North Sea Oil and Gas Reservoirs-II ORGANIZATION Host and Organizers University of Trondheim The Norwegian Institute of Technology (NTH) Scientific Organizing and Editorial Com.m.ittee Antony T. Buller Research Adviser, Statoil, Trondheim (ChairlDan) Adjunct Professor of Petroleum Geology, NTH Eivind Berg Senior Research Geophysicist, Statoil, Trondheim Odd HjellDeland Managing Director, Reservoir Laboratories AS (ResLab), Trondheim Jon Kleppe Professor of Reservoir Engineering, NTH Ole Tors;eter Associate Professor of Reservoir Engineering, NTH Jan Ole Aasen Research Manager, The Continental Shelf and Petroleum Technology Research Institute A/S (IKU), Trondheim Adjunct Professor of Numerical Mathematics, NTH Secretariat Vibeke Halaas Ann Kristin Engan Madelein Wold The Norwegian Institute of Technology Sponsors Norwegian Research Council for Science and Humanities (NAVF), Norway Phillips Petroleum Company, Norway Saga Petroleum a.s., Norway Statoil, Norway The Royal Norwegian Council for Scientific and Industrial Research (NTNF), Norway North Sea Oil and Gas Reservoirs II Proceedings of the 2nd North Sea Oil and Gas Reservoirs Conference organized and hosted by the Norwegian Institute of Technology (NTH), Trondheim, Norway, May 8-11, 1989 Edited by A.T. Buller, E. Berg, O. Hjehneland, J. Kleppe, O. Tors2ter and J.O. Aasen Graham &f Trotman A member of the Kluwer Academic Publishl"rs Group lONDON/DORDRECHT/BOSTON Graham & Trotman Limited Kluwer Academic Publishers Group Sterling House 10 I Philip Drive 66 Wilton Road Assinippi Park London SWIV IDE Norwell, MA 02061 UK USA ISBN-13: 978-94-OlO-6843-7 e-ISBN-13: 978-94-009-0791-1 DOl: lO.1 007/978-94-009-0791-1 © Norwegian Institute of Technology, 1990 First published in 1990 Softcover reprint of the hardcover 1st edition 1990 British Library Cataloguing in Publication Data North Sea Oil and Gas Reservoirs, International Conference, 2nd (1989), Trondheim, Norway North Sea oil and gas reservoirs II. I. North Sea. Natural gas deposits and petroleum deposits I. Title II. Buller, A.T. (Antony T) III. Norges tekniske hogskole 553.280916336 Library of Congress Cataloging-in-Publication Data International Conference North Sea Oil and Gas Reservoirs (2nd: 1989: Trondheim, Norway) North sea oil and gas reservoirs II: proceedings of the 2nd International Conference North Sea Oil and Gas Reservoirs organized by the Norwegian Institute of Technology and held in Trondheim, Norway, 8-11 May, 1989/ edited by A.T. Buller. .. [et at.]. p. cm. Includes bibliographical references. I. Petroleum-Geology-North Sea-Congresses. 2. Gas, Natural-Geology- North Sea-Congresses. 3. Oil fields-North Sea-Congresses. 4. Gas reservoirs North Sea-Congresses. I. Buller, A. T. (An tony Treverton) I I. N orges tekniske hogskole. III. Title. TN870.5.149 1989 553.2'8'091 6336-dc20 90-2870 CIP This publication is protected by international copyright law. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior permission of the publisher. Typeset in Baskerville and printed and bound in Malta by Interprint Ltd. Contents Foreword A.T. Buller ix Field Review I The Forties Field and the evolution of a reservoir DlaDagelDent strategy J.M. Wills and D.K. Peattie Field Review 2 FulJDar: the developlDent of a large North Sea field 25 A.A. van der Helm, D.1. Gray, M.A. Cook and A.M. Schulte Field Review 3 The Tyra Field 47 M.C. Doyle and ].M. Conlin Field Review 4 The Gullfaks Field: geology and reservoir 67 developlDent O. Petterson, A. Storli, E. Ljosland and I. Massie Field Review 5 I. Stochastic lDodelling of reservoir sand bodies 91 for input to reservoir siIDuiation, Snorre Field, northern North Sea, Norway K.O. Stanley, K. Jorde, N. Ra:stad and c.P. Stockbridge D. Reservoir siIDuiation of the Snorre Field 103 G. Nybraten, E. Skolem and K. 0stby Keynote Reservoir characterization frOID crosshole seislDic: 115 Address I new directions W.B. Beydoun Chapter I Use of seislDic attributes in reservoir 125 characterization L. Simneland and O. Barkved Chapter 2 Experiences gained using a seislDic