Reliable predictive models for hydrocarbon production in the high cost Norwegian Offshore have required the development and application to full field simulation of a range of improved 3D geological reservoir modelling techniques. The authors' philosophy for reservoir modelling is outlined and attention focused on the acquisition of the correct type of data, understanding their frequently large inherent uncertainties, and the use of stochastic geological modelling, which, if handling advanced geological concepts and analogue data, enable a proper investigation of the effects of reservoir heterogeneity on fluid flow. Quicker and more trustworthy history matching and 'repairs' of models are made possible. There are challenges to be faced within areas such as geological concepts, estimation of Sw, Sor, and permeability, upscaling, ranking of realizations, the use of 3D seismic to determine lithological and fluid compositions and the conditioning of stochastic models using seismic attributes, well test and production data.
The Statfjord Field, the largest oil field in the Northern North Sea, straddles the Norway/UK boundary and is located on the southwestern part of the Tampen Spur within the East Shetland Basin. The accunmlation is trapped in a 6-8 ~ W-NW dipping rotated fault block comprised of Jurassic-Triassic strata sealed by Middle to Upper Jurassic and Cretaceous shales.Reserves are located in three separate reservoirs: Middle Jurassic deltaic sediments of the Brent Group, Lower Jurassic marine-shelf sandstones and siltstones of the Dunlin Group; and Upper Triassic-lowermost Jurassic fluviatile sediments of the Statfjord Formation. The majority of reserves are contained within the Brent Group; and Statfjord Formation sediments which exhibit good to excellent reservoir properties with porosities ranging from 20-30% permeabilities ranging up to several darcies, and an average net-to-gross of 60-75%. The sandstones and siltstones of the Dunlin Group have poorer reservoir properties where the best reservoir unit exhibits an average porosity of 22%, an average permeability 300 mD and net-to-gross of 45%.Structurally, the field is subdivided into a main field area characterized by relatively undeformed W-NW dipping strata, and a heavily deformed east flank area characterized by several phases of 'eastward' gravitational collapse.Production from the field commenced in 1979 and as of January 2000, 176 wells have been drilled. The oil is undersaturated and no natural gas-cap is present. The drainage strategy has been to develop the Brent and Dunlin Group reservoir with pressure maintenance using water injection and the Statfjord Formation reservoir by miscible gas flood. However, a strategy to improve recovery by implemeiating water alternating gas (WAG) methods is gradually being implemented for both the Brent and Statfjord reservoirs. Current estimates indicate that by 2015 a total of 666 x 106Sm 3 (4192
Th#s paper was selectee for preswlat,on by an SPE Program CcfnmMee following mvlew of ,nformakm contained In an abstract subnulted by the atihor(s) Contents of the paper as presented, have mt &n rev!swed by the Soc,ety of Petroleum Eng,neers and are subfecl to ccxrectwn by IJIM author(s) The maternal, as pfesentad, does not necessary reflect any posnnn of the Socmly of PelrOIWm Engmeefs 11scffK%ls or members Papers pfe5.3fltã l SPE meelmgs Me subject to publcatlw rewew by Ed!torfal Comm Mees of the Scwety of Petrcleum Engneers Permlssmn to copy is restrcted to an abstract of not more than 3Cnl wwds Illustrations may not tm cooled The abslracl should umtam consixxous acknolwedgment of where ati by whom the paper was presented Wmle Llbrartan, SPE P O Oox 833836 Ruhardson TX 750833836, U S A fax 01 .214 -952 -943S AbstractHigh oil recoveries are being obtained in North Sea oil fields by water injection. Favorable mobility due to low viscous oils and the need for pressure maintenance have made this the preferred recovery method. However, a potential for even higher recoveries exists by complementing water injection with gas injection, in different schemes adapted to the geological structure of the reservoirs. A correct description of the heterogeneities is crucial to the success of these efforts. The higher mobility and lower density of the injection gas touches areas not exposed to the injected water. At the same time, an early gas-breakthrough will negate the possible positive effects and even reduce the oil recovery. The probability of the presence of barriers, permeability contrasts, faults and structural traps has to be quantified in order to asses the risks associated with gas injection. This may be done within the quickly developing discipline of modelling heterogeneities with stochastic methods.An overview of field tests planned and performed by Statoil will be given. Amongst these examples are (1) a study of a down-dip miscible WAG in a heterogeneous reservoir, (2) gas injection after water flooding, which bas been successful in displacing attic oil in the Gulifaks field, (3) gas injection in oil producers with subsequent oil production and (4) plans for injection of gaseous water for mobility control by use of the non-newtonian theological properties of the gaseous water.Recognizing the high potential and the challenges present in gas-based IOR, Statoil has initiated a 3-years, US$ 6 million research project which will be summarily presented.
Based on detailed analyses of seismic and well data, the Statfjord Field, northern North Sea, can be separated into a relatively undeformed main field area and an east flank heavily deformed by rotational block slides. The main tectonic event is related to the opening of the Viking Graben which started in the middle Jurassic and which increasingly affected deposition of the Bathonian-Oxfordian Heather Formation. This rift phase peaked before the deposition of the Draupne Formation causing gravitational instabilities along the crest of the field and subsequent rotational block slides in the Upper Triassic and Jurassic sections. Compared to fields closer to the graben centre only limited erosion affected the Statfjord Field reservoirs during the Oxfordian-Ryazanian. Here, the Draupne Formation still drapes a partly degraded Jurassic sequence and is thickest on the west flank and within topographic depressions on the east flank. Minor tectonic activity occurred during the Cretaceous and Tertiary.
Copyr@ 1296, Olfshom Technobgy G&!rence lhi~PSPW was PI@s01710dat the 28th AnrIud OTC INIHCIU$lCWI,Tox8s.U.S A., 6.9 May 1S96 l%,s pSpEI ws SW.3ed for presentauon Dy Ihe OTC P[W8m C0Mmt19e fol!own!g WJWU Of Infammtlca crmlarwd HI m abstract Submmea by the authcf$. C4mlenfs 01 the paper, m presented, have not M rewmved by the Oflsiwre Technobgy Con forenca and are subIac4 lo correction by the authors The material. as pmsanted does not necasmrily rdecl any posrtion of ma Oifshore Technology Caifemnce or M olfuars. Permlss@x m WY IS restrtied to an abstra.a 01 not mom than 200 wads. Itlustratcm may not bm rxwmd. The aburgcl should umtam con$PIcwus knowk%d d whera and by tiun tfM wwr was preaenwd AbstractThe current position within appraisal and production reservoir model Iing is reviewed with respect to the data input, work processes and overall philosophy necessaty to achieve optimal, economic hydrocarbon recovery. Modern applications of 3D seismic and time lapse (4D) seismic are making increasingly significant contributions to the generation and verification of dynamic reservoir models incorporating the required levels of geological realism. Reference is made to examples from the North Sea and Norwegian Shelf to illustrate how these seismic data are being used to optimise structural and stratigraphic mapping, rock property mapping, identification of reservoir heterogeneities and the determination of fluid distributions with time. However, due to the uncertainties involved in data acquisition and processing and the inherent subsurface complexity caution should always be exercised when applying the results of seismic studies. This to avoid invalidating the model being constructed. The potential for the seismic conditioning of stochastic geological models and the use of marine shear wave data is also considered.
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