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`F-01 : THE' USE OF 40 'SEISMIC IN RESERVOIR.
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`~MANAGEMENT
`1
`',' ~-"" .~ .. I ,_,' ~~.~j
`l. ..... _ ..... _,_~ ___ ~·.-. .... _~N ._ . _ __ ._ " " ' __ ............... __ _ .~._ < _~ ____ • • ;,. __ • • _ . __ • .....; .,_'''~
`J MARCUS MARSH, GRAEME BAGLEY, ANDREW LEWIS, JOHN MCGARRITY,
`TIM NASH, RONNIE PARR, IAN SAXBY and DAVID WHITCOMBE
`BP Exploration, Farbum Industrial Estate, Dyce, Aberdeen AB21 7PB, UK
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`.. -
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`-
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`Summary
`In mid 1999, BP acquired a 3-D survey over the West of Shetland area specifically for time-lapse
`interpretation of the reservoir dynamic behavior over the component fields. This was described at the
`EAGE meeting last yeari. This formed a turning point in BP's use of 4-D from experimentation to
`commercial use. The results were very useful in field management and well planning. Before that, it had
`acquired a number of repeat 3-D seismic surveys which in addition to their prime objective, were found to
`be useful for time lapse interpretation. Since then, BP has in 2000 acquired a further five '4-D' datasets,
`covering nine producing fields. These have all been undertaken with the express purpose of obtaining
`data that will allow a tighter control of reservoir management, and thereby enhance commercial value.
`
`In all cases a major objective has been to influence infill well locations. Specific problems differ from
`In the West of Shetland area, the 4-D is a powerful tool in assessing hydraulic
`field to field.
`communication. Forties is using it to identify unswept oil and to improve reservoir characterisation for
`proposed EOR.
`In Montrose, the data will help determine remaining options for field development.
`Accurate determination of the movement of the gas-oil contact is important in Harding, and in Mamock
`the data will be used to improve the reservoir description.
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`60
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`Context
`The increase in the use of 4-D seismic within BP has reflected that in the industry (Figure xx). Although
`the 1999 West of Shetland survey was the first acquired by BP for 'commercial' rather than experimental
`purposes, repeat survey data collected for other reasons had been used for time lapse interpretation in
`Prior to conducting a survey, a number of
`Magnus, Gulfaks and Forties.
`important questions must be answered:
`• Are the rock and fluid
`properties suitable?
`Is the seismic data quality
`adequate?
`• How will the reservoir
`characteristics affect
`interpretation?
`Has there been sufficient
`disturbance of the initial
`conditions as a result of
`production?
`Will the seismic answer the
`reservoir management
`questions in a useful timescale?
`What will its value be, and the
`risks involved?
`
`Q)
`
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`'1
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`198J 1985 1987 1989 1991 1993 1995 1997 1999 loot
`
`0
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`•
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`•
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`•
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`•
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`Figure 1: Growth in Use of 4-D in the North Sea.
`(Industry data supplied by JNOC)
`EAGE 63rd Conference & Technical Exhibition - Amsterdam, The Netherlands, 11 - 15 June 2001
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`Ex. PGS 1079
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`The candidate areas for 4-D acquisition in 2000 were selected on this basis, and are summarized in the
`table below.
`
`Table 1:
`
`Forties
`
`Hardin
`Arbroath
`Arkwright
`Montrose
`Mamock
`
`Foinaven
`Loyal
`Schiehallion
`
`Operator
`
`uired in 2000.
`Question?
`
`65
`440
`
`180
`
`BP , Shell, Esso, Agip
`
`390
`
`BP , Marathon
`Bp·, Shell
`Bp·, Shell, Murphy, Statoil,
`Amerada Hess, OMV
`
`Detection of un swept oil for
`in fill well
`lacement.
`Oil-water contact movement.
`Identification of remaining
`development options.
`
`Reservoir description;
`locatin un accessed areas.
`Optimisation of production
`strategy and drilling
`opportunities.
`
`Case Stlldies
`
`Forties
`
`Forties infill drilling commenced in 1992. By 1995 it was apparent that the process of identifying infill
`targets from simulation models would fail to deliver economic targets beyond 1997 due to a lack of
`resolution in these models. A second 3D survey was shot in 1996 to complement the first Forties 3-D
`volume acquired in 1988, thirteen years after the start of production. Since 1997 Forties infill targeting
`has relied on images of lithology, fluid, and fluid saturation change derived from these seismic volumes.
