Recovery of Oil Resources from the Residual and Transitional Oil Zones of the Permian Basin

Because of their low to moderate oil saturations, the immobile (but significant) oil resources in the transition and residual oil zones (TZ/ROZ) are not economic to produce using primary/secondary oil recovery techniques. As such, the great majority of domestic oil wells are completed at the first evidence of mobile water (oil-water contact) above the TZ/ROZ. Outside of a small group of forward-looking operators, little is known about the ability to target and produce this large TZ/ROZ resource. However, in the current economic climate and with technology advances, the TZ/ROZ resource offers a new source of domestic oil production. The relatively thick (100s of feet) TZ/ROZ located beneath the traditional main pay zones of Permian Basin oil reservoirs provides an attractive domestic development resource.

Carbon dioxide (CO₂) enhanced oil recovery (EOR) is a viable technique for recovering immobile residual oil left behind ("stranded") after waterflooding. Further, the oil saturation in the TZ/ROZ portion of a reservoir is often similar to the oil saturation left after waterflooding. As such, with progress in CO₂ flooding technology and availability of affordable supplies of CO2, the oil in the TZ/ROZ could become an economically viable target.

This paper will provide updates for four key TZ/ROZ projects (two in the Wasson oil field, one in the Seminole San Andres Unit and a fourth at Salt Creek). These pilot projects plus reservoir simulation work by the authors show that this residual oil can be efficiently recovered using CO₂-EOR. The paper discusses how two CO₂-EOR practices, selective completions and joint development with the main pay zone, would enable the recovery of the TZ/ROZ resource to be optimized. The paper concludes with an assessment of how much resource potential in the TZ/ROZ may hold for future oil development with the Permian Basin.