Online Journal for E&P Geoscientists

Faulkner, Andrew ^*1; Shaikh, Mohammed R.¹; Busaid, Yousuf ¹; Afifi, Mohamed ¹
(1) Petroleum Developement Oman, Muscat, Oman.

The AB field located in south Oman is a north south elongate feature consisting of four structural crests and produces from the Permo-Carboniferous Al Khlata and Gharif Formations and Mesozoic Clastics. The Gharif formation is currently under waterflood development

The undersaturated Gharif oil reservoirs were initially developed under primary depletion mode. Drilling and reservoir performance history have shown that the drive mechanism is solution gas drive with some limited aquifer and rock compaction support. Production performance has shown a rapid drop in reservoir pressure with low oil recovery. Subsequently, waterflooding was planned to mitigate the pressure decline increase oil recovery. After waterflood start-up and operation, poor connectivity was observed between producers and injectors in different areas of the field. In one area, reservoir pressure increased substantially, whereas in another area of the reservoir the response was minimal.

Due to the poor initial flood response, a re-look at the field’s geology and the performance history has shown that the reservoir is more faulted than previously interpreted. The lateral distribution of the Middle and Upper Gharif reservoirs is less than originally assumed, whereas the areal distribution of the Lower Gharif reservoirs is more continuous.

The Upper and Middle Gharif reservoir properties are well developed in the northern area where injection rates and pressure data indicates there is good lateral continuity. However, there is poor vertical connectivity between layers.

Conversely pressure and injection rate data in the central and southern area of the field shows that the Middle Gharif reservoirs are isolated and deplete rapidly.

Well control, production and injection data have shown that the Lower Gharif reservoir properties vary laterally across the field. The lateral continuity/connectivity between all the lower Gharif units is good; however production and injection rates and pressures are strongly facies dependent. The reservoir has a weak aquifer support and recent wells have drilled into depleted sections of the reservoir indicating that reservoir connectivity is better than expected.

Well pressure history and production performance have shown that depletion can occur across once thought to be isolated Lower Gharif fault blocks. When this data is analyzed at a field scale, there appears to be a strong facies dependency on pressure distribution and reservoir quality.