The delineation and characterization of fracturing is important in the successful exploitation of many hydrocarbon reservoirs. Such fracturing often occurs in preferentially aligned sets; if the fractures are of subseismic scale, this may result in seismic anisotropy. Thus, measurements of anisotropy from seismic data may be used to delineate fracture patterns and investigate their properties. Here fracture-induced anisotropy is investigated in the Valhall field, which lies in the Norwegian sector of the North Sea. This field is a chalk reservoir with good porosity but variable permeability, where fractures may significantly impact production, e.g., during waterflooding. To investigate the nature of fracturing in this reservoir, P-wave amplitude variation with offset and azimuth (AVOA) is analyzed in a 3D ocean-bottom cable (OBC) data set. In general, 3D ocean-bottom seismic (OBS) acquisition leads to patchy coverage in offset and azimuth, and this must be addressed when considering such data. To overcome this challenge and others associated with 3D OBS acquisition, a new method for processing and analysis is presented. For example, a surface fitting approach, which involves analyzing azimuthal variations in AVO gradients, is used to estimate the orientation and magnitude of the fracture-induced anisotropy. This approach is also more widely applicable to offset-azimuth analysis of other attributes (e.g., traveltimes) and any data set where there has been true 3D data acquisition, land or marine. Using this new methodology, we derive high-resolution maps of P-wave anisotropy from the AVOA analysis for the top-chalk reflection at Valhall. These anisotropy maps show coherent but laterally varying trends. Synthetic AVOA modeling, using effective medium models, indicates that if this anisotropy is from aligned fracturing, the fractures are likely liquid filled with small aspect ratios and the fracture density must be high. Furthermore, we show that the fracture-normal direction is parallel to the direction of most positive AVO gradient. In other situations the reverse can be true, i.e., the fracture-normal direction can be parallel to the direction of the most negative AVO gradient. Effective medium modeling or comparisons with anisotropy estimates from other approaches (e.g., azimuthal variations in velocity) must therefore be used to resolve this ambiguity. The inferred fracture orientations and anisotropy magnitudes show a degree of correlation with the positions and alignments of larger scale faults, which are estimated from 3D coherency analysis. Overall, this work demonstrates that significant insight may be gained into the alignment and character of fracturing and the stress field variations throughout a field using this high-resolution AVOA method.

/pdf/fracture-characterization-at-valhall-application-of-p-wave-2hu3te6n4g.pdf

Fracture characterization at Valhall: Application of P-wave amplitude variation with offset and azimuth (AVOA) analysis to a 3D ocean-bottom data set

Wide-azimuth seismic data can be used to derive anisotropic parameters on the subsurface by observing variation in subsurface seismic response along different azimuths. Layer-based high-resolution estimates of components of the subsurface anisotropic elastic tensor can be reconstructed by using wide-azimuth P-wave data by combining the kinematic information derived from anisotropic velocity analysis with dynamic information obtained from amplitude versus angle and azimuth analysis of wide-azimuth seismic data. Interval P-impedance, S-impedance and anisotropic parameters associated with anisotropic fracture media are being reconstructed using linearized analysis assuming horizontal transverse anisotropy symmetry. In this paper it is shown how additional assumptions, such as the rock model, can be used to reduce the degrees of freedom in the estimation problem and recover all five anisotropic parameters. Because the use of a rock model is needed, the derived elastic parameters are consistent with the rock model and are used to infer fractured rock properties using stochastic rock physics inversion. The inversion is based on stochastic rock physics modelling and maximum a posteriori estimate of both porosity and crack density parameters associated with the observed elastic parameters derived from both velocity and amplitude versus angle and azimuth analysis. While the focus of this study is on the use of P-wave reflection data, we also show how additional information such as shear wave splitting and/or anisotropic well log data can reduce the assumptions needed to derive elastic parameter and rock properties.

Reconstruction of the layer anisotropic elastic parameters and high-resolution fracture characterization from P-wave data: a case study using seismic inversion and Bayesian rock physics parameter estimation

This paper investigates Q-anisotropy for characterizing fractured reservoirs — specifically, the variation of the seismic quality factor Q versus offset and azimuth QVOA .W e derive an analytical expression for P-wave attenuation in a transversely isotropic medium with horizontal symmetry axis HTI and provide a method QVOA for estimating fracture direction from azimuthally varying Q in PP-wave reflection data. The QVOA formula is similar to Ruger’s approximation for PP-wave reflection coefficients, the theoretical basis for amplitude variation with angle offset AVOA analysis. The technique for QVOA analysis is similar to azimuthal AVO analysis. We introduce two new seismic attributes: Q versus offset QVO gradient and intercept. QVO gradient inversion not only indicates fracture orientation but also characterizes Q-anisotropy. We relate the Q-anisotropy parameter Q to fractured-medium parameters and invert the QVO gradient to estimate Q. The attenuation parameter Q and Thomsen-style anisotropy parameter V are found to be interdependent. The attenuation anisotropy magnitude strongly depends on the host rock’s VS/VP parameter, whereas the dependence on fracture parameters is weak. This complicates the QVO gradient inversion for the fracture parameters. This result is independent of the attenuation mechanism. To illustrate the QVOA method in synthetic data, we use Hudson’s first-order effective-medium model of a dissipative fractured reservoir with fluid flow between aligned cracks and random pores as a possible mechanism for P-wave attenuation.

