OCEAN BOTTOM SEISMICS: Impact on field development Mark Thompson, Børge Arntsen* and Lasse Amundsen Statoil R&D During the last 10 years Statoil has acquired a Large number of Ocean Bottom Seismic (or OBS for short ) surveys. In almost all cases have the quality of the subsurface images been much better than the images from corresponding streamer surveys. This has been mostly attributed to the larger cross-line offsets and the larger azimuth angles between source and receiver used in OBS surveys, relative to conventional streamer surveys where the cross-line offsets and azimuths are comparatively smaller. Recently attempts have been made to increase the cross-line offsets in streamer surveys, so-called wide-azimuth towed streamer (WATS) surveys. I thought it would be interesting to share some of the experienec we have had with OBS surveys, which have geometries quite close to some of the proposed wide-azimuth streamer surveys. SEG 2007

Experiences with Multi-Azimuth Acquisition in Ocean Bottom Seismics. Introduction Experiences from North Sea Fields Statfjord Snorre Volve Kvitebjørn Conclusions The outline of the talk is straightforward: A small historical introduction of OBS experiences, followed by examples derived from four Norh Sea oil- and gas fields emphasizing different aspects of the OBS data quality and also some implications for interpretation.

Experiences with Multi-Azimuth Acquisition in Ocean Bottom Seismics Experiences with Multi-Azimuth Acquisition in Ocean Bottom Seismics. The history. Late 1980's: Statoil developed "SUMIC", thus pioneering the marine shear wave (4C) recording technology 1993: 2D-4C TOMMELITEN SURVEY Objective: Demonstrate potential of 4C technology to image through and below gas chimneys 1997: Statoil acquired the first 3D/4C survey in the North Sea 1999: Kauffman Gold Medal Award (to the inventors for an outstanding contribution to the development of new geophysical methods) 2002: Statoil acquire worlds first 4D/4C dataset acquired at Statfjord 2002: Statoil acquire one of the worlds largest 4C datasets at Statfjord Here is a short historical summary of our OBS experience, which started with the pioneering 2D 4C Tommeliten survey with the objective of imaging through a gas cloud in 1993. One of the world’s first 3D OBS surveys was then acquired on the Statfjord field in 1997, followed by one of the first OBS surveys for 4D purposes in 2002.

Experiences with Multi-Azimuth Acquisition in Ocean Bottom Seismics Experiences with Multi-Azimuth Acquisition in Ocean Bottom Seismics. The experience. Offshore tests (SUMIC) Gullfaks, 1989 Gullfaks and Troll, 1993 Tommeliten 2D, 1993 Gullfaks 2D, 1995 Statfjord 3D, 1997 Block 24/12 (PL 204) 2D, 1997 Sleipner Øst 2D, 1997 Faeroes/Shetland Basin 2D, 1997 Huldra 2D, 1998 MN4C98-2, Møre Basin (Bl. 6303) MN4C98-3, Fles (Bl. 6605) MN4C98-4, Helland Hansen (Bl. 6505) MN4C98-5, Modgunn Arch (Bl. 6403) Gullfaks South 3D, 2000 Gullfaks 3D, 2001 Gullfaks South 3D, 2002 Statfjord 3D, 2002 Statfjord Øst 3D, 2002 Volve 3D, 2002 Gullfaks 3D, 2003 Visund 3D, 2003 Tyrihans 2.5D, 2003 (eField Evaluation) Heidrun 3D, 2003 Exploration 2D, 2003 (5 lines ?) Kvitebjørn 3D, 2003 Vigdis/Borg 3D, 2004 Snorre 3D, 2004 Kvitebjørn 3D, 2004 Gullfaks 3D(4D), 2005 Valemon 3D, 2006 Snøhvit 2.5D, 2006 This is a quick overview of most of Statoil’s OBS surveys from the pioneering survey in 1993 on Tommeliten and up to last years surveys.There is about 18 3D surveys and a number of 2D exploration surveys

Experiences with Multi-Azimuth Acquisition in Ocean Bottom Seismics Experiences with Multi-Azimuth Acquisition in Ocean Bottom Seismics. The experience. Imaging through a gas cloud Tommeliten 2D (1993), Gullfaks South 3D (2000), Gullfaks South 3D (2002) Structural imaging of a complex structure Statfjord 3D (1997), Gullfaks 3D (2001), Statfjord 3D (2002), Statfjord Øst 3D (2002), Volve 3D (2002),Gullfaks 3D (2003), Visund 3D (2003), Heidrun 3D (2003), Kvitebjørn 3D (2003), Vigdis/Borg 3D (2004), Snorre 3D (2004), Kvitebjørn 3D (2004) Exploration – Lithology and fluid prediction Block 24/12 (PL 204) 2D (1997), Faeroes/Shetland Basin 2D (1997), MN4C98-2, Møre Basin (Bl. 6303), MN4C98-3 Fles (Bl. 6605), MN4C98-4 Helland Hansen (Bl. 6505), MN4C98-5 Modgunn Arch (Bl. 6403), Exploration 2D, 2003 Reservoir characterisation Sleipner Øst 2D (1997), Huldra 2D (1998) Reservoir monitoring Statfjord (2002), Gullfaks (2001, 2003, 2005) If we split the surveys according to their objectives we get the grouping shown here: Four surveys were designed for gas-cloud imaging, the largest number were designed for improved structural imaging using P-waves. Some exploration surveys have been performed with lithology and fluid-prediction objectives while only two surveys have been optimized for reservoir characterization and monitoring purposes.

