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SPE 25054
I Society of Petroleum Engineers I

The Gullfaks Field Development: Challenges and Perspectives
Svein Tollefsen,' Eirik Graue, and Stein Svinddal, Statoil AIS

'SPE Member

Copyright 1992, Society of Petroleum Engineers Inc.

This paper was prepared for presentation at the European Petroleum Conference held in Cannes, France, 18-18 November 1992.

This paper was selected for presentation by an SPE Program Committee following review of information contained in an abstract submined by the [email protected]). Contents of the paper,
as presented, have not been reviewed by the Society of Petroleum Engineers and are subject to correction by the author(s). The material, as presented, does not necessarily reflect
any position of the Society of Petroleum Engineers, its officers, or members. Papers presented at SPE meetings are subject to publication review by Editorial Committees of the Society
of Petroleum Engineers. Permission to copy is restricted to an ebstract of not more than 300 words. Illustrations may not be copied. The abstract should contain conspicuous acknowledgment
of where and by whom the paper is presented. Write Librarian, SPE, P.O. Box 833836, Richardson, TX 750833836, U.S.A. Telex, 163245 SPEUT.

ABSTRACT

The Gullfaks Field is located in the Norwegian Sector of
the ~orth ' Sea, block 34/10, and currently has the
capability of producing more than 70,000 S d / d of oil
(440,000 stb/dl from three CBS platforms. The
reservoir san& comprise shallow marine to fluvial
sediments of the Cook Formation, Statfiord Formation
and Brent Group, ranging in age from Early to Middle
Jurassic. Water injection is the major drive mechanism
for maintaining reservoir pressure above bubble point.

Development wells have confirmed a complicated
structural picture with numerous faults beyond seismic
resolution, causing major impacts on predicting field
reserves and flow patterns. Reverse faulting in an area
of predominantly normal faulting further emphasises the
structural complexity.

Complex geology along with field performance after
water breakthrough resulted in several changes in the
initial development strategy. Production from the highly
productive Tarbert and Staffjord sands was accelerated
in order to compensate for the loss of production from
the Lower Brent sands caused by sandproduction after
waterbreakthrough. Development of complex Ness and
low productivity Cook sands have recently commenced.

Gravel packing, implemented in the Upper Brent field
development, provided sand control and increased
production rates. Various types of chemical sand
control are currently being evaluated and tested in the
field. Lowering pressure in gravel packed wellbore
region below bubble point may increase the production
rates even further.

Following advances in drilling technology, highly
deviateoP~orimntal wells improve recovery and
accelerate field development by combining production
from several reservoirs in one single well. A test
programme for surfactant and WAG flooding has been
implemented on the field. Other EOR methods, such as
gel and polymer flooding, are currently being
investigated for potential use.

Referencon and illustrations at the end of paper.

377

INTRODUCTION

The Gullfaks Field, situated in the Norwegian Sector of
the North Sea, block 3411 0 (Fig. 1 ), is the first licence
ever run by a fully Norwegian joint venture corporation.
The licence group consists of Statoil (operator), Norsk
Hydro and Saga Petroleum. The field currently has the
capability of producing more than 70,000 Sm3/d of oil
(440,000 stbld) on a stream day basis from three main
reservoirs of Jurassic age. These are the Cook Fm.,
Statfjord Fm. and Brent Group, with total recoverable
reserves estimated to 230 mill. Sm3 of oil (1.5 bill. stb)
and 25 bill. Sm3 of associated gas (890 bill. scf). The
Brent Group has been subdivided into the Upper Brent,
Tarbert and Ness fms. and Lower Brent, Etive and
Mnnoch fms.

The field produces from three separate CBS platforms,
the Gullfaks A, B and C. Gullfaks A and C are fully
independent processing platforms, with three
separation stages. The crude oil is stored in the
concrete base of the platforms and loaded into tankers
via two separate mooring buoys. Associated gas mainly
passes into the Statpipe network. Some gas is,
however, recompressed and reinjected into the
reservoir. The Gullfaks B platform provides processing
facilities for single stage separation only, and transports
partly stabilised crude to both the A and C platforms
for further processing. The particular option of further
processing on two different platforms has ensured a
high degree of flexibility for oil delivery from the
Gullfaks B platform. A separate pipeline transports the
associated gas to Gullfaks A. The platform infra-
structure and capacities are shown in Fig. 2.

This paper presents the history, current status and
Mure perspectives of the Gullfaks Field Development,
the technology available, as well as new technology
required for optimising such complex field develop-
ments. By comparing early development plans to the
present situation, emphasising in particular all the
adjustments required to get there, this paper provides
a broad base of experience which may be of great
value to both further development of this particular
field, as well as to other field developments of similar
complexity in the years ahead.

