L TDS

Microgel systems [4, 64–68] up to 90°C insensitive to salinity variations

**Formation water salinity**

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up to 90°C up to 30,000 mg/L TDS

Hydrogels Applied for Conformance-Improvement Treatment of Oil Reservoirs

30–180°C 30,000–100,000 mg/LTDS

up to 90°C up to 30,000 mg/L TDS

high-permeability zones, and (iii) relatively low oil viscosities [16].

**Table 1.** Summary of gelling polymer systems used for conformance control of oil reservoirs.

**Groups Gelling polymer systems Reservoir** 

metal ions [11, 20–35]

[58, 59]

Gelling systems cross-linked with

Gelling systems cross-linked with organic compounds [19, 42–57]

Gelling systems with no cross-linker

treatments5

*In situ* cross-linked polymer systems

Pre–cross-linked polymer systems

.

of the reservoir [19–22].

5

6

zones of a broad range of reservoir conditions<sup>6</sup>

**2.1.** *In situ* **cross-linked polymer systems**

between 4 and 8, and permeability from 10 to 450 mD [18].

• formation of an *in situ* hydrogel (Section 2.1); or

These polymers can be cross-linked by organic and inorganic cross-linkers forming chemical hydrogels through covalent bonding or physical hydrogels through ionic complexing, hydrogen bonding, polymer entanglements, van der Waals interactions, and/or hydrophilic interactions [13, 14].

The formed hydrogel structure and properties depend on the gelling system components (polymer, cross-linker, and solvent), the concentration of reagents, and the reaction conditions (pH value, salinity, and temperature). An increase in the polymer and/or cross-linker concentration in the gelling system results in a hydrogel with a rigid structure due to a larger number of cross-links per unit of chain length [13].

For conformance-improvement treatment of heterogeneous oil reservoirs, the gelling system must [11, 12, 15]:



**Table 1.** Summary of gelling polymer systems used for conformance control of oil reservoirs.

Furthermore, the porous matrix should present: (i) zones with high-permeability contrasts (e.g. 10:1), (ii) high thickness ratios (e.g. less-permeable zones being 10 times thicker than high-permeability zones, and (iii) relatively low oil viscosities [16].

Moreover, hydrogels that suffer severe dehydration and syneresis, greatly reducing their volume over time under reservoir conditions, are not suitable for reservoir permeability profile treatments5 .

Several gelling systems have been developed (**Table 1**) for the treatment of high-permeability zones of a broad range of reservoir conditions<sup>6</sup> by means of the:


#### **2.1.** *In situ* **cross-linked polymer systems**

treatments of oil reservoirs **Figure 2**. Biopolymers such as xanthan gum, guar gum, chitosan,

These polymers can be cross-linked by organic and inorganic cross-linkers forming chemical hydrogels through covalent bonding or physical hydrogels through ionic complexing, hydrogen bonding, polymer entanglements, van der Waals interactions, and/or hydrophilic

The formed hydrogel structure and properties depend on the gelling system components (polymer, cross-linker, and solvent), the concentration of reagents, and the reaction conditions (pH value, salinity, and temperature). An increase in the polymer and/or cross-linker concentration in the gelling system results in a hydrogel with a rigid structure due to a larger

For conformance-improvement treatment of heterogeneous oil reservoirs, the gelling system

• be formulated with low concentrations of relatively inexpensive environmentally accept-

• have good injectivity and propagation within the matrix reservoir rock;

• have thermal, mechanical, and biological stability under reservoir conditions; and

starch, cellulose, and scleroglucan have also been studied [7, 11, 12].

**Figure 2.** Acrylamide-based polymers used for conformance control of oil reservoirs.

number of cross-links per unit of chain length [13].

• have controllable and predictable gelation time;

• provide a broad range of gel strengths, including rigid gels.

interactions [13, 14].

72 Hydrogels

must [11, 12, 15]:

able and friendly chemicals;

In *in situ* cross-linked polymer systems, a polymer solution and a cross-linking agent are injected together into the porous matrix forming an *in situ* hydrogel within the high-permeability zones of the reservoir [19–22].

<sup>5</sup> Gel dehydration can occur in fractures subjected to high pressure gradients. Gel syneresis can result from the increase in the cross-linking density due to excessive cross-linker agent addition and/or excessive cross-linking sites developed on the polymer chain overtime (e.g. autohydrolysis of acrylamide polymers at high temperatures) [17].