Experiences with Multi-Azimuth Acquisition in Ocean Bottom Seismics. Introduction Experiences From North Sea Fields Statfjord 1997 & 2002 Volve 2003 Snorre 2004 Kvitebjørn Conclusions Let’s now have a look at four OBS cases from the North Sea

Statfjord OBS 1997 – One of Worlds First 3D OBS The Statfjord Field is located in the northern part of the Viking Graben. The right figure rough outline of the main parts of the Statfjord. The structural simple Main Field inside the red poly. And structurally complex East Flank inside the yellow poly.. Several processing attemps have been made to improve the structural imaging over the East Flank area, but a big leap was not obtained before acquiring a pilot 4C survey late 1997. The main objective was to improve the structural imaging in the East Flank area and thereby find optimal location for production wells drilled in the area. The survey layout is shown to the left 8 receiver cables and covered a very small area of a few square kilometers. Each receiver cable was 5 Km long, spaced at 300 meters. Receiever spacing was 25 meters, with maximum cross-line offset of 3Km. The shotgrid was effectively 25 by 25 meters.

Courtesy of Maren Dybwad Statfjord Unit Brent Gp. Statfjord Fm. Dunlin Gp. BSF A B C SFC SFB SFA Main field consists of a rotated fault block with the Brent Gp., Dunlin Gp. and the Statfjord Fm. Here is again a map of the east-flank and we show a cross section along the blue line. The main Statfjord field is almost depleted but consists of the Brent, Dunlin and Stafjord formations. The east flank holds additional reserves and is comprised of slump blocks named A,B and C which have been difficult to image with conventional streamer seismics. Gravitational failure at the crest of the field generated the 3 slump blocks termed A-, B- and C-block. Courtesy of Maren Dybwad

Statfjord OBS 1997 – Worlds First 3D OBS Streamer (1997) OBC (1997) This shows a comparison between dip attribute maps from the east flank based on surface seismics to the left and OBC data to the right. The OBC seismic has improved lateral resolution since faults are much easier to define on the OBC data. We will see how important this is on the next couple of slides.

Statfjord OBS 1997 Model 2000 Model 2001 Top Dunlin EF depth maps This is a picture of the old geomodel of parts of the east-flank. 3 wells are marked in red. Following the old interpretation, based on streamer seismics, they are all in the same fault block with no mapped internal faults. The new model based on the OBS data shows a much more detailed image of the fault pattern. The area is now divided into several smaller fault blocks with many internal faults Top Dunlin EF depth maps

Statfjord OBS 1997 The new fault pattern based on OBS seismic puts B-14 A and B-29 T2 in different fault blocks The injector was moved north of B-29 T2 Production has increased from 200 Sm3/d to 520 Sm3/d Model 2001 B - 14 C B – 29 T2 B - 14 A The B-29 well was supposed to receive pressure support from the B-14A injector to the south , but was still producing too much water. The reason is evident on the new model based on the OBS seismics, the injector and producer are separated by a fault. To get good pressure support a new injector, the 14C well, was positioned north of B-29 T2 in the same fault block. Production from B-29 T2 has increased from 200 Sm3/d to 520 Sm3/d and the water cut has decreased from 75% to 14%. So far the results from B-14 C are as expected and after water breakthrough it will be converted for WAG injection to avoid lifting problems.   Production history B-29 T2

Courtesy of Maren Dybwad 1997 Surface 1997 OBS 1 2 3 4 5 1 2 3 4 5 Another example from the same survey is well positioning. Looking at the vertical cross-section along the black line Courtesy of Maren Dybwad

Improved Seismic Resolution: Well 1 Planned wellpath based on streamer data (red). Planned wellpath based on OBS seismic data (purple). Streamer Eiriksson Raude 146 m OBS well no 11 was originally planned along the red path based on the streamer seismics, but a fault was discovered when the OBS data became available and the well path was subsequently changed to the purple line. This ensured that the target was drilled in a better position. OBS seismic suggest fault in this position 146 m

Statfjord 2002 – One of the worlds largest 3D OBS Areal extent: 120 km2 Receiver line spacing: 300m Receiver line length: 6000m Receiver station spacing: 25m Source line length: 3000m Max source line offset: 1200m Source interval 25m (flip – flop) Based on the results from the 1997 survey, a new large survey was shot in 2002. It covered 120 Km^2 , which is very large for an OBS survey. Key parameters are listed here: 300 meter cable spacing, receiver spacing of 25 meters and maximum cross-line offset of 1200 meters. Shot pattern was effectively a 50 by 50 meter grid.

Statfjord OBS 2002 – One of the worlds largest 3D OBS Streamer Better definiton of onset A-block (Fault between MF and EF) More internal information in the C-block OBC The new survey gave a significant improvement in image quality, and in this case better definition of the onset of the A-block and more internal details of the C-block as shown on the slide.

