Mobility ratio and polymer recovery mechanisms

In order to appreciate how the situation for waterflooding may be remedied using polymer, it is necessary to introduce the idea of mobility ratio, M, defined as  

Now consider the role of polymer flooding in more realistic areal and vertical reservoir sections which are taken to be approximately two-dimensional (2-D) in nature. In a homogeneous areal system, the situation is similar in certain respects to the 1-D case discussed above. At lower M values, there is very good microscopic displacement and good areal sweep. 

in summary, note that poor linear, areal and vertical waterflooding efficiency may occur for unfavourable M, and that the role of polymer is basically to remedy this situation by reducing M. However, for linear and areal (homogeneous) sweep improvement it is necessary only to reduce M to approximately unity. In cases where larger scale heterogeneities occur, e.g. vertical stratification or areal channel sands, it may be necessary to reduce M to a value considerably below unity. The desirable M for such cases will depend on the details of the particular system of interest. This may also be constrained by certain practical problems such as the magnitude of additional pressure build-up that is acceptable when using viscous polymers in the field. 

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