The effect of pH on the flow behavior of xanthan solution through porous media

It has been shown previously that polymer/porous medium interactions have a direct effect on both the transport and in situ rheology of a polymer solution. In this paper, experimental results and theoretical analysis are presented on the effect of pH on the rheology and transport of xanthan biopolymer solution through porous media. This pH variation may affect certain conformational properties of the xanthan molecule, as well as changing the charge density on the rock surface itself, and hence will mediate the interaction between the xanthan molecule and the surface. A series of xanthan solutions with various controlled pH values, but the same bulk concentration, were used in the porous medium flow experiments presented in this work. The results for both polymer transport and the in situ rheology of the xanthan solution demonstrate clearly the presence of a molecular surface exclusion phenomenon. This effect leads to an apparent slip effect on the polymer in situ rheology, since the apparent viscosity of the xanthan in the porous medium is below that of the bulk solution. The apparent depleted layer thickness was estimated using a recently developed linear layer model for the form of the concentration distribution close to the pore wall, and this has been compared with a simpler two-fluid model. The results show that the relative depleted layer thickness, ( d r), increases markedly with increasing pH value over the range studied from pH = 1 to 10. This observation can be interpreted by considering the system behavior over three pH ranges which lead to different combinations of xanthan conformation and porous medium surface charge. The larger contribution to the thickness of the depleted layer is from the extension of the xanthan molecule over the lower pH range (~1-5) rather than the additional electrostatic repulsion at high pH. © 1992.