The Brent Field consists of a major, westerly dipping fault block, with a crestal area broken up into a series of slumped fault blocks. Development of these complex slumped blocks started in 1993 and to date a series of 9 roughly parallel horizontal east-west oil producers have been drilled, some of which have subsequently been converted into water injectors. Oil production to date, at a total rate of up to 100,000 b/d, exceeds the 1993 Development Plan forecast by 11%, despite poorer than expected east-west and north-south communication in some parts of the field. Key conclusions from the first phase of the development are:-–The Development Plan concept of multi-purpose horizontal wells (appraisal, production, injection) has been successful mainly because of the validity of the geological model. This has allowed optimal positioning of wells, with sand-sand juxtaposition initially providing sufficient pressure support via the West Flank.–Controlled reservoir depletion can be used as a method to establish fault block connectivity and to steer the flow of injection water. Uncontrolled depletion will lead to unwanted crossflow, ultimate recovery loss due to oil shrinkage and hazardous drilling due to large pressure differences.–The introduction of 7" monobore completions, combined with consecutive perforation and isolation of 150–250m reservoir sections, has increased ultimate recovery and reduced cost. Multi-straddle assemblies have proved to be ineffective, because communication patterns cannot be predicted accurately enough. An extensive data gathering programme has now been largely completed, consisting of new 3D seismic, borehole imaging logs, radioactive tracer injection, interference testing, advanced PLT logging and permanent downhole gauge output. This has so far resulted in an adjustment to the Development Plan in the compartmentalised Brent Horst and Graben areas, successfully targeting one water injector to support two producers 1500m apart, and in one infill well to appraise the quantity and quality of reservoir rock between the top Etive and the X-unconformity erosional surface. In combination with advanced drilling techniques a recovery factor of 50% is now considered achievable. Introduction The Brent Field consists of a major, westerly dipping fault block, with a crestal area broken up into a series of slumped fault blocks. Development of the Brent slumps started in 1993 and to date a series of 9 roughly parallel, horizontal east-west oil producers have been drilled, two of which have subsequently been converted into water injectors (see Figure 1). The wells in question are listed (in chronological order) in Table 1. They are all part of the first stage of the 1993 Development Plan (ref. 1), eventually comprising of 6 horizontal producers, interspersed with 6 horizontal water injectors. P. 135^

The Brent Field consists of a major, westerly dipping fault block, with a crestal area broken up into a series of slumped fault blocks. The primary phase of development has concentrated on water flooding the West Flank, but effort is now being applied to recover the oil in the slumped blocks. This paper describes the various studies undertaken to formulate a development plan for the estimated 13 x 10 6 m 3 (80 MMSTB) of oil reserves in the slumps of the Brent Reservoir and presents initial well results. A valid geological interpretation was crucial to successful development planning for the slumps. Structural interpretation is difficult due to limited seismic resolution, but the geological model has been supported by a review of structural analogues and by the limited well performance data. The slump faults are interpreted as running predominantly north-south. As such, they provide a significant barrier to west-east flow, while allowing better communication in the north-south direction. Development options have been evaluated by reservoir simulation based on this geological interpretation. East-west communication will be too poor to enable the slumps to be developed at reasonable rates by relying solely on pressure support from West Flank water injection. In addition, the use of horizontal wells, as compared with conventional deviated wells, has been shown to give significant benefits by providing offtake points in several slump blocks with one well, thereby improving the sweep efficiency and increasing the recovery whilst optimizing rig usage. A development plan has been formulated on this basis with a series of roughly parallel east-west horizontal oil producers, interspersed with horizontal water injectors. This plan is now being implemented and the first wells are now on production.