inversion 129 lDethod for detailed reservoir studies E. Berg, I. Brevik and A.T. Buller Chapter 3 SOlDe requirelDents frOID seislDic lDethods for use 139 in reservoir siIDuiation lDodels R.H. Seymour and ].S. Archer Chapter 4 Stratigraphic processing of 3D seislDic by 147 concurrent analysis of surface and borehole data P.S. Schultz, R. Gir, D. Pajot and S.S.C. Wu Chapter 5 Frigg Field reservoir lDanagelDent 155 R. Maritvold Chapter 6 Analysis of fOrlDation water salDpled frOID core 165 M.L. Coleman, M.R.O. Jones and M.A. Cox Chapter 7 EstablishlDent of a geological fracture lDodel for 173 dual porosity siIDuiations on the Ekofisk Field A. Fritsen and T. Corrigan Chapter 8 A new approach to production log analysis 185 G. Nerby Chapter 9 The geotechnical characteristics of weak North Sea 201 reservoir rocks M.E. Jones, M.J. Leddra, A. Goldsmith, O.P. Berget and I. Tappel vi Contents Chapter 10 TIae e8'eet of low perDleabiUty layen on oil 213 prodaedon fl'OlD vertical and horizoatal weUs ia the TroU FieJ4 L. Sundal. C.A. Kossack and J. Kleppe Chapter 11 Stati&tic:al approach to the computer modeUiag 223 of hyddalic fraetariag ia hydrocarbon reservoin R. Romagnoli and R. Varvelli Keynote <ba-titative cleterm:iaatioa of pore structure and 229 Address 2 appHeatioa to Aaid elisplacement ia reservoir rocks N.C. Wardlaw Chapter 12 Comparison between laboratory ellperiments and 245 detailed simulations of _stable miscible elisplacement iaftueneed by gravity M.A. Christie, A.D.W. Jones and A.H. Muggeridge Chapter 13 R.elationship between azimuths of Aood aaisotropy 251 and local earth stresses ia oil reservoirs K.J. Heffer and A.B. Dowokpor Chapter 14 Tracer tests ia oil appraisal and reservoir evaluatioa: 261 state of the art T. Bj0TDstad, K. Garder, I. Hundere, and O.B. Michelsen Chapter 15 Immiscible elisplacement ia a porous medium 271 simulated by a statistical model D. Zhou and E.H. Stenby Chapter 16 Ekofi.sk Field enlumced recovery 281 R.M. Sulak, G.R. Nossa and D.A. Thompson Keynote PseudofunetiODS - the key to practical use of 297 Address 3 reservoir descriptioD L.W. Lake, E. Kasap and M. Shook Chapter 17 A method for estimation of the density of fault 309 elisplacements below the Hmits of seismic resolutioD ia reservoir formatioDs C. Childs, J.J. Walsh and J. Watterson Chapter 18 Determiaation of petroleum accumuiatiOD histories: 319 ezamples from the U1a Field, Central Grabeu, Norwegian North Sea S.R. Larter, K.O. Bj0rlykke, D.A. Karlsen, T. Nedkvitne, T. Eglinton, P.E. Johansen, D. Leythaeuser, P.C. Mason, A.W. Mitchell and G.A. Newcombe Chapter 19 Lateral ellteDt of caieite-ceDlented ZODes ia shallow 331 DlariDe sandstoDes P.A. Bj0rkum and 0. Walderhaug Chapter 20 Sand body cIiDleDsiODs and iafi.ll sequences of stable, 337 humid-cHmate delta plaia channels T. Dreyer Chapter 21 The cODDectivity and cODductivity of overlappiag 353 sand bodies P.R. King Chapter 22 SESIMIRA-a Dew geological tool for 3D Dlodelliag 363 of heterogeneous reservoirs R. Gundes0 and O. Egeland Chapter 23 A computer prograDl for evaluatioD of Auvial 373 reservoirs R. Clemetsen, A.R. Hurst, R. Knarud and H. Omre Chapter 24 HEX-a tool for iategrated Auvial architecture 387 Dlodelliag and Dumerical simuiatioD of recovery processes A.W. Wadsley, S. Erlandsen and H.W. Goemans Chapter 25 An iategrated software for stochastic Dlodelliag 399 of reservoir Hthology and property with an ezample fl'OlD the Yorkshire Middle Jurassic J.L. Rudkiewicz, D. Guerillot, A. Galli and Group Heresi Contents vii Chapter 26 How leostadsdes help decisioas oa complemeatary 407 developmeat O. Ricard Chapter 27 Hybrid DlodeWag of heterogeaeity to iDlprove 3D 415 siDlulatioa results: case studies I and 2 011 a North Sea gas field A.M. Haak and E.F.M. Elewaut Chapter 28 A stochastic: reservoir Dlodel and its lise ia 425 evaluatioas of ..c ertainties ia the results of recovery processes J.O. Aasen, J.K. Silseth, L. Holden, H. Omre, K.B. Halvorsen and J. H0iberg Chapter 29 The DlOVeJlleat of oil iaitially bypassed belaiad 437 stochastic: shale barriers