<sup>6</sup> Typical conditions found in oil reservoirs are: temperature ranging from 50 to 150°C, pressure between 10 and 50 MPa, salinity from 2000 up to 300,000 mg/L of Total Dissolved Solids (TDS) (with 1 to 35 mg/L of divalent ions), pH value between 4 and 8, and permeability from 10 to 450 mD [18].

Several cases of success of field applications of *in situ* cross-linked gelling systems have been reported for the conformance-improvement treatment of high-permeability zones located near-wellbore (with radial penetration into the matrix lower than 5 m) and far-wellbore (with radial penetration into the matrix greater than 5 m). However, *in situ* cross-linked gelling systems still face a number of operational difficulties regarding the control of the gelation kinetics, the efficient mixing of the polymer and cross-linker within the reservoir, the prevention of undesirable separation of the gelant components into the heterogeneous matrix, and the risk of plugging the whole formation or areas containing oil [23].

• the encapsulation of the cross-linker by polyelectrolyte nanogels of polyethylenimine (PEI) and dextran sulfate (DS) to control the release of the metal ions into the porous matrix, forming hydrogels with gelation time from 5 to 30 days, depending on the reservoir tem-

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• the use of biopolymers less shear-sensitive than the polyacrylamide in chromium (III) cross-linked formulations to reduce the polymer degradation when subjected to prolonged shear into the reservoir, for example, xanthan gum, scleroglucan, *Alcaligenes* bacteriaproduced biopolymer and lignosulfonate by-product from paper industry [38, 39]; and • the addition of nanoclays (e.g. kaolinite, laponite, and montmorillonite) to the PHPA/PAM-AMPS-chromium (III) acetate formulations forming nanocomposite hydrogels with good mechanical strength (modulus, elasticity, and deformability) and improved syneresis re-

Various organic compounds (e.g. phenol, resorcinol and formaldehyde, hydroquinone, hexamethylenetetramine, chitosan, modified starch and polyethylenimine) can be used to crosslink water-soluble acrylamide-based polymers (e.g. PAM, PHPA, PAtBA, PAM-AMPS, and

In these systems, a chemical bulk hydrogel with continuous semi-solid three-dimensional network is formed by the reaction of the cross-linker functional groups with the polymer chain functional groups through covalent bonding. These systems have been extensively studied and tested in laboratory and field, especially for harsh reservoir environments (high temperature, salinity, and/or pH value). They have higher gelation times and form hydrogels with higher thermal stability when compared to metal ion–cross-linked systems and are used for the conformanceimprovement treatment of reservoirs with temperatures around 30 up to 180°C [43, 47–49].

Polyacrylamide cross-linked with the product of phenol-formaldehyde reaction formed a hydrogel that remained stable for 13 years at 121°C in a laboratory aging test. This gelling system was successfully applied for the treatment of oil reservoir conformance problems [7, 50]. Despite the good thermal stability shown under harsh conditions, phenol-formaldehyde cross-linked gelling systems have great disadvantages, the phenol toxicity and the formaldehyde carcinogenicity. Thus, less toxic and more environmentally friendly organic cross-

Ortho and para-aminobenzoic acids, m-aminophenol, phenyl acetate, phenyl salicylate, salicylamide, salicylic acid, and furfuryl alcohol were identified as substitutes for phenol and hexamethylenetetramine (HMTA) proved to be suitable to replace formaldehyde, forming

Gelling systems based on polyacrylamide cross-linked with hydroquinone (HQ), hexamethylenetetramine (HMTA), and sodium bicarbonate-formed hydrogels that remained stable for

PAM-NVP) and biopolymers (e.g. modified starch and chitosan) [11, 20, 42–46].

perature [36, 37];

sistance [40, 41].

linking agents have been studied.

stable hydrogels with polyacrylamide [51].

12 months, at 149°C, and for 5 months, at 176.7°C [52, 53].

*2.1.2. Gelling systems cross-linked with organic compounds*

The major *in situ* cross-linked polymer systems commercially available or under academic development are presented below.

## *2.1.1. Gelling systems cross-linked with metal ions*

A wide variety of metal ions (e.g. chromium (III), aluminum (III), zirconium (IV), and titanium (III)) can be used to cross-link water-soluble polymers with carboxylates groups such as: polyacrylamide copolymers (e.g. PHPA, PAtBA, and PAM-AMPS) and biopolymers (e.g. xanthan gum, starch, guar, carboxymethylcellulose, scleroglucan, and lignosulfonates) [11, 20].