Page 2

2 THE GULLFAKS FIELD DEVELOPMENT - CHALLENGES AND PERSPECTIVES SPE 25054

HISTORICAL REVIEW

Exploration history

Block 3411 0 was awarded to the licence group in 1978
during the 4th concession round. The first exploration
well, 3411 0-1 (Fig. 31, encountered a 160 m oil column
in the Brent Group and penetrated water bearing Cook
and Statfjord fms. Exploration well 3411 0-3 established
the oil-water contact (OWC) in the Brent Group. The
next three wells confirmed the OWC and appraised the
western part of the field.

Early use of 3D seismic introduced a new era of
exploration and appraisal drilling in the eastern part of
the field. Well 34110-7 proved a deeper hydrocarbon
system in the Cook Fm., whereas well 34110-1 1 in the
north-eastern corner of the block succeeded in proving
a deep OWC and a new oil system in the Statfjord Fm.
The exploration and appraisal phase was completed by
the end of 1983 with a total of 14 wells.

Development, Phase I

Based on well information and seismics a field
development plan in two phases was proposed.
Following the Commerciality Report in late 1980, the
authorities approved a Field Development Plan (FDP) for
the western part, Phase I in October 198 1. This plan
included the production of the Brent Group reserves
west of an assumed sealing northsouth trending fault
(Fig. 1). Two CBS platforms were required for
producing these reserves.

The overall production philosophy was to produce the
reserves successively shallower within the Brent Group
Water injection would be the major drive mechanism,
maintaining reservoir pressure above bubble point. In
order to avoid potential sand problems during produc-
tion, selective perforating would be used. At this stage,
the total Gullfaks recoverable reserves were estimated
to approximately 230 mill. Sm3 of oil (Table 1).

Development, Phase I1

Based on an updated FDP, which included the eastern
part of the Gullfaks Field, the authorities approved the
Phase II development in 1985. Another CBS platform
was required for producing the Brent, Cook and
Statfjord reserves in the complex Phase II area. A 60"
deviation was considered a limit in drilling platform
wells. The overall production strategy in Phase II was
unchanged compared to the FDP in Phase I.

Production Start

The Gullfaks A platform came on stream on December
22nd. 1986. In order to accelerate production start, a
total of five subsea wells were predrilled and put on

stream initially. A 6th subsea well drilled on a separate
segment was tied back some months later (Fig. 2).
Gullfaks B started production in February 1988, whilst
Gullfaks C came on stream in December 1989.

During the first production period, it became clear that
the process and water injection capacity had to be
upgraded in order to optimise production. This is now
implemented. Fig. 4 shows production profiles as
planned in 1988 compared to current estimates.

GEOLOGY

Geophysics And Structural Geology

Structural mapping of the Gullfaks Field is complicated
due to poor seismic data and complex structural
geology. The first 3D survey was shot in 1979,
providing a great improvement compared to previous
available 2D data. A second 3 0 survey collected in
1985 further improved the data. Recent processing
techniques made it possible to improve seismic quality
even more. As such, a reprocessing of the survey was
completed during the spring of 1992. This resulted in
higher resolution data, particularly towards the eastern
parts of the field.

The Gullfaks Field was highly deformed during the late
Jurassic extensional period. This resulted in rotated
fault blocks with low angle normal faults, dipping
approximately 60' to the east and typically spaced 1 -
1.5 km apart. Strata within the fault blocks normally
dip 15"-25" to the west (Fig. 5). In addition, many
small-scale, east-west trending normal faults occur on
the field. While most faults were non-sealing during
hydrocarbon migration and filling of the reservoir,
several faults act as barriers during production, limiting
horizontal flow and pressure communication across the
field.

In the eastern part of the field a horst block dominates
the structural picture, resulting in a graben west of the
horst. Coexistence of westerly and easterly dipping
faults may have caused spatial problems accompanied
by local reverse faulting. Strata within the graben and
horst blocks are mostly horizontal or even slightly
dipping to the east. Large scale normal faults as well as
a major drop in the Base Cretaceous Unconformity
define the northern, eastern and southern limits of the
Gullfaks structure (Fig. 5).

The stratigraphy of the Gullfaks Field show progressive
erosion towards the east. Whereas the Middle Jurassic
Brent Group is not eroded in the western part of the
field, some 600-800 m of Middle and Early Jurassic
sediments are eroded on the horst block to the east
(Fig. 5). Structural depth maps of top Brent Group, top
Cook Fm. and top Statfjord Fm. are shown in Fig. 3.

