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- Volume 1, Issue 1, 1995
Petroleum Geoscience - Volume 1, Issue 1, 1995
Volume 1, Issue 1, 1995
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The use of well seismic profiles and LWD to steer a horizontal well
Authors A. Dobb, G. Robertson, J. M. Reilly and A. J. PurpichThe 9/13a-A53 sub-horizontal well was drilled from the Beryl Alpha platform in order to target bypassed oil reserves and produce low GOR oil. This paper outlines the work of the multi disciplinary team from the planning through to production. The primary producing unit in the Beryl Field is the Middle Jurassic Beryl Formation. In the Central Area of Beryl Alpha, re-injected gas has preferentially moved down structure in the higher permeability upper Beryl Formation sandstones, overriding a substantial volume of oil. Well 9/13a-A53 was designed to target this oil. Innovative data acquisition strategies were employed during the drilling of the well. The approach to the well target necessitated crossing a 150 ft fault and sub-seismic faults/fractures were anticipated within the wellbore. No pilot hole was planned, instead an intermediate look-ahead' well seismic profile (WSP) was run to determine the position of the fault prior to drilling. Real-time logging while drilling (LWD) was incorporated in the drilling assembly to pick stratigraphic markers and react to possible directional drilling changes. This detailed planning resulted in the well being drilled on prognosis. In the well completion, a cemented liner was landed to allow selective perforation, away from potentially gas-conductive fracture zones. A monobore completion was utilized to improve future gas isolation operations and to optimize deliverability. The well came on-line in late 1992 and is currently producing low GOR oil at commercial rates.
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Challenges in reservoir engineering from prospects for horizontal wells
Authors F. J. Fayers, S. Arbabi and K. AzizIn this review paper, a variety of reservoir applications are illustrated where horizontal wells can have advantages over the use of conventional vertical wells. It is stressed that one of the key advantages relates to the opportunities to optimize the orientation and position of horizontal wells with respect to the principal directions for the reservoir depositional environment, but this may interact with natural fracture or fault directions, and the principal stress direction if hydraulic fracturing is to be considered. Analytical methods for calculating critical coning rates in homogeneous reservoirs are reviewed, and shown to give a very large range of results for horizontal wells. The potential significance of two-phase pressure drop within the wellbore on GOR performance is discussed, and a range of uncertainty by a factor of six is indicated between the use of various correlations for calculating the well pressure drop. In the final section studies are summarized for a gas coning application using ECLIPSE, a commercial simulator. The simulation results indicated an apparently invariant behaviour on GOR history with respect to the effects of wellbore twophase pressure drop when the horizontal well was produced at constant rate in a homogeneous reservoir. However, when the controlling conditions on the horizontal well were made more representative, and reservoir non-uniformity was introduced, it was then found that the two-phase pressure drop became very significant. Some areas for further research are indicated.
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Prediction and evaluation of horizontal well performance; North Slope, Alaska
More LessSeven years of field production data from a horizontal well programme in Alaska are reviewed. Expectations and analyses prior to drilling are compared with extensive efforts to understand well performance once on production. Evaluation of well performance is considered through well testing, production logging and reservoir simulation. The review concludes that accurate reservoir description is critical to successful forecasting; simplistic description can lead to overestimation of productivity. Furthermore, innovative reservoir characterization techniques may be necessary to utilize a complex description properly in predictive mode. Production logging is highlighted as a critical diagnostic tool, despite the need to run on coiled tubing. In contrast, pressure transient analysis, for which theory is well developed, can be problematic and may require modification to traditional field practices.
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Crystalline basement focusing of heat versus fluid flow/compaction effects; a case study of the I-1 well in the Danish North Sea
Authors Z. Yu, R. O. Thomsen and I. LercheThe temperature distribution in the Danish Central Graben contains many local thermal anomalies, with the thermal gradient varying throughout the area from around 20 degrees Ckm (super -1) to about 55 degrees Ckm (super -1) . The presence of sub-surface salt structures of various shapes and sizes is responsible for many of the anomalies. In the case of the 1-1 well, however, four components can contribute to the observed thermal anomaly. (1) crystalline basement focusing of heat; (2) convection of heat from the deeper basin; (3) a local thermal anomaly due to salt or structure effects; and (4) the possible spatial variation of basement heat flux with position relative to the graben flank. While components (1), (2) and (3) do contribute to the thermal anomaly at the 1-1 well, they are not large enough to account for the anomaly without the presence of a local increase in basement heat flux of around 0.5 HFU (c. 21mWm (super -2) ).