These gelling systems form physical bulk hydrogels with continuous semi-solid three-dimensional structures by means of ionic interactions between the multivalent metal cations of the cross-linker and the carboxylate/anions of the polymer chains. The cross-linking rates of these systems can be controlled by varying the polymer and cross-linker concentration, the polymer hydrolysis degree, the solution pH value, and by adding cross-linker's retardants (e.g. acetates, propionates, malonates, and ascorbates) [11, 24–26].

PHPA-chromium (III) ions-based formulations are one of the most commonly used systems applied for the permeability profile treatment of oil reservoirs. These systems have been studied extensively as well as tested in laboratory and field for in-depth conformance control of reservoirs with temperatures up to 90°C [27, 28].

Routson and Caldwell [29] were the first to observe that the injection of a dilute solution of chromium hydroxide and PHPA into a porous medium formed a gel that reduced the matrix permeability. Several authors have proposed changes in the formulation developed by Routson and Caldwell [29] in order to reduce the gelant environmental impact, to increase the gelation time, as well as to improve the formed hydrogel strength and stability such as:


#### *2.1.2. Gelling systems cross-linked with organic compounds*

Several cases of success of field applications of *in situ* cross-linked gelling systems have been reported for the conformance-improvement treatment of high-permeability zones located near-wellbore (with radial penetration into the matrix lower than 5 m) and far-wellbore (with radial penetration into the matrix greater than 5 m). However, *in situ* cross-linked gelling systems still face a number of operational difficulties regarding the control of the gelation kinetics, the efficient mixing of the polymer and cross-linker within the reservoir, the prevention of undesirable separation of the gelant components into the heterogeneous matrix, and the risk

The major *in situ* cross-linked polymer systems commercially available or under academic

A wide variety of metal ions (e.g. chromium (III), aluminum (III), zirconium (IV), and titanium (III)) can be used to cross-link water-soluble polymers with carboxylates groups such as: polyacrylamide copolymers (e.g. PHPA, PAtBA, and PAM-AMPS) and biopolymers (e.g. xanthan gum, starch, guar, carboxymethylcellulose, scleroglucan, and lignosulfonates) [11, 20]. These gelling systems form physical bulk hydrogels with continuous semi-solid three-dimensional structures by means of ionic interactions between the multivalent metal cations of the cross-linker and the carboxylate/anions of the polymer chains. The cross-linking rates of these systems can be controlled by varying the polymer and cross-linker concentration, the polymer hydrolysis degree, the solution pH value, and by adding cross-linker's retardants (e.g.

PHPA-chromium (III) ions-based formulations are one of the most commonly used systems applied for the permeability profile treatment of oil reservoirs. These systems have been studied extensively as well as tested in laboratory and field for in-depth conformance control of

Routson and Caldwell [29] were the first to observe that the injection of a dilute solution of chromium hydroxide and PHPA into a porous medium formed a gel that reduced the matrix permeability. Several authors have proposed changes in the formulation developed by Routson and Caldwell [29] in order to reduce the gelant environmental impact, to increase the gelation time, as well as to improve the formed hydrogel strength and stability

• the use of alternative metal ions with lower toxicity when compared to chromium (III) ions,

• the use of metal ions retardants to increase the gelant gelation time, for example, acetates, propionates, malonates, and ascorbates [17, 32–35]. Malonate ions are 33 times slower than acetate ions in the gelation of PAM-AMPS at 120°C. Under the same conditions, ascorbate ions are 51 times slower than acetate ions. Nevertheless, the use of complexes of metal ions

for example, aluminum (III), zirconium (IV), and titanium (III) ions [11, 30–32];

of plugging the whole formation or areas containing oil [23].

acetates, propionates, malonates, and ascorbates) [11, 24–26].

reservoirs with temperatures up to 90°C [27, 28].

results in the formation of weak hydrogels [34];

such as:

development are presented below.

74 Hydrogels

*2.1.1. Gelling systems cross-linked with metal ions*

Various organic compounds (e.g. phenol, resorcinol and formaldehyde, hydroquinone, hexamethylenetetramine, chitosan, modified starch and polyethylenimine) can be used to crosslink water-soluble acrylamide-based polymers (e.g. PAM, PHPA, PAtBA, PAM-AMPS, and PAM-NVP) and biopolymers (e.g. modified starch and chitosan) [11, 20, 42–46].

