This document summarizes geological controls on waterflood performance in an oil field in South Oman. The field produces from multiple formations, including the Gharif formation which is currently under waterflood development. Initial production performance showed a rapid pressure decline with low oil recovery. After waterflooding started, poor connectivity was observed between some producers and injectors. A re-evaluation of the field's geology found the reservoirs to be more faulted than previously thought. Specifically, the Upper and Middle Gharif reservoirs have poor vertical but good lateral connectivity in the north, but are isolated in the central and southern areas. The Lower Gharif reservoir properties vary laterally, but production data shows connectivity between units is better than expected, with

1. 1178244 Geological Controls on Waterflood Performance, an Example from a Field in South Oman
1178244 Geological Controls on Waterflood Performance, an Example from a Field in South Oman
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.
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