Vissers R.L.M.,University Utrecht |
van Hinsbergen D.J.J.,University Utrecht |
van der Meer D.G.,University Utrecht |
van der Meer D.G.,Nexen Petroleum UK Ltd |
And 2 more authors.
Gondwana Research | Year: 2016
The Pyrenees at the Iberia-Europe collision zone contain sediments showing Albian-Cenomanian high-temperature metamorphism, and coeval alkaline magmatic rocks. Stemming from different views on Jurassic-Cretaceous Iberian microplate kinematics, two schools of thought exist on the trigger of this thermal pulse: one invoking hyperextension of the Iberian and Eurasian margins, the other suggesting slab break-off. Competing scenarios for Mesozoic Iberian motion compatible with Pyrenean geology, comprise (1) transtensional eastward motion of Iberia versus Eurasia, or (2) strike-slip motion followed by orthogonal extension, both favoring hyperextension-related heating, and (3) scissor-style opening of the Bay of Biscay coupled with subduction in the Pyrenean realm, favoring the slab break-off hypothesis. We test these kinematic scenarios for Iberia against a newly compiled paleomagnetic dataset and conclude that the scissor-type scenario is the only one consistent with a well-defined ~. 35° counterclockwise rotation of Iberia during the Early Aptian. We proceed to show that when taking absolute plate motions into account, Aptian oceanic subduction in the Pyrenees followed by Late Aptian-Early Albian slab break-off should leave a slab remnant in the present-day mid-mantle below NW Africa. Mantle tomography shows the Reggane anomaly that matches the predicted position and dimension of such a slab remnant between 1900 and 1500 km depth below southern Algeria. Mantle tomography is therefore consistent with the scissor-type opening of the Bay of Biscay coupled with subduction in the Pyrenean realm. Slab break-off may thus explain high-temperature metamorphism and alkaline magmatism during the Albian-Cenomanian in the Pyrenees, whereas hyperextension that exhumed Pyrenean mantle bodies occurred much earlier, in the Jurassic. © 2016 The Authors.
Biggin A.J.,University of Liverpool |
Steinberger B.,Helmholtz Center Potsdam |
Steinberger B.,University of Oslo |
Aubert J.,CNRS Paris Institute of Global Physics |
And 8 more authors.
Nature Geoscience | Year: 2012
The Earth's internal magnetic field varies on timescales of months to billions of years. The field is generated by convection in the liquid outer core, which in turn is influenced by the heat flowing from the core into the base of the overlying mantle. Much of the magnetic field's variation is thought to be stochastic, but over very long timescales, this variability may be related to changes in heat flow associated with mantle convection processes. Over the past 500 Myr, correlations between palaeomagnetic behaviour and surface processes were particularly striking during the middle to late Mesozoic era, beginning about 180 Myr ago. Simulations of the geodynamo suggest that transitions from periods of rapid polarity reversals to periods of prolonged stability - such as occurred between the Middle Jurassic and Middle Cretaceous periods - may have been triggered by a decrease in core-mantle boundary heat flow either globally or in equatorial regions. This decrease in heat flow could have been linked to reduced mantle-plume-head production at the core-mantle boundary, an episode of true polar wander, or a combination of the two. © 2012 Macmillan Publishers Limited. All rights reserved.
Van Der Meer D.G.,University Utrecht |
Van Der Meer D.G.,Nexen Petroleum UK Ltd. |
Torsvik T.H.,University of Oslo |
Torsvik T.H.,Geological Survey of Norway |
And 7 more authors.