In these systems, a chemical bulk hydrogel with continuous semi-solid three-dimensional network is formed by the reaction of the cross-linker functional groups with the polymer chain functional groups through covalent bonding. These systems have been extensively studied and tested in laboratory and field, especially for harsh reservoir environments (high temperature, salinity, and/or pH value). They have higher gelation times and form hydrogels with higher thermal stability when compared to metal ion–cross-linked systems and are used for the conformanceimprovement treatment of reservoirs with temperatures around 30 up to 180°C [43, 47–49].

Polyacrylamide cross-linked with the product of phenol-formaldehyde reaction formed a hydrogel that remained stable for 13 years at 121°C in a laboratory aging test. This gelling system was successfully applied for the treatment of oil reservoir conformance problems [7, 50].

Despite the good thermal stability shown under harsh conditions, phenol-formaldehyde cross-linked gelling systems have great disadvantages, the phenol toxicity and the formaldehyde carcinogenicity. Thus, less toxic and more environmentally friendly organic crosslinking agents have been studied.

Ortho and para-aminobenzoic acids, m-aminophenol, phenyl acetate, phenyl salicylate, salicylamide, salicylic acid, and furfuryl alcohol were identified as substitutes for phenol and hexamethylenetetramine (HMTA) proved to be suitable to replace formaldehyde, forming stable hydrogels with polyacrylamide [51].

Gelling systems based on polyacrylamide cross-linked with hydroquinone (HQ), hexamethylenetetramine (HMTA), and sodium bicarbonate-formed hydrogels that remained stable for 12 months, at 149°C, and for 5 months, at 176.7°C [52, 53].

The performance of gelling systems based on acrylamide copolymers and biopolymers (starch and chitosan) cross-linked with chitosan, starch, and polyethylenimine (PEI) has been evaluated for the conformance control of reservoir with temperatures above 80°C. Laboratory tests showed that PAtBA cross-linked with PEI formed a stable hydrogel at 156°C for 3 months. Furthermore, acrylamide-based polymers-PEI systems were successfully applied to treat conformance problems of carbonate and sandstone reservoirs with temperatures around 130 and 80°C, respectively [47, 49, 54–57].

citrate or chromium citrate). The low polymer concentrations used in these systems are not enough to form a continuous three-dimensional network, and thus, they produce a dispersion

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CDG treatments involve the injection of large volumes of microgel aggregates into the formation. Nevertheless, the low polymer concentrations used in their formulation make them

The mechanism by which CDG treatment generates incremental oil production is not fully understood. Laboratory and field tests carried out in the USA and China showed good results for the in-depth conformance control of high-permeability zones of sandstone reservoirs [62].

Microgel systems are micrometer-sized particle gels that are prepared cross-linking a acrylamide terpolymer (with 2% of acrylate and 2% of sulfonate groups) with zirconium (IV) ions and lactate (used as a chelater) or with chromium (III) ion and acetate, under defined shear

In these systems, the reservoir permeability control is accomplished by the adsorption of the flexible microgel globules onto the pore walls of the reservoir rock, forming monolayers with thickness equal to the diameter of the gel microspheres. The plugging of the porous medium can be controlled by the appropriate selection of the dimension of the microspheres used for

Some successful field tests were conducted with these flexible microgels for the permeability

BrightWater™ systems are commercial submicro-sized particle gels based on sulfonate-acrylamide-polymers (PAM-AMPS) with both labile and stable cross-links. These systems are prepared using an inverse emulsion polymerization process in order to control the particles size distribution in a sub-micron range (<1 μm). Surfactants are also added into the formulation to

These pre–cross-linked systems, developed by MobPTeCh and Ondeo Nalco Energy Services, can be applied for in-depth conformance treatment of reservoirs with temperatures between

When BrightWater™ particles are injected into the reservoir, they encounter high temperatures and swell irreversibly (pop up) through the cleavage of the labile cross-links. The cross-linking density of the particle gels decreases allowing them to expand aggressively by absorbing additional water, blocking the high-permeability zones. After the labile cross-links dissociation, the

After thermal activation, the particles of the BrightWater™ system expand approximately 10 times their original size. The cleavage rate control of the labile cross-links at different temperatures can be made through the appropriate selection of the cross-linking agent used for

fully disperse the micro- or nano-sized particles avoiding them to agglomerate.

stable cross-links maintain the particle gel network and prevent hydrolysis [69, 70].

of separate gel bundles in which predominate intramolecular cross-links [61–63].

conditions, to control the microgel globule size (1–10 μm) [4, 64–67].

cost-effective.

