bluuebunny/crossref_metadata_2025 huggingface dataset:
The Development of Superconducting Magnets for Use in Particle Accelerators: From the Tevatron to the LHC
Authors: Tollestrup Alvin , Todesco Ezio
Published: October 2008

Superconducting magnets have played a key role in advancing the energy reach of proton synchrotrons and enabling them to play a major role in defining the Standard Model. The problems encountered and solved at the Tevatron are described and used as an introduction to the many challenges posed by the use of this technology. The LHC is being prepared to answer the many questions beyond the Standard Model and in itself is at the cutting edge of technology. A description of its magnets and their properties is given to illustrate the advances that have been made in the use of superconducting magnets over the past 30 years.
Integrating production history into reservoir models using streamline-based time-of-flight ranking
Authors: Wang Y. , Kovscek A. R.
Published: October 2008

The reservoir models generated by geostatistical techniques, but unconstrained to production history, provide equally probable reservoir descriptions that honour observed geology. However, flow simulation results on these models may vary widely where there is geological uncertainty. Constraining geostatistical models to known production history reduces this uncertainty. To this end, a streamline-based algorithm is proposed for ranking geostatistical realizations with regard to production history. First, a rapid, streamline-based inversion method is applied to obtain a history-matched reservoir model. Then the streamline geometries and properties, such as the time-of-flight, are computed without full flow simulation for the history-matched model and the geostatistical models examined. Each model is compared to the history-matched model with regard to streamline properties. In this way, reservoir models that match production history and honour known geological information are obtained. Synthetic examples using up to 600 distinct reservoir models demonstrate computational efficiency and also show that the method readily selects the most appropriate permeability fields. Flow simulations confirm that the selected permeability fields are satisfactory. The technique also appears to be appropriate for downscaling history-matched reservoir models from coarse to fine grids.
Controls on facies architecture in the Brent Group, Strathspey Field, UK North Sea: implications for reservoir characterization
Authors: Morris Jenny , Hampson Gary J. , Maxwell Gregor
Published: October 2008

The Brent Group reservoir in the Strathspey Field, UK North Sea comprises shallow marine, marginal marine and non-marine strata of variable reservoir character. Controls on the distribution, geometry, connectivity and orientation of sandbodies within the reservoir have been assessed via the application of high-resolution sequence stratigraphic methods to an integrated dataset comprising core, wireline log, 3D seismic and reservoir production data. The base of the Brent Group is a major sequence boundary overlain by weakly wave-influenced, shallow marine sandstones of the lowstand-to-transgressive Broom Formation. The overlying Rannoch Formation represents highstand progradation of a wave-dominated shoreface, which is truncated across a sequence boundary by a near-field-wide fluvio-estuarine complex (Etive Formation). Marginal-marine strata of the overlying Ness Formation are divided into field-wide units bounded by mudstones that form vertical pressure barriers. Major, oil-bearing fluvial sandbodies overlie sequence boundaries and the widths and orientations of two such sandbodiesare controlled by syn-depositional, intra-reservoir faults. The distribution of these sandbodies is therefore predictable. The Ness Formation is truncated by a major sequence boundary overlain by stacked, tidally influenced, shallow marine sandstones of the Tarbert Formation. The resulting facies architecture has been successfully used to refine reservoir models, identify bypassed oil and target infill wells.
A 3D outcrop analogue model for Ypresian nummulitic carbonate reservoirs: Jebel Ousselat, northern Tunisia
Authors: Vennin E. , van Buchem F. S. P. , Joseph P. , Gaumet F. , Sonnenfeld Marc , Rebelle M. , Fakhfakh-Ben Jemia H. , Zijlstra H.
Published: October 2008

A three-dimensional high resolution sequence stratigraphic model of an Ypresian nummulitic carbonate ramp and organic-rich basin is presented based on outcrops in Central Tunisia. The sedimentation pattern is influenced by the interplay of different orders of variations in eustatic sea-level (third to fifth order), the pre-existing palaeotopography, and probably some synsedimentary tectonism (differential subsidence). Time-equivalent rocks deposited in a comparable structural and depositional setting along the northern margin of the African plate are hydrocarbon bearing (Tunisia and Libya). This example may thus serve as an outcrop analogue for this petroleum system, providing valuable information on the sub-seismic-scale distribution pattern, geometries and heterogeneities of both the reservoir and source rock facies.

