An integrated PE review of Zauliyah field was necessitated by the poor field performance under depletion drive. The review resulted in a new reservoir model conducive to a water injection scheme. Consequently, the field is currently being re-developed by waterflooding. This project phase is accelerated, and will develop 4.0 million m3 of reserves at an average unit technical cost of less than $2/bbl.
The Zauliyah field is a NW-SE trending anticlinal structure (Figure 1) that contains light oil (44 API) in five stacked reservoir units of the Permian Upper and Middle Gharif fluvial sandstones. A 25-meter thick shale layer separates the Upper Gharif [UG] reservoirs (20-23% porosity, 8-20% net/gross) from the Middle Gharif [MG] reservoirs (19-23% porosity, 25-51% net/gross).
Discovered in 1981 and with a booked STOIIP of 56 mln m3, the field came on stream in 1984. 14 vertical wells have been drilled (ZL-11 was cored, while ZL-12 has FMI data). The initial development by depletion drive recovered only 6% of the STOIIP before reservoir pressure in most parts of the reservoir units declined to below bubble point. A waterflood programme was considered but could not be embarked upon for risk that the reservoirs were isolated and highly compartmentalised (Figure 2). Therefore, the field continued to produce by depletion drive, but most wells had to be closed-in intermittently due to excessive gas production and flaring. Under these circumstances, the development of booked reserves was not economically attractive, making reserves de-booking imminent at the time. However, this view changed following a multi-disciplinary PE study and water injection pilot schemes. It is now planned to book additional reserves in 2001, and the re-development of the Zauliyah field by water injection has been put on fast track.