Polymers for conformance improvement

Early application of polymers for use during oilfield conformance improvement operations was focused on improving volumetric sweep efficiency of waterfloods. More recently, polymers have been used extensively in disproportionate permeability reduction (DPR) and relative permeability modification (RPM) treatments for water shutoff and in conformance improvement polymer-gel treatments. This page discusses polymers used in oilfield operations and how they contribute to conformance improvement.

Polymer fundamentals

Polymers are large molecules and chemical entities referred to as macromolecules. Polymer molecules are the resultant chemical specie when a large number of relatively small and repeating molecular entities, called monomers, are joined together chemically. The chemical process of joining together the monomers and forming polymer molecules is referred to as the polymerization reaction process. Polymers, both natural and man made, have numerous beneficial uses and applications in modern society (everything from wood, to plastics, to man-made thickening agents added to milk shakes). Polymers can come in pure solid and liquid forms. Some polymers can be dissolved in liquids. This article is limited to polymers that can be dissolved (or dispersed) in an aqueous solution and that usually increase the viscosity of the aqueous solution.

Basic oilfield polymer types

There are two fundamentally different types of water-soluble and viscosity-enhancing polymer chemistries that have been used during polymer waterflooding and conformance improvement treatments. The first type is biopolymers, such as Xanthan gum polymer. The second type is man-made synthetic polymers, such as acrylamide-based polymers.

Biopolymers of the type used in conformance improvement are polysaccharides (poly sugars) in which the monomer chemical linkages of the polymer backbone are glycoside linkages, involving carbon-oxygen-carbon chemical bonds. In aqueous solution, the multistranded molecular complexes of xanthan polymer are fairly rigid molecular species, causing the polymer molecules to take on an extended molecular conformation.

Synthetic polymers of the type used in conformance improvement are usually highly flexible molecules in which the polymer backbone consists of a relatively chemically stable carbon molecular chain with single and flexible carbon-carbon bonds. Pendant water-soluble chemical groups (e.g., amide groups) on the molecule render the polymer molecule to be soluble in water.

Synthetic polymers have emerged to become the predominant and preferred polymer type for use in commercial oilfield conformance improvement operations because of the inherent chemical and biological stability of synthetic polymers, along with injectivity and cost issues.

Historically, the two types of biopolymers primarily used in polymer waterflooding have been xanthan and scleroglucan polymers. The only synthetic polymers that have been used extensively during polymer waterflooding (and in polymer-gel conformance treatments) are those based on acrylamide-polymer chemistry.

Viscosity enhancement and permeability reduction

Water-soluble polymers used in conformance-improvement operations operate by reducing fluid mobility by increasing viscosity of the oil-recovery drive fluid (primarily the flood water) and/or by reducing permeability, by which the polymers, directly or indirectly, act as a fluid-flow blocking agent. Reducing permeability is the conformance improvement mechanism by which conformance treatments operate when polymers and polymer gels are used for imparting DPR. Polymers used in waterfloods often have a secondary component in their conformance improvement mechanism that involves permeability reduction within the flooded volume of a matrix rock reservoir.

Chemistry of polymers used in conformance improvement

The chemistry of polymers used in conformance improvement is reviewed before discussing polymer waterflooding and DPR polymer treatments because several of the polymer chemistries that are reviewed are used in both of these oilfield conformance-improvement applications. In fact, some of the polymer chemistries discussed in this section are also used in conformance-improvement polymer gels.

Biopolymers

In addition to being environmentally friendly and readily available, advantages of biopolymers are their relative insensitivity to salinity and mechanical shear degradation. The two major concerns relating to the use of biopolymers are their susceptibility to biological and chemical degradation, and injectability issues resulting from cell debris that usually remain in the biopolymer solutions that are derived from microorganism fermentation processes.

Xanthan has been the most widely used biopolymer for polymer waterflooding. Fig. 1 shows the chemical structure of the xanthan biopolymer molecule. For a xanthan molecule with a molecular weight of 4 million daltons (atomic mass units), the xanthan molecule comprises on the order of 20,000 repeating sugar monomer units. Xanthan polymer is derived from a microorganism fermentation process that usually leaves a substantial amount of cell debris in the final polymer solution.

