Experimental Investigation of Production Characteristics of the Gravity-Assisted Inert Gas Injection (GAIGI) Process for Recovery of Waterflood Residual Oil: Effects of Wettability Heterogeneity

Abstract

The impact of reservoir heterogeneities in terms of wettability at the macroscopic scale on the recovery efficiency of the gravity-assisted inert gas injection (GAIGI) process was investigated through a systematic experimental study for tertiary recovery of waterflood residual oil. Isolated inclusions of oil-wet consolidated glass beads were embedded in a continuum of unconsolidated water-wet glass beads to create heterogeneous packed columns. In comparison to water-wet homogeneous porous media, such heterogeneous media allowed us to establish higher waterflood residual oil saturation whose amount varied linearly with the volume fraction of oil-wet heterogeneities in the packing. Experimental results obtained from tertiary gravity drainage experiments demonstrated that the continuity of water-wet portions of the heterogeneous porous medium facilitates the tertiary oil recovery through the film flow mechanism, provided that the oil-spreading coefficient is positive. In addition, owing to the high waterflood residual oil content of the heterogeneous media tested, the oil bank formation occurred much earlier and grew faster, resulting in a higher oil recovery factor. However, having favorable wettability conditions in homogeneous porous media resulted in slightly lower reduced residual oil saturation after the GAIGI process compared to that in the heterogeneous media with the same condition of withdrawal rate. A relationship between the oil recovery factor at gas breakthrough and gravity number was developed for the heterogeneous and homogeneous media. The recovery factor at gas breakthrough versus gravity number in homogeneous media follows a similar trend as that of homogeneous water-wet porous media. However, at a given gravity number, the recovery factor of heterogeneous media was greater than that of the homogeneous media.