Samik Sil

Fluid substitution effects on seismic anisotropy

We derive equations for HTI and orthorhombic symmetries to analyze fluid substitution effects in porous fractured media. The derivations are based on the anisotropic Gassmann equation and linear slip theory. We assess the influence of fluid substitution (gas, brine, and oil) on elastic moduli, velocities, anisotropy, and azimuthal amplitude variations. We find that in the direction normal to fractures, P-wave moduli increase as much as 56% and P-wave velocity increases up to 19% for gas-to-brine substitution. For the direction parallel to fractures, P-wave velocity remains almost constant when porosity is low (5%) but can increase up to 4% if porosity is high (25%). Since P-waves in two different directions have different sensitivities to fluids and fractures, the Thomsens parameters (defined for HTI and orthorhombic symmetries), ϵ and δ, are sensitive to fluid types and fractures. We also found that δ is sensitive to porosity for liquid saturation but insensitive to porosity for the case of gas saturation. Gassmann assumes (and as has been observed) that shear modulus does not depend on fluids. And we observe no changes in shear-wave splitting (γ) for different fluids. The azimuthal amplitude variation is dependent on fluid types, fractures, and porosity. We observe up to 12% increase in azimuthal amplitude variation for low porosity gas sands after brine saturation and 6% decrease for high porosity gas sands. We find that the percentage changes in gas-to-oil substitution are about half that of the gas-to-brine case. The equations we have derived provide a useful tool to quantitatively evaluate the effects of fluid substitution on seismic anisotropy.

Stochastic simulation of fracture strikes using seismic anisotropy induced velocity anomalies

Availability of a fracture map of a producing reservoir aids in increasing productivity. Generally, accurate information related to fracture orientation is only available at a few sparse well log locations. However, fractures introduce velocity anomalies in seismic data by making the medium azimuthally anisotropic. When multi-azimuth data is available then it is possible to map the fracture attributes in the entire reservoir zone by analysing the anisotropy induced velocity anomalies in the seismic data. In the absence of 3D data, seismic anisotropy induced velocity anomaly from 2D data (as fracture strikes are not constant and data contains multi-azimuthal effect even when it is 2D) can still be used as a secondary source of information for the purpose of fracture strike simulation. To validate the above hypothesis, fracture strike information in a reservoir from the Mexican part of the Gulf of Mexico is derived using Markov-Bayes stochastic simulation. In this simulation process, accurate well log derived fracture information is used as hard or primary data and seismic velocity anomaly/uncertainty based fracture information is used as soft or secondary data. The Markov-Bayes Stochastic simulation provides multiple realisations of the fracture patterns and thus helps to estimate the uncertainty associated with the fracture strikes of the reservoir. Accuracy of the simulation process is also estimated and the simulation result is compared with simple and ordinary kriging methods of fracture strike simulation.

Analysis of fluid substitution in a porous and fractured medium

To improve quantitative interpretation of seismic data, we analyze the effect of fluid substitution in a porous and fractured medium on elastic properties and reflection coefficients. This analysis uses closed-form expressions suitable for fluid substitution in transversely isotropic media with a horizontal symmetry axis (HTI). For the HTI medium, the effect of changing porosity and water saturation on (1) P-wave moduli, (2) horizontal and vertical velocities, (3) anisotropic parameters, and (4) reflection coefficients are examined. The effects of fracture density on these four parameters are also studied. For the model used in this study, a 35% increase in porosity lowers the value of P-wave moduli by maximum of 45%. Consistent with the reduction in P-wave moduli, P-wave velocities also decrease by maximum of 17% with a similar increment in porosity. The reduction is always larger for the horizontal P-wave modulus than for the vertical one and is nearly independent of fracture density. The magnitude of the anisotropic parameters of the fractured medium also changes with increased porosity depending on the changes in the value of P-wave moduli. The reflection coefficients at an interface of the fractured medium with an isotropic medium change in accordance with the above observations and lead to an increase in anisotropic amplitude variation with offset (AVO) gradient with porosity. Additionally, we observe a maximum increase in P-wave modulus and velocity by 30% and 8%, respectively, with a 100% increase in water saturation. Water saturation also changes the anisotropic parameters and reflection coefficients. Increase in water saturation considerably increases the magnitude of the anisotropic AVO gradient irrespective of fracture density. From this study, we conclude that porosity and water saturation have a significant impact on the four studied parameters and the impacts are seismically detectable.

Effect of near-surface anisotropy on a deep anisotropic target layer

Summary Near-surface anisotropy can distort P-wave traveltime and amplitude analysis from deep target layers. When the target layer is azimuthally anisotropic, the traveltime/velocity variation with azimuth (VVAZ) or amplitude variation with azimuth (AVAZ) from the target layer may show anomalous behavior due to the influence of the near-surface anisotropy. This study uses two synthetic cases to analyze the effect of near-surface anisotropy on a deep anisotropic target. Our results suggest that the traveltime data (or VVAZ signals) from the target layers can be distorted significantly due to the presence of near-surface anisotropy; but the near-surface anisotropy influence may be negligible on the AVAZ signals from the deep target layer.