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  Fig. 1 – Chemical structure of a xanthan polymer molecule.

Flooding reservoir matrix rock with fully filtered xanthan polymer solutions tends to result in much less permeability reduction than the comparable flooding with appropriate acrylamide-polymer solutions. Xanthan polymers and the resultant solution viscosity are relatively insensitive to the salinity of the brine into which the Xanthan polymers are dissolved, and Xanthan polymers tend to be relatively insensitive to mechanical shear degradation. Xanthan polymers are quite susceptible to biological degradation.

Scleroglucan, with a triple-stranded molecular configuration, has been suggested to be a biopolymer possessing more favorable stability and performance properties for use during high-temperature polymer waterflooding (e.g., 195°F).

Synthetic polymers

Acrylamide polymers have emerged to be the most widely used synthetic polymer family for application in polymer flooding and in polymer and polymer-gel conformance improvement treatments. This has come about in large part because of cost and availability issues and because of the favorable chemical robustness and biological stability.

Polyacrylamide (PAM) is the simplest and most basic form of acrylamide polymers. Fig. 2 shows the chemical structure of polyacrylamide and partially hydrolyzed polyacrylamide. For polyacrylamide with a molecular weight of 7 million daltons (a representative molecular weight of an acrylamide polymer to be used in polymer waterflooding), the value of n in Fig. 2 and the number of repeating monomer units is on the order of 100,000. When all other factors are equal and when dissolved in brine with a relatively low salinity, polyacrylamide is not as good a viscosity-enhancing agent and is not propagated as well through sand reservoirs when compared to partially hydrolyzed polyacrylamide (HPAM). Because pure polyacrylamide is slightly positively charged (cationic) in an acidic or "neutral" pH environment, polyacrylamide tends to adsorb onto reservoir rock surfaces, especially sands and sandstone pore surfaces. For these reasons, partially HPAM is most often favored for use in polymer flooding.

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  Fig. 2 – Chemical structure of PAM and HPAM polymer molecules.

When polyacrylamide is manufactured commercially, it normally contains 1 to 2 mole percent hydrolyzed (carboxylate) content that is inadvertently imparted during the manufacturing process. This carboxylate "impurity" in many of the commercial polyacrylamide polymers is enough carboxylate content to render such polyacrylamide to be a good candidate for use in conformance improvement polymer gels that involve chemical crosslinking reactions occurring through the polymer’s carboxylate groups. A specialized polyacrylamide polymer is available commercially that contains essentially no carboxylate groups (less than 1 carboxylate groups in 1,000 acrylamide groups). Such polyacrylamide is referred to as ultra-low hydrolysis polyacrylamide. To manufacture ultra-low-hydrolysis polyacrylamide, normal acrylamide monomer feed stock is polymerized as usual, except the polymerization conditions (pH and temperature) are more tightly controlled. Polyacrylamide is normally not referred to as HPAM until the carboxylate content exceeds approximately 2 mole percent.

HPAM is the most widely employed water-soluble polymer for use in both polymer waterflooding and in oilfield conformance polymer-gel treatments. As compared with polyacrylamide, HPAM polymer tends to be a better viscosity enhancing agent in low-salinity brines and tends to adsorb less onto the rock surfaces of reservoirs that are good polymer waterflooding candidates. Thus, the use of the salt form of HPAM is favored over the use of straight polyacrylamide in most polymer flooding applications. A number of practitioners of polymer flooding believe that because of the salt sensitivity of HPAM, this polymer performs best during polymer flooding conducted in reservoirs with low-salinity reservoir brines. However, there have been some instances in which HPAM, when dissolved in a fresh flooding brine, has performed well when flooded in a reservoir with a saline brine. Sohn, Maitin, and Votz cite such an example.

