Polymers for enhanced oil recovery: A paradigm for structure–property relationship in aqueous solution

Dilute solutions of high molecular weight polyisobutylene dissolved in kerosene and low molecular weight polybutene have previously been reported to behave as ideal elastic liquids (“Boger fluids”). We report here rheological properties for similar solutions, having, however, higher molecular weights for the polyisobutylene. At low shear rates, these solutions exhibit the expected Boger-type rheological behavior, and approximately obey the Oldroyd-B constitutive equation. However, above a critical shear rate that depends upon molecular weight, prolonged shearing in a cone-and-plate or parallel-plate rheometer induces a time-dependent increase in the solution viscosity and elasticity. We find that the dependence of this transition on the Weissenberg number and the gap width or cone angle is consistent with a viscoelastic instability predicted by Phan-Thien for Oldroyd-B fluids. This instability appears to be of some generality for Boger fluids, since we have also observed it in a new monodisperse Boger fluid (polystyrene in low molecular weight polystyrene and dioctyl phthalate). Furthermore, this transition may have previously been observed (though not identified) by Jackson et al., using the Boger fluid polyacrylamide in maltose.

A transition occurring in ideal elastic liquids during shear flow

An elongational rheometer is used to measure the relaxation times λ and the steady terminal elongational viscosities ηE,t of aqueous solutions of ionic and nonionic polymers at various concentrations in Θ and in good solvents. In the corresponding ηE,t/λ nomogram the results yield two typical limiting curves—one for solutions of nonionic polymers and another for solutions of ionic polymers. The curves characterize the elongational behavior of the polymer solutions. The elongational flow results in data that are intermediate to the two limiting curves for polymers in aqueous salt solutions and for aqueous solutions of mixtures of ionic and nonionic polymers. By means of an empirical correlation, the relaxation time and the steady terminal elongational viscosity are normalized such that the nomogram can be applied to polymers in various solvents. Thus a simple and reliable method for determining the behavior of polymer solutions in elongational flows is achieved.

Investigation of the elongational behavior of polymer solutions by means of an elongational rheometer

Chemical enhanced oil recovery and the role of chemical product design

Polymer flooding is one of the most successful chemical EOR (enhanced oil recovery) methods, and is primarily implemented to accelerate oil production by sweep improvement. However, additional benefits have extended the utility of polymer flooding. During the last decade, it has been evaluated for use in an increasing number of fields, both offshore and onshore. This is a consequence of (1) improved polymer properties, which extend their use to HTHS (high temperature high salinity) conditions and (2) increased understanding of flow mechanisms such as those for heavy oilmobilization. A key requirement for studying polymer performance is the control and prediction of in-situ porous medium rheology. The first part of this paper reviews recent developments in polymer flow in porous medium, with a focus on polymer in-situ rheology and injectivity. The second part of this paper reports polymer flow experiments conducted using the most widely applied polymer for EOR processes, HPAM (partially hydrolyzed polyacrylamide). The experiments addressed highrate, near-wellbore behavior (radial flow), reservoir rate steady-state flow (linear flow) and the differences observed in terms of flow conditions. In addition, the impact of oil on polymer rheology was investigated and compared to single-phase polymer flow in Bentheimer sandstone rock material. Results show that the presence of oil leads to a reduction in apparent viscosity.

Polymer Flow in Porous Media: Relevance to Enhanced Oil Recovery

Threshold-type shear-thickening has been reported in the literature for relatively complex mixtures and one of the most famous is a solution of poly(vinylalcohol) in water in which a substantial amount of sodium borate is added. As far as we know, relatively few polymer solutions have been shown to be shear-thickening, even less of the threshold type. Two cases of threshold-type shear-thickening are examined. One of them is reversible [HPAM Pusher 700 or partially hydrolyzed poly(acrylamide) aqueous solution], the other one (in vitro-synthesized native dextran aqueous solution) leads to the formation of an irreversible structure provided sufficient strain is applied. In both cases, the occurrence of shear-thickening above a critical value of shear rate is related with a change in macromolecular conformation. This conformational change (reversible) can be followed by the formation of intermolecular bonds (irreversible).

J. Sabatié

Papers

Threshold-type shear-thickening in polymeric solutions