The effect of mobility gradients on viscous instabilities in miscible flows in porous media

The onset of viscous instability and the subsequent fingering have been mos tly studied under conditions of a sharp mobility contrast. In stable miscib le flows in porous media, however, mobility gradients of variable extent ca n develop due to hydrodynamic dispersion. Graded mobility and density profi les will affect instabilities, either by mitigating (in the case of a monot onic variation) or by enhancing (nonmonotonic variation) the rates of growt h. The mitigating effect was exploited by Claridge and Gorrel and Homsy for the optimal design of graded mobility banks. Hickernell and Yortsos showed that in the linear stability limit, the growth rate is controlled by the m aximum of the logarithmic derivative of the mobility profile. We present an experimental study of miscible displacements in porous media to study effe cts of mobility gradients in viscous instability. We take advantage of mole cular diffusion in miscible displacements to create a mobility gradient zon e and subsequently initiate the instability by increasing the displacement velocity. A theoretical analysis based on linear stability is used to analy ze the experimental results. (C) 1999 American Institute of Physics. [S1070 -6631(99)02402-2].