Geofyzika Torun

Transcription

Geofyzika Torun

LAND C-WAVES SEISMIC SURVEYS IN POLAND
by
Michał Podolak
Marian Wilk
Geofizyka Toruń Sp. z o.o.
INTRODUCTION
Multicomponent seismic characterization of the subsurface, implemented as an ocean
bottom cable technique, OBC, has been applied successfully since a decade. For several
years, the onshore equivalent of OBC has been considered too expensive and facing
discouraging obstacles. It took few years to collect experience and develop equipment robust
enough to make onshore multicomponent seismic projects feasible and economically
affordable.
With introduction of the converted-wave (C-wave) technology, requirement of special
equipment, necessary to deal with vector wavefield at its generation and at the recording stage
was reduced to the receiver, which in its novel, digital incarnation, equipped with a built-in,
chip-based computing system, successfully meets expectation of engineers. At the same time,
commercially available software packages, both for processing and interpretation
complemented the toolkit of geophysicist working with multicomponent seismics within land
reality.
GT, Torun-based geophysical company offers now the industrial-scale, onshore
converted-wave seismic services. After the first in Poland, c-wave acquisition in 2002,
followed by successful processing, it continuously upgraded its tools. Today, with several
hundred digital stations, the latest-generation instrument recording three-component data, and
state-of-the-art software packages, GT began routine onshore MC (multi-component)
services. Fourth 3C 2D project was recently acquired and the data seems to contain interesting
geologic information, as it can be seen from processing results and first stage of
interpretation.
Early experience
The adventure with vector wavefields in real, unisotropic media, for Polish
geophysicists, began in late 19th century with foundation of the first in the world geophysical
profesorship by Maurice P. Rudzki at the Jagiellonian University of Krakow (commentary by
K. Helbig and M. Slawinski available from a website). For many years it remained merely
academic theory. The advances of applied geophysics enabled larger experiments. They had
been carried out in 60s and 70s in Russia, France and US. That eventually led to the birth of
marine converted-wave application called OBC. Land MC surveys remained limited-scale,
occasional experiments pioneered by few companies or organizations of which CREWES
Research Project contributed the outstanding amount of experience.
Few years ago GT recorded a 9C onshore single seismic line, but due to some
difficulties, mainly with high level noise and very difficult statics estimation it was decided to
suspend that work. The shear wave source was used (9C experiment) so initial static
corrections made troubles both for source and for receiver. Analog geophones, threecomponent type, had 30 Hz natural frequency, and shear wave light vibrator, owned by GT,
generated sweep from 20 Hz up. Lack of low frequeces made shear-wave statics a hard task.
The target was only few hundred meters deep, but probably deep enough to attenuate S-waves
from the light vibrator, and make them too weak to be recorded by that generation of
geophones.
GT with over 35 years of experience, more than half of that international, had already
been proven seismic company and renowned for its quality and economic services. Natural
consequency was to face the next challenge: land c-wave seismic technology. Fortunately, it
coincided with appearance of new generation, digital receiver named VectorSeis™.
Combination of this sensor with converted-wave approach creates effective, economically
acceptable seismic technology.
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Breakthrough with new seismometer
New hope appeared with the digital accelerometer launched in 2001 by Input/Output
Inc. It turned out to be a breakthrough, but before an examination, it had been a question.
The experiment was located in the south-west of Poland on border of the European Permian
basin. It is the main sedimentary basin in Poland, where the largest hydrocarbon discoveries
in that part of Europe were made. In that basin, GT equipped with the latest seismic
technology discovered BMB, Miedzychod, Koscian, and many more oil and gas fields. One
of the recent findings, Lubiatow, is lithological type. Seismic attributes played important role
in that discovery. It supported an idea that application of multicomponent technology can add
new value to the seismic image.
It was just here, in that hot spot of seismic prospecting, where Geofizyka Torun has
mapped over 30 bcm of recoverable hydrocarbon reserves: first Polish converted-wave
seismic survey has been pioneered in April 2002.
The target consisted of reefal bodies Koscian and Bronsko at over 2000 m depth,
burried under Zechstein salt and anhydrite layers.
Brońsko Reef
Kościan Reef
Bronsko-10
3C line
Fig.1. Bronsko-Koscian reef along with location of the 3C line (red one), and Bronsko-10 well with
the 3C VSP. Irregular shape of the reefal bodies made structural approach problematic.
Importance of lithologic parameters became growing.
