Seismic low-frequency shadows and their application to detect CO2 anomalies on time-lapse seismic data: a case study from Sleipner field, North sea

Geophysics ◽  
2021 ◽  
pp. 1-45
Author(s):  
Emmanuel Anthony ◽  
Nimisha Vedanti
Keyword(s):  
North Sea ◽  
Seismic Data ◽  
High Frequency ◽  
Seismic Waves ◽  
Decomposition Analysis ◽  
Low Frequency ◽  
Time Lapse ◽  
Underlying Mechanism ◽  
Real Field ◽  
Saturated Medium

The detection and underlying mechanism of prospect-scale seismic low-frequency shadows (LFS) has been an issue of debate. Even though the concept of LFS is widely accepted, the practical applicability of the method remains limited due to few real field case studies and little understanding of the underlying attenuation mechanism. To characterize the attenuation phenomenon responsible for the occurrence of LFS in CO2 saturated formations, we use the diffusivity and viscosity of the fluid saturated medium to derive a complex velocity function that characterizes a high-frequency attenuation phenomenon responsible for the occurrence of LFS in a CO2 saturated formation. Synthetic seismic data sets representing pre- and post- CO2 injection scenarios were generated using 2D diffusive viscous equations to model the LFS and understand its occurrence mechanism. Furthermore, to demonstrate the applicability of LFS in a real field, a spectral decomposition analysis of time-lapse 3D seismic data of the Sleipner field, North Sea, was carried out using the continuous wavelet transform. LFSs were clearly detected below the reservoir base at frequencies lower than 30 Hz in the post- CO2 injection surveys. It is shown that the seismic low-frequency shadows are not artefacts but occur due to attenuation of the high frequency components of the propagating seismic waves in the CO2 saturated Utsira Formation. The attenuation of these frequencies is a result of the diffusivity and viscosity of the fluid saturated medium. The low-frequency shadows are localized anomalies at the base of the formation; hence with the present approach, these anomalies cannot be related to the migration of the CO2 plume in the Utsira Formation.

Numerical simulation of seismic waves in porous media components

2021 ◽  
Vol 9 (1) ◽  
pp. 4-30
Author(s):  
M. Azeredo ◽  
◽  
V. Priimenko ◽  
Keyword(s):  
Numerical Simulation ◽  
Porous Medium ◽  
High Frequency ◽  
Seismic Waves ◽  
Computational Algorithm ◽  
Low Frequency ◽  
Physical Parameters ◽  
Mathematical Algorithm ◽  
Biot Equations ◽  
Attenuation And Dispersion

This work presents a mathematical algorithm for modeling the propagation of poroelastic waves. We have shown how the classical Biot equations can be put into Ursin’s form in a plane-layered 3D porous medium. Using this form, we have derived explicit for- mulas that can be used as the basis of an efficient computational algorithm. To validate the algorithm, numerical simulations were performed using both the poroelastic and equivalent elastic models. The results obtained confirmed the proposed algorithm’s reliability, identify- ing the main wave events in both low-frequency and high-frequency regimes in the reservoir and laboratory scales, respectively. We have also illustrated the influence of some physical parameters on the attenuation and dispersion of the slow wave.

A time-lapse seismic repeatability test using the P-Cable high-resolution 3D marine acquisition system

2020 ◽  
Vol 39 (7) ◽  
pp. 480-487
Author(s):  
Patrick Smith ◽  
Brandon Mattox
Keyword(s):  
High Resolution ◽  
Seismic Data ◽  
Low Cost ◽  
Low Frequency ◽  
Time Lapse ◽  
Seismic Signal ◽  
Residual Energy ◽  
Acquisition System ◽  
Positioning Errors ◽  
Seismic Surveying

The P-Cable high-resolution 3D marine acquisition system tows many short, closely separated streamers behind a small source. It can provide 3D seismic data of very high temporal and spatial resolution. Since the system is containerized and has small dimensions, it can be deployed at short notice and relatively low cost, making it attractive for time-lapse seismic reservoir monitoring. During acquisition of a 3D high-resolution survey in the Gulf of Mexico in 2014, a pair of sail lines were repeated to form a time-lapse seismic test. We processed these in 2019 to evaluate their geometric and seismic repeatability. Geometric repetition accuracy was excellent, with source repositioning errors below 10 m and bin-based receiver positioning errors below 6.25 m. Seismic data comparisons showed normalized root-mean-square difference values below 10% between 40 and 150 Hz. Refinements to the acquisition system since 2014 are expected to further improve repeatability of the low-frequency components. Residual energy on 4D difference seismic data was low, and timing stability was good. We conclude that the acquisition system is well suited to time-lapse seismic surveying in areas where the reservoir and time-lapse seismic signal can be adequately imaged by small-source, short-offset, low-fold data.

