Prediction of Xanthan Rheology in Porous Media

Abstract

SPE Members Abstract The flow behavior of xanthan in porous media has been investigated experimentally, and also theoretically using effective medium theory. In the experimental portion of this study, the rheology of a commercially-available xanthan broth was characterized in porous media and viscometers and compared for a wide range of polymer concentrations (300 to 1600 ppm), effective brine permeabilities (40 to 800 md), ppm), effective brine permeabilities (40 to 800 md), residual oil saturations (0 to 29%), temperatures (25 degrees and 80 degrees C), and rock lithologies (sandstones and carbonates). An apparent shear rate equation having no adjustable parameters was developed and proved to be effective in relating the flow behavior of a given polymer solution in porous media at one set of conditions to the behavior at all other porous media conditions tested as well as to the rheology in a viscometer. Although the shear rate dependence on flow velocity (first order) and effective permeability (negative one-half order) agrees with that predicted by traditional capillary bundle model approaches, the value of the experimentally determined constant coefficient is larger than those predicted by the models. The basis for the shear rate equation employed above has been studied theoretically with the assumption that the xanthan solution rheologies approximately follow the power-law relation. The apparent viscosity for a power-law fluid flowing in a porous medium is derived employing the effective medium approximation of percolation theory. In this approach, a porous medium is modeled as a network of capillary tubes, in which the radii of tubes are randomly distributed using a prescribed probability distribution. The apparent viscosity expression obtained is similar to that from the capillary bundle model, but the coefficient values are different, as observed experimentally. This difference is a consequence of the connectivity of flow channels and their variable cross-section. Due to its shear-thinning nature, a power-law fluid flows mainly through the wide power-law fluid flows mainly through the wide channels of porous media, and largely bypasses smallscale pore channels of the porous body. The capillary bundle model cannot describe this tendency of a shear-thinning fluid. Introduction Process models developed to predict production characteristics of enhanced oil recovery projects using polymers for mobility control require information about the rheological behavior of the polymer solution in porous media. One method to obtain this information involves performing time-consuming coreflood studies using the polymer solution and reservoir core material at each specific reservoir condition of interest, such as permeability, porosity, and fluid saturations. Development of an equation that can be used to predict the flow behavior in porous media from easy-to-obtain viscometer data and/or extend the results from one coreflood to other conditions is highly desirable. An equation relating rheology in viscometers to flow behavior in porous media is most suitable for pseudoplastic (" shear-thinning") fluids. A pseudoplastic (" shear-thinning") fluids. A correlation is not needed for Newtonian fluids since the viscosity is independent of shear rate, while the polymers is not observed in conventional viscometers. polymers is not observed in conventional viscometers. Xanthan biopolymer is one excellent candidate for such a correlation because it is almost purely pseudoplastic (" shear thinning") with negligible pseudoplastic (" shear thinning") with negligible elastic effects. Interest in the use of this polymer for oil recovery operations has been high due to favorable properties which include:good injectivity resulting from pseudoplasticity,relative insensitivity of viscosity to salinity, andexcellent resistance to shear degradation over the shear rate range of interest in typical oilfield applications. P. 353