Koscian 3C, converted-wave survey has been limited to a single 2D line. It was just an
experiment before starting production-size service. While being a challenge to acquisition
team, it created also an occasion to apply and check processing package dedicated to
converted wave data.
Apart of 3C line, a classical, reference vertical-geophone line was recorded exactly at
the same position. Moreover, an offset 3C VSP was recorded in the well Bronsko-10, crossed
by the surface line. That enabled tying surface 3C seismic to the well image and validate with
well-established vertical-geophone data.
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Fig.2. Radial component. Offset VSP section spliced into surface seismic helped in identification of cwave events.
Vp/Vs
Zechstein
interval
1 km
Fig. 3. Vp/Vs calculated for Zechstein interval (marked with a brace on the right). Saturated reefs are
clearly visible.
The Koscian 3C was a test of C-wave seismics for Polish Geophysicists. Despite of
complex receiver statics, low S/N for horizontal components, and simplistic software,
correlation of γ anomalies with already known saturation is evident (Fig. 3), and 3C VSP data
derived from Bronsko-10 well (Fig. 2) ensures that event identification is reliable. Correlation
with well-established vertical-geophone data made an important aspect of the validation
process as well.
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GT’s proprietary, interactive residual static correction software was invaluable for
shear wave statics estimation.
Comparison of the vertical component, digital section to classical, analogue one
revealed that qualities (width and flatteness) of digital signal (single VectorSeis
accelerometer per station) are superior to those of analogue signal received with 12 geophones
groupped per channel.
In that situation GT offers recording of C-wave data at the price close to price of
classical P-wave data. When the vertical component is processed, client can order processing
of selected C-wave components, so a risk of useless C-waves is reduced.
Development of the software tools
Processing of Koscian 3C survey was carried out with basic set of tools dedicated to cwaves. Traces were groupped according to asymptotic conversion point approximation.
Velocity analysis was performed with a bit time-consuming method of constant velocity scans
combined with layer stripping. Fig. 4 illustrates an idea of that analysis.
Vp/Vs estimation
Idea of estimation γ = Vp/Vs with layer stripping approach
ACP binning
γ1 scans
selection !
ACP binning
γ2 scans
selection !
ACP binning
γ3 scans
selection !
.... γ scanning for consecutive intervals ........>
γ1 repeated for
ACP binnings
with different γ
Vp/Vs field
is created
Fig. 4. Idea of the layer-stripping analysis of γ = Vp/Vs.
An attempt to estimate anisotropy orientation and apply respective rotation of
horizontal components failed probably because the only single C-wave (and not 9C) 2D line
was available.
Collected experience served to define targets in software development. GT offers now
many state-of-the-art MC processing modules: correction of receiver vertical orientation, true
CCP stacking, interactive γ semblance / scan analysis, and soon, C-wave migration is
expected.
Moreover, interpretation software is also available now on the market. That enables
efficient and comprehensive analysis of the data taking into account well-based data, i.e. well
logs and multicomponent VSP. From analysis of available data it became obvious that simple
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γ coefficient is not a straightforward indicator of saturation. It combines information about
lithology and saturation. To meet interpretation challenge not only dedicated MC software is
needed, but advances in understanding of transformations and mathematical operations
crucial for extraction of information geologist or reservoir engineer are looking for.
Theoretical support from scientific organizations has been valuable.
To understand behaviour of elastic wave gets easier with modeling software. It is
another advancement which GT has made on its way to master seismic method at the level of
reservoir characterization. At present, the interactive software is available for finite difference
simulation of elastic wavefield in 2D approximation. The anisotropic option is included.
More experiments
After very encouraging results got from the Koscian 3C experiment (# 1 in Fig. 5), GT was
awarded next C-wave projects. A map in Fig. 5 shows location of those surveys.
2,3
Torun
1
4
Fig. 5: Four areas of 3C 2D surveys recorded in Poland: 1 – Koscian – gas in lower Permian reefs, 2
– Bartoszyce – oil in Cambrian sandstone, 3 - Dzwirzuty – post-glacial, shallow geology, 4 Nowa Deba – gas in Miocen sediments. Range of Rotliegend is at the background in brown.
Every project sets different target and different problems which are to be solved. Bartoszyce
and Dzwirzuty are limited-volume projects aimed at testing what can be achieved with Cwaves there. However, Nowa Deba is quite large, multi-line survey. In that case, it is
important to keep data consistency at all processing stages. Proprietary software tools
developped in GT perform that task.
