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- Volume 35, Issue 1, 2017
First Break - Volume 35, Issue 1, 2017
Volume 35, Issue 1, 2017
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Statics: from imaging to interpretation pitfalls and an efficient way to overcome them
Authors N. Nosjean, F. Hanot, J.P. Gruffeille and F. MiquelisThe only purpose of static correction is to compensate for wave propagation heterogeneities in the near-surface. Significant progress has been made in seismic imaging and velocity model building, e.g. surface wave inversion, and one might think that problems related to statics were now behind us. In this paper, we will first describe key remaining statics-related issues, and then we will present, through a 2D seismic line case study, a reliable and efficient way to solve those issues. We will detail a statics method computation strongly supported by geological modelling for primary statics and geophysical tools for residual statics.
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A new methodology for estimating field horizontal stress from microseismic focal mechanisms
Authors Alireza Agharazi, Peter Duncan and Michael ThorntonThe full stress state in a formation is characterized by the directions and magnitudes of the three principal stresses. It is common practice to take the vertical stress, ϭv, and the minimum and maximum horizontal stresses, ϭhmin and ϭHmax, as the principal stresses in unconventional reservoirs. Considering the relatively high depth of most unconventional reservoirs, which results in a very high vertical stress magnitude, this assumption holds true in most cases unless a geological feature such as a fault or fold changes the stresses locally. The magnitude of vertical stress can easily be determined by integrating the density of overburden rocks from density logs. A good estimate of ϭhmin magnitude can be obtained from well test results such as a mini-frac test or a diagnostic fracture injection test. The magnitude of ϭHmax, however, remains the most challenging stress component to determine.
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Examination of a broadband sweep with and without Force Control
Authors Spencer L. Rowse and Anhtony TinkleThis article proposes that this known significant discrepancy between the Weighted Sum Ground Force (WSGF) prediction of the vibrator output (the Sallas Approximation; see Sallas, 1984) and downhole measurements of the output is not so much a ‘lie’, as it is simply the natural consequence of the limitations and inaccuracies of the WSGF model. The authors believe that these model deficiencies arise from a woefully incomplete understanding of the complex interaction of the vibrator mechanism with the ground. If this is correct, than an improved understanding of the interactions of the vibrator mechanism and the ground during a sweep should lead to a more complete model of the vibrator/earth interactions, with significant improvements in both the actual energy content of the propagating wavelet as well as more ‘truthful’ quality control reporting thereof. Modern vibrator control systems use both phase and force control systems in controlling the output motions of the reaction mass (RM) and baseplate (BP). Feedback from various sensors on the vibrator mechanism is used to adjust the input hydraulic controls so that the WSGF ‘mimics’ the desired reference sweep. In this paper we examine the response of a downhole vertical geophone to a broadband sweep generated on the surface, initially with both force and phase control operating, and then with force control disabled, to observe what effects force control has on the vibrator mechanism and the propagating seismic wavelet, as well as to develop an improved understanding of the interaction of the vibrator mechanism with the ground during a sweep.
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Understanding MEMS-based digital seismic sensors
Authors Nicolas Tellier and Jérôme LainéOver the last few decades almost all the electronic devices we use in our daily lives have switched from analog to digital owing to the numerous benefits they offer, such as miniaturization, enhanced functionalities or reduced power consumption, etc. This revolution also reached the seismic industry, primarily when fully digital recorders and telemetries became available in the late 1970s. However, measurement of the Earth’s displacement by sensors remained analog. The last step towards full digital recording was made in the early 2000s with the launch of digital seismic sensors based on MEMS (Micro Electro Mechanical Systems) accelerometers that had the potential to replace analog geophones that had been used since the early days of seismic dating back to the 1930s. However, although digital sensors have established a foothold and indeed even become a reference for a number of different applications, the anticipated revolution has however not lived up to the expectations of the seismic industry, the current situation being perhaps analogous to using audio cassette tapes alongside MP3 music files. More than a decade after the release of the first digital seismic sensors, several explanations for such a situation have become clear: • the operating principles of digital sensors are somehow perceived to be more complicated than those of geophones, and trickier to understand; • the quality of data acquired with digital sensors is sometimes judged to be no better than equivalent to data acquired with geophones (more often than not, this is owing to a comparison of single digital sensors with strings of geophones at the same trace interval); • the cost of digital sensors is often seen to be higher (at least for one-component – 1C – acquisition) than the cost of a conventional Field Digitizing Unit (FDU) connected to a string of geophones. This article aims firstly to review the main operating characteristics of MEMS-based Digital Sensor Units (DSU), especially when compared to geophones, and then discuss the quality and cost issues in the light of experience gained after more than a decade of field operations. We conclude that their use with an adapted high-density geometry makes it possible to achieve much better imaging than with geophone arrays, for an equivalent cost.