The studied outcrops cover an area of 10 by 20 km where present-day valleys provide three-dimensional access to the rocks. In addition, the transition from the inner/mid-ramp, with nummulitic reservoir facies, to the carbonate source rocks in the basin is exposed in continuous outcrops. This transition takes place in about 3 km, a distance generally beyond the resolution of well spacing. Based on the physical tracing of beds, and the recognition of three orders of depositional sequences (third to fifth) a high resolution time framework is constructed. The accumulation of large nummulites (best reservoir facies) is stratigraphically controlled, and occurs in the transgressive phases of the landward-stepping fourth order cycles (overall transgression). Carbonate production was at that time so high that aggrading geometries are observed during these transgressive pulses. Our observations show that size, morphology and reproduction strategy of foraminiferal assemblages and, particularly, nummulites and
Discocyclina
, are related to changes in water depth and, consequently, in accommodation space. A regional east–west cross-section shows significant thickness variations of the Ypresian succession that were probably controlled by synsedimentary differential subsidence. The detailed, sub-seismic-scale, geometrical information on stratal patterns and lateral facies change are quantified, and used in a 3D numerical stochastic modelling (HERESIM) of this petroleum system.
Cenozoic basin development and stratigraphy of the Faroes area
Authors: Sørensen Aage Bach
Published: October 2008

The present paper reviews the geological development of the Faroes part of the NE Atlantic Margin in the Cenozoic Era. The Faroes area is located west of the post-Caledonian rift basin system formed in the Devonian after the collapse of the Caledonian mountains. Tectonic movements and plate reorganizations during several phases have strongly influenced sedimentation and erosion along the Caledonian front. During the Cenozoic four main tectonic phases with uplift, non-deposition or erosion, had a major influence on basin development and structural setting. First, the arrival of the Icelandic plume to the Faroes area caused a series of uplift phases, which are recorded in the Selandian–Early Ypressian sedimentary succession. Second, tectonic uplift in the Middle Eocene ended the period of almost continuous subsidence of the Judd Basin since the Danian. In the Middle and Late Eocene new depocentres formed in the northern part of the Faroe–Shetland Trough and in the Faroe Bank Basin. Third, due to uplift and sea-level fall in the Late Oligocene, widespread erosion of the Eocene and Oligocene successions resulted in a major unconformity on the shelf. Finally, in Neogene times, folding and uplift of the Fugloy Ridge occurred contemporaneously with renewed subsidence in the northern part of the Faroe–Shetland Trough and the Faroe Bank Basin.

Maps of Cenozoic key horizons illustrate this geological development from the start of the Palaeogene volcanism to the present day. Four regional maps are presented, two maps showing depths to Top Basalt and Top Palaeogene, and two isochore maps of the corresponding post-Basalt sediment sequences, the Palaeogene and the Neogene.

The petroleum system in the Judd Basin was strongly influenced by inversion in Middle Eocene to Recent times. Reservoirs and source rocks were uplifted up to about 600 m, with gas flushing, seal breakage and interruption in petroleum generation as consequences. Petroleum systems may also exist in the Faroe Bank Basin and in the northern part of the Faroe–Shetland Trough.
A stratigraphical and palaeoenvironmental analysis of the sub-basaltic Palaeogene sediments of East Greenland
Authors: Jolley David W. , Whitham Andrew G.
Published: October 2008

Palaeogene sedimentary strata from beneath the basaltic lavas ofEast Greenland have yielded a suite of palynofloras which determine the ageand environment of deposition of these rock units. These palynofloras include
Apectodinium augustum
,
Deflandrea oebisfeldensis
and a pollen assemblage containing common
Caryapollenites veripites
with
Camerozonosporites heskemensis
and
Stereisporites
in assemblages characteristic of the latest Paleocene to earliest Eocene Sequence T40. This allows a correlation to be established, complementing an already established framework based on lava geochemistry, which shows that lava eruption in East Greenland was initiated somewhat later than in the Faroe Islands. It also highlights the presence of a widespread major unconformity in the East Greenland succession, spanning the late Maastrichtian to the end of the Paleocene. The lack of sedimentation throughout the Early Paleocene, and majority of the Late Paleocene, highlights the possibility of Paleocene sedimentary systems bypassing the East Greenland shelf, with clastic deposition forced eastwards towards the area of the Faroe Islands and the Møre and Vøring basins.
Improved reservoir modelling with time-lapse seismic data in a Gulf of Mexico gas condensate reservoir
Authors: Waggoner J. R. , Cominelli A. , Seymour R. H. , Stradiotti A.
Published: October 2008

Time-lapse 3D – or 4D – seismic data have been tried in several fields to date, with some good case studies published to demonstrate the utility of the 4D seismic information. While another 4D case study would be useful, this paper describes two novel aspects of a recent application in the Gulf of Mexico. First, the target reservoir contains a gas condensate fluid under primary depletion, so pressure, rather than saturation, changes create the observed 4D acoustic response. Further, the primary impact of the pressure change is to the fluid composition as the initially dense gas phase lightens as condensate drops out below the dew point. The result was a 2.8% change in acoustic impedance predicted in the feasibility study.

