Studies sponsored by the Research Partnership to Secure Energy for America (RPSEA) and the U.S. Department of Energy (DOE) have opened the door to a whole new set of opportunities for commercial development of residual oil zones (ROZs) throughout the world. The historical understanding of the zones beneath the oil/water contacts in reservoirs had been relegated to the category of transition zones wherein capillary pressure and surface tension controlled the bottom of a reservoir, where there exists a mixed oiland-water zone of several meters to tens of meters.

Recent research has shown that the zones below many oil fields are not only hundreds of meters thick but often possess an interval of nearly constant residual oil saturation sandwiched between the upward transition to mobile oil (main pay zone) and a lower transition to zero oil saturation below (Figure 1). These ROZs also are commonly observed to possess 20% to 50% oil saturation but with only mobile water. The research shows that they are present in extensive regions between oil fields.

With these ROZ observations as the basis, three types or origins of ROZs have been identified. All of these can be thought of as natural waterflood of a paleo oil entrapment. All come after the subsidence, oil generation and oil migration stage of a basin. They owe their presence to a post-entrapment tilt of the paleo trap, which can be a local or basinwide event; leakage or breach of the seal above the reservoir; or tectonic event on one side of a basin that creates an outcrop of the reservoir formation(s) and causes a lateral movement of meteorologically derived water through the basin to outcrops on the downdip side of the
basin. This last type is being studied extensively today in the Permian Basin’s San Andres Formation and in the Tensleep Formation in the Bighorn Basin of Wyoming.

Permian study
The Permian Basin work sponsored by RPSEA has just completed a four-county study of the area between oil fields in what has been dubbed greenfields (Figure 1). The areas have no well infrastructure in place since all of the wells drilled there were dry holes. The RPSEA-sponsored work has identified 76 Bbbl of high-quality target ROZ reservoirs where the oil saturations exceed 25% and the formation porosity exceeds 8%. ROZ resources have to be produced by processes that alter the properties of the oil to render it mobile. The four-county region encompasses approximately 10,620 sq km (4,100 sq miles) and includes 12 ROZ CO2 EOR projects.

Today the Permian Basin plays host to a total of 15 individual CO2 EOR projects exploiting the ROZ. By conservative
measures, these newly implemented projects are producing more than 12,000 bbl/d of formerly immobile ROZ oil. Most of these projects involve deepening the wells in an oil field into the ROZ (brownfields), but two have recently been completed in greenfield areas. The Tall Cotton project in Gaines County, put into operation late last year by Kinder Morgan CO2 Co., is especially notable in its potential size. Today the project consists of nine injectors and 14 producers on a 40-acre five-spot configuration. While industry is leading the deployments, the ROZ work sponsored by RPSEA clearly has provided much of the scientific platform for the ongoing commercial work.

One of the characteristics of ROZs, especially the upper portions, is that when drilling through the formation, the oil and gas “shows” from those intervals of immobile oil are equally as enticing as oil shows from the main pay zones. Some drilling includes coring, but even this often gives false indicators of mobile oil. As a result, many vertical well completions have been tried, making very large volumes of sulfur water with noncommercial volumes of oil.

Very recently, some operators have utilized the evolving technologies of horizontal drilling and staged hydrofracturing
completions to attempt to establish commercial production in these intervals. There are several examples of 1.6-km- (1-mile-) long laterals with peak production levels of 250 bbl/d or more of oil and projected ultimate recoveries of 250,000 bbl or more of oil. Total well costs are such that the economics are working at $60 oil prices if water disposal costs can be controlled. The typical initial water production rate is 1,500 bbl/d to 2,000 bbl/d accompanied with no oil for 30 to 40 days. After that period oil cuts rise to 10% to 25% as the well matures.

Understanding ROZs
The production concept is not yet widely recognized as the industry’s grasp of ROZs and their origins is not widespread, and understanding of the mostly oil-wet ROZs is still immature. Some believe the oil is coming from the mobile portion of the fluids in the transition zone. However, time should prove that the oil is actually residual oil that has sufficient gas in solution to cause the immobile oil of the ROZ to break out as the pressures decay, analogous to the solution gas drive mechanism long witnessed and modeled in the mobile oil portions of reservoirs. Figure 2 shows the principles of this process, tentatively dubbed depressuring EOR (DEOR) or depressuring the upper ROZ (DUROZ).

The ROZ researchers are beginning to believe that such discoveries are in no way exclusive to the Permian Basin but will soon see deployment in a variety of basins throughout the world. In fact, it is highly likely that most of the so-called dewatering plays around the U.S. are not connecting up mobile oil compartments in reservoirs but are, in fact, an ROZ depressuring play, at least in part.

Finally, it is interesting to reflect on the amazing explosion of new ideas and production occurring in the U.S. today. It is clearly a product of a number of factors, but the availability of research-sponsoring organizations such as RPSEA and DOE is very significant. Just as important are private mineral ownership, the availability of capital and the wealth of entrepreneurs who are willing to experiment on new things in the oil and gas producing and service sectors. Taken collectively, they have all combined to be the accelerant for the innovation being witnessed today. Producing the ROZs by CO2 EOR and DEOR/DUROZ are two examples.

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