Abstract

It is a common practice to use the diffusion-dispersion model for the determination of optimum solvent slug size. This model assumes that the entire pore space is readily available for flow. However. in most carbonate reservoirs, trapping of solvent and oil also plays a significant role in determination of solvent slug size. This trapping is as a result of dead end pore structure and microscopic to macroscopic permeability heterogeneities.

This paper presents the slug size determination for carbonate reservoirs in an ideal displacement where the effect of viscous fingering, gravity override, etc. are neglegible. Based on the dead-end-pore model, several type curves have been developed to determine the solvent slug size for carbonate reservoirs. Field and laboratory tests were conducted to measure the three parameters required for using the type curves.

Determination of solvent slug size is a key factor in the design of a hydrocarbon miscible flood. For economic reasons the size of slug should be as small as possible. The cost of the project will increase without any increase for the oil recovery if the slug size is larger than the optimum volume. However, from a technical point of view the slug size should be large enough to maintain miscibility throughout the flood. If a solvent slug smaller than the optimum volume is injected, the solvent dilutes to the extent that results in less oil recovery.

Dilution of solvent in the reservoir is mainly due to diffusion and dispersion. In carbonate reservoirs trapping of solvent and oil also plays a significant role in determination of solvent slug size. This trapping is as a result of dead end pore structure and microscopic to macroscopic permeability heterogeneities. Other factors such as wettability, the nature of the miscibility process, shape of the displacement front, geometry of the swept area, interwell spacing, mobility ratio, etc. also contribute to the slug size. The effects of these factors have been discussed in detail in the literature. 1–5

Asgarpour et al2 concluded that for a water wet system additional solvent is required due to the increase in the mixing zone length caused by water trapping of oil. For an oil wet system the influence is negligible. Chen et a13 showed that if oil is displaced via a condensing multiple-contact miscible process, the solvent slug size must account for losses due to dilution and the enrichment of the oil that has occurred. The effect of pattern geometry and flood propagation was presented by Abbaszdeh-Dehghni and Brigham4. These authors demonstrated a method of disseminating a pattern into a system of streamlines, and then calculating the concentration profile along each streamline. The effect of flood geometry can also be investigated as a set of converging - diverging linear flow elements. The concentration profile, and thus the minimum slug size required, can be calculated for each element from the mixing equation derived by Brigham1,5

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