Abstract

An entirely new set of CO2 EOR targets has exploded on the scene in the Permian Basin in recent years. The commercial success of this group of Residual Oil Zone ROZ) reservoirs has prompted a systematic reexamination of the science and distribution of their origins. Three types of ROZs have been identified. Much of the work to date has focused on San Andres carbonate reservoirs and the lateral sweep of a paleo-oil entrapment of dramatically larger thickness and lateral extent that would be suggested by looking only at the “conventional” Main Pay Zone (MPZ) fields.

The work is demonstrating that meteoric derived sweep of oil from the ROZ portion of San Andres reservoirs was probable. Modeled pore volume flushing in one particular “fairway of sweep” ranges from 20 to 50 pore volumes over 15 million years (Late Oligocene through Middle Miocene), with flow rates of 10 to 25 cm/year. This hydrodynamic origin can also explain the Oil/Water contact tilts seen in many of the San Andres reservoirs at the time of discovery.

As a result of this long term flow and associated late-stage diagenesis, the ROZ rock properties display a slightly higher porosity and permeability when compared with nearby or overlying MPZ’s. Other associated reservoir characteristics present in the ROZ’s include: native sulfur near the base of the ROZ; increasing variability in oil saturation downward; small scale solution enhanced voids; water chemistry variations, and isolated “pay zone” thicknesses. Observation of free sulfur in vugs, sour oils and gas in the ROZ vs MPZ, and sulfur-rich formation waters adds additional credence to the presence of microbial processes associated with the sweep that contribute to diagenesis. A characteristic “bow” shape on the resistivity and porosity logs is generally present in the ROZs with decreasing resistivity and increasing porosity below the oil/water contacts.

Presently, there are eleven projects in the Permian Basin underway demonstrating EOR flooding of the ROZ. The combination of understanding the lateral sweep origins, successful modeling of the hydrodynamics, the rock property changes, and the new observations of success of CO2 EOR ROZ in these projects is leading to a belief that the opportunities for CCUS may be much larger than previously believed including greenfields that are not associated with overlying main payzones. The large reservoir targets for CO2 storage and oil production, as identified by NETL and DOE in previous studies, may be understated when the ROZ targets are more fully defined and characterized.