Second, the 4D seismic result was used to constrain an optimized history-matching procedure, along with the production data. After describing the method used, the paper will discuss the changes to the reservoir model that resulted. While the results should not be considered unique, they do give some insight intothe structure of the reservoir that should be considered for optimal reservoir management.
Successful application of time-lapse seismic data in Shell Expro's Gannet Fields, Central North Sea, UKCS
Authors: Kloosterman H. J. , Kelly R. S. , Stammeijer J. , Hartung M. , van Waarde J. , Chajecki C.
Published: October 2008

The Central North Sea contains a large variety of oil and gas fields at different stages of maturity within the life cycle of an asset, at varying depths, in varied geology and at widely differing pressure and temperature conditions. Over the last three years Shell UK Exploration and Production (Shell Expro) has acquired new 3D surveys over many of these fields which, following careful attention to detail in acquisition and processing, have been quantitatively compared to pre-production surveys. Differences between these time-lapse seismic datasets have then been interpreted in terms of changes in reservoir fluid movement and changed pressure/temperature conditions. These interpretations have proven useful for reservoir management by identifying swept versus unswept zones, sealing versus non-sealing faults, efficiency of drive mechanisms and connected volumes to specific wells

Four case studies from the Gannet Development illustrate how these observations have impacted reservoir management and show how application of this relatively new, yet rapidly maturing technology has had a positive impact on the remaining value of these fields. Importantly, the detectability of changes in the reservoir has been seen to be greater than predicted prior to data acquisition. Indeed, 4D seismic data have become an established part of the long-term plans for all of Shell Expro's subsurface assets. With these successes have come fresh challenges and future efforts will focus on reducing costs in proven 4D technology whilst pushing to introduce new techniques for data gathering and for interpretation of large volumes of new information.
4D integrated technologies for deep-water turbidite reservoirs:from petrophysics to fluid flow simulation
Authors: Santos Rogerio A. , Sobrinho Alcides A. , Pinto Antonio C. C. , Dillon Lucia D. , Ritter Gerson L. S. , Silva Eduardo F. F. , Pimentel Marcia B. , Backheuser Yeda , Rosalba Jose F. , Guimarães Marcos A. G. , Prais Fabio , Abreu Carlos E. B. S. , Castellani Ricardo T. , Burns Scott
Published: October 2008

Petrobras has been developing its 4D seismic technological programme since 1998, focused on the Brazilian deep-water fields in the Campos Basin, and considering the technical, operational and economic challenges involved in the development plan and reservoir management in this environment.

The first step was to align the objectives of the project with the company goals for the following 15 years, in terms of earnings growth, production growth and reserves replacement. This information guided how the 4D reservoir management should be employed: as hedging technology to ensure that production targets would be achieved in several key fields at once, or as a direct technology investment to increase the production of individual, independent fields. The mission of reservoir management for each field involved was understood and new deep-water seismic technologies were developed to face the global operational and economic targets.

3D seismic reservoir monitoring, or 4D seismic study, was defined as an ‘integration of multidisciplinary technologies that includes the time-lapse monitoring of the drainage efficiency, using cores, well logs, seismic data, production history and pressure management’.

Water injection is the preferable recovery method for the deep-water reservoirs in Brazil. Therefore, seismic monitoring should be able to distinguish contrasts of both fluids – injected water and remaining oil – that normally produce small seismic impedance values. This characteristic has brought the first technical challenge: the use of the 3D P-wave surface legacy data from the 1980s and 1990s, when the major fields started production, as 4D base-volumes to be correlated with future recommended 3D seismic data (surface or ocean bottom systems) as 4D monitor-volumes.

In addition to all developed seismic technologies for data processing, a general 4D work flow was designed and the concept of the integrated reservoir model was adapted to relate all such technologies to the reservoir engineering needs and to the field economics, generating reliable 4D images for each reservoir study.

This paper summarizes the multidisciplinary technical integration, including geological and seismic modelling, petrophysical simulations, seismic processing and interpretation, and reservoir simulation. A 4D methodology was implemented to integrate all such technical development and economic analysis, identifying where, when and how seismic monitoring can contribute to the reservoir management. This methodology has been applied to the Campos Basin deep-water reservoir, Rio de Janeiro State, Brazil.
Integrated time-lapse reservoir monitoring and characterization of the Cervia Field: a case study
Authors: Kovacic Luciano , Poggiagliomi Elio
Published: October 2008

Seismic time-lapse reservoir monitoring (4D) was used to assess the performance of a gas field (Cervia), located offshore Italy, in the central part ofthe gas-producing area of the Adriatic Sea. The chief objective of this work was the characterization of the field producing intervals by applying an innovative methodology based on a high level of integration of seismic data with borehole data. The work, carried out on two ‘legacy’ 3D seismic surveys and all available borehole data, consisted of three main steps.

Characterization of the physical properties of the main reservoir, by extensive measurement and analysis of the core samples. This was in order to implement a petroacoustic model for calibration of seismic acoustic impedance volumes to reservoir description parameters. In addition all borehole data were employed during the implementation, calibration and testing of the petroacoustic model.

Integrated time-lapse processing of the two seismic volumes to achieve high repeatability. Seismic processing parameters were determined independently and objectively for each of the two surveys using well data. Spectral equalization of wavelets in the seismic traces was performed to further improve repeatability.

Integrated Multidimensional Petroacoustic Calibration (IMPAC) of the two seismic volumes. Here the microscopic properties of the reservoir rocks, obtained from the petroacoustic model outlined in step 1 above, were used, together with log data, to derive the main reservoir description parameters (i.e. lithology, porosity, saturation and pressure).

