RESIDUAL OIL ZONES: THE LONG TERM FUTURE OF ENHANCED OIL RECOVERY IN THE PERMIAN BASIN AND ELSEWHERE

Residual Oil Zones (ROZ’s) have for 60 years intrigued both explorationists and production engineers and geologists. ROZ’s have very similar core, cuttings, fluid and log properties as producing wells in modern waterfloods, and typically produce large volumes of water but little if any oil on tests. They are the result of “Mother Nature’s Waterflood”, as tectonically driven lateral flushing has swept oil and connate fluids out of the lower portions of Permian Basin oil reservoirs.

Over the past two decades, a number of operators have demonstrated that ROZ’s beneath existing fields are economically viable targets for EOR. These ROZ’s are only producible with tertiary recovery methods (both CO₂ and Chemical) and it is estimated that there are >10 Billion barrels of recoverable reserves in the ROZ’s in the upper Guadalupian carbonate reservoirs in the Permian Basin alone. “Greenfields” are areas with no associated Main Pay zones where significant thicknesses of ROZ’s exist. Large areas on the northern Central Basin Platform and the Northwest Shelf are underlain by San Andres age Greenfields. These areas also have multi-billion barrel potential and will serve as sites for combined CO₂ EOR and Carbon Sequestration (Carbon Capture Utilization and Storage, CCUS).

There are believed to be ROZ’s in other producing intervals in the Permian Basin which await documentation. Substantial ROZ’s have been shown to exist in the Tensleep Formation in the Bighorn Basin in Wyoming and are believed to also exist in other U. S. basins. Regional reservoir modeling of this natural waterflood process is ongoing and has resulted in confirmation of the magnitude of sweep proposed to have occurred during “Mother Nature’s Waterflood”.