A robust workflow for detecting azimuthal anisotropy

Summary The prestack analysis of amplitude variation with offset/angle (AVO/AVA) has been a useful hydrocarbon exploration tool for a number of years. As this technology matured, forward modeling became an important tool to understand the seismic response, to ensure acquisition parameters were adequate for its detection, and to calibrate and validate the data processing sequence and analysis tools. There has been significant recent interest in using azimuthal variations in seismic amplitudes to detect high productivity sweet spots related to in situ natural fractures. The role of forward modeling has not been as significant in azimuthal AVO as in conventional AVO. We describe a robust azimuthal processing analysis workflow based on a series of 1D and 3D elastic modeling exercises designed to validate the azimuthal processing flow and extracted azimuthal attributes. Our results indicate that forward modeling is necessary to understand the offset range that might exhibit an azimuthal amplitude variation and the magnitude of the variation, and its detectability in the presence of noise.

Seismic critical-angle anisotropy analysis in the τ -p domain

Seismic critical-angle reflectometry is a relatively new field for estimating seismic anisotropy parameters. The theory relates changes in the critical angle with azimuth of the seismic line to the principal axis and anisotropy parameters. Current implementation of the critical-angle reflectometry process has certain shortcomings in that the critical angle is determined from critical offset and the process is vulnerable to different approximation errors. Seismic critical-angle analysis in the plane-wave (τ-p) domain can handle these issues and has the potential to become an independent tool for estimating anisotropy parameters. The theory of seismic critical-angle reflectometry is modified to make it suitable for τ-p domain analysis. Then using full-wave synthetic seismograms at three different azimuths for a transversely isotropic medium with a horizontal axis of symmetry (HTI), the effectiveness of anisotropy parameter estimation is demonstrated.

Sensitivity analysis of fluid substitution in a porous medium with aligned fractures

We study the effect of fluid substitution in a porous fractured medium using explicit expressions developed for aligned fractured medium. We investigate the effect of porosity and water saturation on (1) P-wave moduli, (2) horizontal and vertical velocities, (3) anisotropic parameters, and (4) reflection coefficients. Effects of fracture density on these four parameters are also analyzed. The systematic variations of the moduli and reflection coefficients reported in this paper can thus be used in developing AVO with azimuth in a porous fractured reservoir.

Physical modeling of anisotropic domains: Ultrasonic imaging of laser‐etched fractures in glass

Physical modeling, using ultrasonic sources and receivers over scaled exploration structures, plays a useful role in wave propagation and elastic property investigations. This paper explores the anisotropic response of novel fractured glass blocks created with a laser-etching technique. We compare transmitted and reflected signals for Pand Swaves from fractured and unfractured zones in a suite of ultrasonic experiments. The unaltered glass velocities are 5801 m/s and 3448 m/s for P and S waves, respectively, with fractured zones showing a small decrease (about 1%). Signals propagating through the fractured zone have decreased amplitudes and increased coda signatures. Reflection surveys (zero-offset and variable polarization and offset gathers) record significant scatter from the fractured zones. The glass specimens with laser-etched fractures display a rich anisotropic response.

Observation of azimuthal anisotropy on multicomponent Atlantis node seismic data

Summary In this paper we report on the analysis of a multicomponent ocean bottom node dataset collected over Atlantis field in the deep water Gulf of Mexico. Our primary goal is to determine shallow velocity structure in the area. The unique true 3D shooting geometry allows us to carry our azimuthal traveltime analysis of compressional and converted shear wave data recorded on hydrophones and horizontal geophones. The P-wave data show systematic azimuthal variation of traveltime which is analyzed using NMO ellipse. After successful rotation of the horizontal geophones, we notice negligible direct Pwave energy in the transverse component. However, significant energy is observed at later times that can be interpreted as converted S2 mode. Joint analysis of S1 and S2 phases as a function of azimuth reveals systematic shear wave splitting that is consistent with the observation on Pwave data. We determine a NS oriented stress pattern that is consistent with general understanding of stress field in the area.

Azimuthal t‐p analysis in HTI media

Incorporating the effect of anisotropy during seismic processing and estimating anisotropic parameters is an active area of research. In general five elastic coefficients are needed to describe a traveltime curve in a HTI medium. The problem of estimating five elastic parameters by iterative fitting of travel time data from a single azimuth recording is highly non-unique. This can possibly be achieved by simultaneous fitting of multiple azimuth travel time data. However that would require picking travel time and accurate estimation will require numerical ray tracing for multi-layered media. To circumvent these difficulties we propose analysis of plane wave transformed azimuthal gathers interactively using a single azimuth data at a time and a new delay time equation which is a function of two parameters at each azimuth. Results from independently estimated multi-azimuth gathers can be combined to estimate stiffness or Thomsen coefficients. Azimuthal τ-p analysis also avoids numerical ray tracing resulting in a rapid algorithm. We demonstrate the applicability of our method using a set of P wave synthetic seismograms from a multi-layered medium consisting of isotropic and HTI layers. Azimuth dependent anisotropy parameters are derived by delay time fitting and NMO correction. The reflections from the bottom interface of an isotropic layer with an anisotropic overburden show apparent anisotropic travel time behavior which is easily accounted for by our layer-stripping based azimuthal NMO analysis.