TECHNOLOGY
DRILLING &
PRODUCTION
L. Stephen Melzer
Melzer Consulting
Midland, Tex.
Enhanced oil recovery (EOR), specifically CO2 injection, and horizontal well
depressuring are tapping into residual
oil zones (ROZs) in the Permian basin of
West Texas and southeastern New Mexico.
This article summarizes findings of
10 years of research into ROZs, which
increase the thickness and redefine the
lateral extent of oil accumulations. The
studies document 200-400 ft residual oil sections beneath
many San Andres-formation oil fields.
But ROZs also can be found elsewhere. They are a part of
original (paleo) oil traps that have undergone natural water
flooding over geological time. Some ROZs subsequently became separate, distant greenfields without an overlying trap
while others exist as aquifers with residual oil under an oilfield (brownfield) (OGJ, Nov. 3, 2014, p. 78).
Most basins worldwide have experienced multiple stages of tectonic activity.
ROZs result when a tectonic stage moves
oil and water following the original entrapment stage. Industry now commercially produces this residual (immobile)
oil despite it remaining inaccessible by
primary or waterflood methods. Both
EOR and reservoir depressuring allow solution gas in the oil to expand, liberating
the oil for production. Depressuring is also called dewatering.
ROZs are portions of a reservoir where nature swept wa-
ter through the paleo oil trap. Three types of late-stage tec-
tonics can cause ROZs:
• Type 1, local, regional, or basinwide tilt. 1
• Type 2, leaky or breached seals.
• Type 3, flushing caused by a lateral hydrodynamic
Residual oil zones changing
commercial reservoir models
PERMIAN ROZ CO2 PROJECTS Table 1
Operator Project County, state Top MPZ depth, ft Pay zone MPZ start ROZ start
Chevron Corp. Vacuum, San Andres,
Grayburg unit Lea, NM 4,550 San Andres 2007 2007
Fasken Oil & Ranch Hanford Gaines, Tex. 5,500 San Andres July 1986 August 2009
Hess Seminole unit,
ROZ Phase 1 Gaines, Tex. 5,500 San Andres July 1983 July 1996
Hess Seminole unit,
ROZ Phase 2 Gaines, Tex. 5,500 San Andres July 1983 April 2004
Hess Seminole unit, ROZ
Stage 1, full feld Gaines, Tex. 5,500 San Andres July 1983 October 2007
Hess Seminole unit, ROZ
Stage 2, full feld Gaines, Tex. 5,500 San Andres July 1983 May 2011
Hess Seminole unit, ROZ
Stage 3, full feld Gaines, Tex. 4,200 San Andres July 1983 July 2013
Kinder Morgan Goldsmith-Landreth
unit Ector, Tex. 4,200 San Andres August 2009 August 2009
Kinder Morgan Tall Cotton, pure greenfeld Gaines, Tex. 5,200 San Andres — November 2014
Occidental Petroleum Corp. Wassen, Bennett Ranch unit Yoakum, Tex. 5,250 San Andres June 1995 2000
Occidental Petroleum Wassen, Denver unit Yoakum, Tex. 5,200 San Andres April 1983 1995
Occidental Petroleum Wassen ODC Gaines, Tex. 5,200 San Andres November 1984 2005
XTO Energy Inc.,
ExxonMobil Corp. Means Andrews, Tex. 4,500 Grayburg,
San Andres November 1983 November 2011
Tabula Rasa East Seminole Gaines, Tex. 5,400 San Andres January 2013 January 2013
Trinity CO2 George Allen; Brownfeld, greenfeld Yoakum, Tex. 4,900 San Andres December 2012 December 2012
Chevron Central Vacuum Lea, NM 5,000 San Andres July 1997 2014
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