Page 8

8 THE GULLFAKS FIELD DEVELOPMENT - CHALLENGES AND PERSPECTIVES SPE 25054
Tarbert Fm. Instable hole conditions during drilling and
completing the well with satisfactory sand control are
aspects which require careful planning and sophisti-
cated technical solutions. Horizontal cased hole gravel
packing across a 400 m perforated interval is currently
being planned for sand control purposes in well 8-23
(which will be the first horizontal Upper Brent well on
Gullfaks).

The efforts of reducing injection water override effects
in the Lower Brent and Ness reservoirs will continue.
Established methods, such as zone isolations,
perforating wells structurally deep, varying perforation
density and drilling horizontal wells in low permeable
sands will be applied and further improved in the
future. The approaches of WAG injection and various
high productivity reservoir blocking techniques, will be
also be considered for application on a field scale basis.

Production With Reservoir Pressure In Near-
Wellbore Region Below Bubble Point

The Gullfaks Field is classified as an undersaturated oil
reservoir. Upon proposing the original field development
plans, attention was therefore paid to potential prob-
lems of reduced recovery and productivity due to the
creation and production of secondary gas. As such, the
field was planned exploited by a complete reservoir
pressure maintenance scheme, maintaining the
reservoir pressure above bubble point.

Detailed reservoir simulation studies have, however,
identified a potential of accelerated oil production in
some wells by reducing near-wellbore pressure below
bubble point. This increases pressure drawdown and
hence production rate. By producing the wells at a rate
corresponding to a particular pressure drawdown
profile, aided by detailed reservoir simulation, it is
possible to control secondary gas movement such that
all liberated solution gas may be produced directly and
evenly distributed into the well. A pilot programme was
initiated in April 1992 in the horizontal well A-34A,
including the careful monitoring of productivity develop-
ment and GOR/wellhead pressure in this well and the
stratigraphically higher surrounding wells B 4 and 8-7.
The programme resulted in an immediate increase in
production rate by approximately 40 % in well A-34A.
No decline in productivity due to gas blocking or re-
duced relative permeability to oil is observed yet. Other
wells are currently being evaluated for potential produc-
tion with near-wellbore pressure below bubble point.

The creation of secondary gas caps for the purpose of
draining additional oil reserves structurally updip of the
wells (attic oil), may be a possible approach to
enhanced recovery provided it is possible to control gas
movement in the reservoir. Producing oil wells downdip
below bubble point may as such represent an
alternative to secondary gas injection.

Enhanced Oil Recovery - Advanced Methods

Motivation

Reservoir simulations based on the current field
development strategy predict that more than 300 mill.
Sm3 of oil will remain in the Gullfaks reservoirs at the
end of field life. A substantial part of this oil is
contained in rocks with excellent reservoir properties.
All the major platforms are installed, and uncertainties
concerning reservoir architecture, fluid distributions and
flow performance decrease as more field history
becomes available. It is an obvious challenge to recover
as much of this oil as possible, which is one of the
main goals of the Current reservoir management efforts.

An extensive programme has been initiated to verify
techniques or methods capable of improving recovery
on the Gullfaks Field. The stages in a process of
selecting and verifying EOR methods are as follow:

1. Identify the EOR potential.

2. Identify applicable EOR methods.

3. Conduct laboratory testing and simulation
studies.

4. Perform well tests and field pilots.

5. Implement on field scale.

Potentials

Remaining oil in the Gullfaks reservoirs are mainly
associated with:

high residual oil saturation
poor vertical sweep efficiency
attic oil residuals
low permeability regions

Method selection

Three systems are identified to have an EOR potential
specifically for the Gullfaks Field (Fig. 11 1. These are:

Water-alternating-gas injection I WA GI,
improving vertical sweep efficiency and
recovering attic oil.

Thin polymer gels, - blocking high permeable
reservoirs to improve vertical sweep efficiency
in low permeable reservoirs.

Surfactant flooding, reducing residual oil
saturation by improving microscopic sweep
efficiency.

Page 9

SPE 25054 SVElN TOLLEFSEN, ElRlK GRAUE AND STEIN SVINDDAL 9

Proiect status

The WAG project started in 1990. Simulation studies
were performed in order to identify the improved oil
recovery potential. A pilot test was designed and the
necessary modifications on the water injector ended in
January 1991. The pilot test commenced in March
1991, and up to June 1992 three gas injection periods
were performed in the central area of the field (fault
block 3F). Gas breakthroughs were observed in the
main target wells. Maximum water cut decreased from
54 % to 42 % in the main target well. Post test
simulation studies show improved recovery of
approximately 100,000 Sm3 of oil. A more extensive
WAG implementation, including more water injectors,
is currently evaluated.