This approach produced good repeatability between the two processed seismic volumes. In addition, time-lapse analysis of the acoustic impedance volumes, calibrated to reservoir description parameters, enabled the visualization of fluid movement within the main reservoirs. This, in turn, led to the identification of areas where the gas may only have been partially depleted. These significant results will provide an important contribution for an improved and more efficient gas production from the field.
New tools for 4D seismic analysis in compacting reservoirs
Authors: Nickel M. , Schlaf J. , Sønneland L.
Published: October 2008

In order to successfully use time-lapse seismic data to monitor dynamic effects occurring in carbonate reservoirs, the development and adaptation of tools for seismic attribute analysis are necessary. In particular, since carbonate reservoirs tend to compact and subside under production, it is desirable to estimate this effect from the time-lapse seismic data and to separate this response from fluid effects that influence the seismic response. Furthermore, it is very important to identify and classify the fault network of carbonate reservoirs in detail due to its strong influence on the fluid flow. New tools are presented that address these problems. These tools are applied to seismic time-lapse data from the Ekofisk Field to demonstrate how compaction distributions can be quantified in 3D and how migration paths of injected water along fault and fracture zones can be identified.
BP's increasing systematic use of time-lapse seismic technology
Authors: Marsh J. Marcus , Whitcombe David N. , Raikes Sue A. , Parr Ronnie S. , Nash Tim
Published: October 2008

Following experimental success in the application of 4D technology in the Magnus and Foinaven fields and the delivery of commercial value from the application of time-lapse seismic (4D) studies in the Forties and West of Shetland fields and in the Pompano Field in the Gulf of Mexico, BP has embarked on the systematic use of 4D monitoring in its NW Europe operations to enhance value in the light of an increasing production challenge. Within the past two years, 12 datasets have been acquired over 19 fields of varied reservoir age, type and property. Although specific questions differ from field to field, in all cases a major objective is to influence the location and design of infill wells.

Useful time-lapse information has been gained from post-stack matched ‘legacy’ surveys of varying data quality. However, it is clear that more value is added by custom-designed 4D surveys: seismic quality and repeatability are critical to the successful delivery of reliable time-lapse seismic monitoring. Integration of seismic data with dynamic data is still relatively ‘low-tech’ and further benefits will be gained by improvement in this area.

The paradigm is shifting to a position where 4D monitoring in the North Sea is the rule rather than the exception. Now that there is clear evidence of added value in the most obvious target reservoirs, surveys are being carried out in areas that are technically more challenging – and the early results indicate that these too will be of real commercial benefit. Use of 4D data is also being progressed in other regions, including the Gulf of Mexico, and application in other basins around the world is being planned. On the technology development front, BP is seeking continuous improvements in acquisition and processing, both in quality and turnaround time, and richer integration with production and conventional well-based surveillance data. The ultimate goal is for effective reservoir management based on a range of surveillance methods.
Reservoir stress path during depletion of Norwegian chalk oilfields
Authors: Goulty N. R.
Published: October 2008

Pore pressure drawdown during reservoir depletion results in reduced horizontal principal stresses within a reservoir due to three distinct mechanisms: normal compaction, poroelastic behaviour and normal faulting. Established relationships, based on simplifying assumptions, give the ratio of the change in minimum horizontal stress,
S
h
, to the change in pore pressure,
P
, in terms of sediment properties for each mechanism. In spite of the approximations introduced by the assumptions, these relationships may be useful for discriminating between the mechanisms that control the reservoir stress path. For the Norwegian chalk oilfields, it is important to know whether normal faulting, in particular, is the governing mechanism because slip on active faults can shear well casings, and active faulting and fracturing can increase reservoir permeability. Previously reported field observations and laboratory measurements on chalk samples are compared to infer the mechanisms governing the reservoir stress path for the Ekofisk and Valhall fields. The amount of subsidence at the seabed observed at Ekofisk is evidence that the weaker horizons within the reservoirs are yielding plastically through pore collapse. Nevertheless, the reservoir stress path corresponds to that expected for poroelastic behaviour or normal faulting, and not that expected for plastic yielding.
Seismic characteristics of fluid leakage from an underfilled and overpressured Jurassic fault trap in the Norwegian North Sea
Authors: Teige Gunn M. G. , Hermanrud Christian
Published: October 2008

Pre-drill estimates of hydrocarbon column heights are often uncertain, particularly where filling to structural spill point is questioned. Identification of locations where vertical leakage is concentrated can allow more reliable hydrocarbon column height predictions. The locations of vertical leakage were sought on seismic data over an underfilled and overpressured trap (35/10-2) in the Norwegian North Sea. It was hoped that one single location or one single narrow leakage zone coinciding with the gas–water contact would be found. If such a location or zone existed, it was expected to be in or above a fault plane, as leakage in the 35/10-area is thought to mainly result from shear failure along faults.