Bartoszyce, the second experimental line recorded by GT with C-wave equipment was
basically the first-in-the-world industrial performance of advanced SystemFour™ recording
instrument. Also, much improved since Koscian 3C acquisition, digital receivers contributed
to continuation of C-wave success. Figures 6a and 6b make comparison of classical analogue
section and new-technology digital one.
The experimental conclusion can be made, that quality of vertical component of digital Cwave data is at least equal, and even superior to quality of analogue classical P-wave data.
Geologic task was here to look for oil reservoirs in sandstones of middle Cambrian.
Recognition of lithological changes within Zechstein which overlies Cambrian, is crucial in
this case. It allows for correct depth transformation.
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a
b
Fig.6. Comparison of analog (a) to ditigitally recorded section (b) from Bartoszyce area.
Braces point at the target interval where differences were examined.
Dzwirzuty is a shallow seismic project executed for Polish Geological Institute. Survey
located in the same area as Bartoszyce, yet recorded for different purpose. Geologic task is to
image structure and lithology of post-glacial sediments, namely Quaternary and Tertiary.
The challenge here is to get higher resolution from converted waves than from P-waves –
target is shallow. Another challenge is to calibrate static corrections properly. At shallow
times, seismic image is sensitive to statics.
Production-scale project: Nowa Deba
The survey was carried out in the province of shallow gas accumulations in southeastern
Poland. Miocene with the upper part of underlying Jurassic are here the investigated intervals.
Geologic target: gas in Miocene
Reflection from the bottom of Miocene
Fig. 7. Target horizons in the Nowa Deba project are in Miocene sediments. Gas-saturated sandstones
are looked for.
Nowa Deba is a large-scale industrial application of converted-wave seismics. It was
recorded mainly for the P-wave image, with the secondary target to collect converted wave
data. Location map of the 16 lines recorded within the survey can be seen in Fig. 8.
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L01
L02
L03
L04
L05
L06
L07
L09
L12
L13
L15
Fig. 8. Location map of the Nowa Deba 3C 2D project. Radial component processing of the lines in
black was performed at first.
Surface and LVL are relatively smooth in the area, and do not create processing
challenges. However, the shallow refraction S-wave survey was carried out. That allowed to
get near surface γ-coefficient model to calibrate reflection seismics. Maps of the γ for two
shallow layers are shown in Fig. 9. While the shallowest layer is homogeneous, the deeper
one exhibits variability of γ. One possible explanation can be existance of the S-wave
anisotropy in near-surface. Investigation of that could be a subject of a separated project. The
S-wave soundings were probably not enough dense to image complex distribution of γ in that
layer. S-wave static corrections for receivers were estimated through scaling P-wave receiver
statics with average coefficient 4.5 (compare Fig. 9), followed by residual interactive and
automatic corrections. Fig. 10 displays an example of P-wave weathering model from first
breaks, while Fig. 11 is an example of total static correction for the radial component. The
solution of statics estimation was accepted as satisfactory.
Good quality of the raw pre-stack data (Fig.12) suggests no troubles with basic
processing. In that situation, more advanced, c-wave algorithms can perform well.
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First, 2-meter thick layer
Second layer till D.P.
Fig. 9. Distribution of γ coefficient estimated from S-wave shallow velocity survey.
Interval velocities
Interval thickness
Fig. 10. Weathered layer model derived from P-wave first breaks inversion.
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Fig.11. Improvement in the stack due to total PS statics.
a
b
c
Fig.12. Sample of a raw shot from Nowa Deba 3C. Three components can be seen: (a) vertical, (b)
radial, and (c) transverse. The only processing applied to the displayed data was amplitude
recovery for the display purpose.
At the interpretation stage, two lines from the survey recorded in 1999 were used as a
reference. Location of these lines can be seen in Fig. 13.
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Reference lines
from the 1999
acquisition
Fig. 13. Location map of two lines (in blue) from the 1999 survey. The two lines were used as a
reference for interpretation of the Nowa Deba project 2003. Comparison displayed hereunder
is for the blue lines and respective sections of the violet line closest to them.
In Fig. 14 there is a comparison of acquisition parameters of the Nowa Deba 2003
survey to the reference one from 1999. Frequency spectra of the two surveys do not differ
significantly. The processing of P-wave component was fairly similar.
Figures 15 to 18 make comparison of the P-wave sections.
Good, high resolution image achieved in the Nowa Deba 2003 project is attributed to the
application of digital receiver: it records reliably high frequency content of the wavefield.
Especially, phase spectrum is recorded with high fidelity, what greatly helps in efficient
processing of high frequences.
