A Residual Oil Zone (ROZ) is a naturally waterflooded reservoir at residual or near residual oil saturation, technically recoverable only through unconventional methods. The development of ROZs is extensively pursued in the Texas Permian Basin. Several successful ROZ CO2 Enhanced Oil Recovery (CO2-EOR) projects indicate enormous resource potential for these emerging oil plays. Another approach, called Depressurizing the Upper ROZ (DUROZ), was recently proposed and is currently under extensive investigation. This study offers a mechanistic understanding of DUROZ and investigates the factors affecting its viability and performance.
DUROZ refers to the progressive reduction of reservoir pressure through the withdrawal of large volumes of water from a horizontal well in the upper section of ROZ. When reservoir pressure falls below the saturation pressure, gas bubbles liberate from capillary-trapped oil and develop into a continuous gas phase. Consequently, the oil phase may also become mobile beyond waterflood residual oil saturation. We history-match the oil and water production from two DUROZ producers in the upper San Andres dolomite using a tuned reservoir simulation model. To properly capture the rock and fluid interaction, the relative permeability data are tuned with the experimental core data from the San Andres ROZ. Extensive experimental studies have highlighted a substantial difference between relative permeabilities measured under external drive and solution gas drive. We define two sets of relative permeabilities: an original set based on the external drive experiments and the relevant correlations, and a modified set for modeling production under solution gas drive.
Our results show that even for the most optimistic relative permeabilities, the reservoir initial oil saturation should be at least 20% above the residual oil saturation in order to match the oil cuts of the reported DUROZ producers. This shows inconsistencies in use of the term "residual oil" zone by the industry in the Permian Basin. In other words, although DUROZ shows potential for production beyond waterflood residual oil saturation, the documented cases are unlikely to be true ROZ producers and are more likely completed in the Transition Zone (TZ) or near-ROZ. Finally, a discussion on the operational risks and technical considerations associated with DUROZ, including water disposal, infrastructure and facility requirements, and modeling limitations, is presented.