Shiguang Guo

Noise suppression of time-migrated gathers using prestack structure-oriented filtering

AbstractPrestack seismic analysis provides information on rock properties, lithology, fluid content, and the orientation and intensity of anisotropy. However, such analysis demands high-quality seismic data. Unfortunately, noise is always present in seismic data even after careful processing. Noise in the prestack gathers may not only contaminate the seismic image, thereby lowering the quality of seismic interpretation, but it may also bias the seismic prestack inversion for rock properties, such as acoustic- and shear-impedance estimation. Common postmigration data conditioning includes running window median and Radon filters that are applied to the flattened common reflection point gathers. We have combined filters across the offset and azimuth with edge-preserving filters along the structure to construct a true “5D” filter that preserves amplitude, thereby preconditioning the data for subsequent quantitative analysis. We have evaluated our workflow by applying it to a prestack seismic volume acquired o...

Vector correlation of amplitude variation with azimuth and curvature in a post-hydraulic-fracture Barnett Shale survey

AbstractKnowledge of induced fractures can help to evaluate the success of reservoir stimulation. Seismic P-waves through fracturing media can exhibit azimuthal variation in traveltime, amplitude, and thin-bed tuning, so amplitude variation with azimuth (AVAz) can be used to evaluate the hydraulic-fracturing-caused anisotropy. The Barnett Shale of the Fort Worth Basin was the first large-scale commercial shale gas play. We have analyzed two adjacent Barnett Shale seismic surveys: one acquired before hydraulic fracturing and the other acquired after hydraulic fracturing by more than 400 wells. Although not a rigorous time-lapse experiment, comparison of AVAz anisotropy of these two surveys provided valuable insight into the possible effects of hydraulic fracturing. We found that in the survey acquired prior to hydraulic fracturing, AVAz anomalies were stronger and highly correlated with major structural lineaments measured by curvature. In contrast, AVAz anomalies in the survey acquired after hydraulic fra...

The effect of shale properties on the anisotropic brittleness criterion index from laboratory study

Shale anisotropy has a significant impact on the brittleness index and hydraulic fracturing. First of all, we improve the Rickmans brittleness index by introducing the anisotropy parameters and angle parameters based on Rickmans brittleness index (BI). Then the anisotropic brittleness criterion index (BCI) is proposed by introducing failure criteria parameters to balance the shortcomings of the conventional BI index. The new brittleness index is applied to the laboratory measurements of samples of Santa Maria Basin shale formation. The anisotropic BCI might be closer to the actual value of brittleness in the fracturing process by considering the stress of crack expansion. The value of the BCI increases as porosity decreases, thus the porosity log is an effective parameter and can help us establish a good BI or BCI predication rock models. We show that total organic carbon (TOC) is not reliable for predicting BI or BCI because there is no linear relationship between BI, BCI and TOC. It is interesting that delta (δ) is more sensitive to BCI than epsilon (e) and gamma (γ). It appears to have a good linear relationship with the BCI. It might be reliable to predict the BCI of shale formation of the Santa Maria Basin by δ. By introducing the relationship between pressure and BCI, we find that shale samples with different TOC show regular changes of brittleness index under different pressures. The effect of stress needs to be considered in the analysis of BCI and one must choose the appropriate stress for fracturing.

Noise suppression using preconditioned least-squares prestack time migration: Application to the Mississippian Limestone

Conventional Kirchhoff migration often suffers from artifacts such as aliasing and acquisition footprint, which come from sub-optimal seismic acquisition. The footprint can mask faults and fractures, while aliased noise can focus into false coherent events which affect interpretation and contaminate amplitude variation with offset, amplitude variation with azimuth and elastic inversion. Preconditioned least-squares migration minimizes these artifacts. We implement least-squares migration by minimizing the difference between the original data and the modeled demigrated data using an iterative conjugate gradient scheme. Unpreconditioned least-squares migration better estimates the subsurface amplitude, but does not suppress aliasing. In this work, we precondition the results by applying a 3D prestack structure-oriented LUM (lower–upper–middle) filter to each common offset and common azimuth gather at each iteration. The preconditioning algorithm not only suppresses aliasing of both signal and noise, but also improves the convergence rate. We apply the new preconditioned least-squares migration to the Marmousi model and demonstrate how it can improve the seismic image compared with conventional migration, and then apply it to one survey acquired over a new resource play in the Mid-Continent, USA. The acquisition footprint from the targets is attenuated and the signal to noise ratio is enhanced. To demonstrate the impact on interpretation, we generate a suite of seismic attributes to image the Mississippian limestone, and show that the karst-enhanced fractures in the Mississippian limestone can be better illuminated.

Data conditioning of legacy seismic using migration-driven 5D interpolation

AbstractLegacy seismic surveys cover much of the midcontinent USA and Texas, with almost all 3D surveys acquired in the 1990s considered today to be low fold. Fortunately, recent advances in 5D interpolation have not only enhanced the quality of structural and stratigraphic images, but they have also improved the data sufficiently to allow more quantitative interpretation, such as impedance inversion. Although normal-moveout-corrected, common-midpoint-based 5D interpolation does an excellent job of amplitude balancing and the suppression of acquisition footprint, it appears to misinterpolate undercorrected diffractions, thus smearing fault and stratigraphic edges. We described a least-squares migration-driven 5D interpolation workflow, in which data were interpolated by demigrating the current subsurface image to the missing offsets and azimuths. Such demigration accurately interpolates fault edges and other diffractors, thereby preserving lateral discontinuities, while suppressing footprint and balancing...

Sensitivity of shale anisotropic parameters to core cutting rotation error

In laboratories, core cannot always be cut exactly along the axis of symmetry (normal to the bedding plane), which leads to a minor core cutting rotation (CCR) error. The presence of a small CCR error can give rise to an error in computation of anisotropic parameters, which will result in erroneous P-wave and S-wave velocities (VP, VSV and VSH). In this study, we test the sensitivity of Thomson’s anisotropic parameters, epsilon (ε), gamma (γ) and delta (δ), to the CCR error. In the vertical transverse isotropy (VTI) system where no rotation exists, values of ε, γ and δ will not vary with azimuth. Similarly, in VTI the values of VP, VSV and VSH, measured at orientations with respect to the axis of symmetry, will also not vary with azimuth. We modelled and analysed the error generated, in anisotropic parameters and phase velocities, due to rotation error of 5° on VTI ANNIE model data, and the Niobrara Shale core sample measurements. For these two cases, we obtained values of P-wave and S-wave velocities along with anisotropy parameters ε, γ and δ, at 0°, 45° and 90° orientations with respect to the axis of symmetry, before and after a rotation of 5°, for different azimuth directions. This study shows that, when the CCR error exists, ε, γ and δ vary with azimuth. For Niobrara Shale samples, we observed that, among all three Thomsen’s parameters δ is the most sensitive parameter to the CCR error; when we varied azimuth, we observed a sign change of δ from positive to negative. For ε and γ, variation in azimuth lead to only slight changes in values without any change of sign. The CCR error affects VP and Vs measurements the most at 45° orientation, and the least at 90° orientation. The maximum error occurred at 0° azimuth, whereas the minimum error occurs at 90° azimuth. This analysis suggests that, if core cutting rotation exists, phase velocities should be measured at 90° azimuth for accurate results. In laboratories, core cannot always be cut exactly along the axis of symmetry (normal to the bedding plane), which leads to a minor core cutting rotation error. This paper investigates the sensitivity of Thomson’s anisotropic parameters, epsilon (ε), gamma (γ) and delta (δ), to the cutting error.