RESIDUAL OIL ZONE TYPES
Original oil hypothetical, static aquifer
Base of oil saturation
(BOSO) Spill point
BOSO Aquifer Spill point
Type 3 Greenfeld ROZ, no overlying mobile oil zone
Aquifer BOSO Spill point
sweep accompanied by oil-water contact tilt.
Fig. 1a-d illustrates a hypothetical example and the three
ROZ types. The first two types generally are limited to one
pore volume (PV) of sweep, but Type 3 can result in multiple
PVs passing through the reservoir. One situation was hydrologically modeled at 17-50 PV sweeps over 15 million years. 2
In some freshwater or brackish high-permeability ROZs,
PV sweep can exceed 100 in time frames less than 10 million years. Freshwater ROZ examples exist in the US Rocky
Mountains and other sites with nearby source water and
Solubility (diffusivity) can influence residual oil properties,
demonstrating the importance of PV sweep. Low PV sweep
retains oil properties very much like the unaffected primary
oil traps above the ROZ or nearby fields.
Identification of situations where PV sweep is limited
could prove critical to finding oil with the necessary light
ends to allow the gas-expansion drive or low hydrocarbon
minimum miscibility pressures with CO2.
Recent San Andres ROZ characterizations and accumulating producing-case histories show changes in oil properties often are minor and can allow commercial production
via either EOR or depressuring.
Studies over the last 30 years show average percent ROZ
saturations are very similar to the swept zones in MPZ water
floods. Recognition of these similarities prompted CO2 EOR
projects in the ROZs just as CO2 is used in mature MPZ waterflood zones.
In the late 1990s, Hess Corp. initiated CO2 EOR ROZ pilots in the Seminole San Andres Unit while Shell Oil Co.
conducted a pilot to the north in Wasson field. These successful pilots led to full field execution of ROZ flooding in
Operators of other Permian basin fields also investigated
the zones beneath the oil-water contract. Many extended
their floods deeper and into the ROZ. The table outlines 16
current ROZ CO2 EOR projects.
The multiple ROZ EOR projects along with the ubiquity
and thickness of ROZs prompted a new theory to replace
the long-held transition-zone concept. The US Department
of Energy sponsored the first research proposing the three
ROZ origins. 1, 4, 5 DOE also sponsored work to quantify ROZ
resources beneath 50+ fields in the Permian basin.
The Research Partnership to Secure Energy for America,
meanwhile, sponsored two critical projects in which re-
• Modeled ROZ Type 3 laterally swept origins at a par-
ticular site in west Texas. 2
• Investigated the science of ROZ formation, mapped ROZ
fairways, and estimated the oil resource available within the
ROZ fairways. The resource compilation also involved an integration of the oil in place over a volume of the formation. 3