*2.2.2. Microgel systems*

the reservoir treatment.

*2.2.3. BrightWater™ systems*

the submicrogel synthesis [71].

profile control of Daqing reservoir (China) [68].

35 and 140°C and salinity up to 120,000 mg/L TDS [69, 70].

#### *2.1.3. Gelling systems with no cross-linker*

Kansas University Super Polymer One (KUSP1) is a gelling system developed by the University of Kansas based on a biopolymer−polysaccharide (β-1,3-polyglucane)—produced via fermentation by *Alcaligenes faecalis* and *Agrobacterium* bacteria.

This nontoxic biopolymer gels in the absence of cross-linker when the pH value is reduced below 10.8. This gelation process is reversible so that the formed hydrogel can be dissolved by increasing the pH of the solution and can be re-gelled by reducing it.

Field tests were carried out with KUSP1 gelling system to control the permeability profile of oil reservoirs [58, 59].

## **2.2. Pre–cross-linked polymer systems**

In pre–cross-linked polymer systems, the polymer chains are cross-linked in the surface facilities prior to be injected into the reservoir, or at least, are partially gelled in the wellbore. These systems are injected into the reservoir in the form of dispersed aggregates or particles that swell in water within the matrix plugging thief zones with high permeability.

These dispersed aggregates or preformed particles do not have the same drawbacks faced by the *in situ* cross-linked systems, such as: reaction control problems, change of gelant composition or dilution by the formation water. However, the pre–cross-linked gelling systems may undergo high mechanical retention and filtration through the pores of the reservoir, increasing the pressure losses of the system, and may face operational issues associated with poor injectivity of the swollen particles and risk of plugging nondesired regions of the reservoir matrix.

Several cases of success of field applications of pre–cross-linked polymer systems have been reported for the conformance control of high-permeability zones and anomalies located nearwellbore or deeply into the matrix reservoir rock. The injection of preformed gel particles into the reservoir prevents these systems from substantially invading and damaging the matrix rock adjacent to the treated zone [4, 60].

The major pre–cross-linked polymer systems commercially available or under academic development are presented below. The main differences between them are related to particle sizes, swelling rates, and reservoir conditions in which they can be applied.

#### *2.2.1. Colloidal dispersion gels (CDG)*

Colloidal dispersions gels (CDGs) are microgel aggregates that are formed cross-linking a low concentration solution of polymer (e.g. PHPA and PAM-AMPS) with metal ions (e.g. aluminum citrate or chromium citrate). The low polymer concentrations used in these systems are not enough to form a continuous three-dimensional network, and thus, they produce a dispersion of separate gel bundles in which predominate intramolecular cross-links [61–63].

CDG treatments involve the injection of large volumes of microgel aggregates into the formation. Nevertheless, the low polymer concentrations used in their formulation make them cost-effective.

The mechanism by which CDG treatment generates incremental oil production is not fully understood. Laboratory and field tests carried out in the USA and China showed good results for the in-depth conformance control of high-permeability zones of sandstone reservoirs [62].

## *2.2.2. Microgel systems*

The performance of gelling systems based on acrylamide copolymers and biopolymers (starch and chitosan) cross-linked with chitosan, starch, and polyethylenimine (PEI) has been evaluated for the conformance control of reservoir with temperatures above 80°C. Laboratory tests showed that PAtBA cross-linked with PEI formed a stable hydrogel at 156°C for 3 months. Furthermore, acrylamide-based polymers-PEI systems were successfully applied to treat conformance problems of carbonate and sandstone reservoirs with temperatures around 130 and

Kansas University Super Polymer One (KUSP1) is a gelling system developed by the University of Kansas based on a biopolymer−polysaccharide (β-1,3-polyglucane)—produced

This nontoxic biopolymer gels in the absence of cross-linker when the pH value is reduced below 10.8. This gelation process is reversible so that the formed hydrogel can be dissolved by

Field tests were carried out with KUSP1 gelling system to control the permeability profile of

In pre–cross-linked polymer systems, the polymer chains are cross-linked in the surface facilities prior to be injected into the reservoir, or at least, are partially gelled in the wellbore. These systems are injected into the reservoir in the form of dispersed aggregates or particles that