The investigation found a zone with pronounced dimming above a triple fault intersection bounding the 35/10-2 structure. This zone stretches further downdip and is positioned above a fault plane. The shallowest part of this zone intersects the top of the reservoir at the depth of the proven gas–water contact. These observations suggest that a discrete leakage zone, of which the shallowest part controls the column height of the structure, has been identified and that the column height is limited by leakage resulting from shear failure. The occurrence of similar seismic features over undrilled structures can lead to safer assessments of hydrocarbon column heights, especially if the observations are consistent with the general knowledge of stress state and leakage processes in the area.
Hydrocarbon leakage processes and trap retention capacities offshore Norway
Authors: Nordgård Bolås Hege M. , Hermanrud Christian
Published: October 2008

Associations between high overpressures and sparse hydrocarbon occurrence are commonly ascribed to hydrocarbon leakage through pressure-induced fractures in the cap rock. However, several hydrocarbon traps in the North Viking Graben area in the North Sea still contain abundant commercial volumes of hydrocarbons at very high pore pressures. By contrast, a majority of the overpressured structures at the Halten Terrace further to the north have leaked hydrocarbons, even at considerably lower overpressures.

A selection of wells in the North Viking Graben and the Halten Terrace areas was investigated to find possible explanations for these observations. Distinct regional differences emerged, as the emptied reservoirs at the Halten Terrace generally have higher retention capacities than the overpressured discoveries in the North Viking Graben area. Thus, there appears to be a lack of any clear relationship between structures emptied of hydrocarbons and low retention capacities, which could be expected if pressure-induced fracturing of the cap rock was the main process of the hydrocarbon leakage.

The regional differences in retention capacities were mainly attributed to different leakage processes in the two basins. Stress history variations are suggested to be the main controlling factor of these leakage processes.
Sand-grade density flow evolution on a shelf-edge–slope–basin-floor complex in the Upper Jurassic Olympen Formation, East Greenland
Authors: Bruhn Rikke , Surlyk Finn
Published: October 2008

The uppermost Callovian–Middle Oxfordian Olympen Formation is part of the early syn-rift succession of Jameson Land and Traill Ø, East Greenland and was deposited in prograding shelf-edge, slope and basin-floor environments. The formation consists of: (1) coarsening-upward units of sand-rich, prograding shelf-edge wedges; (2) slope and basin-floor mudstone; and (3) lenticular elongate and sheet-like, massive sandstone bodies deposited by sedimentary density flows on the slope, base-of-slope and basin floor. Systematic spatial and temporal variations in sandstone body geometry reflect down-slope transformations of the density flows. Lenticular and down-slope elongate sandstones mainly occur randomly scattered within prograding shelf-edge sandstone and slope mudstone. They represent short-travelled, hyperconcentrated density flows, filling pre-existing gullies that formed by retrogressive slumping. Sheet-like sandstones encased in mudstone were deposited from unconfined, concentrated density flows transitional to turbidity flows at the base-of-slope and on the basin floor. The Olympen Formation provides a well-exposed example of a sand-dominated shelf-edge–slope–basin-floor dispersal system in which the down-slope transformation of sand-grade density flows and resultant sandbody geometries can be demonstrated effectively. It may serve as a useful field analogue for a poorly understood but important type of hydrocarbon reservoir.
Quantitative assessment of mudstone lithology using geophysical wireline logs and artificial neural networks
Authors: Yang Yunlai , Aplin Andrew C. , Larter Steve R.
Published: October 2008

Using a dataset of 530 analysed mudstone samples from 19 North Sea and 9 Gulf of Mexico wells, back propagation artificial neural networks (ANNs) have been trained to estimate the clay content (proportion of particles smaller than 2 μm diameter), grain density and total organic content (TOC) of mudstones from standard wireline log data (gamma, resistivity, sonic, density, calliper). ANNs have also been trained to discriminate carbonates from clastic mudstones and also give a preliminary indication of the extent to which mudstones are lithified or cemented. Results show that for clay content, 85% of predictions are within ±10% of the measured value; for TOC, 92% of predictions are within ±1% of the measured value; for grain density, 91% of predictions are within ±0.07 g cm
−3
of the measured value; for the discrimination of carbonates from clastics, 98.3% of carbonate samples and 99.9% of non-carbonate samples are classified correctly. The ANNs work well not only in the areas from where training data were measured, but also (as an example) in offshore West Africa. Potential applications of the technique include (1) the possibility to define the 3D sedimentary architecture of mudstone sequences from wireline data and, because both the porosity–effective stress and porosity–permeability relationships of mudstones are strongly influenced by clay content, (2) more accurate, basin-scale fluid flow modelling.
Indicators of hot fluid migration in sedimentary basins: evidence from the UK Atlantic Margin
Authors: Wycherley H. L. , Parnell J. , Watt G. R. , Chen H. , Boyce A. J.
Published: October 2008