J.M.D. Thomas Chapter 30 Practic:al applic:ati01l of SllperC:ODlPllUag ia 445 blac:k-oil reservoir siDlulatioa A. Henriquez, T. Karstad and T. Steihaug Foreword The first North Sea Oil and Gas Reservoirs Conference was held in Trondheim in 1985 as part of the Norwegian Institute of Technology's 75th anniversary celebrations. Favourable reactions from the delegates prompted the Committee to re-run the event some three and a half years later, and it is now intended that the Confe rence be held on a regular basis as long as there is a demand for this type of gathering. The objectives of the 1989 Conference, which were broadly similar to those of the previous one, were: (a) to bring together those engaged in various geoscientific and reservoir engineering aspects of North Sea Oil and gas reservoirs in one forum; (b) to demonstrate wherever poSsible the interdependence oft he various disciplines and specializations; (c) to promote innovative, synergistic approaches to research and development programmes aimed at North Sea conditions; and (d) to reflect current trends in the reservoir sciences. Naturally there was no place for specialist parallel sessions in a Conference aimed at encouraging interdisciplinary integration and awareness. The Conference format was essentially the same as before. It contained a blend of North Sea field reviews (by B.P., Shell, Saga, Statoil and Maersk), keynote addresses (by Professor Larry Lake, Professor Norman Wardlaw and Dr. Wafik Beydoun) and 31 research papers. There was, however, one significant difference. All of the presentations at the 1985 Conference were by invitation only. This time the research papers were selected from an overwhelming response to a Call for Papers in accordance with the aims of the Conference and their technical contents. The programme, which attracted 230 scientists from II countries, embraced a wide variety of topics within improved seismic methods for detailed reservoir description, characterization of the near-well environment, reservoir mechanisms and processes, and reservoir heterogeneities, modelling and simulation. These themes have been maintained in the structuring of this refereed Proceedings Volume. Any successful conference requires the generous assistance ofa great many people. We would like to extend a special "Thank you" to the devoted ladies of our Secretariat (Vibeke Halaas, Ann Kristin Engan and Madelein Wold) who shouldered the main burden of the administration in a highly professional and willing manner. We would also like to thank: our speakers, co-authors and those companies and institutions who sanctioned the release of proprietary information; our honoured guests, Professor Dag Kavlie (Rector of the Norwegian Institute of Technology) who made the Welcoming Address and Professor Jan 0verli (Director of Statoil's Research and Development Centre in Trondheim) who made the main speech at the Conference Dinner; and our chairmen and referees-I. Akervoll, J. Alvestad, L. Amundsen, B. Arntsen, H.O. Augedal, K.A. Berteussen, K. Bj0rlykke, R.K. Bratli, J.D. Collinson, V. Dalen, S. Ehrenberg, A. Espedal, L. Fiilt, J. Feder, G. Fladmark, R. Gabrielsen, C.D. Griffiths, S. Hagen, H.H. Haldorsen, J. Hauge, A. Henriquez, P. Horsrud, O.K. Jensen, R. Kanestmm, A.G. Koestler, C. Kossack, A. Krill, O. Langeland, A. MacDonald, D. MaInes, J.H. Martin, T. Meyer, J.P. Nystuen, H. Omre, L. Rasmussen, R. Risnes, T. Rudshaug, H.G. Rueshitten, N. Saigal, R. Sinding-Larsen, P. Skalle, A. Skauge, S.M. Skjreveland, S. Snow, O. Steinert, L. S0nneland, S.B. Torp, R.H. Utseth, O. Vi kane and C.H. Whitson. I t is also important at a conference to generate a pleasant social atmosphere amongst the delegates. This was helped to a considerable degree by the introduction of a little local culture into the social events. In this respect, we would like to thank the Baroque Ensemble of the Ringve Museum of Music History in Trondheim, the NTH student glee club--Pirum-and the young people of the Byasen School Band, dressed in uniforms based on the old Trondheim