Introduction to the Gullfaks Field
by Arild Hesjedal- Statoil
The Gullfaks oil field is located in the
Norwegian sector of the North Sea,
in block 34/10, approximately 175 km
northwest of Bergen, see Figure 1.
The reservoir units are sandstones of early and middle Jurassic age,
around 2000m sub sea and measure several hundred meters thick (Ref.1).
Reservoir quality is generally very high, with permeability ranging
from
few tens of mD to several Darcys depending on layer and location.
Figure 2 shows a cross section
indicating the quality and variability of
the reservoirs.
The reservoirs are over pressured, with
an initial pressure of 310 bar
at datum depth of 1850 m below mean sea
level, and a temperature
of 70 degrees C. The oil is undersaturated, with a
saturation pressure
of approximately 245 bar, depending on formation depth
and location.
The GOR ranges between 90 and 180 Sm 3 /Sm 3 , with stock
tank
oil gravity around 860 kg/m 3 . Structurally, the field is very
complex
and can be divided into three regions (Ref. 2): the socalled
'Domino Area' with rotated fault blocks in the west, and a Horst area
in the east; in between is a complex 'Adaptation Zone', characterized
by folding structures. The north-south faults that divide up the field
have throw up to 300 meters. In the western part the faults slope
typically around 28 degrees downward to the east, whereas in the
eastern horst they slope 60-65 degrees downwards to the west.
The field
is further cut by smaller faults, with throws of zero to few
tens of
meters, both in the dominant north-south as well as east-west
direction.
Many of these lesser faults have slopes of 50-80 degrees.
This results in
complex reservoir communication and drainage patterns,
and is a major
challenge in optimally placing wells in the reservoir.
Figure 3 shows a structural map at the
Statfjord level, and a typical
cross section showing structural
aspects.
Production from the field is now on
decline, reduced by a third from
the peak year 1994, when oil pro-duction
exceeded 30 MSm 3.
Recoverable reserves are currently estimated at 319 MSm
3 , of which
approx-imately 250 MSm 3 have been produced to date.
The
uppermost Brent sequence contains roughly 80% of the reserves,
with the
deeper Cook and Statfjord formations contributing the remainder.
The field has been produced with pressure
maintenance, mostly through
water injection, but natural water influx has
also contributed.
Gas injection has been employed in the past to drain
attic oil, but also
to avoid reducing oil production during periods of
restricted gas export.
Gas flaring as a production control mechanism was
eliminated in 1998.
WAG injection is also being employed in parts of the
field to improve
vertical sweep. Large differences in reservoir quality
between adjacent
layers have in some parts of the field resulted in water
override and
inefficient vertical sweep. The dense fault pattern has
necessitated
close well spacing in some areas, which again; often combined
with
good internal reservoir quality, has resulted in rapid water and gas
breakthrough in producers. A few wells are currently shut in due to high
H 2 S levels.
The field was discovered in 1978 and put
on production in 1986, with sub
sea wells producing to the GF-A platform,
the first of the three gravity
base concrete platforms. Water depth is
between 130 and 180 m.
The GF-B and GF-C platforms were installed and
started production in
1988 and 1990 respectively. GF-A and GF-C have
integrated production
and drilling, as well as water and gas injection,
facilities. GF-B has 1st
stage separation only, with further fluid
processing on GF-A and GF-C,
and is without gas injection facilities.
Following a three-stage separation
process, the field gas production is
exported by sub sea pipeline to shore,
where NGLs are removed, while the
produced oil is stored offshore and
exported by tankers.
Field production is currently limited by
well potential and runs at 30-35%
above initial design processing capacity.
In order to further utilize the
processing capacity of the Gullfaks
installations, third party processing
on GF-C of fluids from the nearby
Tordis Field commenced in 1994.
Later the Vigdis and Visund fields have
been hooked up the GF-A for oil
storage and export. Production from the
Gullfaks South sub sea
development, operated by the Gullfaks license group,
started in 1998 to
GF-A. Preparations are now under way for Phase-2 of the
this
development, with upgrading of facilities on GF-C to receive
additional oil
and gas volumes. Both enterprises will significantly boost
the oil and gas
output from Gullfaks in the next cen-tury. Nevertheless,
capacity is still
available for the processing increased volumes from the
main field. The
third party processing contributes to the IOR potential by
extending the
economic life of the installations, as well as offering a
welcome source of
injection gas.
Primary drilling is now practically
complete on GF-A and GF-B, and
identification is under way of infill
drilling targets to secure continued,
efficient, oil extraction. On GF-C
only a few primary drilling targets remain.
The platforms have in total 136
drilling slots in addition to initial 6 sub sea
wells tied back to GF-A. A
total of 106 platform wells, 79 producers and 27
injectors, are currently
in operation. Many of the wells are "designer wells",
with multiple
reservoir targets and long high angle and/or horizontal sections
(Ref. 3).
Several of the reservoir sections require sand control, and to date,
47
wells have been gravel packed. Propped, hydraulic fracturing is also an
important tool for effecting reservoir management strategy, with 18 wells
fractured to date (Ref. 4).
An important goal for the Gullfaks
license is to increase the recoverable
reserves by 40 MSm 3 , of which the
Brent Group is expected to yield 30
MSm 3 through a launched technology
programme. Locating the remaining
oil is an important part of this
programme. The project was named "Gullfaks
Oil Drive", and aptly
abbreviated "GOD" and is fully funded by the Gullfaks
Business Unit (Ref
5). The allocated resources amount to 28 manyears over
the two year period.
The project draws upon expertise from across the
organization, where
different organizational units contribute team members
on a continuous
basis over periods of months, rather than individual tasks
being submitted
to the resource unit to be solved by incidentally available
personnel.
Petroleum Engineering, G&G, Drilling, Well Technology, Production,
Health and Safety and Economics are all represented in the project team.
Time lapse (4D) seismic is considered to
be a key element in the process of
locating the remaining oil. So far one
well has successfully been drilled based
on this technology, and more
similar wells will be drilled in the near future.
1. H.M. Ånes, O. Haga, R.
Instefjord and K.G. Jakobsen: "The Gullfaks Lower
Brent Waterflood
Performance" paper presented at The 6th European Symposium
on Improved Oil
Recovery, 21-23 May 1991 in Stavanger, Norway
2. Fossen H. & Hesthammer J:
"Structural Geology of the Gullfaks Field, northern
North Sea", In Coward
M.P., Daltaban T.S. & Johnson H. (editors) Structural
Geology in
Reservoir Characterization. Geological Society, London, Special
Publications, 127, 231-261.
3. Samsonsen B., Jacobsen B.G.,
Skagestad T. & Kerr S., Statoil a.s.: "Drilling and
Completing a High
Angle Well with Coiled Tubing Technology" paper SPE 48941
presented at the
1998 SPE Annual Technical Conference and Exhibition held in
New Orleans,
Louisiana 27-30 September
4. Bale A., Owren K., and Smith M.B.,:
"Propped Fracture as a Tool for Sand Control
and Reservoir Management" SPE
24992, presented at EUROPEC, Cannes, France
16-18 November 1992.
5. Agustsson H., Stroenen L.K., and
Solheim, O.A.,: "The Gullfaks Field: "Creating
Value by means of a
Multidisciplinary Reservoir Management Approach", paper OTC
10739 presented
at the 1999 Offshore Technology Conference, Houston, Texas,
3-6 May 1999.