These dispersed aggregates or preformed particles do not have the same drawbacks faced by the *in situ* cross-linked systems, such as: reaction control problems, change of gelant composition or dilution by the formation water. However, the pre–cross-linked gelling systems may undergo high mechanical retention and filtration through the pores of the reservoir, increasing the pressure losses of the system, and may face operational issues associated with poor injectivity of the swollen particles and risk of plugging nondesired regions of the reservoir matrix. Several cases of success of field applications of pre–cross-linked polymer systems have been reported for the conformance control of high-permeability zones and anomalies located nearwellbore or deeply into the matrix reservoir rock. The injection of preformed gel particles into the reservoir prevents these systems from substantially invading and damaging the matrix

The major pre–cross-linked polymer systems commercially available or under academic development are presented below. The main differences between them are related to particle

Colloidal dispersions gels (CDGs) are microgel aggregates that are formed cross-linking a low concentration solution of polymer (e.g. PHPA and PAM-AMPS) with metal ions (e.g. aluminum

sizes, swelling rates, and reservoir conditions in which they can be applied.

swell in water within the matrix plugging thief zones with high permeability.

via fermentation by *Alcaligenes faecalis* and *Agrobacterium* bacteria.

increasing the pH of the solution and can be re-gelled by reducing it.

80°C, respectively [47, 49, 54–57].

76 Hydrogels

oil reservoirs [58, 59].

*2.1.3. Gelling systems with no cross-linker*

**2.2. Pre–cross-linked polymer systems**

rock adjacent to the treated zone [4, 60].

*2.2.1. Colloidal dispersion gels (CDG)*

Microgel systems are micrometer-sized particle gels that are prepared cross-linking a acrylamide terpolymer (with 2% of acrylate and 2% of sulfonate groups) with zirconium (IV) ions and lactate (used as a chelater) or with chromium (III) ion and acetate, under defined shear conditions, to control the microgel globule size (1–10 μm) [4, 64–67].

In these systems, the reservoir permeability control is accomplished by the adsorption of the flexible microgel globules onto the pore walls of the reservoir rock, forming monolayers with thickness equal to the diameter of the gel microspheres. The plugging of the porous medium can be controlled by the appropriate selection of the dimension of the microspheres used for the reservoir treatment.

Some successful field tests were conducted with these flexible microgels for the permeability profile control of Daqing reservoir (China) [68].

#### *2.2.3. BrightWater™ systems*

BrightWater™ systems are commercial submicro-sized particle gels based on sulfonate-acrylamide-polymers (PAM-AMPS) with both labile and stable cross-links. These systems are prepared using an inverse emulsion polymerization process in order to control the particles size distribution in a sub-micron range (<1 μm). Surfactants are also added into the formulation to fully disperse the micro- or nano-sized particles avoiding them to agglomerate.

These pre–cross-linked systems, developed by MobPTeCh and Ondeo Nalco Energy Services, can be applied for in-depth conformance treatment of reservoirs with temperatures between 35 and 140°C and salinity up to 120,000 mg/L TDS [69, 70].

When BrightWater™ particles are injected into the reservoir, they encounter high temperatures and swell irreversibly (pop up) through the cleavage of the labile cross-links. The cross-linking density of the particle gels decreases allowing them to expand aggressively by absorbing additional water, blocking the high-permeability zones. After the labile cross-links dissociation, the stable cross-links maintain the particle gel network and prevent hydrolysis [69, 70].

After thermal activation, the particles of the BrightWater™ system expand approximately 10 times their original size. The cleavage rate control of the labile cross-links at different temperatures can be made through the appropriate selection of the cross-linking agent used for the submicrogel synthesis [71].

Field tests, both onshore and offshore, carried out in North America, Asia, Africa, Europe, and South America, showed that BrightWater™ submicrogels can travel long distances, allowing the permeability control of the reservoir at great depths [69, 71–73].

formation water) due to the increase of the pH caused by geochemical reactions between the

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The microgel swelling-deswelling process occurs due to electrostatic effects caused by adjacent anionic groups that change the hydrodynamic volume of the macromolecules and their

Laboratory and field tests with pH-sensitive gel systems were successfully carried out for in-

The success of a hydrogel-conformance-improvement treatment depends on the correct assessment of the nature of the conformance problem and on the selection of an effective

