Effect of Mixing Mechanisms on Recovery by Enriched-Gas Injection above the Minimum Miscible Enrichment (MME)

by
Reinel Solano, MSE

University of Texas at Austin, 2000
Supervisor: Dr. Russell T. Johns

Gas enrichment is an important variable to optimize oil recovery in enriched-gas drives. The minimum miscible enrichment (MME) is the maximum enrichment level at which recovery increases significantly for slim-tube experiments. For field applications, however, large levels of mixing, caused by dispersion, crossflow, gravity tonguing, or channeling, can affect the optimum enrichment. In reservoir simulations, numerical dispersion can cloud the interpretation of the results by artificially increasing the level of mixing in the simulated reservoir.

This work investigates the interplay between various mixing mechanisms, enrichment levels, and numerical dispersion. The mixing mechanisms examined are dispersion, gravity crossflow and viscous crossflow. The main objective of this work is to identify the impact of the mixing mechanisms and numerical dispersion on local displacement efficiency and sweep efficiency when the injected gas is enriched above the MME.

The results show that for one-dimensional (1-D) displacements the recovery difference between two enrichments above the MME changes significantly with dispersion and obtains a maximum at a particular value of dispersivity. This maximum in the recovery difference is the maximum possible increment in oil recovery obtained by enriching the gas above the MME.

For two-dimensional (2-D) displacements, the recovery difference is always smaller than the 1-D local displacement efficiency. The sensitivity of recovery difference to horizontal grid refinement and numerical derivative methods is greatly reduced when mixing by gravity and viscous crossflow becomes important, such as during WAG injection or when layered reservoirs exist. The magnitude of the recovery difference may be affected in some cases by heterogeneity. Nevertheless, the additional recovery obtained by enrichment above the MME seems more influenced by changes in the local displacement efficiency than by changes in sweep efficiency.

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