Summary
The main objective of this study was to extract fracture data from multiple
sources and present it in a form suitable for reservoir simulation in a
fractured carbonate field in Oman. Production is by water injection. A
combination of borehole image (BHI) logs and openhole logs from horizontal
wells revealed that water encroachment occurs mostly through fracture corridors
and appears as sharp saturation spikes across fracture clusters. Dispersed
background joints have little flow potential because of cementation, lack of
connectivity, or small size. Image logs indicate that fracture corridors are
oriented dominantly in the west/northwest direction. Most of the several
injector/producer short cuts are also oriented in the west/northwest direction,
supporting the view that fracture corridors are responsible for the short
cuts.
Flowmeter logs from vertical injector or producer wells intersecting a
fracture corridor show a step profile. A comparison of the injection or
production history of wells with or without a step profile provided a means to
calculate permeability enhancement by fracture corridors. The field has more
than 300 vertical wells and nearly 20 horizontal wells, which allowed us to
generate detailed fracture-permeability enhancement and fracture-corridor
density maps based on injector and producer data, short cuts, mud losses,
openhole logs, and BHI logs. We also were able to build stochastic 3D
fracture-corridor models using corridor density from dynamic data and
orientation from BHI logs and seismic data. Fracture-corridor length and width
were tied to fracture-permeability enhancement using wells with both image logs
and production data. The fracture-permeability enhancement maps were verified
independently by waterflood-front maps. Notwithstanding the uncertainties, the
fracture data were sufficiently accurate and detailed to generate both single-
and dual-porosity simulation results with good field-scale history match.
Introduction
The field was discovered in 1968 in Oman (Fig. 1). Production is from the
Shua’iba and Kharaib reservoirs of the Lower Cretaceous age. The Kharaib is a
poorly bedded stack of repetitive shoaling cycles. The Shua’iba reservoir
consists of a deepening upward sequence. The thick-bedded massive Lower
Shua’iba-B gives way to the well-layered Lower Shua’iba-A and Upper Shua’iba
units. The Kharaib and the Lower Shua’iba are separated by a horizon of tight
argillaceous limestone (Hawar or Kharaib-K1, Fig. 2). The Shua’iba reservoir is
directly overlain unconformably by Paleocene Umr er Radhuma in this field.
The field consists of two low-relief eastern-A and western-B domes. Both the
Kharaib and Shua’iba units are oil-bearing in the A and B fields. The A
structure is subdivided into northern and southern fields. The southern field
is located on a west/northwest fault zone through the southern flanks of the A
field (Fig. 3). A major west/northwest graben connects the A south to B.
Another major fault zone is located on the northern flanks of the A field.
Production from the field started in 1976. Waterflooding was started in 1986
following a series of pilot projects. The original development involved
waterflooding by means of an inverted nine-spot vertical well. Soon after the
initiation of the waterflooding program, early water breakthrough made it clear
that the field was more faulted and fractured than originally anticipated. The
drilling pattern was subsequently changed in 1994–95 to a vertical line drive
oriented parallel to the dominant northwest/southeast-trending fault/fracture
pattern, following an assessment of 3D seismic faults and fractures, fault
cutouts, BHI logs, and early production data (Arnott and Van Wunnik 1996).
The fractures and faults of the field were studied repeatedly before and
after the drilling pattern was converted from an inverted nine-spot pattern
into a vertical line drive. Approximately 37 BHI logs were obtained from
horizontal wells, which provided valuable information and paved the way toward
a comprehensive understanding of fractures. The present study is the latest
phase in the ongoing appraisal of fractures, which is aimed at approaching a
more predictive fracture model by integrating previous findings with all
available BHI logs and production and seismic data.
© 2006. Society of Petroleum Engineers
View full textPDF
(
3,467 KB
)
History
- Original manuscript received:
8 January 2005
- Revised manuscript received:
9 February 2006
- Manuscript approved:
1 March 2006
- Version of record:
20 June 2006
View Cart
