Reservoir Fine Structure and its Effect on Miscible Fluid Displacements

Microstructure comprising features of <1m dimension can be very influential in the mixing of fluids such as EOR solvents with oil or aquifer waters of differing salinity. Microstructure such as fine layering is rarely modelled and cannot normally be included in reservoir computer simulations. Analytical models are useful in order to make rapid predictions. Here we use analytical solutions to the problems of viscous and gravitational crossflow and derive the resultant relative velocity with which fluid is displaced within each layer. The method extends previous proven solutions to include gravity. The transverse mixing effects are also added using well accepted “Taylor Mixing” theory. Mixing is especially rapid between thin layers corresponding to a dimensionless mixing time which is inversely proportional to thickness squared, is of order 1.

This emphasises the critical effect of reservoir fine structure in promoting fluid mixing. Applications are demonstrated for the design of miscible EOR slugs showing the sensitivity to fluid mobility ratio, flow rate and dip angle. Geological description of fine structure such as micro layering of thickness order 0.1m is concluded to be crucial if reservoir mixing of miscible fluids is to be successfully modelled.