Broadband seismic over/under sources and their designature-deghosting

Geophysics ◽  
2017 ◽  
Vol 82 (5) ◽  
pp. P61-P73 ◽  
Author(s):  
Lasse Amundsen ◽  
Ørjan Pedersen ◽  
Are Osen ◽  
Johan O. A. Robertsson ◽  
Martin Landrø
Keyword(s):  
Seismic Data ◽  
High Frequency ◽  
Low Frequency ◽  
Point Sources ◽  
High Frequency Side ◽  
Frequency Content ◽  
Source Depth ◽  
Deep Sources ◽  
The Cost ◽  
High Frequency Content

The source depth influences the frequency band of seismic data. Due to the source ghost effect, it is advantageous to deploy sources deep to enhance the low-frequency content of seismic data. But, for a given source volume, the bubble period decreases with the source depth, thereby degrading the low-frequency content. At the same time, deep sources reduce the seismic bandwidth. Deploying sources at shallower depths has the opposite effects. A shallow source provides improved high-frequency content at the cost of degraded low-frequency content due to the ghosting effect, whereas the bubble period increases with a lesser source depth, thereby slightly improving the low-frequency content. A solution to the challenge of extending the bandwidth on the low- and high-frequency side is to deploy over/under sources, in which sources are towed at two depths. We have developed a mathematical ghost model for over/under point sources fired in sequential and simultaneous modes, and we have found an inverse model, which on common receiver gathers can jointly perform designature and deghosting of the over/under source measurements. We relate the model for simultaneous mode shooting to recent work on general multidepth level array sources, with previous known solutions. Two numerical examples related to over/under sequential shooting develop the main principles and the viability of the method.

Frequency-dependent velocity analysis and offset-dependent low-frequency amplitude anomalies from hydrocarbon-bearing reservoirs in the southern North Sea, Norwegian sector

Geophysics ◽  
2017 ◽  
Vol 82 (6) ◽  
pp. N51-N60 ◽  
Author(s):  
Sayyid Suhail Ahmad ◽  
R. James Brown ◽  
Alejandro Escalona ◽  
Børge O. Rosland
Keyword(s):  
North Sea ◽  
High Frequency ◽  
Spectral Decomposition ◽  
Low Frequency ◽  
Velocity Analysis ◽  
Frequency Dependent ◽  
Lower Velocity ◽  
The North ◽  
Significant Difference ◽  
The North Sea

Our aim was to identify some of the characteristics of low-frequency anomalies. Specifically, we have looked, in 3D broadband data from the North Sea, for any offset dependence in these anomalies and any frequency-related change in normal moveout (NMO) velocity that could influence stacking power over different frequencies. After high-resolution spectral decomposition, two types of low-frequency anomaly have been identified associated with hydrocarbon-bearing reservoirs: (1) at the reservoir top and (2) below the reservoir, with a time delay of approximately 100–200 ms. Both types of anomalies indicate offset dependence. On the near-offset stacks, they are relatively strong, but they tend to be absent on the far-offset stacks. In addition, horizon velocity analysis, which was performed along the horizons picked at the tops of reservoir and nonreservoir intervals, has revealed frequency-dependent NMO velocity. For nonreservoir events, we found no significant difference between the NMO velocities for the low-frequency and high-frequency filtered common-midpoint gathers. However, along the anomalously low-frequency events observed at the tops of, and below, oil-bearing reservoirs, lower velocity is observed for low-frequency and higher velocity for high-frequency filtered gathers. If these properties turn out to be universally typical, increased understanding and inclusion of them could lead to improved workflows and help increase the reliability of low-frequency analysis as a hydrocarbon indicator.

Time-lapse seismic analysis of overburden water injection at the Ekofisk field, southern North Sea

Geophysics ◽  
2020 ◽  
Vol 85 (1) ◽  
pp. B9-B21
Author(s):  
Filipe Borges ◽  
Martin Landrø ◽  
Kenneth Duffaut
Keyword(s):  
North Sea ◽  
Seismic Data ◽  
Seismic Event ◽  
Seismic Analysis ◽  
Water Injection ◽  
Time Lapse ◽  
Reservoir Compaction ◽  
Southern North Sea ◽  
Stress Changes ◽  
Before And After

On 7 May 2001, a seismic event occurred in the southern North Sea in the vicinity of the Ekofisk platform area. Analysis of seismological recordings of this event indicated that the epicenter is likely within the northern part of the field and its hypocenter lies in the shallow sedimentary layer. Further investigation in this same area revealed a small seabed uplift and identified an unintentional water injection in the overburden. The injection presumably caused the seabed uplift in addition to stress changes in the overburden. To better understand the consequences of this water injection, we analyze marine seismic data acquired before and after the seismological event. The 4D analysis reveals a clear traveltime shift close to the injection well, as well as a weak amplitude difference. We find that these measured time shifts correspond reasonably well with modeled time shifts based on a simple geomechanical model. The modeling also correlates well with the observed bathymetry changes at the seabed and with global positioning system measurements at the platforms. Although no explicit amplitude sign of the seismic event could be detected in the seismic data, the modeled stress changes, combined with the effect of decades of production-induced reservoir compaction, suggest a source mechanism for the far-field seismological recordings of the May 7th event.

scholarly journals Effect of grain-scale gas patches on the seismic properties of double porosity rocks