Nature Geoscience | Year: 2012
The vast Panthalassa Ocean once surrounded the supercontinent Pangaea. Subduction has since consumed most of the oceanic plates that formed the ocean floor, so classic plate reconstructions based on magnetic anomalies can be used only to constrain the ocean's history since the Cretaceous period 1,2, and the Triassic-Jurassic plate tectonic evolution of the Panthalassa Ocean remains largely unresolved 3,4. Geological clues come from extinct intra-oceanic volcanic arcs that formed above ancient subduction zones, but have since been accreted to the North American and Asian continental margins 4. Here we compile data on the composition, the timing of formation and accretion, and the present-day locations of these volcanic arcs and show that intra-oceanic subduction zones must have once been situated in a central Panthalassa location in our plate tectonic reconstructions 5-7. To constrain the palaeoposition of the extinct arcs, we correlate them with remnants of subducted slabs that have been identified in the mantle using seismic-wave tomographic models 8,9. We suggest that a series of subduction zones, together called Telkhinia, may have defined two separate palaeo-oceanic plate systems - the Pontus and Thalassa oceans. Our reconstruction provides constraints on the palaeolongitude and tectonic evolution of the Telkhinia subduction zones and Panthalassa Ocean that are crucial for global plate tectonic reconstructions and models of mantle dynamics. © 2012 Macmillan Publishers Limited. All rights reserved.
Tambach T.J.,TNO |
Koenen M.,TNO |
Wasch L.J.,TNO |
van Bergen F.,TNO |
van Bergen F.,Nexen Petroleum UK Ltd
International Journal of Greenhouse Gas Control | Year: 2015
The short- and long-term geochemical impact of CO2 injection into a depleted gas reservoir (DGR) is investigated using reservoir/geochemical modeling with TOUGH2/TOUGHREACT and 1D kinetic diffusion modeling with PHREEQC (caprock/well-cement). Simulations of CO2 injection into the reservoir predict displacement and buoyancy of post-production CH4, as well as dry-out of the near-well zone. We computed that the areal extent of the CH4/brine dominated zone and the dry-out zone are relatively small compared to the CO2/brine dominated zone after well-closure. For the current DGR model we therefore conclude that it is reasonable to model geochemical reactions in the reservoir without taking into account post-production CH4. Although the CO2 dissolution capacity of the studied DGR is smaller compared to a deep saline aquifer of similar size, the modeling predicts that dissolution and subsequent CO2 mineral trapping proceed faster. Precipitation of dawsonite and magnesite were yet predicted at initial CO2 partial pressure (PCO2) of 9.3bar, while these minerals were not identified in reservoir samples. This could indicate that their tendency of precipitation is overestimated by the model and hence the database used. This has significant impact on long-term modeled bulk porosity and PCO2. Simulations of CO2 diffusion through the caprock show that mineral reactions significantly retard the total dissolved carbon (TDC) plume. After 10,000 years, 99% of the TDC is present within the first 6.4m above the reservoir contact. The progression of the TDC plume in the caprock is sensitive to the composition, kinetic rates, and surface area of primary and secondary minerals. Cement alteration modeling shows progressive carbonation of cement phases, resulting in three zones of distinct mineralogy and porosity. The three zones are predominantly characterized by: (i) unaltered cement, (ii) portlandite dissolution, and (iii) calcite precipitation. The simulated thickness of the affected zone is 3.8cm after 100 years. This distance is sensitive to kinetic rate constants of C-S-H phases, but less sensitive to kinetic rate constant of portlandite. In summary, our applied methodology provides quantitative predictions of the geochemical impact of CO2 on the DGR storage complex. The methodology can be used for screening of potential DGR storage locations and to define criteria for minimal caprock and cement sheet thickness, for assuring short- and long-term integrity of the storage location. © 2014 Elsevier Ltd.