As the Brent Field approaches the 25th anniversary of its discovery and the 20th anniversary of first production, three of the four platforms are being completely refurbished in preparation for a major change in the operation of the field. Starting in 1997, the reservoir pressure will be gradually reduced in order to recover the large volumes of gas dissolved in the residual and bypassed oil. Initially depressurisation will be accomplished by stopping water injection, but, from the year 2000, it will be enhanced by using Electric Submersible Pumps (ESPs) to back produce aquifer water. Despite the lower wellhead pressures after refurbishment, the declining reservoir pressure will cause oil wells to loose lift from 2000, and by 2004 oil production will virtually cease. This sets the time frame for optimising production of the remaining oil reserves (360 MMstb at 1.1.96). A major project has been undertaken to Locate The Remaining Oil (LTRO) in unswept and bypassed areas of the Brent Field. This involves a multi-disciplinary effort encompassing petroleum engineering, well engineering and research. A detailed geological assessment, followed by inventorisation and quantification of the remaining oil pools. formed the basis of the project. Once drilling targets were identified, the integrated team approach was used to optimise the well paths, usually requiring a compromise between the desired and the practical approach. As the Brent Field matures, the development of the remaining reserves becomes more challenging. Bypassed accumulations tend to be smaller than traditional development targets. have a greater range in volume uncertainty and are generally more difficult to access. The latest developments in horizontal and extended reach drilling have been used to drill complicated well paths, connecting up targets in several reservoir layers at minimum cost. The LTRO project in Brent is well advanced and reserves of over 70 MMstb have been identified. The first LTRO wells have been drilled and the successes to date have proved the concepts which were applied to identify and to economically develop the remaining oil. Introduction The Brent Field was discovered in July 1971 and brought on stream just over five years later on 11th November 1976. In the year of the 25th anniversary of discovery and the 20th anniversary of first production, the Brent Field is undergoing major surgery in preparation for a change of life. Located 186 km north-east of Lerwick, Shetlands Islands, the Brent Field has a STOIIP of 3.8 MMMstb and a GIIP of 7.5 Tscf in two reservoirs; Brent and Statfjord. The field consists of a westerly dipping monocline, the West Flank, and, at both reservoir levels, the slumps, which are a zone of tilted and rotated blocks created by crestal failure when the structure began to form (Fig. 1). The field reached a maximum annual average oil production of 410 Mb/d in 1984. Since the mid-80s, oil production has been declining but, because of the high solution GOR (ranging from 250 to 980 v/v) substantial gas reserves still remain, dissolved in the residual and bypassed oil. Since the outset of field development there have been conceptual ideas on how to recover these large gas volumes and give this giant oil field a second lease of life. As oil production started to decline, IOR/EOR options were systematically evaluated. In 1992 the decision was taken to go ahead with the largest ever offshore brownfield development. The ageing facilities on three of the four Brent platforms are being completely refurbished in order to optimise recovery by lowering the first stage separator pressure, to implement Cullen safety upgrades, to extend the platform life and to reduce operating costs. Each platform is being shut down for up to a year while field production is maintained from the other platforms. The justification for depressurisation has been extensively reported. P. 171

Despite 21 years of production, the Brent field still holds the largest remaining hydrocarbon reserves of any field in the UK North Sea. The drive to optimise ultimate recovery of oil and gas has led Shell UK Exploration and Production to invest 20 man-years in a thorough revamping of the Brent Full Field Model to ensure the availability of an up-to-date reservoir management tool for the upcoming depressurisation of the field. For the first time a three-dimensional modelling approach was used to create the geological model, with a significantly greater level of detail and more geological layers than were modelled previously. Reprocessed seismic data and revised geological interpretations were used to assign rock and fluid properties to the 70 million building blocks that make up the new "static" model of the field, while pseudo relative permeability and capillary pressure curves were derived to conserve as much as possible of this detail after upscaling to the simulation model. The corresponding "dynamic" model consists of 88,060 grid blocks, three times as many as in the previous model. Reservoir fluids were also re-characterised to better represent the depth dependency of the fluid properties. During the project a large number of hardware and software problems had to be overcome. These were related both to the use of new software and the large size of the Brent model. The early results of the Full Field Model provided the confidence to increase the field's by 127 MMstb