Microthermometric, petrographic and isotopic methods have been used to detect evidence for hot fluid flow in Mesozoic and Tertiary sediments from the NW UK continental margin, West of Shetland. New data presented here show that temperatures are hotter by
c
. 40°C in Tertiary samples than in the underlying Jurassic and Cretaceous sediments in wells 204/28-1, 206/5-2, 208/27-1, especially in cements from samples as young as mid–upper Eocene in age. Paleocene samples can be discriminated from older (Jurassic and Cretaceous) and younger (Eocene) sandstones on the basis of silica cement morphology and cathodoluminescence zonation. Jurassic, Cretaceous and Eocene quartz cements show oscillatory zoning as a consequence of relatively slow burial cementation. In direct contrast, rapid precipitation of silica cements from the cooling of hot fluids has produced unzoned cements in all but one Paleocene sample. No evidence for unzoned quartz cements was noted in any pre-Paleocene or Eocene samples. The restriction of hot fluid inclusions and unzoned cements to the Paleocene and post-Paleocene is consistent with lateral focusing of hot fluids. Isotopic data from kaolinites indicate that these fluids are best represented by mixtures of Mesozoic or Tertiary meteoric waters and marine porewaters that have undergone isotopic alteration through interaction with volcanic material. Our results indicate that hot fluid flow occurred over a relatively long time-scale (i.e. several million years), which may have important consequences for the degradation of reservoired hydrocarbons in West of Shetland Paleocene plays.
Modelling of hydraulic leakage by pressure and stress simulations and implications for Biot's constant: an example from the Halten Terrace, offshore Mid-Norway
Authors: Lothe A. E. , Borge H. , Gabrielsen R. H.
Published: October 2008

A coupled pressure and stress simulator has been applied to the reservoir sandstone unit, the Middle Jurassic Garn Formation in the Halten Terrace, offshore Mid-Norway, in order to simulate hydraulic fracturing and leakage due to overpressure. The over- and underlying rocks are used as vertical seals to the compartments. The overpressure generation within the pressure compartments was modelled quantifying the mechanical and chemical compaction. An empirical model for the minimum horizontal stress was applied to the Griffith–Coulomb failure criterion to estimate the pressure levels at which hydraulic fracturing occurs.

Sensitivity tests of Poisson's ratio and Young's modulus, which control Biot's constant, show an effect on the accumulations of overpressure during time. Defining Biot's constant equal to 1 results in too early pressure build up and hydraulic leakage in the western parts of the basin. Lower values of Biot's constant (>0.85) give a present-day pressure distribution closer to the measured pressures in wells. The Kristin compartment is simulated to experience hydraulic failure around 1.5 Ma. High values of the coefficient of internal friction (
μ
) and the coefficient of sliding friction (
μ
′`), which are used in the failure criteria, results in a time-delay in failure and, thereby, less leakage from overpressured compartments.
The influence of intraplate structural accommodation zones on delineating petroleum provinces of the Sub-Andean foreland basins
Authors: Jacques John M.
Published: October 2008

The Sub-Andean foreland region is summarized in a series of palaeogeographical maps. Various time-slices show important basin-forming processes and the extent and evolution of principal depositional elements. Based on spatial and temporal changes in palaeodepositional setting, the Sub-Andean region can be subdivided longitudinally into a number of tectonostratigraphic domains. The differential amount of subsidence between two adjacent tectonostratigraphic provinces or sub-provinces has been taken up across, complex, broad, transverse, transcontinental accommodation zones, which probably represent the multiphase reactivation of pre-existing fault systems in the underlying basement. Two predominant sets of basement lineaments are recognized: ENE–WSW and NW–SE. Both sets of lineaments occur as major structural anisotropies throughout the basement rocks of South America, providing zones of weakness, which were repeatedly reactivated and, at least in part, controlled: (a) the geometry of inter- and intra-cratonic rifting; (b) rates of subsidence and uplift along the Andean depositional axis; (c) the position of basin-bounding and intra-basinal highs/arches; (d) the structural geometry of the Andean Deformation Zone, correlating with changes in deformational style and major deflections; and (e) the location of magmatism.

Based primarily on source rock age, it is suggested that these transverse, structural accommodation zones had a profound effect on source rock distribution and hydrocarbon occurrence; subdividing the Sub-Andean region into five petroleum provinces/megasystems: a ‘Northern’ Late Cretaceous (La Luna Formation and equivalents); a ‘Western’ Late Palaeozoic–Early Mesozoic (Permian and Upper Triassic to Lower Jurassic source rocks); a ‘Central’ Palaeozoic (Late Devonian and Silurian); an ‘Eastern’ Tertiary; and a ‘Southern’ Mesozoic.
Well test simulation through Discrete Fracture Network modelling in a fractured carbonate reservoir
Authors: Casciano Claudio , Ruvo Livio , Volpi Bruno , Masserano Franco
Published: October 2008

A Discrete Fracture Network (DFN) model was used to simulate the results of a production test carried out in a well drilled in a tight, fractured carbonate reservoir. Several static DFN models, depicting different geological scenarios, were built based on data from well logs, core analyses, PLT surveys and structural geology studies. Each of these models underwent a validation procedure, consisting of the simulation of the production test. The comparison between the simulated results and the actual data identified the scenario whose results most closely matched the actual well behaviour.