Van der Merwe W.C.,University of Liverpool |
Van der Merwe W.C.,Nexen Petroleum UK Ltd |
Hodgson D.M.,University of Leeds |
Brunt R.L.,University of Manchester |
Flint S.S.,University of Manchester
Geosphere | Year: 2014
The geomorphology and seismic stratigraphy of deep-water clastic systems from slope valleys through channel-levee systems to basin-floor fans have been observed and described in modern and ancient sub surface examples around the world. However, the distribution of sedimentary facies, grain size, and small-scale architectural elements remains poorly constrained. Extensive exposures (>2500 km2) of four stacked deep-water composite sequences have been mapped from heterolithic channel-levee systems on the slope to sand-rich basin-floor deposits. The data set from Units C-F of the Fort Brown Formation in the Permian Laingsburg depocenter of South Africa permits a unique opportunity to document and compare their depositional architecture at a high resolution for tens of kilometers downdip. Isopach thickness maps indicate that compensational stacking across multiple stratigraphic scales occurs on the basin floor, whereas preferred axial pathways were present on the slope, leading to subvertical stacking patterns. Units C and D are sand-attached systems; slope valley systems are mapped to pass transitionally downslope through leveeconfined channels to lobe complexes over distances of >30 km. The slope valley fills of Units E and F, however, are separated from their downdip sand-rich lobe complexes by a thin, sand-poor tract several kilometers in length and are termed sand detached. Locally, this sand-poor tract is characterized by a distinctive facies association of thin-bedded turbidites with numerous scours mantled with rip-up clasts, and a top surface that includes megaflutes and remobilized sediments. This assemblage is interpreted to indicate a widespread area of sand bypass. This unique data set provides an exploration- scale insight and understanding of how different segments of a prograding slope evolved over time in terms of gradient, physiography, and hence the degree to which sand was stored or bypassed to the basin floor, and the evolution from sand-attached to sand-detached systems. The development of sand-detached systems suggests that a steeper gradient formed, possibly related to developing underlying structure, that led to the development of a stepped slope profile. The study highlights that updip stratigraphic trapping at reservoir scale can occur with minor bathymetric changes. © 2014 Geological Society of America.
Adams A.J.,Nexen Petroleum UK Ltd |
Gibson C.,Nexen Petroleum UK Ltd |
Smith R.,Nexen Petroleum UK Ltd
SPE Drilling and Completion | Year: 2010
Probabilistic estimation of well duration has been common practice for more than a decade; many papers have been written on the subject, and commercial software is available for the purpose. Is the subject, therefore, mature? The authors suggest that this is not the case and show that several essential aspects of both data characterization and probabilistic analysis have been overlooked in previous contributions. In 2007, Nexen began a study with the objective of improving our process for well-time estimation. Its workscope was to Assemble a statistically significant historical well database. Develop a consistent definition of nonproductive time (NPT) as it relates to the original approved-for-expenditure (AFE) timings. Reanalyze the historical well database for NPT from the original daily drilling reports (DDRs), on the basis of the definition above. Decide how NPT is best characterized (that is, determine the correct choice of input variables). Determine the associated occurrence frequencies and probability-density-function (PDF) parameters from the historical database. Validate the probabilistic model, by comparison of program predictions against the historical data set. Work with a software provider to implement any necessary changes in a commercially available product. A database of 118 central North Sea wells was reanalyzed independently for NPT from the original DDRs. These considerably underestimated the true NPT, by 19.7% on average. Train wrecks (mechanical NPT events of more than 2.5 days) were only 4% by number but contributed 50% of NPT by duration. It was found that "ordinary" mechanical NPT, train wrecks, waiting on weather (WOW) in open water, and WOW with riser connected are all statistically distinct, with very different occurrence frequencies and PDFs. Earlier workers did not observe this distinction or properly validate their models against the well database. Therefore, it is impossible to obtain historically accurate probabilistic well-time predictions (i.e., that are consistent with the historical database) using the previous state of the art. Moreover, such predictions will generally be underestimates. This paper describes an accurate method that overcomes previous limitations. While central North Sea data are used, the analysis techniques are not area specific, and the method may be applied easily to other areas in the oil field. Copyright © 2010 Society of Petroleum Engineers.