A 30% hydrolysis level within polyacrylamide is near the optimum in terms of simultaneously promoting maximum viscosity enhancement of the polymer solution and minimizing polymer adsorption onto reservoir rock surfaces during most polymer waterfloods. For crosslinked polymer gels, the optimum hydrolysis level of 5 to 10 mole percent (in this case) simultaneously maximizes gel strength and minimizes unproductive intramolecular crosslinking.

As Fig. 2 shows, HPAM can come in two forms as it relates to the chemistry of the carboxylate groups. The carboxylate groups can be in the acid or salt form. For use in polymer waterflooding and in polymer gels, HPAM is almost always used in the sodium salt form. Unless specifically stated otherwise in this chapter, when referring to HPAM, we are referring to HPAM with its carboxylate groups in the sodium salt form.

In low salinity brines, the electrostatic charge repulsion between carboxylate groups of HPAM molecule tends to cause the flexible-backbone polymer to assume a distended tertiary conformational form that is a more effective for enhancing aqueous-solution viscosity than is the more balled-up form occurring for such polymers dissolved in a high salinity brine and the balled-up molecular conformational form of unhydrolyzed polyacrylamide. Fig. 3 depicts the stretched out and more effective viscosity-enhancing molecular form of HPAM that exists in a low-salinity aqueous environment. Fig. 3 also shows the balled-up conformational form that HPAM assumes in a high salinity brine environment. High salinity causes the electrostatic fields around the carboxylate groups to shrink substantially and allows the HPAM molecule to assume a more balled-up form because of the elimination of a high degree of electrostatic repulsion between the negatively charged carboxylate groups on the polymer’s backbone. The balled-up polymer form does not generate nearly as much viscosity as the distended form in otherwise comparable polymer-waterflood solutions.

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  Fig. 3 – Schematic of the effect of solution ionic strength on the molecular conformation of flexible coil polyelectrolye molecules such as HPAM.

Hydrolyzed acrylamide groups, or equivalently termed carboxylate groups, can be introduced into polyacrylamide polymers by several means. First, polyacrylamide that is dissolved in aqueous solution can be reacted with caustic material, such as sodium hydroxide, to convert a portion of the polymer’s pendant amide groups to carboxylate groups. This form of HPAM is referred to as partially hydrolyzed polyacrylamide. Second, during the polymerization process, acrylamide monomers can be copolymerized with acrylate monomers to form HPAM. This form of HPAM is referred to as being a copolymer of acrylamide and acrylate. All polyacrylamides and all commercially available HPAMs, when heated in aqueous solution, slowly undergo an autohydrolysis reaction in which a portion of the acrylamide polymer’s pendant amide groups spontaneously hydrolyzes to carboxylate groups. The final degree of carboxylate content that is attainable within a polyacrylamide molecule increases with temperature, but does not reach 100 mole% carboxylate groups. That is, the acrylamide polymer cannot be converted in aqueous solution at high temperature (under reservoir conditions) to pure polyacrylate by means of the autohydrolysis reaction. The autohydrolysis reaction of acrylamide polymers is both acid and base catalyzed.

In high-temperature reservoirs after polyacrylamides or HPAMs autohydrolyze to sufficiently high levels, hardness ions, such as calcium or magnesium, in the reservoir brine cause the polymer to undergo a phase change, precipitate, and cause the polymer to lose most of its viscosity-enhancing function. This outcome is the major limitation of acrylamide-polymer flooding in high temperature reservoirs. Fig. 4 shows the degree of polymer hydrolysis vs. time at various selected temperatures for 1,000 ppm PAM polymer dissolved in a brine of 5% salinity.

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  Fig. 4 – Degree of hydrolysis vs. time at various temperatures for 1,000 ppm unhydrolyzed PAM in 5% salinity.

Copolymers containing 2-acrylamido-2-methyl-propanesulfonic acid (AMPS) monomers and acrylamide monomers have been suggested to form acrylamide polymers for use in polymer waterflooding of high-temperature (e.g., 200°F) and high-salinity reservoirs where the AMPS copolymer’s performance and stability will be somewhat better than comparable HPAM.

