Pore Level Investigation of Dispersivity

Understanding hydrodynamic dispersion is important for optimizing miscible displacement processes and analyzing tracer studies. The phenomenon of dispersion has been extensively studied but still continues to present theoretical and conceptual challenges. Despite the importance of dispersivity in modeling dispersion phenomena, we still don’t know what value of dispersivity to use in field scale applications. Simulations of miscible transport processes generally assume Fickian representations of dispersion in which the dispersivity of the medium is considered constant. However, it has been found that dispersivity grows with distance traveled (Freeze and Cherry, 1979; Arya et al., 1988; Gelhar, 1993). Dispersivity values measured in the field are generally found to be larger than those measured in laboratory by several orders of magnitude. Therefore laboratory-measured dispersivities appear to be of little value in application. Dispersivity has a strong dependence on the pore structure and pore level physics. The understanding of the basic mechanism of solute transport at the pore level is important for understanding the phenomenon of dispersion. We intend to understand this mechanism better by measuring dispersivity at microscopic scale. We use a very thin probe, having two thin wires (100 microns in diameter) working as two electrodes, inserted in a needle of 1 mm internal diameter. The probe allows us to measure electrical conductivity at a scale of few 100s of microns. We conducted standard miscible flood experiments (injection of water into a sand-pack followed by brine) and measured microscopic conductivity by inserting the probe through a hole drilled at the top cap of the sand pack. We find a gradual change in microscopic salinity. The results are consistent with the mixing cell theory proposed by Aris and Amundson (1957). Solute particles traveling along different streamlines can jump from one streamline to the neighboring one. For the range of flow rates and the sand grain diameter used for the experiments, any solute particle spends enough time inside the pore body to completely homogenize the solute concentration inside the pore due to diffusion. We measured microscopic salt concentration at several radial and axial positions inside the column at a range of flow rates. The microscopic solute breakthrough curve shows all the qualitative features of macroscopic breakthrough curve e.g. typical S-shaped transition from zero to maximum salt concentration, increase in length of mixing zone with travel distance and with fluid velocity. Quantitatively too, the microscopic dispersivity at points far enough from the wall of the column is equal to the overall dispersivity. Points near the wall show higher microscopic dispersivity because of wider distribution of streamline velocities in that region. The findings allow interpretation of the basic flow mechanism and nature of microscopic mixing and would help in development of better models for macroscopic transport in porous media.Petroleum and Geosystems Engineerin