Estimation of layer thickness and velocity changes using 4D prestack seismic data

Geophysics ◽  
2006 ◽  
Vol 71 (6) ◽  
pp. S219-S234 ◽  
Author(s):  
Thomas Røste ◽  
Alexey Stovas ◽  
Martin Landrø
Keyword(s):  
Layer Thickness ◽  
Seismic Data ◽  
Single Layer ◽  
Time Lapse ◽  
Seismic Velocities ◽  
Seismic Line ◽  
Velocity Increase ◽  
Reservoir Compaction ◽  
Velocity Changes ◽  
Zero Offset

In some hydrocarbon reservoirs, severe compaction of the reservoir rocks is observed. This compaction is caused by production and is often associated with stretching and arching of the overburden rocks. Time-lapse seismic data can be used to monitor these processes. Since compaction and stretching cause changes in layer thickness as well as seismic velocities, it is crucial to develop methods to distinguish between the two effects. We introduce a new method based on detailed analysis of time-lapse prestack seismic data. The equations are derived assuming that the entire model consists of only one single layer with no vertical velocity variations. The method incorporates lateral variations in (relative) velocity changes by utilizing zero-offset and offset-dependent time shifts. To test the method, we design a 2D synthetic model that undergoes severe reservoir compaction as well as stretching of the overburden rocks. Finally, we utilize the method to analyze a real 2D prestack time-lapse seismic line from the Valhall field, acquired in 1992 and 2002. For a horizon at a depth of around [Formula: see text], which is near the top reservoir horizon, a subsidence of [Formula: see text] and a velocity decrease of [Formula: see text] for the sequence from the sea surface to the top reservoir horizon are estimated. By assuming that the base of the reservoir remains constant in depth, a reservoir compaction of 3.6% (corresponding to a subsidence of the top reservoir horizon of [Formula: see text]) and a corresponding reservoir velocity increase of 6.7% (corresponding to a velocity increase of [Formula: see text]) are estimated.

Theory of traveltime shifts around compacting reservoirs: 3D solutions for heterogeneous anisotropic media

Geophysics ◽  
2009 ◽  
Vol 74 (1) ◽  
pp. D25-D36 ◽  
Author(s):  
Rodrigo Felício Fuck ◽  
Andrey Bakulin ◽  
Ilya Tsvankin
Keyword(s):  
Isotropic Medium ◽  
Transversely Isotropic ◽  
Anisotropic Media ◽  
Time Lapse ◽  
Theory Of Elasticity ◽  
Closed Form Expression ◽  
Reservoir Compaction ◽  
Velocity Changes ◽  
Zero Offset ◽  
Induced Velocity

Time-lapse traveltime shifts of reflection events recorded above hydrocarbon reservoirs can be used to monitor production-related compaction and pore-pressure changes. Existing methodology, however, is limited to zero-offset rays and cannot be applied to traveltime shifts measured on prestack seismic data. We give an analytic 3D description of stress-related traveltime shifts for rays propagating along arbitrary trajectories in heterogeneous anisotropic media. The nonlinear theory of elasticity helps to express the velocity changes in and around the reservoir through the excess stresses associated with reservoir compaction. Because this stress-induced velocity field is both heterogeneous and anisotropic, it should be studied using prestack traveltimes or amplitudes. Then we obtain the traveltime shifts by first-order perturbation of traveltimes that accounts not only for the velocity changes but also for 3D deformation of reflectors. The resulting closed-form expression can be used efficiently for numerical modeling of traveltime shifts and, ultimately, for reconstructing the stress distribution around compacting reservoirs. The analytic results are applied to a 2D model of a compacting rectangular reservoir embedded in an initially homogeneous and isotropic medium. The computed velocity changes around the reservoir are caused primarily by deviatoric stresses and produce a transversely isotropic medium with a variable orientation of the symmetry axis and substantial values of the Thomsen parameters [Formula: see text] and [Formula: see text]. The offset dependence of the traveltime shifts should play a crucial role in estimating the anisotropy parameters and compaction-related deviatoric stress components.