Guard. The speakers, chairmen and special guests also received a small token of the Committee's appreciation in the form of a glass ornament. This comprised a model ofa production platform encased in a glass "oil droplet" and set on a base of Berea sandstone. The glasswork was most skilfully carried out by Mr. Kjell Rams0skar-NTH's own glassblower-and the bases were prepared by Lars 0yno, a NTH research student. Finally we would like to acknowledge financial support from the following government institutions and companies: the Norwegian Research Council for Science and Humanities (NA VF), Phillips Petroleum Company (Norway), Saga Petroleum a.s., Statoil, and The Royal Norwegian Council for Scientific and Industrial Research (NTNF). Their generosity enabled us to defray our costs significantly, thereby keeping the Conference registration fee at a reasonable level. Antony T. Buller Conference Chairman Field Review 1 The Forties Field and the Evolution of a Reservoir Management Strategy }.M. Wills and D.K. Peattie BP Petroleum Development Ltd, Farburn Industrial Estate, Aberdeen, UK The Forties Field is located 180 km ENE of Aberdeen, predominantly in UK Licence Block 21/10. It was discovered in 1970, when an exploration well encountered hydrocarbons in Palaeocene sandstone within an anticlinal structure. Four appraisal wells confirmed the existence of a major oilfield, with an area of approximately 90 km2• Initial estimates of the oil-in-place were 4600 million stb, with a recovery factor of 40%. The initial reservoir development plan involved producing the oil from four platforms with a plateau rate of 400000 stbd. Because of uncertainties concerning the performance of the underlying aquifer, water-injection facilities were installed on the platform to provide additional pressure support via peripheral water injectors. Information gained early during the development phase led to the oil-in-place estimates being revised downwards to approximately 4000 million stb, but to the recovery factor being increased upwards to 45% and the plateau rate revised to 500000 stbd. As more geological and engineering data has become available it has been possible gradually to increase the ~stima~e of the recovery factor, although estimates of oil-in-place have remained essentially unchanged. The current estimate IS approximately 57% with artificial lift, with the predominant oil displacement mechanism being bottom water drive. At the end of 1987, 75% of the total reserves had been produced and as a result of prudent reservoir management the annual average offtake rate had only declined to some 330 000 stbd. However, there is increasing evidence available which indicates that the Forties reservoir is more heterogeneous than previously thought. The evolution of a coherent strategy aimed at securing remaining. reserves and recognising, quantifying and recovering any additional incremental oil, represents a considerable challenge in mature field reservoir management. This requires an integrated approach by geophysicists, geologists, reservoir engineers and production engineers and a much more detailed description of the reservoir than is commonly required during the early plateau period of the life of a North Sea field. INTRODUCTION a current rate of 100 mbpd. An important reservoir management issue is to determine the origins of this water Discovered in 1970, the Forties Field has been producing and effectively control its production so as to maximize oil oil since September 1975. Stock Tank Oil Initially in Place recovery. (STOHP) has been calculated as 4343 mmstb*, and At present there are 103 available wells in the field, 81 original reserves estimated as 2470 mmstb, representing an producers and 22 injectors. At anyone time, a number of overall recovery of 57%. The field came off plateau wells may be temporarily shut-in, suspended or dead. production of 500 mstbpd in 1981, and by late 1988 Proper reservoir management has a significant impact on production had declined to about 280 mstbpd. After 13 the economic worth