Summary
This paper presents the success story of waterflooding in a geologically
complex reservoir that contains high-viscosity (90 cp) oil. This reservoir is
part of a large brownfield in south Oman and has been on production for more
than 25 years. The reservoir comprises glacial sandstones of Palaeozoic age and
is highly heterogeneous, with wide variation in reservoir characteristics at
scales smaller than well spacing. The reservoir geology is complicated because
of the presence of faults and fractures.
The reservoir has been subdivided into different areas that we call
"catchments," characterized by their geological setting and dynamic
reservoir behavior. Full-field water injection was started in the field after
approximately 18 years of primary depletion. Different water-injection
philosophies for different areas of the field have been adopted with
consideration to factors such as primary drive mechanism operative in the
reservoir, presence or absence of vertical-flow barriers; pre-existing
field-development pattern; and more importantly, future development beyond
waterflood to maximize the value of the reservoir in its entire life cycle.
The paper illustrates that the success of a waterflood in a complex
reservoir relies on the implementation of a tailor-made development plan with
flexibility to be changed on the basis of data from reservoir surveillance. The
importance of well and reservoir management (WRM) to improve the value of
waterflood is emphasized.
Introduction
The discussed reservoir is in a large (original oil in place > 2 billion
bbl) brownfield on the eastern flank of the south Oman salt basin (Fig. 1). The
field comprises a northeast/southwest trending anticline approximately 14 km
long and 8 km wide. The Al Khlata formation (lower part of the Haushi group of
Carboniferous age) is the principal oil-bearing reservoir in the field. In
addition to Al Khlata, oil-producing reservoirs are present in the Mahwis
formation (the Haima group of Cambro-Ordivician age) and Gharif formations
(Haushi group of Carboniferous/Permian age) at depth ranges of 550 to 675 m
subsea (ss). The field is unconformably overlain by Cretaceous Nahr-Umr shale
that acts as the caprock (Fig. 2).
Oil in Al Khlata has moderate-to-low API gravity (22°API), with viscosity of
90 cp. The original reservoir pressure was 9,300 kPa at datum of 610 m ss.
Porosities range from 25 to 30%. The permeability of the reservoirs varies
widely from approximately 100 md to more than 5 darcies. Table 1 shows the
basic rock and fluid properties of the Al Khlata reservoir.
Regular production from the reservoir began in 1980. The reservoir
production history is shown in Fig. 3. Development of the reservoir has been
carried out in different phases and consists of an initial phase of
vertical-well drilling, enhanced oil recovery (EOR) pilot schemes, horizontal
infills, and later, water injection. A chronological description of the major
events in the reservoir is presented in Table 2.
The reservoir has been divided into several catchment areas on the basis of
reservoir characterization and reservoir drive mechanism. Development strategy
for the reservoir varies from one catchment area to another. This has resulted
in a number of development schemes, such as vertical waterflood development
with use of pattern flood, horizontal development with and without water
injection, and flank water injection in different catchment areas.
Despite the geological complexity of the reservoir and the adverse
mobility-ratio conditions under which waterflood is being implemented, the
reservoir has responded favorably to water injection. This paper presents the
rationale behind implementing a development strategy for each catchment area.
It also illustrates how a tailor-made field-development plan, supported by a
judicious WRM strategy, can result in a successful waterflood in a geologically
complex reservoir with adverse mobility-ratio conditions.
© 2008. Society of Petroleum Engineers
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History
- Original manuscript received:
3 December 2006
- Meeting paper published:
11 March 2007
- Revised manuscript received:
28 September 2007
- Manuscript approved:
14 November 2007
- Version of record:
20 June 2008
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