Investigation of Mixing in Field-Scale Miscible Displacements Using Particle-Tracking Simulations of Tracer Floods With Flow Reversal

Abstract

Summary Dispersivity data compiled over many lengths show that values at typical interwell distances are approximately two to four factors of 10 larger than those measured on cores. Such large dispersivities may represent significant mixing in the reservoir, or they may be a result of convective spreading driven by permeability heterogeneity. The work in this paper uses the idea of flow reversal to resolve the ambiguity between convective spreading and mixing. We simulate flow-reversal tests for tracer transport in several permeability realizations using particle-tracking simulations (free from numerical dispersion) on 3D, high-resolution models at the field scale. We show that convective spreading, even without local mixing, can result in dispersion-like mixing-zone growth with large disper- sivities because of permeability heterogeneity. But, for such cases, the dispersivity estimated on flow reversal is zero. With local mixing (diffusion or core-scale dispersion), the dispersivity value on flow reversal is nonzero and much larger than typical core values. Layering in permeability, while increasing the convective contribution to transport, also enhances mixing by providing larger area in the transverse direction for diffusion to act. This suggests that in-situ mixing is an important phenomenon affecting the transport of solutes in permeable media even at large scales. Dispersivity values increase with scale mainly because of the increase in the correlation in the permeability field, but they could also apparently appear to do so because the Fickian model fails to capture the mixing-zone growth correctly at early times. The results and approach shown here could be used to differentiate between displacement and sweep efficiency in field-scale displacements, to ensure accurate representation of dispersive mixing in reservoir simulation, and to guide upscaling workflows. The flow-reversal concept motivates a new line of inquiry for laboratory- and field-scale experiments.