Estimation of True Dispersivity in Field-Scale Permeable Media

Abstract

Summary: Simulation studies (Solano et al.1; Stalkup2) show that oil recovery for enriched gas drives depends on the amount of dispersion in reservoir media, but the value of dispersion, expressed as dispersivity, is unknown at the field scale. This work focuses on three types of dispersion in permeable media to obtain realistic estimates of dispersive mixing at the field scale. The dispersivity from single-well tracer tests (SWTTs), also known as echo dispersivity, is dispersivity that is unaffected by fluid flow direction. Layering in permeable media tends to increase the dispersivity observed in well-to-well tracer tests, also known as transmission dispersivity, but should not affect the echo dispersivity. We analyzed a collection of SWTT data to estimate echo dis-persivity at the SWTT scale. The estimated echo dispersivities closely match a published trend with length in dispersivities obtained from groundwater tracer tests. This unexpected result—it was thought that transmission dispersivity should be greater than echo dispersivity—is analyzed with numerical simulation. Another type of dispersive mixing is local dispersivity, or the mixing observed at a point as tracer flows past. Numerical simulation results show that the local dispersivity is always less than the transmission dispersivity and greater than the echo dispersivity. It is closer to one limit or the other depending on the amount and type of heterogeneity, the autocorrelation structure of the medium's permeability, and the lateral (vertical) permeability. The agreement between the SWTT echo dispersivities and the field trend suggests that the field data are measuring local dispersivities. All dispersivities appear to grow with length.