of the field. This paper describes years production, the field has produced 1907 mmstb, and significant geological, geophysical and reservoir engineer remaining reserves are 563 mmstb, which includes recovery ing aspects of the field, brought up to date since previous via artificial lift. Field life has been projected to extend publications, and also provides a view of the future beyond the year 2000. management of the reservoir. The emphasis is on a multi Water injection into the aquifer commenced in 1976, disciplinary team approach involving geologists, geo and continues at the present day at an average rate of physicists, and reservoir and production engineers. 390 mbwpd. There is also a significant contribution to The original licencees of the field were BP (100%) in pressure support from the underlying aquifer. Water Block 21/10, and Shell and Esso (50% each) in Block 22/6. production started in 1979 and has steadily increased to In 1983 and 1984 BP assigned certain fixed interests to minor partners, numbering 22 companies. Subsequent company and interest changes have resulted in 23 present ·Prefixes: m= 10'; mm= 106• licencees in the field. Unit Participation Interests have b~en agreed as 94.78% in Block 21./10 (BP83.13%, 20 North S.a Oil and Gas R.servoirs-II mInor partners 11.65%) and 5.22% In Block 22/6a (Shell © The Norwegian Institute of Technology (Graham & Trotman, 1990). 2.61%, Esso 2.61%). 2 North Sea Oil and Gas Restrvoirs - 1/ RESERVOIR GEOLOGY steel drilling/production platforms were installed on the Main Field in Block 21/10 with the facility for 108 wells, Location of field a producing system capable of around 500000 bpd, and a seawater injection system with a capacity of600 000 bwpd. The Forties Field is located in the central North Sea Development drilling began inJune 1975 and production approximately 180 km ENE ofA berdeen, Scotland (Fig. I). started in September 1975, with the plateau being reached Most of the field underlies UKCS Block 21/10, licenced to during 1978. A fifth platform was added over the south BP, with a southeastern extension into ShelllEsso's Block eastern extension (South East Forties) in Block 22/6a 22/6a. Water depth ranges from 104 to 128 m. during 1986, and production commenced through this facility in March, 1987. Regional geological setting Further major seismic data were acquired over the field The field is situated close to the crest of the Forties-Mont in 1980-81 and a regional grid overlapping the periphery rose Ridge, at the northern end of the Central Graben of the field in 1986. A full 3D seismic survey was acquired (Fig. I). over the entire field during the summer of 1988. In the North Sea, the early Tertiary marked the transi Early geological descriptions of the field were published tion from Late Cretaceous and Danian pelagic carbonate by Thomas et at. (1974), Hill and Wood (1980), and sedimentation with low terrigenous input to the accumula Carman and Young (1981). tion of vast thicknesses of clastic sediment deposited in Structural development/trap configuration deltaic, shelf and submarine-fan environments (Stewart, 1987). The distribution of early Tertiary sediments was The field has a broad, four-way dip-closed, anticlinal initially controlled by the pre-Tertiary structural confi structure, about 90 km2 in area, with a vertical relief of guration of the basin, so that early fan sediments accumu about 180 m. Depth to crest is 2030 m subsea. The lated in the axial regions of the Mesozoic grabens, with no structure at top reservoir level has two culminations or only thin sedimentation over the previously emergent (Fig. 3). The main crest is located west of the FA platform, highs such as the Forties-Montrose Ridge. By late Palaeo and a secondary crest lies to the east of the FC platform. cene times, however, clastic sedimentation had smothered There is an approximate coincidence between the structural all topographic features so that the regional structure was spill point and the initial oil-water contact at 2217 m expressed as a series