Billions of barrels of oil in residual zones in the US Permian Basin, Rocky Mountain states and around the world have never been considered for production, but they represent a vast resource available through enhanced oil recovery (EOR).

A paper titled “Stranded Oil in the Residual Oil Zone (ROZ),” written for Advance Resources

The Seminole San Andres Unit field shows a positive reponse to a CO2 flood that includes the residual oil zone. (Graphics courtesy of Melzer Consulting)
International and the US Department of Labor Office of Fossil Energy’s Office of Oil and Natural Gas by L. Stephen Melzer of Melzer Consulting, describes the play and the potential.
Beneath the oil-water contact in every water-drive field a transition zone (TZ) between oil and water contains oil held in place by capillary action. That zone doesn’t contain enough oil to make an EOR project profitable.

But in some basins, under the right geological and hydrodynamic conditions, an ROZ below that TZ contains a vast trove of oil that operators may not even know about, Melzer said in the paper. That zone can be as thick as 300 ft (91.5 m) and contain a lot of oil.

Frequently, it is either ignored or unknown. In a normal operation, an operator will drill to the target zone, find the oil-water contact (OWC) and drill the remaining wells in the field above the OWC. If they know about an oil cut in the lower ROZ, they may not want to fuss with it because their prime targets are the oil in the main producing zone they can get from primary recovery and a waterflood. They want to reduce water production, not increase it.

Melzer’s research found the ROZ may contain the same oil-water ratio as the main producing zone after a waterflood, and it may have the same positive reaction to a CO2 tertiary recovery program as the main producing zone.

He estimated the ROZs in nine fields in the Permian Basin of West Texas alone contain 8 billion bbl of oil in the TZ and ROZ and 3 billion bbl of that oil is potentially recoverable.

Conditions
The conditions are right for those fields. They all meet the requirement of a trap that originally held oil from which some of the oil was displaced. Among those conditions are:
• A regional tilt of the basin;
• A reservoir in which the trap was breach but later healed; or
• Changed conditions in the underlying aquifer.
In the regional tilt situation, a gravity-dominated shift in the oil-water contact could leave a lot of oil in the ROZ. Traps that were breached and later sealed probably are common, but only indirect evidence is available.

The most-likely situation is the changed hydrodynamic condition in the underlying aquifer.
A graph of the Seminole field water saturation shows the extent of oil remaining the residual oil zone.
That happened in the northern and central Permian Basin platforms and in the Panhandle and Hugoton fields in the Panhandle region of Texas.

To find the extent of the oil in the ROZ in fields, Melzer said he first looked for a tilted oil-water contact. Next, he estimated the oil in place in the ROZ below the oil-water contact and calculated the updip thickness of the ROZ wedge, based on the degree of tilt. Finally, he assigned a residual oil saturation in the ROZ consistent with the saturation in the waterflooded main producing zone.

Case studies
Those figures were not guesstimates. The basis for those calculations came from three Permian Basin fields from which operators have intentionally produced from the ROZ. Those are Occidental Petroleum’s Transition Zone project in the Denver Unit of Wasson field, Hess’s ROZ pilot in the San Andres Unit of Seminole field south of Wasson field and Occidental’s Main Pay and Transition Zone project in the Bennett Ranch Unit of Wasson field. Occidental includes the ROZ in its definition of a transition zone.

Shell found an anomalously thick ROZ at the Denver Unit in Yoakum County, Texas. It was oil-saturated from 85% at the top to 0% at the bottom of the 300-ft (92-m) interval. The saturation was around 30% in much of the interval, about the same saturation level as the main producing zone.

It planned a CO2 flood for the upper 150 ft (46 m) of the ROZ but sold the field before the study was complete. Altura took it over, continued the evaluation, expanded the flood and sold it to Occidental. Occidental liked the idea well enough that it expanded to phases 2, 3 and 4 and is looking at three more expansion in the future.

The Denver Unit had 4.5 billion bbl of oil in place. Occidental estimated 103 million bbl of oil in place in the ROZ, and it can produce 14 million bbl of that oil through the existing infrastructure by deepening some wells and added some new wells. The cost of its transition zone project is US $7.14 million, and it will earn a 20% internal rate of return.

The second case study is in the Hess San Andres Unit. It is in the first phase of development with a four-pattern pilot project and is planning a move to the second phase.

The ROZ averages 126 ft (38 m) of net pay under the San Andres Unit and 195 ft (59.5 m) in the main production zone, which held an original estimate of 1 billion bbl of oil. Hess estimated 400 million to 1.1 billion bbl of oil in place in the ROZ with an average oil saturation of 32% compared with an original saturation of 84% in the main production zone.
In the third field, Bennett Ranch, Shell had planned to simultaneously produce the main producing zone and 75 ft (23 m) the ROZ, but low oil prices interrupted plans to deepen existing wells in the field. When it bought the field, Occidental added production from the ROZ in 2003.

The common features that make the conditions right for ROZ recovery under a tilted oil-water contact are that the fields are near an uplift.

The Permian Basin and the Billings Nose in the Williston Basin of Montana show the lowest degree of uplift of the fields Melzer studied. The farthest occurrence of a ROZ from the uplift feature is 150 miles (240 km).

There are other similarities:
• The hydrodynamic flow is always away from the uplift (source of the water);
• The oil-water contact tilts down, away from the uplift; and
• The ROZ is thickest at its nearest point to the uplift.

The economics to produce ROZ oil are in place in many areas, and the key ingredient to a profitable operation may well be the availability of CO2 at a price low enough to trigger projects.