Polymer impact on permeability

Disproportionate permeability reduction (DPR) is a phenomenon whereby many water-soluble polymers and many polymer gels reduce the permeability to water flow to a greater extent than to oil or gas flow. DPR is also referred to as relative permeability modification (RPM). In Disproportionate permeability reduction, a review is presented of the concepts, applicability, limitations, and desirability of the DPR phenomenon as it applies to conformance improvement water-shutoff (and/or water-reduction) treatments. This article focuses specifically on polymer use for DPR and RPM.

As early as 1964, certain polymer flood water soluble polymers were known to impart DPR to water flow in reservoir rock that had been previously flooded with the polymer. Although, in concept, water-soluble permeability reducing polymers can be injected (using appropriate polymers and conditions) into matrix rock to reduce the absolute permeability to all fluids (including water, oil, and gas), the injection of water-soluble permeability reducing polymers into matrix rock is most often performed to impart DPR.

Advantages and issues

Bullhead injection of a simple aqueous solution containing a water-soluble polymer to treat conformance problems, such as excessive water production, in matrix rock (unfractured) reservoirs is a highly appealing concept. Most DPR polymers (also known as RPM polymers) are not usually highly exotic or costly. Thus, polymer alone DPR treatments for reducing excessive water production are much simpler and less risky, in concept, than conducting the same task using a relatively strong total-shutoff gel, particularly a “strong” crosslinked polymer gel. A polymer solution alone poses less risk of totally sealing off the treated reservoir volume compared with injecting a water-shutoff gel. The chemical and operational aspects of injecting a polymer solution for water-shutoff/reduction purposes are substantially less complicated than injecting a comparable polymer gel.

However, polymer DPR water-shutoff and/or water-reduction treatments do have a number of significant limitations, in addition to those already discussed in Disproportionate permeability reduction.

1. How fast will the treatment polymer be desorbed and flow back to the production well?
2. DPR polymer treatments for conformance improvement are normally successful only when applied to matrix rock oil reservoirs with a relatively low permeability (usually less than 1 Darcy). In addition, many existing DPR polymer conformance-improvement treatments are only applicable to sand and sandstone reservoirs. DPR polymer conformance-improvement treatments are not directly applicable within fractures and other high-permeability anomalies.
3. The amount of DPR, which is imparted by available polymer systems, is often quite small as compared with the amount of DPR that can be imparted by DPR polymer gels. Mennella provides guidelines for well-candidate and chemical selection for use when considering the application of polymer DPR water-shutoff/reduction treatments.
4. Performance of polymer DPR treatments has been erratic in both the laboratory and field setting.

Mechanism for imparting DPR

Although the mechanism by which polymers impart DPR to water flow in reservoir porous media is currently under active study, the basic mechanism is thought to involve polymer adsorption onto the pore body walls and/or retention at the pore throats. In most cases, DPR polymers tend to decrease the relative permeability to water with little effect on the oil or gas relative permeability curve.

Fig. 1, taken from Zaitoun and Kohler, depicts the DPR and RPM effect on relative permeability curves imparted in a 4.8 Darcy sand pack that was flooded at 140°F with a 10 g/L solution of biopolymer Polysaccharide G in 10 g/L KCl brine. The figure shows, as a result of flooding the sand pack with the biopolymer solution, how the relative permeability curve to water was substantially reduced while the relative permeability curve to oil was relatively unaffected.

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  Fig. 1 – Relative permeability modification effect induced in a sand pack by Polysaccharide G.

Range of applicability of DPR polymer treatments

While DPR polymer systems for imparting conformance improvement have been targeted primarily at sand and sandstone reservoirs, presumably being water wet, favorable DPR polymer effects have been observed when nonionic polyacrylamide was placed in various carbonate rocks with either water-wet and oil-wet conditions. DPR polymer treatments have been applied to reservoirs with temperatures up to 225°F; however, DPR polymer systems for use in conformance-improvement treatments are only applicable over a limited lower permeability range (approximately 5 md to hundreds of md in most cases). Because the upper permeability limit varies with the specific DPR polymer system and with specific reservoir rock lithologies, it is difficult to provide a universal upper permeability limit for the successful application of DPR polymer systems. The upper permeability limit for the successful application of any specific DPR polymer system is a treatment variable that should be scrutinized closely. The application of polymer DPR treatments to reservoir conformance problems involving flow channels with permeabilities exceeding the upper permeability limit of a DPR polymer is a major cause of failures for such polymer conformance treatments.

Illustrative technologies and field applications

The earliest polymers (uncrosslinked) reported to have DPR properties used in water-shutoff/reduction treatments applied to production wells were polyacrylamides, which is the same general type of water-soluble polymer that has been used extensively in polymer flooding for mobility control and sweep improvement purposes. In 1973, SPE literature reported on a proprietary and commercially available "brush" polymer for "selectively reducing water production."

In 1988, Zaitoun and Kohler reported on how the adsorption of, respectively, polyacrylamide and a polysaccharide onto water-wet sand and sandstone promotes DPR to water flow. Also, it was reported how the adsorption of these polymers increase the irreducible water saturation. The same research group reported on the development of two polyacrylamide-based DPR water-shutoff processes and the field application of one of the processes in an underground gas-storage facility.

Gruenenfelder et al. reported on the application of DPR polymer water-shutoff/reduction treatments to two gravel-packed wells of high-temperature (190 to 200°F) sand reservoirs of the US Gulf Coast. The DPR polymer treatments involved the use of a nonionic triple-stranded polysaccharide biopolymer.

The application of DPR polymer water-shutoff/reduction treatments, involving an amphoteric polymer material, to five wells in a high-permeability and high-temperature (up to 225°F) sandstone reservoir in Indonesia has been reported.

A couple of sources discuss the use of cationic polyacrylamide as a candidate polymer for use in polymer DPR treatments to impart conformance improvement.

In 2001, Eoff et al. reviewed the structure and process optimization of a commercial "brush" polymer that has been used since 1973 in various forms and under various trade names as a RPM polymer in conjunction with conformance-improvement treatments.

Noteworthy papers in OnePetro

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External links

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See also

Conformance improvement

Polymers

Polymer waterflooding

Polymer waterflooding design and implementation

PEH:Polymers,_Gels,_Foams,_and_Resins