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Geology, geometry and effective flow
Authors G. E. Pickup, P. S. Ringrose, P. W. M. Corbett, J. L. Jensen and K. S. SorbieThe effects of small-scale structure are frequently ignored in reservoir simulation, although they may have a significant effect on hydrocarbon recovery. Many sandstones exhibit lamination, and in such rock structures, permeability may vary by an order of magnitude over distances of a centimetre or less. Frequently, laminations are inclined with respect to the pressure gradient, which gives rise to cross-flow within the unit. In this case a tensor may be used to represent the effective permeability of the bed. In this paper the effect of small-scale geological structure on single-phase flow is investigated using a model of a cross-bedded unit to determine which factors have the greatest effect on cross-flow. In addition, a range of other types of bedding are considered, including models with stochastic variation. We have found that, in general, the geometry of the sedimentary structure has a significant effect on flow. The cross-flow is greater (and therefore tensors are more likely to be necessary) when the angle of the laminae is large relative to the pressure gradient (up to 45 degrees ), when the permeability contrast between laminae is large, or when the structure is asymmetric.
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Geological modelling of a reservoir analogue; Cenozoic meander belts, Loranca Basin, Spain
Authors M. Diaz-Molina, J. Arribas, J. J. Gómez and A. TortosaThe reconstruction of meander belts was approached by detailed geological mapping, the interpretation of exposed sandstone bodies and the application of sedimentological concepts. The single reconstructed meander belts were integrated, considering stratigraphic position and thickness in a three-dimensional model of the depositional architecture. Some useful reservoir parameters, such as volume of meander loops, areas of interconnection and fluid flow barriers, are calculated from the obtained model. Sandstone porosity is mainly of primary origin and mechanical compaction is the most intense diagenetic feature. Models of estimated permeability evolution in the point bar sequences have been developed. Gamma ray readings from different lithologies were obtained and grouped according to their palaeoenvironmental classification. The results are comparable to well logs and aid subenvironmental characterization of continental deposits.
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Lithology and fluid prediction from seismic and well data
Authors Andrew W. H. Bunch and Peter W. DromgooleWe can define five measurable properties from surface seismic. Reflection time and continuity give structure and compartmentation information. The third, compressional rock velocity, comes from seismic velocity analysis . The fourth, acoustic impedance, is measured from reflection amplitudes and the fifth, Poisson's ratio, from seismic amplitude variation with offset (AVO) analysis. Petrophysical analysis relates these seismic properties to rock properties such as lithology, fluid content, porosity, net pay and permeability in the reservoir, and is used to predict lateral variations in rock properties. To predict and map reservoir properties from seismic data requires high quality, properly processed and interpreted seismic, core and well log data. Integration of all these data and close co-operation between all disciplines are the keys to success.
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The prediction of high porosity chalks in the East Hod Field
Authors Simon J. D. Campbell and N. GravdalThe East Hod Field in the Norwegian Block 2/11 produces from the Lower Tertiary and Upper Cretaceous Chalk Group. Although at first sight the trapping mechanism of the East Hod Field appears to be structural, increased understanding of porosity preservation, combined with new seismic interpretation, demonstrates that there is also a strong stratigraphic component. Both autochthonous and allochthonous facies occur, the latter forming the principal reservoirs. The extremely homogeneous nature of the matrix means that variations in seismic response directly reflect changes in pore volume, and a good correlation between seismic impedance and porosity is observed. High quality 3D seismic data made it possible to undertake seismic inversion which, together with the correlation between seismic impedance and porosity, allows prediction of porosity variations within the inter-well volume. The results indicate that within the allochthonous reservoir units, porosity is maintained in down-flank areas at depths previously believed to be predominantly tight. Seismic inversion is particularly accurate for predicting porosity because of the exceptionally uniform nature of the chalk matrix.