QVOA analysis: P-wave attenuation anisotropy for fracture characterization

A system of aligned vertical fractures produces azimuthal variations in stacking velocity and amplitude variation with offset, characteristics often reported in seismic reflection data for hydrocarbon exploration. Studies of associated attenuation anisotropy have been mostly theoretical, laboratory or vertical seismic profiling based. We used an 11 common-midpoint-long portion of each of four marine surface-seismic reflection profiles, intersecting each other at 45° within circa 100 m of a common location, to measure the azimuthal variation of effective attenuation, Q−1eff and stacking velocity, in a shallow interval, about 100 m thick, in which consistently orientated vertical fracturing was expected due to an underlying salt diapirism. We found qualitative and quantitative consistency between the azimuthal variation in the attenuation and stacking velocity, and published amplitude variation with offset results. The 135° azimuth line showed the least apparent attenuation (1000 Q−1eff= 16 ± 7) and the fastest stacking velocity, hence we infer it to be closest to the fracture trend: the orthogonal 45° line showed the most apparent attenuation (1000Q−1eff= 52 ± 15) and slowest stacking velocity. The variation of Q−1eff with azimuth φ is well fitted by 1000Q−1eff= 34 − 18cos[2(φ+40°)] giving a fracture direction of 140 ± 23° (±1SD, derived from ‘bootstrapping’ fits to all 114 combinations of individual common-midpoint/azimuth measurements), compared to 134 ± 47° from published amplitude variation with offset data. The effects of short-window spectral estimation and choices of spectral ratio bandwidth and offset ranges used in attenuation analysis, individually give uncertainties of up to ±13° in fracture direction. This magnitude of azimuthal variation can be produced by credible crack geometries (e.g., dry cracks, radius 6.5 m, aspect ratio 3 × 10−5, crack density 0.2) but we do not claim these to be the actual properties of the interval studied, because of the lack of well control (and its consequences for the choice of theoretical model and host rock physical properties) and the small number of azimuths available here.

/pdf/anisotropic-p-wave-attenuation-measured-from-a-multi-azimuth-197jbfqwi3.pdf

Anisotropic P‐wave attenuation measured from a multi‐azimuth surface seismic reflection survey

An algorithm is proposed for determining the fracture orientation based on the azimuthal variations in the P-wave reflection moveout for a target interval. The differential moveout between orthogonal survey lines from the bottom of a given target shows cos 2ϕ variations with the line azimuth ϕ measured from the fracture strike for a fixed offset. A configuration of four intersecting survey lines may be used to quantify the fracture strike. The four lines form two orthogonal pairs, and the fracture strike can be obtained by analyzing the crossplot of the two corresponding pairs of the differential moveouts. An offset‐depth ratio (x/z) of 1.0 or greater (up to 1.5) is often required to quantify the moveout difference reliably. The sensitivity of the method is further enhanced by low/high impedance contrast at the top target interface but is greatly reduced by high/low impedance contrast. The method may be particularly useful in marine exploration with repeated surveys of various vintages where continuous az...

/pdf/fracture-detection-using-azimuthal-variation-of-p-wave-3yfv9yoirc.pdf

Fracture detection using azimuthal variation of P-wave moveout from orthogonal seismic survey lines

Azimuthal Anisotropy Analysis From P-wave Seismic Traveltime Data

Methods and apparatus for processing dual sensor (e.g., hydrophone and vertical geophone) data that includes intrinsic removal of noise as well as enhancing the wavefield separation are provided. The methods disclosed herein are based on a decomposition of data simultaneously into dip and frequency while retaining temporal locality. The noise removed may be mainly coherent geophone noise from the vertical geophone, also known as V(z) noise.

Geophone noise attenuation and wavefield separation using a multi-dimensional decomposition technique

Principal directions for an azimuthally anisotropic medium are the directions along which the quasi P and quasi S waves propagate as pure P and S modes. For azimuthal anisotropy, induced by a single set of oriented vertical fractures, two of these principal directions are the directions parallel to and perpendicular to the fractures. To investigate if conventional P-wave data could be used in fracture detection, we computed synthetic data for an isotropic Taylor shale over a fractured Austin chalk model. We found that the modelled P-wave reflection amplitude variation with offset has different slopes along different directions. We also found that the reflection amplitude at fixed offsets is periodic in 2f where f is the orientation angle of the shooting direction with respect to one of the principal directions. For fracture induced anisotropy, this direction corresponds to the direction parallel to or perpendicular to the fractures. We use this periodic azimuthal dependence to obtain fracture orientation and a qualitative measure of the fracture density from the azimuthal P-wave data. We applied our technique to real P-wave data, collected over a wide source-to-receiver azimuth. Computations using our method gave an orientation of the principal direction consistent with the general fracture orientation in the area as inferred from other geological and geophysical evidence.

Computation of principal directions of azimuthal anisotropy from P‐wave seismic data

Computation of principal directions of azimuthal anisotropy from P-wave seismic data

Methods and apparatus for removing noise from signal traces collected during a seismic gather, particularly removing the aliased energy in the time-slowness (tau-P) domain so that aliasing noise does not lead to high noise levels in the inverse transformed data are provided. Removal of the aliasing noise may provide for quieter seismic traces and may increase the likelihood for successful three-dimensional (3-D) tau-P interpolation and detection of seismic events.

3-d tau-p interpolation

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