In order to compensate for the lack of geological data, an iterative loop was performed between the static model and the dynamic simulation. Constraints-added flow simulations provided new information for use in modifying the DFN model, resulting in a step-by-step updating of the static model itself. Finally, a geologically sound model accurately matching the results of the production test was obtained. The final DFN model was used to calculate the equivalent petrophysical parameters that were transferred to the corresponding region of the full field dual-porosity fluid flow model.
Influence of burial history on microstructure and compaction behaviour of Kimmeridge clay
Authors: Nygård Runar , Gutierrez Marte , Høeg Kaare , Bjørlykke Knut
Published: October 2008

The paper presents the results of laboratory compaction tests on two samples of the lower Kimmeridge Clay Formation obtained from two different outcrop localities in the UK. The purpose of the testing programme is to study the compaction behaviour of an argillaceous sediment from the same parent material but with different diagenetic histories. Since most available data are for clays subjected to moderate effective stresses (less than 20 MPa), the compaction laboratory tests were carried out to as high as 120 MPa, corresponding to about 9 km burial. The microstructure of the tested materials was studied in a scanning electron microscope before and after compaction testing to evaluate the effects of diagenesis on the microstructure of argillaceous sediments. The two tested materials have, in the past, been subjected to different burial depths and degrees of chemical precipitationand cementation. Several pieces of evidence are shown of the effects of mechanical and chemical processes on the compaction behaviour of argillaceous materials. Mechanical compaction can only account for part of the porosity loss and increase in stiffness of argillaceous materials during burial. Mechanical compaction produces an anisotropic rearrangement of clay particles in uncemented sediments. Once chemical diagenesis has taken place, chemical cementation preserves the microstructure of the sediment and only minor changes in clay particle arrangements take place during further mechanical compaction, even when very high stresses are used. Also, it is seen that chemical processes reduce the permeability more strongly than mechanical compaction.
Uncertainty analysis of fluvial outcrop data for stochastic reservoir modelling
Authors: Martinius A. W. , Næss A.
Published: October 2008

Uncertainty analysis and reduction is a crucial part of stochastic reservoir modelling and fluid flow simulation studies. Outcrop analogue studies are often employed to define reservoir model parameters but the analysis of uncertainties associated with sedimentological information is often neglected. In order to define uncertainty inherent in outcrop data more accurately, this paper presents geometrical and dimensional data from individual point bars and braid bars, from part of the low net:gross outcropping Tortóla fluvial system (Spain) that has been subjected to a quantitative and qualitative assessment. Four types of primary outcrop uncertainties are discussed: (1) the definition of the conceptual depositional model; (2) the number of observations on sandstone body dimensions; (3) the accuracy and representativeness of observed three-dimensional (3D) sandstone body size data; and (4) sandstone body orientation. Uncertainties related to the depositional model are the most difficult to quantify but can be appreciated qualitatively if processes of deposition related to scales of time and the general lack of information are considered. Application of the
N
0
measure is suggested to assess quantitatively whether a statistically sufficient number of dimensional observations is obtained to reduce uncertainty to an acceptable level. The third type of uncertainty is evaluated in a qualitative sense and determined by accurate facies analysis. The orientation of sandstone bodies is shown to influence spatial connectivity. As a result, an insufficient number or quality of observations may have important consequences for estimated connected volumes. This study will give improved estimations for reservoir modelling.
Capillary resistance and trapping of hydrocarbons: a laboratory experiment
Authors: Teige Gunn M. G. , Hermanrud Christian , Thomas Wibeke Hammervold , Wilson Ove Bjørn , Nordgård Bolås Hege M.
Published: October 2008

Low permeability cap rocks retain oil by capillary forces when the pore throats of the seals are sufficiently small to prevent a flux of oil into the cap rock. In order to investigate the influence of aquifer overpressures on oil retention, water pressure was applied to a water-wet, highly permeable (1988 mD) core sample, which was oil-saturated to irreducible water saturation
S
wi
and mounted with a low-permeability and water-wet membrane at the outlet. A water pressure difference of 0.5 MPa was applied across the core. This pressure was high enough to ensure fluid flow through the sample. The experiment was designed to see whether the water pressure would force oil through the membrane or if capillary forces at the sandstone–membrane interface would retain the oil, in which case water flow might take place in the (residual) water in the core and through the membrane.

