- Home
- A-Z Publications
- First Break
- Previous Issues
- Volume 32, Issue 9, 2014
First Break - Volume 32, Issue 9, 2014
Volume 32, Issue 9, 2014
-
-
Seismic quality monitoring during processing
Authors Alexandre Araman and Benoit PaternosterReservoir characterization is often used to guide field developments and therefore requires the ‘best’ seismic data possible. Furthermore, at the end of the data processing, one should make sure that the propagating wavelet has constant properties: its amplitude, phase and bandwidth should remain as stable as possible across the area and target depth interval. There is therefore a trade-off between the wavelet’s ‘best’ characteristics required and its stability. These conditions also apply to the incidence and azimuthal angle dimensions of the dataset for dedicated reservoir characterization workflows. To insure that a processing sequence will lead to a dataset that meets the above-stated requirements, adequate quality control should be performed at numerous key processing stages. First, desired directions for improvement should be defined with the interpreter and translated into the relevant seismic attributes. Attributes which can be mapped are privileged, so that the lateral variations of the wavelet characteristics can be visualized, confronted with other interpretative information and, hopefully, its stationary behavior quantified. Then, milestones should be set at relevant steps of the processing sequence to quantify the selected attributes with intermediate migrated 3D seismic volumes. Finally, relative scores can be established to monitor the ongoing processing quality improvement and to eventually compare it with a vintage dataset if available.
-
-
-
Using full-azimuth imaging and inversion in a Belarus salt dome tectonic regime to analyze fracturing in Upper Devonian intersalt and subsalt carbonate reservoirs
An analysis of recent publications, together with the results of the European Association of Geoscientists & Engineers (EAGE) and EurAsian Geophysical Society (EAGO) exhibitions, reveals an unequivocal trend towards an increase in information content and accuracy of fracture predictions made using full-azimuth seismic acquisition and processing technology. This trend is notable in oil and gas fields in carbonate and terrigenous reservoirs within the countries of the Commonwealth of Independent States (CIS), where major oil companies such as Gazprom, Gazpromneft, Lukoil, Novatek, etc., are already conducting work of this type. This trend has also been observed abroad, but on a larger scale. The number of successfully completed projects is sufficient to confirm a surge in the use of higher density and richer seismic azimuth surveys in the coming decades. Belorusneft PA RUE places great importance on the modernization of field surveys and on hardware systems that are capable of supporting the implementation of super-dense, full-azimuth imaging and inversion systems. While these surveys are considerably more expensive, they enable a comprehensive full-azimuth study of deep targets and oil and gas reservoirs. However, these surveys require special software technologies that are capable of correctly processing full-azimuth data and extracting the greatest possible amount of information from them.
-
-
-
Joint Impedance and Facies Inversion – Seismic inversion redefined
Authors Michael Kemper and James GunningIn this paper we will first review the industry-standard simultaneous inversion method (which derives continuous impedances) and subsequently identify some pitfalls. We will then introduce our new Joint Impedance and Facies Inversion technology (which we call Ji-Fi for short in this paper), which overcomes these pitfalls by recasting the seismic inverse problem as mixed discrete/continuous. Having so captured the correct physics, we apply this first on a wedge model, followed by a case study, before drawing some conclusions. Note that in this paper, it is assumed that the seismic to be inverted is an ensemble of true amplitude partial angle stacks with corresponding wavelets derived from well ties.
-
-
-
Broadband processing of West of Shetland data
Authors Rob Telling, Nick Riddalls, Ahmad Azmi, Sergio Grion and R. Gareth WilliamsRob Telling, Nick Riddalls, Ahmad Azmi, Sergio Grion and R. Gareth Williams present broadband processing of 2D data in a configuration that enables demultiple algorithms, designed for processing conventional data, to be used as part of a standard prestack time sequence. The example dataset described here forms part of a 2D broadband well-tie survey conducted within the Shetland-Faroe basin. The water-bottom within the survey area is hard and varies in depth between 120 m and 1700 m. The deeper geology throughout is characterized by a prominent layering of Paleogene flood basalt, varying in thickness from a few hundred metres to over a kilometre. In terms of seismic response, the high impedance contrasts at the water-bottom and at the top of the basalt gives rise to prominent reflections and strong multiples. Additionally, severe attenuation takes place within the alternating layers of basalt and silt and clay stones which limits deeper penetration of energy. It is important for successful imaging to maintain good signal-to-noise ratio below this layer, particularly at the more penetrating low frequencies, and this survey is therefore well suited to a broadband acquisition and processing solution. The high signal-to-noise ratio offered by towing streamers deep below the source of wave noise is important for maximizing the processing bandwidth (Williams and Pollatos, 2012). A central feature of the processing of this dataset is the use of algorithms designed for processing conventional data, made possible by the adoption of a flat or, as used here, a very mildly slanted streamer configuration during acquisition. In contrast, where strongly slanted or curved profiles are used, algorithms require modification to account for the strong variability of the receiver ghost response with offset (Sablon et al., 2012), due to a breakdown of the stationary wavelet assumption that many processing algorithms require (e.g. semblance velocity analysis, SRME) or alternatively deghosting and re-datuming must be carried out at a much earlier stage in the processing sequence.
