Presentation on theme: "OCEAN BOTTOM SEISMICS: Impact on field development"— Presentation transcript:
1OCEAN BOTTOM SEISMICS: Impact on field development Mark Thompson, Børge Arntsen* and Lasse AmundsenStatoil R&DDuring 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
2Experiences with Multi-Azimuth Acquisition in Ocean Bottom Seismics. IntroductionExperiences from North Sea FieldsStatfjordSnorreVolveKvitebjørnConclusionsThe 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.
3Experiences 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 technology1993: 2D-4C TOMMELITEN SURVEYObjective: Demonstrate potential of 4C technology to image through and below gas chimneys1997: Statoil acquired the first 3D/4C survey in the North Sea1999: 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 Statfjord2002: Statoil acquire one of the worlds largest 4C datasets at StatfjordHere 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 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.
4Experiences with Multi-Azimuth Acquisition in Ocean Bottom Seismics Experiences with Multi-Azimuth Acquisition in Ocean Bottom Seismics. The experience.Offshore tests (SUMIC)Gullfaks, 1989Gullfaks and Troll, 1993Tommeliten 2D, 1993Gullfaks 2D, 1995Statfjord 3D, 1997Block 24/12 (PL 204) 2D, 1997Sleipner Øst 2D, 1997Faeroes/Shetland Basin 2D, 1997Huldra 2D, 1998MN4C98-2, Møre Basin (Bl. 6303)MN4C98-3, Fles (Bl. 6605)MN4C98-4, Helland Hansen (Bl. 6505)MN4C98-5, Modgunn Arch (Bl. 6403)Gullfaks South 3D, 2000Gullfaks 3D, 2001Gullfaks South 3D, 2002Statfjord 3D, 2002Statfjord Øst 3D, 2002Volve 3D, 2002Gullfaks 3D, 2003Visund 3D, 2003Tyrihans 2.5D, 2003 (eField Evaluation)Heidrun 3D, 2003Exploration 2D, 2003 (5 lines ?)Kvitebjørn 3D, 2003Vigdis/Borg 3D, 2004Snorre 3D, 2004Kvitebjørn 3D, 2004Gullfaks 3D(4D), 2005Valemon 3D, 2006Snøhvit 2.5D, 2006This 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
5Experiences with Multi-Azimuth Acquisition in Ocean Bottom Seismics Experiences with Multi-Azimuth Acquisition in Ocean Bottom Seismics. The experience.Imaging through a gas cloudTommeliten 2D (1993), Gullfaks South 3D (2000), Gullfaks South 3D (2002)Structural imaging of a complex structureStatfjord 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 predictionBlock 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, 2003Reservoir characterisationSleipner Øst 2D (1997), Huldra 2D (1998)Reservoir monitoringStatfjord (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.
6Experiences with Multi-Azimuth Acquisition in Ocean Bottom Seismics. IntroductionExperiences From North Sea FieldsStatfjord 1997 & 2002Volve 2003Snorre 2004KvitebjørnConclusionsLet’s now have a look at four OBS cases from the North Sea
7Statfjord 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 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.
8Courtesy of Maren Dybwad Statfjord UnitBrent Gp.Statfjord Fm.Dunlin Gp.BSFABCSFCSFBSFAMain 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
9Statfjord 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.
10Statfjord 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 faultsTop Dunlin EF depth maps
11Statfjord OBS 1997The new fault pattern based on OBS seismic puts B-14 A and B-29 T2 in different fault blocksThe injector was moved north of B-29 T2Production has increased from 200 Sm3/d to 520 Sm3/dModel 2001B - 14 CB – 29 T2B - 14 AThe 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
12Courtesy of Maren Dybwad 1997 Surface1997 OBSAnother example from the same survey is well positioning.Looking at the vertical cross-section along the black lineCourtesy of Maren Dybwad
13Improved Seismic Resolution: Well 1 Planned wellpath based on streamer data (red).Planned wellpath based on OBS seismic data (purple).StreamerEirikssonRaude146 mOBSwell 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 position146 m
14Statfjord 2002 – One of the worlds largest 3D OBS Areal extent: 120 km2Receiver line spacing: 300mReceiver line length: 6000mReceiver station spacing: 25mSource line length: 3000mMax source line offset: 1200mSource interval 25m (flip – flop)Based on the results from the 1997 survey, a new large survey was shot in 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.
15Statfjord OBS 2002 – One of the worlds largest 3D OBS StreamerBetter definiton of onset A-block (Fault between MF and EF)More internal information in the C-blockOBCThe 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.
16Experiences with Multi-Azimuth Acquisition in Ocean Bottom Seismics. IntroductionExperiences From North Sea FieldsStatfjord 1997 & 2002Volve 2003Snorre 2004KvitebjørnConclusionsVolve is the next field where we are going to have a look at the OBS experience
17Structural 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 trapAge : Jurassic & Triassic - Hugin formationThick sands - Strongly faulted
18Volve 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.GoalNew well or high quality seismic data in order to decrease the structural uncertainties and hence increase the proven volumes in the fieldAgree upon the decision basis for development of the fieldThe seimic data was ambiguosand 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.
19Acquisition Geometry Swath geometry with inline shooting 2 x 6km receiver lines per swath400m receiver line separation24 sail 12km per swath3km inline offset; 975m x-line offset (max.)100m sail line separationDual source, 50m source separation, 25m flip/flopCa. 27 km² covering 5 Wells, one verticalThe 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.
20streamerOBSPlan for development and production (PDO) approved April 2005ResourcesOil 11.4 Million Sm3Gas 1.5 Billion. Sm3Field developmentJackup process and drilling facilityInterim storage on shipOn stream spring 2007Here 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 beConstructed.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
21Experiences with Multi-Azimuth Acquisition in Ocean Bottom Seismics. IntroductionExperiences From North Sea FieldsStatfjord 1997 & 2002Volve 2003Snorre 2004KvitebjørnConclusionsThe Snorre field is one of the major fields in the North Sea
22Snorre 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.
23Snorre OBS 2004 Streamer OBS 34/7-334/7-3BCUBCUSN 11.4SN 11.4SN 10.4SN 10.4SN 10.1SN 10.1SN 9.3SN 9.3Here is a zoom of the previous section which shows the much better resolution of the OBS data relative to the streamer data.lfu916StreamerOBS
24Experiences with Multi-Azimuth Acquisition in Ocean Bottom Seismics. IntroductionExperiences From North Sea FieldsStatfjord 1997 & 2002Volve 2003Snorre 2004KvitebjørnConclusionsKvitebjørn is a North Sea gas field.
25PSDM data (arb. line along cable 3) Kvitebjørn OBS Test 2003OBS data (Cable 3)PSDM data (arb. line along cable 3)BCUT.BrentT.EtiveA small OBS test was shot across the Kvitebjørn field in 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 NECourtesy of Edel Areklett
26Experiences with Multi-Azimuth Acquisition in Ocean Bottom Seismics. IntroductionExperiences From North Sea FieldsStatfjord 1997 & 2002Volve 2003Snorre 2004KvitebjørnConclusionsSo for the conclusion…
27Ocean 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 OBSIn spite of :-OBS costs a lot more than 3D streamer seismic.OBS data is more challenging to process and interpret.