The thin polymer gels project was initiated in 1989 for
the development of a chemical system capable of
selectively blocking override zones some 50 to 100
meters into the reservoir. Three different systems are
identified with interesting properties. A field test is
planned in 1993.

wells drilled from the B-platform. Natural water influx
is expected to provide sufficient pressure support.
Drilling is due in 1993.

The Tordis Field, located in block 3417 (Fig. 1) and
operated by Saga Petroleum A/S, was discovered in
1987. Pressure data showed depletion of oil in the
Brent Group caused by the Gullfaks Field production.
The FDP involves a subsea development connected to
the C-platform with pressure maintenance by water
injection. Production start is planned for 1994.

Production well C-7 proved oil in the Lunde Fm. in the
eastern part of the Gullfaks Field (fault block 1). Oil
was earlier observed in exploration well 3411 0-1 3. The
Lunde reservoir was production tested in C-7, but
showed an immediate decline in reservoir pressure,
indicating a somewhat limited communication in parts
of the formation. Evaluation of development strategy
will start in near future.

The surfactant flooding project started in 1989 by
screening studies and optimising of a surfactant system
for the Gullfaks Brent oil. The system reduced residual
oil saturation in core flooding experiments from 3540
% to 5 %. Two field tests have been performed in two
different wells to verify laboratory results.
Measurements of residual oil saturation were carried
out using the Partitioning Tracer Test Technique, along
with an extensive logging programme. Data are
currently being evaluated. Depending on the results, a
further pilot field test will be considered.

Discoveriiw and prospects

A number of recent discoveries and promising
prospects are located in the vicinity of the Gullfaks
Field. Three discoveries with proven significant reserves
are the "Gullfaks Snr", the "Gullfaks Vest" and the
"Tordis" Fields. In addition, the Lunde Fm. in fault block
1 was found oil bearing in well C-7 (Fig. 1).

Exploration well 34110-2, drilled in 1978, led to the
discovery of the Gullfaks Ssr Field (Fig. 1). Further
appraisal drilling proved oil and gas in both the Brent
Group and the Statfjord Fm. The field is structurally
complex with many different OWC's and pressure
regimes. An FDP has not yet been approved by the
licence, but preliminary plans indicate a separate
platform with flowlines to the main field for processing
and transport. One or two long range wells may be
drilled from the,P-platform, accelerating production
start from the oil ;one.

Exploration well 3411 0-34, drilled in 1991, led to the
discovery of the Gullfaks Vest Field (Fig. I), proving oil
in the Tarbert and Ness fms. According to current

The Gullfaks Field area is structurally complex. 70
% of the wells prove minor or large faults not
previously seen on seismics.

Continuous production drilling has resulted in a re-
distribution of the estimated reserves. The Tarbert
Fm. is now the main reservoir on the field.

Accelerating the development of the highly
productive Tarbert and Statfjord fms. increased
the plateau rate of the field.

Advances in drilling and well completion,
commingled productiontinjection and high capacity
water injectors accelerate oil production and
reduce the total number of wells required.

Gravel packing is currently the main technique of
sand control on the Gullfaks Field. Recent
improvements have reduced costs of implemen-
tation and improved production behaviour.

Squeezehnjection of resin coated proppants has
proved a successful and cost effective method of
chemical sand control in "old" production wells.

plans, the field will be woduced from two horizontal
385

Page 16

YULE PROOELNi

DEEP MARINE SHALES

RESERVUR

cam
F I W E C R U W NEARWORE

DEEP MARINE %ALES

Fig.6 A composite log display. Composed of wells 8-9 (Brent),
C-3 (Cook) grid C-2 (Statfjord).

Page 17

BEFORE DRILLING

AFTER DRILLING
0-3

Fig.7 Segmentation of a fault leaves more of the
reservoir above the OWC, compared to a "one big
fault" situation. ( Modifiedafter Petterson et.al. )

m 5 1/2" TUBING COMPLETION r MONOBORE C o M P m a
llNlMUM ID: 3.7" MINIMUM ID: 6"

Fig.8 5 112 Tubing and 7" Monobore completion
configurations

Fig.9 Shematic perforation tunnel in gravel packed
wells showing various pressure regimes.

SPE

ig. 10 Production potential in gravelpacked wells.

SURFACTAN T

O I L

GEL WATER

0 GAS SURFACTANTS+ MOBlLlSED O I L
g.11 Improved vertical (GELIWAG) and microscopic

(surfactant) sweep efficiency.

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