Pore-pressure detection sensitivities tested with time-lapse seismic data

Geophysics ◽  
2005 ◽  
Vol 70 (6) ◽  
pp. O39-O50 ◽  
Author(s):  
Øyvind Kvam ◽  
Martin Landrø
Keyword(s):  
Pore Pressure ◽  
Seismic Data ◽  
Time Lapse ◽  
Pressure Increase ◽  
Burial Depth ◽  
Velocity Analysis ◽  
Seismic Velocities ◽  
Data Set ◽  
Pore Pressure Prediction ◽  
Velocity Changes

In an exploration context, pore-pressure prediction from seismic data relies on the fact that seismic velocities depend on pore pressure. Conventional velocity analysis is a tool that may form the basis for obtaining interval velocities for this purpose. However, velocity analysis is inaccurate, and in this paper we focus on the possibilities and limitations of using velocity analysis for pore-pressure prediction. A time-lapse seismic data set from a segment that has undergone a pore-pressure increase of 5 to 7 MPa between the two surveys is analyzed for velocity changes using detailed velocity analysis. A synthetic time-lapse survey is used to test the sensitivity of the velocity analysis with respect to noise. The analysis shows that the pore-pressure increase cannot be detected by conventional velocity analysis because the uncertainty is much greater than the expected velocity change for a reservoir of the given thickness and burial depth. Finally, by applying amplitude-variation-with-offset (AVO) analysis to the same data, we demonstrate that seismic amplitude analysis may yield more precise information about velocity changes than velocity analysis.

Quantitative estimation of compaction and velocity changes using 4D impedance and traveltime changes

Geophysics ◽  
2004 ◽  
Vol 69 (4) ◽  
pp. 949-957 ◽  
Author(s):  
Martin Landrø ◽  
Jan Stammeijer
Keyword(s):  
Seismic Data ◽  
Quantitative Estimation ◽  
Time Lapse ◽  
Seismic Velocities ◽  
Compaction Process ◽  
Empirical Relationships ◽  
Velocity Change ◽  
Reservoir Rocks ◽  
Velocity Changes ◽  
4D Seismic

In some hydrocarbon reservoirs, severe compaction of the reservoir rocks is observed. This compaction is caused by production, and it is often associated with changes in the overburden. Time‐lapse (or 4D) seismic data are used to monitor this compaction process. Since the compaction causes changes in both layer thickness and seismic velocities, it is crucial to distinguish between the two effects. Two new seismic methods for monitoring compacting reservoirs are introduced, one based on measured seismic prestack traveltime changes, and the other based on poststack traveltime and amplitude changes. In contrast to earlier methods, these methods do not require additional empirical relationships, such as, for instance, a velocity‐porosity relationship. The uncertainties in estimates for compaction and velocity change are expressed in terms of errors in the traveltime and amplitude measurements. These errors are directly related to the quality and repeatability of time‐lapse seismic data. For a reservoir at 3000‐m depth with 9 m of compaction, and assuming a 4D timeshift error of 0.5 ms at near offset and 2 ms at far offset, we find relative uncertainty in the compaction estimate of approximately 50–60% using traveltime information only.

Using sub-daily timelapse imagery to investigate the behaviour of Narsap Sermia, SW Greenland.

2021 ◽  
Author(s):  
Dominik Fahrner ◽  
James Lea ◽  
Stephen Brough ◽  
Jakob Abermann
Keyword(s):  
Feature Tracking ◽  
Greenland Ice Sheet ◽  
Time Lapse ◽  
High Temporal Resolution ◽  
Time Intervals ◽  
Velocity Increase ◽  
Tidewater Glaciers ◽  
Time Period ◽  
Velocity Changes ◽  
General Velocity