Reservoir conformance problems that can be treated with gelling polymer systems are basi-

• **Matrix conformance problems**–high-permeability flow path (with no cross-flow) occurring through an unfractured matrix rock with permeability lower than 2 Da. For those, the gelling system is injected into the reservoir, preferably in the gelant state, from the injection well side, improving the overall flood sweep efficiency and generating incremental oil production; • **Anomaly conformance problems**–high-permeability flow path and/or water/gas coning occurring through anomalies such as: fracture networks (both natural and hydraulically induced), faults, interconnected vugular porosity, caverns, and localized matrix reservoir rock with permeabilities greater than 2 Da. For those, the gelling system is injected into the reservoir, preferably in a matured or partially matured state, from: (i) the production well side for water/gas-shutoff treatments near-wellbore purely blocking the fluid flow, or (ii) the injection well side for the placement of the hydrogels in some significant distance into the fracture or other anomaly surrounding the injection well, functioning as a plugging and diverting agent.

The screening and selection of the most appropriate gelling polymer system for the conformance-improvement treatment of an oil reservoir can be done using different laboratory tests (i.e. bottle tests, continuous and oscillatory rheological measures, and core flooding experimental tests) to access information on the gelation time and final gel strength, as well as the short- and long-term stability of gelling polymer systems under specific reservoir conditions − temperature, salinity, pH value of the formation water, and the presence of either carbon

S) [20, 35, 53, 80–83]. Gelling polymer systems used to control the anisotropic permeability profile of oil reservoir

• behave as moderately pseudoplastic fluids, with viscosity between 10 and 30 mPa.s at a constant shear rate of 7 s−1 to ensure good injectivity and propagation in the porous medium.

acid (injected with the microgel) and the mineral components of the matrix rock [77].

conformation in solution [77, 78].

gelling system.

cally [21, 22]:

dioxide (CO<sup>2</sup>

should:

) or hydrogen sulfide (H<sup>2</sup>

depth conformance control of reservoirs [60, 79].

**3. Gelling polymer system screening**

## *2.2.4. Preformed particle gels (PPG)*

Preformed particle gels (PPG) and partially preformed gel systems are millimeter-sized preformed gels (10 μm to mm) based on acrylamide, polyacrylamide copolymers (e.g. PAM, PHPA, PAtBA, and PAM-AMPS), modified superabsorbent polymers (SAP), or biopolymers (e.g. chitosan and starch) cross-linked with metal ions (e.g. chromium acetate), organic compounds (e.g. N-N′-methylene-bis-acrylamide and PEI) or biopolymers (e.g. chitosan and starch) [74, 75].

The PPGs are prepared by solution polymerization/cross-linking followed by drying, crushing, and sieving the preformed bulk gels to the desired particle size. In the field, prior to be injected into the reservoir, these preformed particles swell in water forming strong and stable hydrogels in surface facilities [74–76].

PPGs were applied in China for more than 2000 wells to reduce the water production in mature water-flooded reservoirs. Since PPGs particles are relatively large, their application is limited to reservoirs with fractures or fracture-like channels.

The PPGs have many advantages over other conformance-improvement treatments, such as [74, 76]:


In order to improve the PPG swellability, thermal stability and mechanical strength (modulus, elasticity, and deformability) nanoclays (e.g. kaolinite, laponite, and montmorillonite) can be added to their formulation [75].

#### *2.2.5. pH-sensitive gel systems*

pH-sensitive gel systems are micrometer-sized particle gels (1–10 μm) that are based on anionic polyelectrolytes (e.g. PHPA and poly(acrylic acid)) cross-linked by allyl ethers of polyols (e.g. allyl pentaerythritol).

In these systems, the pre–cross-linked microgels injection is carried out at low pH values. The pre-addition of hydrochloric acid or citric acid is necessary to reduce the viscosity of the microgels before injection − the low-pH coils polymer chains reducing the gelant viscosity. Once deep into the reservoir, the gel microglobules swell (polymer chains uncoil absorbing formation water) due to the increase of the pH caused by geochemical reactions between the acid (injected with the microgel) and the mineral components of the matrix rock [77].

The microgel swelling-deswelling process occurs due to electrostatic effects caused by adjacent anionic groups that change the hydrodynamic volume of the macromolecules and their conformation in solution [77, 78].

Laboratory and field tests with pH-sensitive gel systems were successfully carried out for indepth conformance control of reservoirs [60, 79].