of oil and an additional 200 bscf of gas. Furthermore, the tool allows numerous "what if" questions to be tested, resulting in further optimisation of field development. In investment terms, the value added to Brent by the Full Field Model has already vastly exceeded its building cost and many further benefits are foreseen. Introduction The Brent Field, located in the UK sector of the northern North Sea (Fig. 1), is Shell Expro's largest Field and has been producing since end 1976. The Field consists of two westerly dipping formations, the Brent and the underlying Statfjord formation, separated by the non-reservoir Dunlin Group. To the East both reservoirs are present in a number of slump blocks which are in limited communication with the major part of the Field. The initial development plan called for oil and gas production together with down dip water injection in both reservoirs for pressure maintenance purposes. Current plans are to depressurise the field in order to develop the gas dissolved in the by-passed and residual oil. At present (1.1.1997) the remaining oil reserves after 20 years of production are estimated at 400 MMstb of which approximately 30% are located in the Slumps and 34 MMstb are related to depressurisation. Gas reserves are currently 2.6 Tscf of which 1.5 Tscf is related to the depressurisation of the Field. Since 1988 the Brent Full Field Model (FFM) has been an invaluable tool in the management of the Field and for the preparation of the development plan for depressurisation. However, it had become more difficult to match the production from the crestal wells and the Slumps due to the simplified way of modelling the crestal volumes and permeability barriers. In August 1991 the decision was taken to upgrade the Full Field Model using the latest technology and modelling tools. It was argued that where the existing simulation model was adequate for the conceptual design of the depressurisation development plan, the new model should be suitable for the detailed design. The objectives of the Full Field Model Upgrade project were:–To incorporate the latest geological and petrophysical interpretation of the Field and in particular to improve the modelling of the crestal area and the Slumps.–To improve the representation of the depth dependent fluid properties in the simulation model. P. 115^

C+y.ght 199S, SOCIW oi Petcol..m Engmewa I.. Thm pqw was pmpamc l.< prese.lat ton .1 the 19??S SPE E.rc$man P.trob.m Co.ler.nc. haki m Mire, ilaly 22-24 Od&r 19% Ths LMPU was sebcied for presentalton by an SPE Program CQmmnee follovw!g renew d mformakm mntatnad m ran aimtrac( subm'tted~tha autfwr(s ) Contents of the papa, as presented Favo not b+en revmwed by the SCX$ety of Petrobum Engmeors and are subject to CYXW31011 W the auihw(s) Tha matorml, as prmented dms nd nocessanly reflect any pwtion d NW SouetY G4 Pdrobum Engnwets, & off-m. or nwwbers Papcm presented at SPE nmattngs we s.b)ecf to pubkatmn rewew ty Ed#ona! C-amtwt.m d the Sw8fY cd Pdrob.m E"gme.m Pwmss,on to .xpq IS r.stnckl to an abstmci c4 nti m. than 2CU words Uhstrat,o.s may not bo copmd Tn. I&l rat! should cont.). conspac.ous ackowiedgemenl of n'twr. qnd bI whom the paper was pmsenl%i Wute Lbra.an, SPE P O Ebx S33S3S I%4urd=n, TX 7-2S36 U S A lax 01.214 -952-W35 AbstractAs the Brent Field approaches the 25th anniversary of its discovery and the 20th annive~of fmt production, three of the four platforms are being completely refurbished in preparation for a major change in the operation of the field. Starting in 1997, the reservoir pressure will be graduafly reduced in order to recover the large volumes of ga.. dissolved in tJre residuaf and bypassed oil. Initially depressurisation will be accomplished by stopping water injection, but, from the year 2000, it will be enhanced by using Electric Submersible Pumps @.SPs) to back produce aquifer water. Despite the lower weUhead pressures after refurbishment. the declining reservoir pressure will cause oil wells 10 loose lif[ from 2000, and by 2004 oil production will virtuafly cease. This sets the time frame for optimizing production of the remaining oil reserves (360 MMs(b at 1.1.96).A major project has been undertaken to Locate The Remaining Oil (LTRO) in unswept and bypassed areas of the Brent Field. This involves a multi-disciplinary effort encompassing petroleum engineering, well engineering and research. A detailed geological assessment, followed by inventorisation and quantification of the remaining oil pools, formed the basis of the project. Once &llling targets were iden'rlfied,the intcgrated team approach was used to optimise the well paths, usuafly requiring a compromise between [he desired and the practical approach.As the Brent Field matures, the development of USC remaining reserves becomes more challenging. BypMsed accumulations tend to be smaller than traditional development targets, have a greater range in voiumc uncertainty and ,are generafly more difficult [o access. The latest developments in horizontal and cx[cnded reach drilling have been used to drill complicated well paths, connecting up targets in severaf reservoir layers at minimum cost.The LTRO projtxt in Brent is well advanced and reserves of over 70 MMstb have been identified. The first LTRO wells have been drilled and the successes to date have proved the concepts which were applied to identify and to economically develop t...