The objectives of this project are to perform a novel fundamental study of the mechanism of dispersion, to develop an improved multiscale statistical model of dispersion, and to use this advance in understanding to optimize field-scale displacements.

University of Texas at Austin, Austin, TX
University of California, Merced, CA

The ultimate oil recovery efficiency from carbon dioxide (CO2) injection is low. The CO2 utilization rate—the amount of CO2 injected to recover a barrel of oil—is high. The performance of a CO2 process is strongly influenced by hydrodynamic dispersion, but the appropriate level of dispersivity to use at the scale of reservoir simulation is largely unknown. That dispersivity increases with scale (travel distance) has been known for more than 30 years. There is substantial ambiguity about what causes the scale dependence. Part of the work being proposed here is to investigate the scale dependence downward—from the core or column scale to the pore scale—as opposed to upward, as has been done previously. This approach has never been tried as part of an effort to understand the fundamental mechanism of dispersion.

Results
Large-scale high resolution 3D simulation studies show that measuring dispersion in flow reversal tests (also referred to as push-pull tests or echo tests) is the single most powerful technique for establishing the relative contributions of diffusion (mixing at molecular scale) and heterogeneity (the variability of flow speeds on streamlines resulting from natural spatial variability of the permeability of a formation.) If diffusion is small, then a flow reversal experiment shows the dispersion is largely reversible. The time required to reach “vertical equilibrium” depends on the magnitude of diffusion and the correlation length of the permeability field in the primary direction of flow. If diffusion is large, then flow reversal does not reverse the spreading of the tracer front. However the dispersion coefficient, as measured by the variance of the location of tracer particles in the simulations, does not increase during the period of reverse flow.

Transmission and echo dispersivities were estimated for a wide range of reservoir descriptions (varying heterogeneity, correlation lengths and anisotropy). The simulated data fall within the overall trend of the dispersivity-length plot (Gelhar et al., 1992). High values of echo dispersivities (up to 10m) which are comparable to the corresponding transmission value indicate significant levels of mixing occurring in such displacements. The magnitude of the echo dispersivity and the nature of the mixing zone growth are largely dependent on the correlation lengths of the permeability field.

The essential grain-scale phenomena giving rise to hydrodynamic dispersion observed in porous media are (i) stream splitting of the solute front at every pore, thus causing independence of particle velocities purely by convection, (ii) a velocity gradient within throats and (iii) diffusion. Taylor’s dispersion in a capillary tube accounts for only the second and third of these phenomena, yielding a quadratic dependence of dispersion on Peclet number. Plug flow in the bonds of a physically representative network accounts for the only the first and third phenomena, resulting in a linear dependence of dispersion upon Peclet number.

Benefits
This project is developing improved models and simulators to establish a better grasp of the parameters that control optimal performance in miscible and near-miscible displacements. This capability will also enable better translation of laboratory results to field projects.

Summary
The most striking accomplishment of this project was the solution of a long-standing problem in transport through porous media, namely, the determination of the pore-scale origin of macroscopic hydrodynamic dispersion. Using a physically representative network model of a simple but geometrically realistic porous medium, the researchers developed the first algorithm for completely reversible (no diffusion) particle motion at network junctions. This enabled the first analysis of the contribution of convective spreading at the pore-scale. With this model, the researchers demonstrated that the combination of flow splitting (as fluid enters a pore then goes around grains), parabolic velocity profiles within pore throats (due to viscous momentum transfer to pore walls) and diffusion within pore throats (enabling particle to sample different streamlines within a throat) are necessary and sufficient to explain (predict without adjustable parameters) the long-standing observation that dispersion coefficient scales nonlinearly (the power-law exponent is 1.2) with pore-scale Peclet number.

The project researchers also:

• Developed a PVC-based fabrication technique for creating cost-effective nitrate sensors to better support transport studies in porous media
• Demonstrated the utility of particle tracking simulations of flow reversal (echo) tests to identify the nature of transport.
• Demonstrated the possibility of significant levels of mixing occurring in field scale miscible displacements for typical reservoir heterogeneity conditions.
• Validated streamline particle tracking algorithm by comparing simulated results with the experimental data.

(January 2009) 
This project has been completed and the final report is available below under "Additional Information".

Funding 
This project was selected in response to DOE’s Office of Fossil Energy Research and Development solicitation DE-PS2604NT15450-3F.

$800,000

$200,000 (20 percent of total)

NETL - Chandra Nautiyal (chandra.nautiyal@netl.doe.gov or 918-699-2021)
U. of Texas - Steve Bryant (steven_bryan@mail.utexas.edu or 512-471-3250)

Final Project Report [PDF-10.0MB]

Publications
Jha, R., Bryant, S., Lake, L. and John, A. “Investigation of Pore-Scale (Local) Mixing” SPE 99782, Proceedings of 2006 SPE/DOE Symposium on Improved Oil Recovery held in Tulsa, Oklahoma, U.S.A., 22–26 April 2006.

Jha, Raman, Pore Level Investigation of Dispersivity, University of Texas master's thesis, May 2005.

Jha, R.K., John, A.K., et al., ”Flow reversal and mixing,” 2006 SPE Annual Technical Conference and Exhibition, San Antonio, TX.

Harmon, T.C., Jursich, N.L., Davidson, M.J., and Haux, J.E., Scale-able Nitrate Microsensors in the Form of a Plant Root, fall meeting of the American Geophysical Union, December 13-17, 2004, San Francisco, CA.

Harmon, T.C., Jursich, N.L., and Davidson M.J., Fabricating Scale-able Potentiometric Nitrate Microsensors in Environmentally Interesting Forms: Early Successes and Challenges Ahead, Symposium on Nitrogen Eutrophication in Xexic Wildland and Agricultural Systems, January 19-20, 2005, Riverside, CA.

Rat’ko, A.A., Y. H. Wijsboom, C.A. Butler, M. Bendikov, and T.C. Harmon, Selectivity and Longevity of Nitrate-Selective Microsensors Based on Modified Polypyrrole films in Model Environmental Systems, in preparation for submittal to Journal of Solid State Electrochemistry.