Estimation of the anisotropy parameters of transversely isotropic shales with a tilted symmetry axis (original) (raw)
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Estimation of anisotropy parameters using the P‐wave velocities on a cylindrical shale sample
SEG Technical Program Expanded Abstracts 2011, 2011
In this paper we present a new approach to the estimation of the Thomsen anisotropy parameters and symmetry axis coordinates from the P-wave traveltime measurements on cylindrical shale samples. Using the tomography-style array of transducers, we measure the ultrasonic P-wave ray velocities to estimate the Thomsen anisotropy parameters for a transversely isotropic shale sample. This approach can be used for core samples cut in any direction with regard to the bedding plane, since we make no assumption about the symmetry axis directions and will estimate it simultaneously with the anisotropy parameters. We use the very fast simulated re-annealing to search for the best possible estimate of the model parameters. The methodology was applied to a synthetic model and an anisotropic shale sample.
73rd EAGE Conference and Exhibition incorporating SPE EUROPEC 2011, 2011
In this paper we present a new approach to the estimation of Thomsen anisotropy parameters from laboratory data on cylindrical rock samples. Using tomography-style transducers array, ultrasonic P-wave ray velocities are measured on a transversely isotropic shale sample. This approach is applied to core samples cut along and normal to the bedding plane. Synthetic and actual laboratory data from an anisotropic shale specimen are used as examples. The fast simulated re-annealing method is used to search for the anisotropy parameters.
P‐wave anisotropy in shales from crosswell data
SEG Technical Program Expanded Abstracts 1994, 1994
A crosswell dataset, collected at Conoco' s Borehole Test Facility in Oklahoma, was first processed using a tomography algorithm based on an isotropic velocity model. The resulting 2-D velocity tomogram was found to have huge artifacts believed to be caused by nonuniform ray coverage and possibly anisotropy. A comparison of the zero offset crosswell velocities (Horizontal Path Log) with the log-derived sonic velocities (Vertical Path Log) indicated nearly 25% P-wave anisotropy in some formations. Despite the artifacts, a good interpretation of the 2-D isotropic tomogram was made with the help of synthetic data. The data were then inverted using an algorithm that incorporates a model of elliptical transverse isotropy. This anisotropy model produced an unambiguous image for two components of velocity that simultaneously match the sonic log and the crosswell data. Both inversions support a final interpretation of 1-D vertical stratification of layers, some of which exhibit significant P-wave anisotropy. This interpretation is consistent with the sonic logs, crosswell data, and available geological information.
Laboratory measurement of P-wave anisotropy in shales with laser ultrasonics
SEG Technical Program Expanded Abstracts 2012, 2012
We evaluated a laser-based noncontacting method to measure the elastic anisotropy of horizontal shale cores. Whereas conventional transducer data contained an ambiguity between phase and group velocity measurements, small laser source and receiver footprints on typical core samples ensured group velocity information in our laboratory measurements. With a single dense acquisition of group velocity versus group angle on a horizontal core, we estimated the elastic constants c 11 , c 33 , and c 55 directly from ultrasonic waveforms, and c 13 from a least-squares fit of modeled to measured group velocities. The observed significant P-wave velocity and attenuation anisotropy in these dry shales were almost surely exaggerated by delamination of clay platelets and microfracturing, but provided an illustration of the new laboratory measurement technique. Although challenges lay ahead to measure preserved shales at in situ conditions in the lab, we evaluated the fundamental advantages of the proposed method over conventional transducer measurements.
Geophysical Prospecting, 2005
Shales are a major component of sedimentary basins, and they play a decisive role in fluid flow and seismic-wave propagation because of their low permeability and anisotropic microstructure. Shale anisotropy needs to be quantified to obtain reliable information on reservoir fluid, lithology and pore pressure from seismic data, and to understand time-to-depth conversion errors and non-hyperbolic moveout. A single anisotropy parameter, Thomsen's δ parameter, is sufficient to explain the difference between the small-offset normal-moveout velocity and vertical velocity, and to interpret the small-offset AVO response.
