During injection of a gelant to miscibly displace water, both diffusion and dispersion will occur. While diffusion is the transport of mass because of spatial concentration differences, dispersion is mixing caused by variations in the velocity within each flow channel and from one channel to another.41In flow through reservoirs, dispersion usually is much more important than diffusion.41 The size of the mixing zone (again, between the 90%-10% concentration levels) created by dispersion can be estimated using Eq. 6,
where α is the dispersivity of the porous medium and L is the distance traveled by the fluid front. Laboratory values for a commonly are in the range from 0.001 to 0.05 ft.38,41,42 However, field dispersivity values are usually significantly greater than laboratory values because of the greater heterogeneity experienced on the larger scale.41
Ref. 42 presents a detailed examination of the impact of dispersion and diffusion on gelant placement. From that analysis, we concluded that dispersion will dilute gelant banks in high-permeability zones by approximately the same factor as in low-permeability zones. Therefore, dispersion is unlikely to aid gelant placement.