-
-
-
Full-azimuth, high-density, 3D point-source/point-receiver seismic survey for shale gas exploration in a loess plateau: a case study from the Ordos basin, China
The Ordos Basin, located in the central part of China, contains abundant oil and gas shale resources. This study took place in an area southwest of Yan’an City, Shaanxi province, and its main exploration targets were the Mesozoic Yanchang Formation, with a maximum depth of around 1500 m, and the Upper Paleozoic Benxi Formation, with a maximum depth of around 3550 m. The tectonic structure at the target levels is almost flat, with dips of only around one degree. The objective of the survey was to identify ‘sweet spots’ in a heterogeneous gas shale reservoir beneath thick deposits of loess, which is sediment formed by the accumulation of wind-blown silt. Variations in the topography, thickness, and seismic velocity of the loess were identified as key factors that had led to inadequate imaging results from previous 2D exploration surveys in the area (Yao and Li, 2004). The new 3D survey, acquired by CNPC Sichuan Geophysical Company (SCGC) during 2013 using the WesternGeco UniQ integrated point-receiver land seismic system, delivered imaging results that improved the characterization of the gas shale. Several features of the project area presented challenges for seismic exploration, many of which are typical of a loess plateau. The area is covered by unconsolidated loess with severe variations in thickness and velocity that can result in serious statics problems. The loess also causes dramatic signal absorption, leading to poor signal-to-noise ratio and potentially contributing to low resolution in the seismic data. As shown in Figure 1, surface conditions include rapid variations in topography that further complicate near-surface statics correction and can be the cause of various types of interference and noise. Survey logistics needed to account for heavily forested areas, hills and gulches. Also, more than 500 oil wells were pumping and some rigs were drilling during survey acquisition, representing additional sources of noise (Figure 2).
-
-
-
Non-linear behaviour for naturally fractured carbonates and frac-stimulated gas-shales
By Nick BartonGas-shales and naturally fractured reservoirs usually produce from several kilometers depth, with fracing-stimulation and eventual water-drive respectively. Due to porosity, the matrix is generally weaker than is typical for basement rocks. The potential pore pressure reduction of tens of MPa during the early life of the fields, may therefore be a significant proportion of the strength of the matrix. Inevitable non-linear rock strength behaviour for the matrix should not then be ignored. It is therefore unrealistic to utilize a linear Mohr-Coulomb strength criterion as so frequently seen. The joints or natural fractures in the shales and carbonates, which are so important for production, will have producing fracture sets with different roughness and aperture, and few of them are planar enough to follow the frequently used linear Mohr-Coulomb behaviour. Non-linearity especially applies to the favourable shear strength-dilation-permeability coupling which is relevant for both NFR and gas-shales, and to the less desirable stress-closure-permeability coupling of a stress-sensitive reservoir. Non-linear constitutive modelling, partly based on the joint- or fracture-roughness coefficient (JRC) used widely in rock mechanics, also applies to the conversion from hydraulically interpreted theoretical smooth-wall apertures (e) to the larger and non-planar-non-smooth-wall physical apertures (E) through which the oil or gas actually flows to the wells. Simple index tests which can also be applied on joints or fractures recovered in occasional and inevitably expensive core, and which can also be estimated when mapping fractured pavement analogues, have been available in rock mechanics for several decades. They were already incorporated in coupled distinct element (jointed) non-linear modelling routines in 1985. However, their implementation in petroleum industry geomechanics seems to be very rare judging by numerous workshops attended in the last seven to eight years on both sides of the Atlantic. Application of non-linear (non Mohr-Coulomb) rock mechanics, using recovered core from Ekofisk in 1986-1987 in order to model fracture shear-dilation coupling with simplified E and e tracking during compaction, may be the earliest example.
-
Volumes & issues
-
Volume 42 (2024)
-
Volume 41 (2023)
-
Volume 40 (2022)
-
Volume 39 (2021)
-
Volume 38 (2020)
-
Volume 37 (2019)
-
Volume 36 (2018)
-
Volume 35 (2017)
-
Volume 34 (2016)
-
Volume 33 (2015)
-
Volume 32 (2014)
-
Volume 31 (2013)
-
Volume 30 (2012)
-
Volume 29 (2011)
-
Volume 28 (2010)
-
Volume 27 (2009)
-
Volume 26 (2008)
-
Volume 25 (2007)
-
Volume 24 (2006)
-
Volume 23 (2005)
-
Volume 22 (2004)
-
Volume 21 (2003)
-
Volume 20 (2002)
-
Volume 19 (2001)
-
Volume 18 (2000)
-
Volume 17 (1999)
-
Volume 16 (1998)
-
Volume 15 (1997)
-
Volume 14 (1996)
-
Volume 13 (1995)
-
Volume 12 (1994)
-
Volume 11 (1993)
-
Volume 10 (1992)
-
Volume 9 (1991)
-
Volume 8 (1990)
-
Volume 7 (1989)
-
Volume 6 (1988)
-
Volume 5 (1987)
-
Volume 4 (1986)
-
Volume 3 (1985)
-
Volume 2 (1984)
-
Volume 1 (1983)