Ultrasonic velocity and attenuation anisotropy of shales, Whitby, United Kingdom
GEOPHYSICS, 2015
We have conducted ultrasonic experiments, between 0.3 and 1 MHz, to measure velocity and attenuation ([Formula: see text]) anisotropy of P- and S-waves in dry Whitby Mudstone samples as a function of stress. We found the degree of anisotropy to be as large as 70% for velocity and attenuation. The sensitivity of P-wave anisotropy change with applied stress is more conspicuous than for S-waves. The closure of large aspect-ratio pores (and/or micro cracks) seems to be a dominant mechanism controlling the change of anisotropy. Generally, the highest attenuation is perceived for samples that have their bed layering perpendicular ([Formula: see text]) to the wave path. The observed attenuation in the samples is partly due to the scattering on the different layers, and it is partly due to the intrinsic attenuation. Changes in attenuation due to crack closure during the loading stage of the experiment are an indication of the intrinsic attenuation. The remaining attenuation can then be attr...
Mathematical modelling of anisotropy of illite-rich shale
Geophysical Journal International, 2009
The estimation of illite-rich shale anisotropy to account for the alignment of clays and gas-or brine-filled cracks is presented via mathematical modelling. Such estimation requires analysis to interpret the dominance of one effect over another. This knowledge can help to evaluate the permeability in the unconventional reservoir, stress orientation, and the seal capacity for the conventional reservoir.
Seismic anisotropy in exploration and reservoir characterization: An overview
GEOPHYSICS, 2010
Recent advances in parameter estimation and seismic processing have allowed incorporation of anisotropic models into a wide range of seismic methods. In particular, vertical and tilted transverse isotropy are currently treated as an integral part of velocity fields employed in prestack depth migration algorithms, especially those based on the wave equation. We briefly review the state of the art in modeling, processing, and inversion of seismic data for anisotropic media. Topics include optimal parameterization, body-wave modeling methods, P-wave velocity analysis and imaging, processing in the [Formula: see text] domain, anisotropy estimation from vertical-seismic-profiling (VSP) surveys, moveout inversion of wide-azimuth data, amplitude-variation-with-offset (AVO) analysis, processing and applications of shear and mode-converted waves, and fracture characterization. When outlining future trends in anisotropy studies, we emphasize that continued progress in data-acquisition technol...
The elastic anisotrophy of shales
Journal of Geophysical Research, 1994
Shales constitute about 75% of the clastic fill of sedimentary basins and have a decisive effect on fluid flow and seismic wave propagation because of their low permeability and anisotropic microstructure. The elastic stiffnesses of a shale with partially oriented clay particles is expressed in terms of the coefficients Wlmn in an expansion of the clay-particle orientation distribution function in generalized Legendre functions. Application is made to the determination of the anellipticity of shales. For transverse isotropy the anellipticity quantifies the deviation of the P wave slowness curve from an ellipse and is shown to depend on a single coefficient W400 in the expansion of the clay-particle orientation distribution function. If W400 is small, the anellipticity may be neglected, as is apparently the case for a near-surface late Tertiary shale studied by Winterstein and Paulson. Strongly aligned clay particles result in a positive value of W400 and a positive anellipticity, in agreement with the majority of field measurements. However, less well ordered shales could have a significantly positive second moment W200 but only a small positive or even negative value of W400. For such shales the anellipticity would be small or negative despite a preferred alignment of clay particles in the bedding plane. Numerical examples of clay particle orientation distribution functions leading to zero or negative anellipticity are given.
This paper describes a method to determine anisotropy parameter by shear wave splitting of crosswell seismic data. The purpose of this work is to present a section of anisotropy to be used in detecting hydrocarbon. Splitted S-waves were detected by picking the traveltimes of S-fast and S-slow on SH and SV components, respectively. Polarization method was done on the 3-C crosswell data using hodogram analysis, so that from these three components we can observe P-, SH-and SV-waves. Tomography of SH-and SV-wave of crosswell seismic were run to produce SH and SV tomograms. Due to the azimuth randomness of the 3-C receivers, an azimuth and inclination correction were implemented on the data, and some preconditioning were applied to enhance the quality of the firstbreaks including deconvolution, bandpass filtering and fx-decon. The traveltimes were picked on SH and SV components, furthermore to be inversed using traveltime tomography algorithm. In this work, a case study was carried out to the seismic data collected on inter-bedded sand-shale layers. The results of this work are SH-and SV-wave tomograms and the anisotropy section. We conclude that this method is effectively presenting an anisotropy section to be used for reservoir recognition.