Origin of Scale-Dependent Dispersivity and Its Implications For Miscible Gas Flooding

Abstract

Dispersive mixing has an important impact on the effectiveness of miscible floods.Simulations routinely assume Fickian dispersion, yet it is well established that dispersivity depends on the scale of measurement. This is one of the main reasons that a satisfactory method for design of field-scale miscible displacement processes is still not available.The main objective of this project was to improve the understanding of the fundamental mechanisms of dispersion and mixing, particularly at the pore scale. To this end, microsensors were developed and used in the laboratory to measure directly the solute concentrations at the scale of individual pores; the origin of hydrodynamic dispersion was evaluated from first principles of laminar flow and diffusion at the grain scale in simple but geometrically completely defined porous media; techniques to use flow reversal to distinguish the contribution to dispersion of convective spreading from that of true mixing; and the field scale impact of permeability heterogeneity on hydrodynamic dispersion was evaluated numerically.This project solved a long-standing problem in solute transport in porous media by quantifying the physical basis for the scaling of dispersion coefficient with the 1.2 power of flow velocity. The researchers also demonstrated that flow reversal uniquely enables a crucial separation of irreversible and reversible contributions to mixing. The interpretation of laboratory and field experiments that include flow reversal provides important insight. Other advances include the miniaturization of long-lasting microprobes for in-situ, pore-scale measurement of tracers, and a scheme to account properly in a reservoir simulator (grid-block scale) for the contributions of convective spreading due to reservoir heterogeneity and of mixing. 3 EXECUTIVE SUMMARYDispersive mixing has an important impact on the effectiveness of miscible floods.Despite decades of research into dispersion, it continues to present theoretical and conceptual challenges. Simulations of miscible transport processes generally assume Fickian representations of dispersion in which the dispersivity of the medium is considered constant. However, dispersivity is found to be dependent on the scale of measurement. A satisfactory method for accurate designing and performance prediction of field scale miscible displacement processes is yet to be developed.The main objective of this project was to perform experimental and computational studies to understand the basic mechanisms of dispersion and mixing at pore scale. The work covered four primary topics: development of microsensors to enable the direct measurement of the solute concentrations at the scale of individual pores; grain-scale evaluation of the origin of hydrodynamic from first principles of laminar flow and diffusion; theoretical evaluation of the effect of flow reversal as a means to distinguish the contribution convective spreading from that of true mixing to dispersion; and numerical evaluation of the field scale impact of permeab...