Summary
When revitalizing mature oil or gas fields, the ability to maximize
productivity while minimizing operating costs is critical. Achieving these
goals becomes more challenging where sand control is required. This is
especially true in connection with a complex reservoir lithology, which leads
to a variety of fracture and pore pressure gradients and premature water
breakthrough. In addition, reduced deliverability resulting from pressure
depletion presents additional complications related to achieving acceptable
project economics.
The Barbara field, located in the central Adriatic Sea, is an example of
this type of reservoir. For this project, a dedicated work team was
assigned to identify and implement the solutions to improve field performance
through continuous operations optimization. The Barbara field consists of
stacked pay sections that have been depleted since the early 1980s. To
achieve favorable economics, techniques had to be used that would do both:
maximize the deliverability of each well and minimize operational costs.
This paper reviews the Barbara field workover program. The evolution of
techniques, equipment, and products leading to enhanced productivity while
substantially reducing costs is described. The fluid systems and operation
procedures designed to optimize frac-pack geometry are discussed. In
addition, the one-trip multizone gravel-pack assemblies are described that
allow multiple zones to be individually fracture packed in a single run, thus
providing significant rig-time reductions without sacrificing completion
efficiency. Finally, the complex nature of this reservoir dictates the
need for a variety of completion techniques. This means that fracture
packing was not necessarily an option for all zones. For the intervals at
which fracturing was not employed, productivity still needed to be
maximized. To accomplish this goal, perforation tunnel-cleanup procedures
have been optimized.
Introduction
The Barbara field (Fig. 1) is situated in the Adriatic offshore area,
approximately 30 km from the coast. It has the form of a very gentle, slightly
asymmetrical anticline, having its eastern limb steeper than its western. It
occupies an area of about 80 km2 and a vertical closure
of about 50 m. Methane-gas-bearing strata are found at 40 different productive
layers and have been identified at depths between 1000 and 1300 meters; the
permeability ranges from a minimum of 5 to 10 md to a maximum of 500 to 600 md.
The reservoir sands are composed of sediments of the Asti formation, which were
deposited in the Pleistocene period in a turbidite environment; the cap rock is
formed by several argillaceous intercalations. Multizone completions are
necessary for efficient reservoir management. Experience and low
unconfined compressive strength ranging from 200 to 320 psi (determined with a
confined stress of 60 psi owing to the unconsolidated nature of the gas-bearing
productive layers) indicate that these unconsolidated sands require sand
control.
The exploitation of the Barbara field started in the early 1980s. In less
than 7 years, the field was completed with over 100 wells in dual,
multiselective, sand-control completions. In the late 1990s, however, because
of the subsidence phenomena, most of the higher-producing layers saw a dramatic
drop in production performance.
To aid in restoring productivity, a major workover program was conducted
between the years 2000 and 2003 on the platforms “F” and “C” of the Barbara
field. The original project scope was only 11 wells; however, the unexpectedly
positive results led to this project being expanded to a total of 77
completions in 16 wells.
© 2005. Society of Petroleum Engineers
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History
- Original manuscript received:
4 February 2004
- Revised manuscript received:
7 June 2005
- Manuscript approved:
18 June 2005
- Version of record:
15 September 2005
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