Experiences with Multi-Azimuth Acquisition in Ocean Bottom Seismics. Introduction Experiences From North Sea Fields Statfjord 1997 & 2002 Volve 2003 Snorre 2004 Kvitebjørn Conclusions Volve is the next field where we are going to have a look at the OBS experience

Structural Setting Key Words. Volve is a small field discovered in 1993, with two appraisal wells drilled in 1997/1998. Here is a geological cross section of Volve. The reservoir is Jurassic and Triassic and consists of thick sands, but is heavily faulted. Key Words. Reservoir : Stratigraphic & structural trap Age : Jurassic & Triassic - Hugin formation Thick sands - Strongly faulted

Volve OBS 2003 History Oil discovered in 1993. Two appraisal wells drilled in 1997/98. Ambiguous seismic data. Challenges. Common structural interpretation. Volume uncertainties. Field development concept. Goal New well or high quality seismic data in order to decrease the structural uncertainties and hence increase the proven volumes in the field Agree upon the decision basis for development of the field The seimic data was ambiguos and the challenge here was to agree on the structural interpretation of the field, decrease volume uncertainties and also agree on a field development concept. It was decided to shoot an OBS survey and the objectives was to decrease structural uncertainties and increase proven reserves.

Acquisition Geometry Swath geometry with inline shooting 2 x 6km receiver lines per swath 400m receiver line separation 24 sail lines @ 12km per swath 3km inline offset; 975m x-line offset (max.) 100m sail line separation Dual source, 50m source separation, 25m flip/flop Ca. 27 km² covering 5 Wells, one vertical The acquisistion geometry is shown here. The receiver cables were 6 km long, 2 cables pr. swath. Cable separation was 400 meters and max cross-line offset was 975 meters.

streamer OBS Plan for development and production (PDO) approved April 2005 Resources Oil 11.4 Million Sm3 Gas 1.5 Billion. Sm3 Field development Jackup process and drilling facility Interim storage on ship On stream spring 2007 Here is an exqample comparing the OBS and streamer seismics with the disputed interpretation overlaid. Note in particular that the bottom and horizontal delineation of the reservoir is easier to define on the OBS data. A map of the sandstones could then be Constructed. With help of the the OBS data, partners agreed on the interpretation and volumes, and the field could be developed. In this case was the increased quality of the OBS data a risk reduction factor. 1 km

Experiences with Multi-Azimuth Acquisition in Ocean Bottom Seismics. Introduction Experiences From North Sea Fields Statfjord 1997 & 2002 Volve 2003 Snorre 2004 Kvitebjørn Conclusions The Snorre field is one of the major fields in the North Sea

Snorre OBS 2004 Streamer OBS and the Snorre OBS survey was shot in 2004, and one of the lines is shown here. In general the OBS data shows better resolution and continuity and is easier to interpret.

Snorre OBS 2004 Streamer OBS 34/7-3 34/7-3 BCU BCU SN 11.4 SN 11.4 SN 10.4 SN 10.4 SN 10.1 SN 10.1 SN 9.3 SN 9.3 Here is a zoom of the previous section which shows the much better resolution of the OBS data relative to the streamer data. lfu916 Streamer OBS

Experiences with Multi-Azimuth Acquisition in Ocean Bottom Seismics. Introduction Experiences From North Sea Fields Statfjord 1997 & 2002 Volve 2003 Snorre 2004 Kvitebjørn Conclusions Kvitebjørn is a North Sea gas field.

PSDM data (arb. line along cable 3) Kvitebjørn OBS Test 2003 OBS data (Cable 3) PSDM data (arb. line along cable 3) BCU T.Brent T.Etive A small OBS test was shot across the Kvitebjørn field in 2003. The quality of the OBS image changed the fault-interpretation quite substantially and the first production well was moved as a result. Later a larger survey has been shot and has had significant impact on positioning of wells. The OBS data is now the standard data set used for interpretation on Kvitebjørn. The OBS test led to a revision of the well targets and the first production well was moved 270 m toward NE Courtesy of Edel Areklett

Experiences with Multi-Azimuth Acquisition in Ocean Bottom Seismics. Introduction Experiences From North Sea Fields Statfjord 1997 & 2002 Volve 2003 Snorre 2004 Kvitebjørn Conclusions So for the conclusion…

Ocean Bottom Seismic. A good business case ! Conclusions. Value has consistently been obtained from OBS in areas of seismically challenged data. Data quality has generally improved when using OBS In spite of :- OBS costs a lot more than 3D streamer seismic. OBS data is more challenging to process and interpret.

Acknowledgements Statoil R&D Statfjord Statoil ASA, ExxonMobil, ConocoPhillips, Norske Shell, ConocoPhillips, Centrica Resources, BP, Enterprise Oil Snorre Petoro, Norsk Hydro, Statoil ASA, ExxonMobil, Idemitsu Petroleum, RWE Dea, Total, Amerada Hess Volve Statoil, ExxonMobil, Total, Norsk Hydro