Effect of Dispersive Mixing and Enrichment on Oil Recovery in One-Dimensional Multicomponent Gas Drives

by
Pashupati Sah, MSE

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

Enriched gas floods are being conducted in a large number of reservoirs worldwide. The objective in such floods is to achieve a multicontact miscible (MCM) displacement by taking advantage of the high local displacement efficiency that is possible when the injection gas is sufficiently enriched with intermediate hydrocarbon components. Experiments and field observations show that increasing the enrichment of the injection gas leads to an increase in the oil recovery. Since greater enrichment implies greater investment one needs to specify an optimum enrichment beyond which further enrichment would not yield greater profits.

One dimensional laboratory experiments, also called slim-tube experiments, often show that by enriching the injection gas, oil recovery increases sharply to the minimum miscibility enrichment (MME) but increases very little with further enrichment (Stalkup, 1998). Numerical and analytical solutions demonstrate that oil recovery may be substantially increased by gas enrichment above the MME in enriched-gas drives. The level of increase is dependent on the level of dispersive mixing and the size and shape of the two-phase region.

Slim-tube displacements are one-dimensional in nature but reservoir floods are not. For real floods, near 100% recovery is not achieved at the MME as is the case for the experiments. Despite these substantial differences it is important to study one-dimensional displacements in order to understand reservoir-scale displacements. Reservoir-scale dispersion is a crucial factor which can have an effect on the displacement efficiency.

In this thesis, the interaction of phase behavior and flow in the presence of dispersive mixing is examined for 1-D multicomponent enriched-gas displacements. The optimum enrichment is defined as the position of the characteristic bend in the recovery curves, also known as the "knee". One-dimensional numerical and analytical solutions are used to study four- and twelve-component gas-oil displacements.

In the four-component displacements, the "knee" is found to occur at an enrichment greater than the MME called the critical gas enrichment (CGE), which is invariant for levels of dispersive mixing greater than those present in slim-tubes. For extremely low levels of dispersion, as might be the case for slim-tubes, the "knee" is found to occur at the MME. The CGE separates the multi-contact miscibility region into two regions: MCM1, when the gas lies within the region of tie-line extensions, and MCM2, when the gas lies outside. For enrichments below the CGE (in MCM1), oil recovery can increase significantly with enrichment. Above the CGE (in MCM2), oil recovery is not as sensitive to enrichment for the cases studied.

For the twelve-component example studied, the position of the "knee" shifts to greater levels of enrichment with an increase in dispersion. The increase in recovery (displacement efficiency) above the MME is found to be as large as 15% OOIP depending on the level of mixing. For the given system the gas tie line remains within the region of tie-line extensions even for injection of pure solvent.

In this research, effects of dispersion on displacement mechanisms are also studied for the four-component system. It is shown that for the case of the four-component model the displacement mechanism changes from a combined condensing/vaporizing displacement (CV) to a strictly condensing one as enrichment increases above the MME. A method of quantifying the percentage of the CV displacement that is vaporizing or condensing by calculating the compositional distances between key tie lines identified from "dispersion-free" theory is also proposed.

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