Acquisition 1999
Source:
Sweep:
Instrument:
Receiver:
# of recs / group:
# of chans:
Xmax:
∆ SP = ∆ x :
PP fold:
dynamite
10 – 120 Hz
I/O System TWO™
analog
32
120
1575 m
25 m
60
Acquisition 2003
vib. MARK IV
8 – 150 Hz
I/O System FOUR™
digital
1
200
1000 m
10 m
100
Fig. 14. Comparison of the acquisition parameters employed for the presented Nowa Deba project,
recorded in 2003 to the parameters of the reference project recorded four years earlier.
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Comparison of processed P-wave sections show quality improvement with respect to the
previous sections received with one-component receivers (group of geophones).
Just finished interpretation of processed P-wave component data allow to indicate few
structures. Additionally, many different types of anomalies (amplitude, acoustic impedance,
AVO- see Fig. 15) are observed on P-wave sections. Some of them are related to gas fields
discovered earlier, while other are good indicators for further development.
Fig.15. Example of typical anomaly associated with gas-saturated sandstone, seen on P-wave
reflectivity derived from AVO analysis. Signal in the left section is rotated 90° with respect to
the signal in the section on the right. Clear anomaly in this place appears also in the pseudoimpedance section estimated during post-stack inversion.
Fig. 16. Line A01 (see map in Fig. 13) with true and false anomalies. Two wells marked with black
ellipse are positive, while the one marked with red is negative.
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Fig. 17. Line from the 2003 project, recorded close to the line A01 (see Fig. 16). Higher resolution
than in the Fig. 16. Anomaly in red disappeared.
Fig. 18. Line A02 (see map in Fig. 13).
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Fig. 19. Line from the 2003 project, recorded close to the line A02 (see Fig. 18). Note very different
image in the ellipse.
1 km
500
ms
Fig. 20. P-wave image. A bright spot can be seen in the box.
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1 km
500
ms
Fig. 21. C-wave image. Radial component. No bright spot appears in the box. Shear waves cannot see
fluids, so a gas presence is possible explanation.
Interpretation of the vertical component has been just completed. It brought new quality
image of the investigated Miocene sediments. Several possible locations of new wells are
suggested. It is expected that further exploration in the area will result in multicomponent
VSP and full acoustic sonic logs recorded in new wells. Such data would enable reliable
interpretation of horizontal components, thus allowing further reduction in drilling uncertainty
with respect to interpretation limited to P-wave images merely.
Conclusions
-
Introduction of VectorSeis - new generation, digital sensor - made a breakthrough in
seismic reservoir characterization onshore.
-
Application of C-wave technology on land became feasible seismic method, from
acquisition to interpretation.
-
Gas saturation can be detected with Vp/Vs indicator as well as by comparison of the
P-wave and C-wave images.
-
GT, being well-known for its land seismic specialization, continues efforts of
contributing new experience of the leading-edge technology to the oil and gas
industry.
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Acknowledgements
Samples of multicomponent data from different Polish projects are shown due to courtesy of
POGC, Polish Oil and Gas Company.
VectorSeis™, SystemTWO™, and SystemFOUR™ are trademarks of Input/Output
Company.
Authors thank associates from Geofizyka Toruń for useful assistance, and namely Grażyna
Burek, Maciej Zarzyka for interpretation, and Robert Dybalak for processing of the data.
References
1. Dohr, C., Seismic Shear Waves, in Handbook of Geophysical Exploration, London –
Amsterdam 1985, Geophysical Press.
2. Garotta, R., Shear Waves from Acquisition to Interpretation, SEG DISC No. 3, Tulsa
1999.
3. Tsvankin, I., 2001, Seismic Signatures and Analysis of Reflection Data in Anisotropic
Media, in Handbook of Geophysical Exploration, vol. 29, Elsevier Science.
4. Helbig, K., Treitel, S., 1997, Foundations of anisotropy for exploration seismics, in
Seismic Exploration, Elsevier Science, on CD-ROM.
5. Tatham R.H., Cormack M.D., 1998, Multicomponent Seismology in Petroleum
Exploration, SEG, Tulsa 1998.
6. Stewart R.R., Gaiser J.E., Brown R.J., Lawton D.C., 1999, Converted-wave seismic
exploration: a tutorial, CREWES Research Report vol. 11.
7. Podolak, M.W., 2003, Koscian – Bronsko reef in the light of C-waves. Available at
www.GTservices.pl.
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Geofyzika Torun

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