A considerable portion of the total oil in place (OIP) present in mature reservoirs cannot be extracted by conventional methods. Therefore, there is an enormous motivation in the development of different techniques to increase the oil recovery. Enhanced Oil Recovery (EOR) is the implementation of several techniques to increase the amount of oil extracted from reservoirs. Its purpose is to mobilize and recover the capillary trapped oil in the reservoirs, and to improve oil sweep efficiency. Dilute aqueous polymer solutions are used, as flooding agents, to improve the mobility index of regular water flooding. Polymers increase water-phase viscosity and reduce the difference between the permeability of the oil and the water phases. Polymers can significantly increase the viscosity of the injected brine by factors up to 20 at very low concentrations. The most used polymer in EOR applications is partially hydrolyzed polyacrylamide (HPAM), a linear copolymer of acrylamide and acrylic acid. Numerous chemical modifications were proposed to HPAM. In this short review, different chemical modifications to the conventional HPAM are summarized and discussed. Furthermore, different synthesis strategies for these water-soluble polymers are analyzed and reviewed.

Keywords: oil, flooding agents, water-phase viscosity, polymer, acrylamide, acrylic acid, water-soluble polymers, aqueous solution,efficiency, aqueous phases

OIP, oil in place; EOR, enhanced oil recovery; HPAM, hydrolyzed polyacrylamide; PAM, polyacrylamide; PAA, polyacrylic acid; DMSO, dimethyl sulfoxide

The use of water soluble polymers for enhanced oil recovery (EOR) provides an additional chance to extract between 5 to 30% of total oil in place from drilled and mature reservoirs ¹. A big number of patents, mainly from multinational companies, remark the importance of polymers for EOR ². The role of the polymer in the aqueous solution is to increase the viscosity of the displacement solution ³. An important parameter to evaluate the polymer flooding efficiency is the mobility index (M) that depends on the relative permeability and the viscosity of each phase; such parameter between the oil and aqueous phases must be close to the unity to obtain high displacement efficiency.¹

Water-soluble polymers for EOR applications have been successfully implemented in several oilfields.^4,5 Given the harsh conditions present in oil reservoirs, new problems and limitations arise with the use of water-soluble polymers. They must withstand high salt concentration, the presence of calcium ions, high temperatures (>70°C) and long injection times.^1,6

The first polymer used as thickening agent for aqueous solutions was Polyacrylamide (PAM). The thickening capability of PAM resides mainly in its high molecular weight, which reaches values of several millions of grams per mole. PAM is used as the reference model polymer for EOR applications. Partially hydrolyzed polyacrylamide (HPAM) is the most used polymer in EOR. HPAM can be obtained by partial hydrolysis of PAM or by copolymerization of acrylic acid with acrylamide. The optimum acrylic acid content between 20 and 35 wt%.⁷ The presence of electrostatic charges along a polymer backbone is responsible for prominent stretching (due to electric repulsion) of the polymeric chains in water and, eventually, results in a viscosity increase compared to the uncharged PAM.⁸ When HPAM is dissolved in salted water, a reduction in viscosity is observed.⁹ Yet, alternative ideas to increase the polymer solution viscosity have been studied over the last years. These chemical modifications constitute an important group of polymers for EOR applications named hydrophobically modified polyacrylamide (HMPAM). The association between hydrophobic groups incorporated in the backbone of the polymers affects positively the viscosity of the aqueous phase.³ A small number of hydrophobic groups distributed along the main backbone generates a hydrophobic association between those segments.^10,11

Hydrophobic micro-domains are formed when the polymer is dissolved in water, as a result of the association of the hydrophobic groups, above a given hydrophobic monomer concentration. Consequently, an increase of the hydrodynamic volume is observed. This yields a solution with much higher viscosity compared with its non-associative analogue.^11,12 Many different hydrophobic comonomers¹³ were used, such as acrylate derivatives, alkyl acrylamides, sulfonates, phenyl methacrylamides, fluorocarbons, vinyl pyrrolidone. The chemical species used as hydrophobic¹⁴ moieties in HMPAM are summarized in Table 1. Although we identified an important number of relevant chemical modifications of HMPAM, we do not claim our final list in the present work to be complete and exhaustive.

Synthesis strategies

Radical polymerization the most used methodology for the synthesis of EOR polymers.² The three main chemical approaches used to synthesize HPAMs are:

1. The direct free-radical polymerization to produce PAM followed by the acid or base hydrolysis of some of the amide groups along the PAM backbone to produce HPAM
2. Co-polymerize chosen proportions of acrylamide and acrylic acid.
3. Polymerize acrylic acid to polyacrylic acid (PAA) followed by the aminolysis of the PAA.

For HPAM or HMPAM, there are different reaction techniques based in the phase scenario of the reactive mixture during the reaction. They can be classified as follows:

1. Solution polymerization.
2. Dispersed-phase polymerization.
3. Precipitation polymerization.

The aqueous solution polymerization is the most common and cheapest method used in the production of HPAM where thermal initiators such as peroxide, persulphate, azo-compounds are commonly used. The pH of the aqueous medium determines the final degree of hydrolysis of the product. This process produces a viscous liquid product containing up to 20 wt% of the copolymer.

Dispersed-phase or latex polymerization is a more expensive process, carried out in a biphasic liquid mixture; the aqueous reactants are dispersed in an inert organic solvent before the reaction begins. In this case the product is obtained as beads, typically containing 50 wt% polymer and 50 wt% water. Another option is to use a surfactant for micellar copolymerization that remains nowadays as the most used method for synthesizing HMPAMs.³²

Precipitation polymerization is synthesis technique that could overcome problems associated to solution polymerization and the expensive emulsion process. In this method, the reaction start in a homogeneous mixture of solvent, monomer and initiator (continuous phase). When polymers chains grow above a critical value, they precipitate, forming a separate polymer-rich solid phase. Thus, the reaction medium changes from homogenous to heterogeneous towards the end of reaction.³³ One of the attractive characteristics of this method is that it produces polymers that can be used without any further purification or separation process; with a simple drying step if organic solvents were used as the reaction medium. A slightly different method is the precipitation polymerization in compressed solvents. As the hydrophobic monomers are not soluble in water or polar organic solvents, a compressed solvent can be used to dissolve the hydrophobic and the hydrophilic monomers at the same time. Carbon dioxide is the most promising new solvent for the compressed precipitation reaction^34,25 The advantage of this solvent is that high purity copolymers can be obtained without drying or separation process, and that the solvent can be recycled by decompression and recompression.

Our group has experience in different synthesis strategies. Traditional water solution polymerization of acrylamide and/or acrylic acid in water, solution polymerization in dimethyl sulfoxide (DMSO), emulsion polymerization in cyclohexane and carbon dioxide precipitation polymerization reactions were carried out in our laboratory. Recently, conventional organic-solvent precipitation polymerization in ethyl acetate and in ethyl acetate/cyclohexane mixtures was successfully carried out and the results are being prepared for publications.

This short review highlights the importance of the design of new water-soluble polymers for EOR applications and the different hydrophobic modifications of HPAM to produce self-association in the aqueous polymer solution used for flooding oil wells. Also, a short description of the synthesis strategies for industrial production of these type of materials, is presented. The polymer chemical architecture, and the synthetic techniques used must fulfill the product application requirements. The increase in viscosity is the most important parameter to consider, as a result of the nature of the dilute solution.

The authors are grateful to CONICET, ANPCyT and Universidad Nacional de Córdoba (Argentine) for their financial support.

The author declares no conflict of interest.

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