Abstract

Two large-scale CO₂ enhanced oil recovery (CO₂-EOR) flooding demon-stration projects in the Permian Basin have been conducted over the past several years and have shown the commercial value of exploiting residual oil zones (ROZs) beneath the main oil pay intervals. Results are presented from these two projects, the Amerada Hes’ ROZ pilot at the Seminole San Andres Unit in Gaines County and Oxy Permian’s Transition Zone projects at the Denver Unit in Yoakum County. The results of the projects set the stage for the coming exploitation of residual oil zones elsewhere in the Permian Basin and other oil provinces.

Three origins for the development of ROZs are postulated (Figure 1) with a stated predominance of one type, hydrodynamic, which can form abnormally thick intervals and be equated to nature’s equivalent to a waterflood. Past work of M. King Hubbert, along with more recent concepts presented by Alton Brown and R. F. Lindsay, are referenced and extrapolated to develop insights as to the expected attributes of Permian Basin residual oil zones. Tilted oil-water contacts and other key features of the underlying aquifer are considered key parameters for discerning the presence of additional produceable oil in place. Contrasts with conventional transition zones, explained by capillary forces alone, are outlined although the coming commercial exploitation of the upper portions of the traditional transition zones are shown to be comparable in principle to residual oil zones.

Theoretical modeling work is presented to show that hydrodynamic effects are consistent with well-documented ROZs. It is also postulated that aquifer pressure variations can account for varying profile shapes (by depth) of the oil saturation curves. Further work, including basinwide geological characterization and modeling is needed to better establish an understanding of the regional distributions of the ROZ and its underlying aquifers to assist in the location of commercial ROZ resources and also to assist in furthering identification of CO₂ geological storage sites.