The experiment showed that oil was kept in place by capillary forces while water flowed through the core and the membrane. Accordingly, residual water can move through sandstones that are saturated to
S
wi
. The experiments also demonstrated that the permeability associated with this residual water is high enough to prevent overpressures in the aquifer below the oil–water contact from pushing oil through a membrane seal. Thus, even for this highly permeable sandstone, the overpressure in the aquifer will not cause capillary seal failure.
The controversy concerning stratigraphic architecture of channelized reservoirs and recovery by waterflooding
Authors: Larue D. K. , Friedmann Francois
Published: October 2008

There is a subtle controversy in the petroleum industry regarding the relationship between reservoir architecture and recovery efficiency from petroleum reservoirs. Stratigraphers tend to believe that facies architecture and geometric shapes strongly govern recovery, whereas engineers stress factors such as permeability heterogeneity and anisotropy. To understand the relationship between architecture and recovery better, a suite of conceptual models of different channelized clastic reservoir architectures was constructed and pore-volume replacement waterflood simulations were performed on each. Three 11-member suites of models were constructed at net: gross values of 35%, 60% and 85%. The reservoir architecture features that were varied were channel width to thickness, thickness, sinuosity, stacking patterns, orientation and reservoir element type (point bar vs. channel). The three sets of models appear visually very different from a reservoir architecture standpoint. Permeability and porosity were simulated using geostatistical techniques but share the same conditioning population. The same water saturation, well count (110 acre spacing), fluid properties and relative permeability were used in each waterflood simulation. In this situation, in which the only significant variable was reservoir architecture, only a few percent spread in recovery was noted for each net: gross suite. Variation in recovery efficiency can be shown to be associated with reservoir connectivity, which is generally high for all models but shows some variation. Additional studies were made to address the influence of well count, mobility ratio, permeability heterogeneity and geostatistical seed number on recovery efficiency.

Two stratigraphic factors are shown to influence recovery efficiency: reservoir connectivity and permeability heterogeneity. Both factors influence the volumetric sweep efficiency of reservoirs. Models that appear to be visually different, but have similar connectivity, well count and permeability heterogeneity, have similar sweep efficiencies and, therefore, recovery efficiencies. The practical importance of this observation for development studies is that building and flow-simulating suites of models with different reservoir architectures may not help to characterize recovery uncertainty unless the models are constructed to emphasize differences in sweep efficiency.
Exhumation of the Corrib Gas Field, Slyne Basin, offshore Ireland
Authors: Corcoran D. V. , Mecklenburgh R.
Published: October 2008

The Corrib Gas Field contains in excess of 10
12
 SCF (28.3×10
9
 Sm
3
) of gas in place in a Triassic fluviatile sandstone reservoir, sealed by late Triassic evaporites and trapped in a relatively simple, but underfilled, NE–SW-trending anticlinal structure. In contrast, the post-Triassic overburden manifests a more complex evolution with indications of flipped polarity, extensional faulting on a crestal detachment structure during the Jurassic, succeeded by alternate episodes of burial and exhumation from the Early Cretaceous onwards. Compactional, thermal and stratigraphic frames of reference have been used to assess the magnitude and timing of the principal exhumation episode, which occurred during the early Cretaceous. Large uncertainty is associated with exhumation estimates derived from individual techniques, but stratigraphic analysis and seismic interpretation support the conclusion that kilometre-scale (800–1700 m) exhumation and erosion of the Jurassic overburden has occurred in the Corrib area. A number of post-Aptian re-burial and exhumation events also occurred but are unlikely to have exceeded the earlier maximum burial of source, reservoir and seal rocks achieved during the Late Jurassic. Continued local extensional reactivation of the listric detachment structure, post-Aptian, has resulted in a pattern of heterogeneous exhumation within the field area. However, the severely attenuated post-rift stratigraphic record throughout the Slyne Basin suggests that long-wavelength processes, such as mantle hotspot activity and shoulder uplift linked to the rifting and thermal subsidence of the Rockall Basin, were the primary drivers of exhumation in the Corrib area during the Cretaceous-Tertiary period.
Sequence stratigraphic-based analysis of reservoir connectivity: influence of depositional architecture – a case study from a marginal marine depositional setting
Authors: Ainsworth R. Bruce
Published: October 2008

This case study of the Sunrise and Troubadour fields (offshore northwest Australia) concentrates on the impact of primary depositional architecture on reservoir connectivity via a sequence stratigraphic-based, 3D reservoir modelling approach. The marginal marine reservoir is composed of fluvial-dominated and wave-dominated depositional environments. The succession is divided into six sequences and 12 systems tracts. Each systems tract is subdivided into parasequences which form the basic building blocks of the 3D model. The connectivity of sandbodies within each parasequence, systems tract and sequence were calculated when the models were palinspastically restored to a depositional datum.

The findings indicate that depositional connectivity trends within a sequence stratigraphic framework are predictable. Connectivity trends can be related directly to depositional and stratigraphic trends and to position in the sequence stratigraphic hierarchy. Therefore, with a good understanding of depositional settings and high resolution sequence stratigraphic subdivision of strata, predictions of depositional connectivity trends at all stratigraphic hierarchical levels can be made. All connectivity trends at sequence and systems tract stratigraphic hierarchical levels remained the same when the area of the model was reduced by a factor of four and the volume of the model was reduced by truncation below the gas–water contact. Hence, the relationships between the stratigraphic trends and the connectivity trends for the thicker stratigraphic units can be said to be scale invariant.

Three-dimensional reservoir models currently provide the best means of quantitatively assessing and predicting reservoir connectivity. However, in low fault density settings, initial screening of reservoir connectivity can be made using connectivity indicators calculated from simple parameters derived from well data such as ‘Thickness divided by Sand/Shale ratio’.