2016 ◽  
Vol 208 (1) ◽  
pp. 432-436 ◽  
Author(s):  
Stanislav Glubokovskikh ◽  
Boris Gurevich
Keyword(s):  
High Frequency ◽  
Low Frequency ◽  
Rock Physics ◽  
Time Lapse ◽  
Double Porosity ◽  
Ultrasonic Velocities ◽  
Seismic Properties ◽  
Partially Saturated ◽  
Frequency Regime ◽  
Wave Induced

Time-lapse ultrasonic measurements constitute a tool to establish and calibrate rock physics models for surface seismic monitoring of partially saturated rocks. This workflow requires one to take into account seismic dispersion caused by frequency-dependent wave-induced fluid flow. We develop a theory of squirt flow in rocks saturated with a viscoelastic material containing isolated gas patches between compliant intergranular contacts. This model is valid for the entire frequency range, from seismic to ultrasonic. In the limit of full saturation the derived equations reduce to the Gassmann equations in the low-frequency regime and traditional squirt theory in the high-frequency regime. The model prediction of ultrasonic velocities versus saturation matches with experimental observations.

The influence of the planted geophone on seismic land data

Geophysics ◽  
1980 ◽  
Vol 45 (8) ◽  
pp. 1239-1253 ◽  
Author(s):  
G. M. Hoover ◽  
J. T. O’Brien
Keyword(s):  
Seismic Data ◽  
High Frequency ◽  
Mineral Exploration ◽  
Low Frequency ◽  
Soil Consolidation ◽  
Pass Filter ◽  
Low Pass Filter ◽  
Strong Damping ◽  
Local Soil ◽  
Oscillatory Coupling

Characteristics of the seismic data acquisition system that previously have been ignored become important as more sophisticated interpretive methods based on broader frequency bandwidths are developed to extract stratigraphic information from land data in hydrocarbon and mineral exploration. Theoretical and experimental results indicate that the geophone plant can be approximated by a damped oscillatory coupling, properties dependent upon the geophone mass, dimension of earth contact, and local soil consolidation. A massive geophone with minimal earth contact exhibits a low‐frequency plant resonance with weak damping and acts as a low‐pass filter to eliminate the high‐frequency components of a recorded signal. A lightweight geophone with large earth contact exhibits a high‐frequency plant resonance with strong damping and, consequently, filtering effects are minimal if the plant resonance is well above the signal bandwidth. Although signal filtering influences are weak for strong damping, phase delays can introduce errors of several milliseconds which resemble static errors. Additional complications arise since these time shifts are frequency dependent and, consequently, not identical for all reflection events in a seismic trace. The resonant frequency of the geophone plant increases with increased soil consolidation; however, damping demonstrates only a weak dependence upon consolidation. All of these factors can limit the effectiveness of common‐depth‐point (CDP) stacking methods if the proper technique is not practiced in the acquisition of broad‐bandwidth seismic data.

scholarly journals Tutorial: Spectral bandwidth extension — Invention versus harmonic extrapolation

Geophysics ◽  
2017 ◽  
Vol 82 (4) ◽  
pp. W1-W16 ◽  
Author(s):  
Chen Liang ◽  
John Castagna ◽  
Ricardo Zavala Torres
Keyword(s):  
Seismic Data ◽  
High Frequency ◽  
Low Frequency ◽  
Original Data ◽  
Spectral Bandwidth ◽  
Bandwidth Extension ◽  
Seismic Resolution ◽  
Earth Structures ◽  
Seismic Sections ◽  
Phase Acceleration

Various postprocessing methods can be applied to seismic data to extend the spectral bandwidth and potentially increase the seismic resolution. Frequency invention techniques, including phase acceleration and loop reconvolution, produce spectrally broadened seismic sections but arbitrarily create high frequencies without a physical basis. Tests in extending the bandwidth of low-frequency synthetics using these methods indicate that the invented frequencies do not tie high-frequency synthetics generated from the same reflectivity series. Furthermore, synthetic wedge models indicate that the invented high-frequency seismic traces do not improve thin-layer resolution. Frequency invention outputs may serve as useful attributes, but they should not be used for quantitative work and do not improve actual resolution. On the other hand, under appropriate circumstances, layer frequency responses can be extrapolated to frequencies outside the band of the original data using spectral periodicities determined from within the original seismic bandwidth. This can be accomplished by harmonic extrapolation. For blocky earth structures, synthetic tests show that such spectral extrapolation can readily double the bandwidth, even in the presence of noise. Wedge models illustrate the resulting resolution improvement. Synthetic tests suggest that the more complicated the earth structure, the less valid the bandwidth extension that harmonic extrapolation can achieve. Tests of the frequency invention methods and harmonic extrapolation on field seismic data demonstrate that (1) the frequency invention methods modify the original seismic band such that the original data cannot be recovered by simple band-pass filtering, whereas harmonic extrapolation can be filtered back to the original band with good fidelity and (2) harmonic extrapolation exhibits acceptable ties between real and synthetic seismic data outside the original seismic band, whereas frequency invention methods have unfavorable well ties in the cases studied.

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