`This has allowed those areas of the field with significant pools of remaining oil us to be identified.
`
`The application of this technology has allowed well targets to be ranked, and as a consequence, there has
`been a progressive improvement in the average size of target accessed and initial well rate achieved. It is
`anticipated that the 2000 data will provide an up to date fluid image and improved reservoir
`characterisation. Together with continuous improvements in processing technology this will enable infill
`drilling to be sustained until at least 2002, and will support planning of the potential EOR project.
`
`Foinaven
`Preliminary examination of the results of the 2000 4D seismic data over Foinaven has shown no major
`It has strengthened confidence in reservoir management strategy, and emphasized the
`surprises.
`importance of water injection.
`
`In common with Schiehallion and· Loyal, amplitude brightening is indicative of increasing gas saturation
`or oil pressure (with no change in phase); dimming is indicative of increasing water saturation or reducing
`gas saturation. In the example shown, the overwhelming observation is a brightening between 1993 and
`1999 and a dimming between 1999 and 2000. Although subtle details remain to be worked, this supports
`the interpretation that the water injection campaign has been effective in raising reservoir pressure. This
`is demonstrated by the drop in production GOR of most of the Lower T34 wells. Other sands in the field
`demonstrate the same phenomenon, but the uppermost sand, which was first produced after the 1999
`survey had been acquired, shows a significant brightening, attributable to pressure reduction below
`bubble point and gas evolution. This is similar to the brightening seen in the underlying sands evident in
`1999. The reduction of gas production is significant in reservoir management, as reservoir energy is
`conserved, and production is at less risk of restriction by facility constraints.
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`Figure 2: Foinaven Example.
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`Schiehallion
`The 2000 4-D largely confirmed the understanding of the field that had developed during 2000. This
`understanding and the increasing trust in the simulation model is rooted in the history matching of both
`production data, and well
`test
`results together with the 1999 40
`interpretation. Strong qualitative
`the 40
`correlations between
`amplitude data and
`simulator
`pressure plots indicate that an
`increased level of confidence in
`the two independent 'measures' of
`reservoir pressure is justified.
`
`+200
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`Figure 3: Schiehallion Example.
`
`Initial processing of the 2000 data
`was carried out on board in a little
`under three weeks, so that data
`would be available for planning
`the 3Q/4Q drilling programme.
`Comprising two wells, this was
`designed to drain a significant
`unswept pool in the centre of the
`field in a deeper sand, which from the 1999 appeared to be largely unaffected by existing wells. This was
`the lower sand shown in O.
`The simulator predicted that the sand would show slight depletion in the east, with weak support from the
`west. When drilled the sand section was depleted by about 120 psi. The overlying sand, already accessed
`by a nearby producer has strong indications of seismic brightening, interpreted to be due to evolved gas as
`a result of production. The simulator predicted depletion of around 550 psi, MDT measurement showed
`this to be around 630 psi. The pressure predictions were good, partly as a result of using the 4-D data in
`matching the simulator. A downdip injector has since been drilled to support both sands in preference to
`an additional producer, which was the original plan. This decision was made as a direct result of the 4-0
`information.
`
`Loyal
`The Loyal field is located in the West of Shetland area adjacent to Foinaven and Schiehallion. It is
`currently being developed as part of the Schiehallion project. Oil production started in 1998 and to date,
`four out ofthe six planned development wells have been drilled. Since start-up, two 3-D seismic surveys
`improving
`have been acquired over the field with the aims of optimizing the remaining wells and
`production forecasts. The time-lapse seismic data shows a strong response to pressure reduction around
`producing wells. This is because the bubble point pressure of the oil is close to intial reservoir pressure.
`As a result, gas evolves and its presence increases seismic reflectivity. In the longer term, seismic images
`of water movement are likely to have a bigger impact on reservoir management. This places bigger
`demands on seismic quality, attribute analysis and interpretation. However, having multiple repeat
`
`EAGE 63rd Conference & Technical Exhibition - Amsterdam, The Netherlands, 11 -15 June 2001
`
`Ex. PGS 1079
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`surveys goes a long way to help separate signal from noise and so aids the interpretation effort. Current
`work programmes are focused on this area and are revealing some very interesting images.
`
`Harding
`Harding field is situated on the western flank of the Crawford ridge. The main development consists two
`heavy oil pools, in massive homogeneous sandstones of the Eocene Balder formation with 35% porosity
`and very high permeability, in excess of IO Darcies.
