A Rheological Model for pH-Sensitive Ionic Polymer Solutions for Optimal Mobility-Control Applications

Abstract

Abstract Polyelectrolytes are water-soluble polymers that form molecular networks by association in solution. The properties of such polymers are very sensitive to the pH and ionic content of the solution. Polyelectrolytes such as poly(acrylic acid) can swell up to ∼1,000 times of its own volume by retaining a huge volume of water, or they may reversibly deswell to their original volume. Their solution viscosity can accordingly be changed by several orders of magnitude in a controlled manner by adjusting the solution pH. This remarkable property of pH-sensitive polymers can be exploited for a number of improved oil recovery (IOR) applications, as proposed by Al-Anazi and Sharma (2002). A key advantage for polyelectrolytes such as poly(acrylic acid) is their very low cost, because immense quantities are manufactured annually for consumer and industrial applications. For IOR processes, such as surfactant flooding, CO2 flooding and alkaline flooding, a major technical challenge is to bring the injected chemicals into contact with the oil and to properly herd the mobilized oil to producers, minimizing the undesirable influence of reservoir heterogeneity. Placement of the mobility control polymer in desired locations in the reservoir for controlled viscosification can significantly alleviate the problem of volumetric sweep. For the optimal control of the polymer viscosity at the desired locations, the dependence of the polymer rheology on pH, salinity, polymer concentration and molecular structure needs to be known accurately. The rheology correlation can then be employed for IOR process simulations for optimal design of mobility-control bank placement and viscosification, and for evaluation of its effectiveness. A comprehensive rheology model is developed by combining the ionic hydrogel swelling theory of Brannon-Peppas and Peppas (1988) with the Mark-Houwink equation that relates the polymer intrinsic viscosity with polymer molecular size; the Martin equation that relates the characteristic Newtonian viscosity with polymer concentration and intrinsic viscosity; and the Carreau equation that relates polymer viscosity with shear rate. The ionic swelling theory had been independently validated with laboratory swelling experiments earlier, and the Mark-Houwink, Martin, and Carreau equations have been employed to characterize aqueous polymer solution rheology. The polymer solution viscosity can, therefore, be reliably predicted as a function of pH, salinity, polymer concentration, and shear rate. The model predictions match the laboratory-measured viscosity data reasonably well, for a range of pH, salinity, polymer concentration, and shear rate. The model is presented in a form that can be implemented in an IOR process simulator for optimal mobility control applications, as described above.