Residual oil zones have been located below oil/water contact of many oil reservoirs (Brownfield’s) in the Permian Basin and elsewhere; they are also present far from main pay zones (Greenfield’s). These ROZ’s were, for a long time, considered an integral part of the aquifer as they have been naturally water flooded at a very low flow rate over geological times.
New thinking is recognizing them as an important hydrocarbon resource with economic oil saturations ranging between 20 and 40 %. They (ROZ’s) are an excellent target for a tertiary recovery process which, in the Permian Basin, is almost synonymous to CO2 injection. ROZ’s are probably the most conventional of unconventional oil resources.
Located right beneath or in the vicinity of fully developed oil fields, these oil zones were a portion of a larger oil entrapment which has been thoroughly water flooded by Mother Nature. Field projects are demonstrating that ROZ’s are both technically and economically viable through a tertiary recovery process.
Several pilots and full field applications are currently being carried out in the Permian Basin and soon to be starting in the Rockies. As their distribution becomes well documented, ROZ’s are expected to account for a substantial increase in oil resources. Those under CO2 injection in the Permian Basin are currently being produced above the estimated rate of 11,000 bopd, keeping in mind that some companies do not report the ROZ zone production separate from the main pay.
Modern oil/water contacts whereas traditionally considered the lowest part of an oil reservoir, ROZ’s are changing that perception and increasing significantly the thickness and lateral extent of several oil fields. In many instances they (ROZ’s) are thicker than the maximum thickness of the known associated primary reservoir. Owing to the large lateral extent of the aquifer flushing process, many green fields are now being recognized at significant distances from fully developed oil fields.