`There is a free gas cap in both pools with a clear GOC seen on seismic as a discrete event in Harding
`Central and as amplitude changes on top reservoir in Harding South. There are ten horizontal producers
`(seven in Central and three in South) located 75 ft below the GOC to minimize gas coning. Waterflood
`reservoir management strategy has been to maintain reservoir pressure, although pressure has in fact
`dropped by about 70 psi since production
`started in 1996.
`In response to production and water injection,
`models predict that the GOG has tilted. This
`could be as much as 80ft down in the East and
`50ft up in the West. Multilateral infill wells are
`planned to target the attic oil between the
`existing wells and the GOC. As these will be
`drilled closer to the GOC than the earlier wells,
`the changes in the location of the GOC need to
`be mapped over the whole field. To this end a
`repeat of the pre-production 3D seismic survey
`was . carried out in September 2000 and is
`currently being processed in parallel with the
`pre-production survey acquired in 1990. A fast
`track dataset has been produced, and results
`froqt this have qualitatively confirmed both the
`gas movements predicted by the reservoir
`model and also the movement of injected water.
`
`Figure 4: South Harding Top Reservoir amplitude
`Difference 1990 - 2000
`
`'MonArb'
`The Montrose and Arbroath fields together with subsea tieback Arkwright are located south of Forties and
`in an equivalent age Paleocene reservoir. The significant rock properties and predicted 4D response is
`expected to be comparable across these three fields and similar to the pre-existing Forties seismic
`reservoir survey. This survey successfully reduced the risk on new well targets.
`
`Field production commenced on Montrose in 1976 and on Arbroath in 1990, whilst the Arkwright field
`was tied in during 1996. Development drilling is currently taking place on Arbroath.
`
`The baseline 'Monarb' 3D survey was acquired in1993 and the 475 km2 4D survey was acquired during
`the summer of2000 Time lapse processing of both surveys is currently underway and data delivery is
`expected in March 2001. Interpretation will focus on understanding sweep and identifying the remaining
`infill and short range sidetrack options.
`
`Marnock
`The Marnock field is part of a development which consists of seven oil and gas fields. It is a gas
`condensate field with approximately 1 TCF gas and 110MMstb condensate in place in a combined
`stratigraphic! structural trap. Triassic fluvial sandstones form the reservoir and have porosities of 18 -
`21 % and permeabilities of 0.1 - 2000mD. Initial reservoir pressure was close to 9000 psi.
`
`Rock property modelling on Marnock shows that observed pressure changes in wells, from 9000 - 5000
`psi, results in large acoustic impedance (AI) changes, which could be detectable using 4D methods. Fluid
`saturation changes give rise to small AI changes and may not be observable.
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`Production data acquired since start-up in 1998, suggests that the field is more compartmentalized than
`originally thought. Detection of pressure changes may allow un-drained compartments to be identified,
`thus optimising the location of any future producers. Successful results from 4-0 may also improve the
`prediction of drilling hazards. Finally, as the field is at an early stage of production, interpretation of the
`40 seismic data can improve understanding of reservoir behaviour and thereby impact reservoir
`management.
`
`The baseline seismic dataset is provided by a 3D survey acquired in 1986, while a survey acquired in
`2000 by Shell, operator ofthe neighbouring block, provides the second dataset. Differences in acquisition
`parameters between the two surveys, most notably the shooting direction, will present a number of
`processing challenges.
`
`Conclusiolls
`4-0 has become an important tool in reservoir interpretation and management in BP. Available results
`have been put to immediate use in solving problems of dynamic performance, and in optimization of the
`design and location of infill wells in a number of fields. Processing and interpretation of the datasets
`acquired in 2000 is not yet complete, but initial results obtained from fast track processing are already
`providing useful results, and the remainder of the results will be obtained in the next few months. Further
`surveys are planned for 2001.
`
`Acknowledgement
`
`The authors thank BP and its coventurers for permission to publish this abstract. The views expressed are
`the opinions of the authors and do not necessarily represent the official position of BP or any of its
`coventurers.
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`EAGE 63rd Conference & Technical Exhibition - Amsterdam, The Netherlands, 11 -15 June 2001
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`Ex. PGS 1079
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`i Parr, R S and Marsh, M. Development of 4-D Reservoir Management, West ofShecland. World Oil, September 2000, pp39 - 4'
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`Ex. PGS 1079
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