<p>Greenland’s tidewater glaciers (TWG) have been retreating since the mid-1990s, contributing to mass loss from the Greenland Ice Sheet and sea level rise. Satellite imagery has been widely used to investigate TWG behaviour and determine the response of TWGs to climate. However, multi-day revisit times make it difficult to determine short-term processes such as calving and shorter-term velocity changes that may condition this. </p><p>Here we present velocity, calving and proglacial plume data derived from hourly time-lapse images of Narsap Sermia, SW Greenland for the period July 2017 to June 2020 (n=13,513). Raw images were orthorectified using the <em>Image GeoRectification And Feature Tracking toolbox</em> (ImGRAFT; Messerli & Grinsted, 2015) using a smoothed ArcticDEM tile from 2016 (RMSE=44.4px). TWG flow velocities were determined using ImGRAFT feature tracking, with post-processing adjusting for varying time intervals between image acquisitions (if >1 hour) and removing outliers (>x2 mean). The high temporal resolution of the imagery also enabled the manual mapping of proglacial plume sizes from the orthorectified images and the recording of individual calving events by visually comparing images.</p><p>Results show a total retreat of approximately 700 m, with a general velocity increase from ~15 m/d to ~20 m/d over the investigated time period and highly variable hourly velocities (±12m/d). The number of calving events and plume sizes remain relatively stable from year to year throughout the observation period. However, later in the record plumes appear earlier in the year and the size of calved icebergs increases significantly, which suggests a change in calving behaviour. </p>

SEISMIC VELOCITIES IN THE SOUTHEASTERN SAN JOAQUIN VALLEY OF CALIFORNIA

Geophysics ◽  
1941 ◽  
Vol 6 (4) ◽  
pp. 327-355
Author(s):  
E. J. Stulken
Keyword(s):  
Seismic Data ◽  
Seismic Velocity ◽  
San Joaquin Valley ◽  
Seismic Velocities ◽  
Velocity Measurements ◽  
Constant Depth ◽  
Entire Area ◽  
Velocity Gradients ◽  
Velocity Changes ◽  
First Time

For the first time, seismic velocity measurements from well surveys have been made intensively enough to justify an analysis of the velocity field in an entire area instead of just along lines between wells. Maps are drawn showing velocity changes in the southeastern San Joaquin Valley of California. A portion of the valley floor in the neighborhood of Bakersfield, about twenty‐five miles wide and thirty‐five miles long, was chosen for study because of the number of wells in the area whose velocities were known. Differences in average velocity of 1700 feet per second for a constant depth are observed, and horizontal velocity gradients averaging over 100 feet per second per mile are computed. Correction schemes for the adjustment of seismic data are suggested, and correction maps shown. An attempt is made to establish a connection between stratigraphy and seismic velocity. Comparative study of the logs of wells and the velocities observed in them yields certain qualitative conclusions, but attempts to express the relation in a quantitative way fail.

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 Influence of microheterogeneity on effective stress law for elastic properties of rocks

Geophysics ◽  
2008 ◽  
Vol 73 (1) ◽  
pp. E7-E14 ◽  
Author(s):  
Radim Ciz ◽  
Anthony F. Siggins ◽  
Boris Gurevich ◽  
Jack Dvorkin
Keyword(s):  
Effective Stress ◽  
Seismic Data ◽  
Elastic Moduli ◽  
Seismic Velocity ◽  
Time Lapse ◽  
Seismic Velocities ◽  
Laboratory Measurements ◽  
Hydrocarbon Production ◽  
Stress Coefficient ◽  
Seismic Measurements

Understanding the effective stress coefficient for seismic velocity is important for geophysical applications such as overpressure prediction from seismic data as well as for hydrocarbon production and monitoring using time-lapse seismic measurements. This quantity is still not completely understood. Laboratory measurements show that the seismic velocities as a function of effective stress yield effective stress coefficients less than one and usually vary between 0.5 and 1. At the same time, theoretical analysis shows that for an idealized monomineral rock, the effective stress coefficient for elastic moduli (and therefore also for seismic velocities) will always equal one. We explore whether this deviation of the effective stress coefficient from unity can be caused by the spatial microheterogeneity of the rock. The results show that only a small amount (less than 1%) of a very soft component is sufficient to cause this effect. Such soft material may be present in grain contact areas of many rocks and may explain the variation observed experimentally.

