Ekofisk Time-Lapse Seismic Monitoring of Injection in Shale

Narongsirikul, Sirikarn

doi:10.3997/2214-4609.201900295

Cited by 3 publications

(1 citation statement)

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“…Long wavelength time-lapse velocity changes close to the reservoir can arise due to the stress arches and pillars induced by reservoir depletion or injection (Hatchell & Bourne, 2005;Røste et al, 2005). Shorter period velocity variations in the overburden can also occur due to, for example, cuttings injection (Narongsirikul, 2019), gas injection (Falahat, 2012), gas blowout (Zadeh & Landrø, 2011) or fault reactivations (Røste et al, 2007). Additionally, time-lapse variations in the reservoir due to pressure and saturation are somewhat mixed in terms of the scale of spatial variation.…”

Section: Pre-stack Variationsmentioning

confidence: 99%

A review and analysis of errors in post‐stack time‐shift interpretation

MacBeth

Izadian

2023

Geophysical Prospecting

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This study complements two earlier papers on the interpretation and estimation of post‐stack time‐shifts that detail the most popular measurement methods to date. In this current work, the focus is on describing the magnitude and identifying the origin of the uncertainty present in these time‐shift values. We also consider how the underlying assumptions behind conventional time‐shift estimation methods can contribute to the inadequate resolution of the subsurface time‐lapse changes. The various errors fall into three broad categories: (1) those related to intrinsic data limits that cannot be avoided; (2) those associated with seismic measurement methods that can be corrected with some effort; and finally (3) those that arise due to approximations to the wave physics made during the design or implementation of the methods. In the first category are limitations due to sampling rate and signal‐to‐noise ratio, and wavelet interferences resulting from the narrow band nature of the seismic data and the heterogeneous nature of the geology itself. The effects produce errors of typically a fraction of a millisecond, but exceptionally in 4D data with poor repeatability up to 1 ms. In the second category are acquisition and processing errors. These are usually of the order of a few milliseconds but can reach up to 10s of milliseconds and are, therefore, essential to correct. It is possible to correct the effects of tides and seawater variations in marine acquisition if these changes are monitored and measured. Navigation and timing are identified as issues to consider carefully. For land data, daily and seasonal near‐surface variations are still problematic, and source and sensors must be buried to deliver interpretable time‐shifts. The impact of choices made during processing can be significant but is specific to the workflow and dataset and thus cannot be generically assessed. However, the effects from residual multiples can be identified and treated. Of moderate importance is the third category of errors, which consists of four items. Of these, the effect of lateral image shifts and amplitude effects are judged to play a minor role. Deviation from the assumption of vertical ray‐paths during post‐stack analysis appears to be of concern only for reservoirs with noticeably dipping structures and strong contrasts. The effect of pre‐stack variations on the post‐stack measurements remains a topic to be more thoroughly examined, and the conclusion is unclear. Finally, it is apparent that uncertainties in all categories are strongly dependent on the field setting and geographical location.

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Section: Pre-stack Variationsmentioning

confidence: 99%

A review and analysis of errors in post‐stack time‐shift interpretation

MacBeth

Izadian

2023

Geophysical Prospecting

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Angle-dependent 4D seismic time-strain inversion for estimating subsurface thickness and velocity changes

Taweesintananon,

Landrø,

Narongsirikul

et al. 2024

GEOPHYSICS

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In a time-lapse (also called 4D) seismic analysis, the time-shift of a certain seismic reflection event is caused by the changes in the seismic velocity and the depth of the event. An interpretation of 4D time-shifts is normally simplified by neglecting displacement changes (strains) or assuming a linear relation between thickness and velocity strains. Here, we go beyond these assumptions and propose a least-squares optimization method to simultaneously estimate the thickness and velocity strains in vertical transverse isotropic (VTI) media from angle-dependent 4D seismic time strains. Through examples from synthetic and field data, we show that the 4D thickness strains, velocity strains, and anisotropic parameter changes can be estimated simultaneously without prior knowledge about the geomechanics of the survey area. Our time-strain inversion method can be applied to any other 4D seismic data set in which angle-stack images are available. We see that our method has high potential in many other applications, because the thickness and velocity strains are the fundamental components of most physical properties used in 4D seismic and geomechanics applications.

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Multi Attribute Approach for Quantifying Competing Time Lapse Effects and Implications for Similarity Indicators in Data Assimilation

Santos

Schiozer

Davólio

2020

SPE Annual Technical Conference and Exhibition

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This study aims to reduce uncertainties related to non-uniqueness in the interpretation of competing 4D effects and their impact to the interpretations and data assimilation into reservoir models. The study is based on synthetic and observed 4D seismic data, tracers, laboratory measurements, production and geophysical well log data. The methodology involves: (1) building and calibrating a petro-elastic model; (2) forward modeling of each separate physical effect such as saturation, pressure, salinity and noise; (3) combining the effects according to simulated production scenarios; and (4) correlating the modeled with observed 4D seismic data. We generate synthetic logs and seismic traces to quantify the time-lapse observations and to analyze how the combination of effects may affect the seismic character. This work demonstrates a major impact of competing 4D effects in the number and types of true possible interpretations. Yet, estimating their influence on the magnitude and polarity of 4D signal is still achievable. We show application examples from a Brazilian turbidite reservoir, in a setting where a combination of 4D effects occur simultaneously and the confidence level on the 4D interpretations is high due to good quality and frequent seismic data from a permanent reservoir monitoring system. In the example presented, a scenario where only water saturation effects occurs (e.g. aquifer water invading oil zone), 12 to 16% P-impedance (IP) increase (hardening effect) is expected. However, according to salinity measurements and tracers, injected sea water with lower salinity than the formation water decreases the hardening effect towards 8 to 10% in IP change. Having identified the impact of salinity changes in the 4D effects, an approach to include such changes in the petro-elastic model is proposed to generate 4D attributes that reproduce this interaction of effects. The outcome is a better match between modeled and observed 4D attributes, as compared to modeled attributes from a conventional petro-elastic model that considers salinity as constant. Additionally, combining these effects with gas going out from solution due to depletion below bubble point, the IP decreases significantly, reversing the polarity of 4D signal, and a 10% increase in gas saturation produces a softening effect (15% IP decrease). Competing 4D seismic effects are often mentioned but rarely quantified, and their resultant non-uniqueness impacts on similarity indicators used in data assimilation processes. As these similarity indicators are generally based on comparisons between observed and modeled seismic, the methodology presented results in an improved confidence on the workflows. Additionally, the methodology proposed is straightforward and adequate to reduce uncertainty related to 4D seismic interpretations.

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Ekofisk Time-Lapse Seismic Monitoring of Injection in Shale

Cited by 3 publications

References 0 publications

A review and analysis of errors in post‐stack time‐shift interpretation

A review and analysis of errors in post‐stack time‐shift interpretation

Angle-dependent 4D seismic time-strain inversion for estimating subsurface thickness and velocity changes

Multi Attribute Approach for Quantifying Competing Time Lapse Effects and Implications for Similarity Indicators in Data Assimilation

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