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Experimental models of strike-slip tectonics
Authors P. D. Richard, M. A. Naylor and A. KoopmanWe present a variety of models for strike-slip tectonics which represent a useful database for seismic interpreters and geologists. In pure strike-slip, the Riedel shear geometry is shown to depend on the initial stress state, interference of parallel basement faults and horizontal layering of the overburden. Above two parallel basement faults, a single wide or two separate fault zones may be mapped, depending on the depth of observation. In a heterogeneous layered sequence, upwards branching of Riedel shears occurs at layer interfaces. In oblique-slip faulting, the sense of vertical displacement and the geometry of the fault pattern are indicative of the tectonic regime. The degree of obliquity of the fault strike can be related to the ratio of dip-slip to strike-slip movement. In the case of relay structures, the ratio of the length of basement-fault offset to the thickness of the overburden controls the geometry of the fault pattern.
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Petroleum potential of the North Hungarian Oligocene sediments
More LessAs part of Eoparatethys, a partly isolated sea existed to the north of the present-day Pannonian Basin during Oligocene time. From Lattorfian times thick shale sequences, rich in organic matter, were deposited. Locally, laminates of euxinic facies formed (Tard Clay Formation) and are the best oil-prone source rocks of the Oligocene sequence. Although oil discoveries were made in the basin in the late 1930s, exploration was prevented by the complicated tectonics. Currently, detailed seismic, geological and geochemical studies have been initiated, the first results of which are summarized in this paper. Rock-Eval pyrolysis data and bituminological analysis show the Tard Clay Formation to be a good source rock. Its thermal maturity has been influenced by subsequent tectonic movements and Neogene heat flux. Migration and accumulation of the generated hydrocarbons is controlled by unconformities and, in particular, by the tectonic elements. Complicated structural and stratigraphical traps are covered by the Pliocene basal marl.
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Petroleum geology of the Carpathian Foredeep and overthrust zones in the Czech Republic
More LessThe area is situated at the contact of two regional megastructures -the Bohemian Massif and the Western Carpathians. The eastern slope of the Bohemian Massif dips under the Carpathian Foredeep and thick Flysch nappes and is covered by Variscan and Neoidic autochthonous Palaeozoic, Mesozoic and Palaeogene sediments. In the southeast, the Vienna basin's Neogene fill overlies the units of the Magura Flysch and a part of the Zdanice unit. The area can be divided into three parts. The south is characterized by a thick Jurassic sedimentary complex that overlies the crystalline basement. The basal elastics of this Jurassic complex are very prospective and in the central part of the area the oil field Uhrice II is situated in these strata. In this central part two deep depressions - the Vranovice and Nesvacilka grabens - are the most important buried geomorphological elements. Both are filled with Palaeogene strata. The north of the area is characterized by uplifted crystalline basement with thin autochthonous sediments, mostly with Miocene strata of the foredeep and with the flysh nappes. Thick Palaeogene and Upper Jurassic pelites are supposed to be the main source rocks. The cap rocks are pelitic sequences of variable thicknesses including Jurassic marls, Palaeogene and Miocene claystones and basal claystones of flysch units. Traps are delimited by faults, lithology changes and unconformities.
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Volumes & issues
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Volume 30 (2024)
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Volume 29 (2023)
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Volume 28 (2022)
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Volume 27 (2021)
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Volume 26 (2020)
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Volume 25 (2019)
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Volume 24 (2018)
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Volume 23 (2017)
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Volume 22 (2016)
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Volume 21 (2015)
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Volume 20 (2014)
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Volume 19 (2013)
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Volume 18 (2012)
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Volume 17 (2011)
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Volume 16 (2010)
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Volume 15 (2009)
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Volume 14 (2008)
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Volume 13 (2007)
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Volume 12 (2006)
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Volume 11 (2005)
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Volume 10 (2004)
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Volume 9 (2003)
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Volume 8 (2002)
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Volume 7 (2001)
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Volume 6 (2000)
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Volume 5 (1999)
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Volume 4 (1998)
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Volume 3 (1997)
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Volume 2 (1996)
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Volume 1 (1995)