Adams A.J.,Nexen Petroleum UK Ltd
SPE Projects, Facilities and Construction | Year: 2011
For more than 30 years, the design of platform and jackup conductors has been based on Stahl and Baur's famous hypothesis that internal string loads do not contribute to buckling (Stahl and Baur 1983). It is a vital result, allowing significant weight and cost savings. However, no derivation was ever given, and the result has remained a folk theorem: widely used, but never proved. The industry has, therefore, been at risk should the result prove to be a severe approximation, or to have unduly restrictive assumptions and/or limitations. This paper provides a rigorous proof of the hypothesis. It shows that it is an approximation, though an acceptable one, and gives a thorough exposition of its meaning, assumptions, and limitations. Finally, it derives the exact counterpart of Stahl and Baurs' result. The improved result gives minor weight and cost savings. Copyright © 2011 Society of Petroleum Engineers.
Adams A.J.,Nexen Petroleum UK Ltd |
Grundy K.C.,Nexen Petroleum UK Ltd |
Kelly C.M.,Nexen Petroleum UK Ltd
SPE Drilling and Completion | Year: 2016
In 2010, Adams et al. published a paper on well-time estimation, which for the first time allowed accurate calculation of probabilistic duration for noncritical wells. The present paper extends the method and data coverage to high stepout and high-pressure- high-temperature (HP/HT) wells. The method has now been in use for 5 years. Actual and predicted durations are given for each year's drilling, showing that the accuracy is historically within 2 to 3%for stable trainwreck rates. The historical well database upon which the statistics are based now stands at 211 wells, 93 more than the previous paper. To the authors' knowledge, it remains the only large-sample timings database published in the open literature. To allow others to use the method, the updated activity timings, mechanical nonproductive time (NPT), and waiting-on-weather (WOW) data for semisubmersible-drilled wells are given in full. It is shown that the commonly used distributed trainwrecks model has too high a sampling uncertainty for accurate time estimation. The lumped trainwrecks approach presented here does not suffer from this limitation, and is therefore (to the authors' knowledge) the only published method that delivers the required accuracy for practical data-set sizes. © 2016 Society of Petroleum Engineers.
Adams A.J.,Nexen Petroleum U.K. Ltd |
Grundy K.C.,Nexen Petroleum U.K. Ltd |
Kelly C.M.,Nexen Petroleum U.K. Ltd
SPE/IADC Drilling Conference, Proceedings | Year: 2015
In 2010, Nexen UK published a paper on well time estimation (Adams et al. 2010), which for the first time allowed accurate calculation of probabilistic duration for non-critical wells. The present paper extends the method and data coverage to high step-out and HPHT wells. The method has now been in use at Nexen for five years. Actual and predicted durations are given for each year's drilling, showing that the accuracy is historically within 2-3% for stable train-wreck rates. The historical well database upon which the statistics are based now stands at 190 wells, 72 more than the previous paper. To the authors' knowledge, it remains the only large-sample timings database published in the open literature. To allow others to use the method, the updated activity timings, mechanical NPT and waiting on weather data for semi-sub drilled wells are given in full. It is shown that the commonly used distributed train-wrecks model has too high a sampling uncertainty for accurate time estimation. The lumped train-wrecks approach presented here does not suffer from this limitation, and is therefore the only method that delivers the required predictive accuracy for practical dataset sizes. Copyright 2015, SPE/IADC Drilling Conference and Exhibition.
Adams A.J.,Nexen Petroleum U.K. Ltd
SPE/IADC Drilling Conference, Proceedings | Year: 2011
For over thirty years, the design of platform and jack-up conductors has been based on Stahl and Baurs' famous hypothesis that internal string loads do not contribute to buckling (Stahl and Baur 1980). It is a vital result, allowing significant weight and cost savings. However, no derivation was ever given, and the result has remained a folk theorem: widely used, but never proved. The industry has therefore been at risk should the result prove to be a severe approximation, or to have unduly restrictive assumptions and/or limitations. This paper provides a rigorous proof of the hypothesis. It shows that it is an approximation, albeit an acceptable one, and gives a thorough exposition of its meaning, assumptions and limitations. Finally, it derives the exact counterpart of Stahl and Baurs' result. The improved result gives further minor weight and cost savings. Copyright 2011, SPE/IADC Drilling Conference and Exhibition.