Wave‐equation datuming before stack

Geophysics ◽  
1984 ◽  
Vol 49 (11) ◽  
pp. 2064-2066 ◽  
Author(s):  
John R. Berryhill
Keyword(s):  
Wave Equation ◽  
Seismic Data ◽  
Arbitrary Shape ◽  
Forward Modeling ◽  
Common Source ◽  
Computationally Efficient ◽  
Seismic Line ◽  
Mathematical Algorithm ◽  
Zero Offset ◽  
Kirchhoff Integral

This note describes the extension to unstacked seismic data of a computationally efficient form of the Kirchhoff integral published several years ago. In the previous paper (Berryhill, 1979), a wave‐equation procedure was developed to change the datum of a collection of zero‐offset seismic traces from one surface of arbitrary shape to another, even when the velocity for wave propagation is not constant. This procedure was designated “wave‐equation datuming,” and its applications to zero‐offset data were shown to include velocity‐replacement datum corrections and multilayer forward modeling. Extending this procedure to unstacked data requires no change in the mathematical algorithm. It is necessary only to recognize that operating on a common‐source group of seismic traces has the effect of extrapolating the receivers from one datum to another, and that, because of reciprocity, operating on a common‐receiver group changes the datum of the sources. Two passes through the data, common‐source computations, then common‐receiver computations, are required to change the datum of an entire seismic line before stack from one surface to another. Common‐source and common‐receiver trace groups must take the form of symmetric split spreads if both directions of dip are to be treated equally; reciprocity allows split spreads to be constructed artificially if the data were not actually recorded in the required form.

scholarly journals Investigation of core data reliability to support time-lapse interpretation in Campos Basin, Brazil

Geophysics ◽  
2008 ◽  
Vol 73 (2) ◽  
pp. E59-E65 ◽  
Author(s):  
Marcos Hexsel Grochau ◽  
Boris Gurevich
Keyword(s):  
Seismic Data ◽  
Low Frequency ◽  
Time Lapse ◽  
Compressional Wave ◽  
Stress Sensitivity ◽  
Pressure Effects ◽  
Seismic Velocities ◽  
Quantitative Interpretation ◽  
Core Damage ◽  
Ultrasonic Measurements

Quantitative interpretation of time-lapse seismic data requires knowledge of saturation and pressure effects on seismic velocities. Although the former can usually be modeled adequately using the Gassmann equation, the latter is obtained mainly by laboratory measurements, which can be affected by core damage. We investigate the magnitude of this effect on compressional-wave velocities by comparing laboratory experiments and log measurements. We use Gassmann fluid substitution to obtain low-frequency saturated velocities from dry core measurements (thus mitigating the dispersion effects) taken at reservoir pressure. The analysis is performed for an unusual densely cored well from which 43 cores were extracted over a [Formula: see text]-thick turbidite reservoir. These computed velocities show very good agreement with the sonic-log measurements. This confirms that, for this particular region, the effect of core damage on ultrasonic measurements is less than the measurement error. Consequently, stress sensitivity of elastic properties as obtained from ultrasonic measurements is adequate for quantitative interpretation of time-lapse seismic data.

Uncertainty in determining interval velocities from surface reflection seismic data

Geophysics ◽  
2002 ◽  
Vol 67 (3) ◽  
pp. 952-963 ◽  
Author(s):  
Dan D. Kosloff ◽  
Yonadav Sudman
Keyword(s):  
Layer Thickness ◽  
Seismic Data ◽  
Single Layer ◽  
Seismic Survey ◽  
Fourier Domain ◽  
Layer Depth ◽  
Surface Reflection ◽  
Reflection Seismic ◽  
Spurious Oscillations ◽  
Interval Velocities

The ability of reflection seismic data to uniquely determine the subsurface velocity has been uncertain. This paper uses a tomographic approach to study the resolution of typical seismic survey configurations. The analysis is first carried out in the spatial Fourier domain for the case of a single horizontal reflector. It is found that for a ratio of maximum offset to layer depth of one, the lateral resolution is very low for velocity and interface depth variations of wavelengths of approximately two‐and‐a‐half times the layer thickness. The resolution improves with an increase in the ratio of maximum offset to layer depth. The results of the analysis in the Fourier domain are confirmed by results from a least‐squares tomographic algorithm. It is found that regularization of the tomography by adding damping terms suppresses the spurious oscillations resulting from the areas of low resolution at the expense of loss of resolution at the shorter spatial wavelengths. Analysis of the single layer response for 3‐D survey geometry shows that a 3‐D acquisition with multiazimuthal coverage has the potential to significantly improve velocity determination.

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