ofmonoclinal ramps. This resulted in subsea. The structure is very gentle and nowhere in the the development ofa thick, extensive sand-rich submarine field are dips in excess of 10° recorded (Fig.4). The fan system (the Forties Fan) which both dips and thins in structural style is similar at all underlying levels within the a southeasterly direction along the Central Graben, and reservoir and aquifer. also thins westward towards the Scottish coast. The Forties The structure results from a combination ofs edimentary reservoir is part of this major fan system. At the end of the drape, synsedimentary faulting, and compaction over the Palaeocene, a relative rise in sea level resulted in the Forties-Montrose Ridge. deposition of the regional basinal mudstones of the Sele Minor faults are believed to penetrate the reservoir, Formation. These fine-grained sediments form the seal to particularly on the southern flank where evidence of the Forties reservoir. syndepositional slumping exists. However, these features The pattern of sedimentation is believed to have been are difficult to resolve with the current seismic database influenced by the relative position of sea-level in the basin and well spacing, and are not believed significantly to (Stewart, 1987). Low sea-level stands corresponded to influence reservoir continuity or production. periods of high coarse clastic input, whereas high sea-level The reservoir seal is provided by the overlying and stands corresponded to quiescent intervals of mudstone conformable Eocene-age mudstones of the Sele Formation. deposition (see chronostratigraphic diagram, Fig. 2). Further fluctuations between highstand mudstones and Field stratigraphy lowstand submarine fan clastic deposition occurred through out the Eocene in the central North Sea. In the Forties The stratigraphic column for the field discovery well, area extensive basin-slope erosional channels were deve 21/10-1, is shown in its local context in Fig.4. loped during the Middle Eocene, some of which are The oil accumulation occurs in thick sandstones of Late Palaeocene age, deposited in two major sand-rich sub sand-filled and oil-bearing. From the Late Eocene to the Holocene, continuing marine fan sequences of the Forties and Andrew Forma mudstone deposition onlapped and overlapped the under tions. They lie beneath a thick, monotonous section of lying shelf-delta-fan system in the Forties area and infilled mudstones, ranging in age from Late Palaeocene to Holocene. Thin, and occasionally oil-bearing, sandstones the entire basin. also occur in the overlying middle Eocene. The fan Exploration and development history sequences overlie the interbedded mudstones, sandstones and thin limestones of the Maureen Formation, which in BP was awarded Block 21/10 in the UK Second Licencing turn overlie the basal Palaeocene (Danian) and Maas Round, in November, 1965. The field was discovered in trichtian micritic limestones of the Ekofisk and Tor October, 1970, by well 21/10-1, drilled on the axis of Formations respectively. a large structural nose recognized on seismic at Base Figure 5 summarizes the lithostratigraphy currently in Tertiary level (Thomas et at., 1974). The well encountered use over the Forties area, which is consistent with the 119 m of oil-bearing Palaeocene sands at a depth of regional nomenclature established by Deegan and Scull 2131 m. Following the acquisition ofa 1.5 x 1.5 km seismic (1977). survey over the structure, a further four appraisal wells were drilled during 1971-72. Shell/Esso also drilled a Seismic-stratigraphic framework successful well in Block 22/6 that demonstrated the southeastern extension of the field. Reservoir subdivision in the field has been achieved by the The development plan evolved during 1972-75, and at use of seismic-stratigraphic techniques. the same time reservoir studies were undertaken to assess Prior to the 1988 3D survey, the most recent seismic likely field performance (Hillier et a/., 1978). Four fixed survey across the field was shot in 1980/81 in a triangular