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Fracture detection and calibration in a tight volcanic gas reservoir, Barmer Basin, India
Authors Sreedurga Somasundaram, Sudeep Bhat, Amian Das, Bineet Mund, Abdhudai Beohar and Pranay ShankarThis paper focuses on the detection of fracture zones within volcanic reservoirs using seismic attributes, and their calibration using well data. The work was done to understand observed field production behaviour, and for the use of these attributes to optimize future well placement during the field development programme. The Raageshwari Deep Gas (RDG) Field is located in the southern Barmer Basin in Rajasthan, India, within the RJ-ON- 90/1 contract area operated by Cairn India Limited (Figure 1). The field was discovered in 2003 with the drilling of exploration well Raageshwari-1. The field contains gas condensate with excellent gas quality of approximately 80% methane within tight volcanic reservoirs. Subsequent to discovery and appraisal drilling, the RDG field is currently being developed. To date, 30 deviated development wells have been drilled, with multi-stage hydraulic fracturing to increase production to economic rates.
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The use of pseudorandom sweeps for vibroseis acquisition
Authors Timothy Dean, John Tulett and Darvin LaneThe use of pseudorandom sweeps (rather than the more conventional sweeps whose instantaneous frequency changes linearly) has received periodic attention over the past 45 years. In a previous article (Dean, 2014) I summarised the work published in this area including the different motivations for their use as well as different methods for generating a variety of different sweeps. At the time, I attributed their lack of use to the difficulty in transmitting them and their relatively low energy levels when compared with conventional, linear, sweeps. In this article I summarise recent work (i.e. not covered in Dean (2014)) involving pseudorandom sweeps before describing some of our own test results. I then discuss the implications of this recent work on the future of the method.
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New plays on the Greater East Shetland Platform (UKCS Quadrants 3, 8-9, 14-16) – part 2: newly reported Permo-Triassic intra-platform basins and their influence on the Devonian-Paleogene prospectivity of the area
Authors Stefano Patruno and William ReidDespite significant discoveries within Palaeogene-age reservoirs (e.g., Mariner) the East Shetland Platform (ESP) is underexplored, with only ten wells per 1000 km2. Mesozoic units are thin or absent whilst Paleozoic reflectors resemble acoustic basement on legacy seismic. Recent 3D dual-sensor broadband surveys (GeoStreamer) covering 17,200 km2 over parts of Quadrants 3, 8-9, 14-16 have allowed for clearer imaging. Here, this dataset is interpreted, leading to new insights into this large frontier region. The ESP petroleum system comprises multiple proven reservoir and source intervals, with viable play fairways. Up to four regional unconformities are interpreted, merging into fewer erosional surfaces on persistent highs. Elsewhere on the ESP, predominantly subsiding Permo-Triassic depocentres contain a nearly continuous Paleozoic-Mesozoic succession. The most prominent of these, to the south and south-west of the Beryl Embayment, is here referred to as the ‘Crawford-Skipper Basin’. Existing hydrocarbon discoveries on the ESP are in the vicinities (<7 km) of intra-platform Permo-Triassic basin margins. Exploration close to such basins is inherently less risky due to possible positive influences of deep-seated structures on the petroleum system. These include: (1) formation of Meso-Cenozoic closures; (2) Devonian source maturity and presence of simple fault-related migration pathways; (3) viability of sub-Cretaceous reservoir-trap-seal configurations.
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Multi-attribute Bayesian risk modification – a case study from the Norwegian Barents Sea
Authors Thomas Rühl and Samuelsson JörgenThe assessment of the probability of geological success (i.e. the chance that a well encounters mobile hydrocarbons) is an important task in the prospect evaluation workflow. After the initial assessment of a geological probability of success, POSg, we use the geophysical direct hydrocarbon indicators (DHI) as extra information in a second Bayesian risk modification step. The Bayesian risk modification (BRM) is a very flexible approach, allowing many data types to be considered. The BRM is extended in this paper to include multiple DHI attributes. If we want to use two seismic attributes (e.g. top reservoir reflection and flat spot amplitudes), then possible interdependences between the attributes must be carefully investigated before application. We suggest a simple approach to determine the degree of attribute dependence and we demonstrate how to compute the scenario conditional probabilities (likelihoods) of these attributes for all possible exploration outcomes. A case study from the Barents Sea shows the feasibility of the multi-attribute BRM to solve a complex risking situation for a prospect with a flat spot. The later drilling and well results confirmed the plausibility of the risking modification outcome.
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Volumes & issues
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Volume 42 (2024)
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Volume 41 (2023)
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Volume 40 (2022)
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Volume 39 (2021)
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Volume 38 (2020)
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Volume 37 (2019)
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Volume 36 (2018)
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Volume 35 (2017)
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Volume 34 (2016)
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Volume 33 (2015)
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Volume 32 (2014)
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Volume 31 (2013)
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Volume 30 (2012)
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Volume 29 (2011)
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Volume 28 (2010)
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Volume 27 (2009)
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Volume 26 (2008)
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Volume 25 (2007)
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Volume 24 (2006)
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Volume 23 (2005)
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Volume 22 (2004)
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Volume 21 (2003)
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Volume 20 (2002)
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Volume 19 (2001)
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Volume 18 (2000)
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Volume 17 (1999)
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Volume 16 (1998)
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Volume 15 (1997)
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Volume 14 (1996)
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Volume 13 (1995)
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Volume 12 (1994)
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Volume 11 (1993)
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Volume 10 (1992)
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Volume 9 (1991)
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Volume 8 (1990)
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Volume 7 (1989)
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Volume 6 (1988)
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Volume 5 (1987)
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Volume 4 (1986)
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Volume 3 (1985)
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Volume 2 (1984)
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Volume 1 (1983)