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.