Copolymers of vinylpyrrolidone and acrylamide, along with ter-polymers of vinylpyrrolidone, acrylamide, and acrylate, have been reported to be candidate polymers for use in polymer floods and conformance polymer-gel treatments that are to be applied to high-temperature reservoirs with harsh environments. Certain vinylpyrrolidone polymers were reported to not precipitate from seawater after aging for six years at 250°F (121°C). Potential concerns regarding these co- and ter-polymers of vinylpyrrolidone are their relatively high cost as compared with more conventional acrylamide polymers and the relatively low molecular weight of the commercially available forms of these co and ter polymers.

Fig. 5 shows the general chemical structure of ter-polymers of vinylpyrrolidone, acrylamide, and acrylate. The primary beneficial function of the incorporated vinylpyrrolidone monomer into an acrylamide polymer is that it prevents the acrylamide monomer content of the polymer from autohydrolyzing at high temperatures to the excessively high levels of hydrolysis whereby the polymer would become susceptible to precipitating out of solution when the polymer encounters hardness divalent ions.

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  Fig. 5 – Chemical structure of a ter-polymer poly (vinylpyrrolidone-acrylamide-acrylate) molecule.

Cationic polyacrylamides are acrylamide polymers that have positively charged chemical groups attached to at least some of the polymer’s pendant amide groups or acrylamide polymers that have been copolymerized with monomers containing positively charged pendant groups. These polymers have an exceptionally strong tendency to adsorb onto reservoir rock surfaces, especially sand and sandstone surfaces.

Cationic acrylamide polymers find specialized applications in conjunction with a variety of conformance improvement treatments. These applications include use as polymer-anchoring agents to help promote conformance polymer-gel adsorption onto reservoir rock surfaces, "bridging-adsorption" and/or "flow-induced-adsorption" polymers for injection before a conformance gel treatment to purportedly promote the selective placement of the gel treatment during the bullheaded treatment-placement mode, and polymer for use in certain polymer DPR conformance treatments. Water-soluble cationic acrylamide polymers come in a wide variety of forms and chemistries. Fig. 6 shows the chemical structure of two cationic acrylamide polymers that have been studied for use in the bridging-adsorption phenomenon.

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  Fig. 6 – Chemical structure of two cationic acrylamide polymers for possible use in the bridging-adsorption selective placement process for conformance gel treatments.

Benefits of polymer for conformance improvement

The application of oilfield polymer technologies, in the form of polymer waterflooding and polymer DPR treatments (and as polymer-gel treatments), can promote conformance improvement during enhanced oil recovery flooding and oil production operations. They do so by the following means.

• Improve sweep efficiency—The application of polymer waterflooding and polymer DPR treatments promote more effective economic use of injected oil recovery drive fluids, such as water during waterflooding. DPR treatments can also be used to reduce the amount of injected oil recovery drive fluid that must be coproduced to yield a given oil recovery factor.
• Accelerate production—Successful polymer waterflooding and polymer DPR treatments accelerate oil production during a waterflood or other oil-recovery flooding operations by reducing the amount of injected oil recovery drive fluid that must be coproduced to attain a given level of oil recovery.
• Promotes incremental oil production—Polymer waterflooding and DPR treatments rarely reduce waterflood residual oil saturations. However, they do promote incremental oil recovery by increasing the amount of oil production before reaching the economic water-oil ratio (WOR) limit of a production well, well pattern, or field during a waterflood or other oil-recovery flooding operation.
• Extend economic lives—Polymer waterflooding and DPR treatments can extend the economic lives of production wells, well patterns, and fields by increasing oil cuts as a function time and deferring the time when the economic WOR limit of a well, well pattern, or field is reached.

Noteworthy papers in OnePetro

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External links

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See also

Polymer waterflooding

Polymer waterflooding design and implementation

Field performance of polymer waterflooding

Conformance improvement

Disproportionate permeability reduction

Polymer impact on permeability

Gels

PEH:Polymers,_Gels,_Foams,_and_Resins