Mike Ammerman

Application of new seismic attributes to collapse chimneys in the Fort Worth Basin

Three-dimensional seismic volumes from the central Fort Worth Basin display roughly circular collapse chimneys that extend vertically about 800 m from the Ordovician Ellenburger Formation to the Atokan (middle Pennsylvanian) Caddo Limestone. Collapse chimneys in carbonates may be caused by subaerial karst, hydrothermal, or tectonic extensional processes. We use 3D multitrace geometric attributes including coherence, volumetric curvature, and energy-weighted, coherent amplitude gradients to investigate details of the origin of these structures. The Ordovician Ellenburger surface resembles a subaerial karst landscape of cockpits, dolines, and frying-pan valleys, while resistivity-based wireline image logs record 50 m of karst breccia facies. However, images from coherence and long-wavelength most-positive and most-negative-curvature volumes show many of the 800-m collapse features are associated with basement faults or with subtle Pennsylvanian and younger tectonic features, rather than with intra-Ellenburg...

Derivation of Anisotropy Parameters In a Shale Using Borehole Sonic Data

Anisotropy plays a key role in the processing and interpretation of seismic data. Failure to account for its presence can lead to errors in prestack depth migration, amplitude variation with offset (AVO), and hydraulic fracture monitoring. A type of anisotropy often observed in shales is vertical transverse isotropy (VTI) or polar anisotropy. This type of anisotropy can be quantified by estimating the three Thomsen parameters epsilon (e), delta (δ), and gamma (γ) (Thomsen, 1986). Methods for estimating these parameters (and in particular epsilon and delta) have utilized trial and error scanning after migrating with the well velocities, tomographic inversion using well misties as a constraint, and measuring these parameters directly from walkaway VSP, multi-offset VSP, or multiazimuthal VSP recordings (Miller, et al, 1994) (Leaney, et al, 1999) The latter method has been used to update anisotropic models for prestack depth migration. Although it is recognized that walkaway VSPs can provide an important calibration point for refining anisotropic models, such data is not always available. Various conditions may limit the ability to record VSPs. The logistics of performing such a survey can be a prohibitive factor as well as the cost of these surveys.

Fracture Characterization Through the Use of Azimuthally Sectored Attribute Volumes

Azimuthal volumes have often been looked at in conjunction with velocity anisotropy to determine the strike of fractures or maximum stress direction. Due to variations in the directions of fractures, attributes calculated on azimuthal volumes may also provide insight into the strike and dip, location, and spatial variation of the fractures. In particular, attributes highlighting the differences in frequency, time, and amplitude can effectively isolate areas of fractures (Tod et al 2007). If these seismic experiments can detect naturally-occurring fractures in reservoirs then why not use the same principles to detect hydraulicallyinduced fractures? The techniques discussed in this paper will show how these azimuthally sectored volumes and their attributes can be used to map the hydraulicallyinduced fractures in the Barnett Shale in the Ft. Worth Basin. Areas of pre-existing hydraulic fractures are typically avoided because of the potential interference with other wells in the area. We demonstrate how prior geologic knowledge of areas of hydraulically-induced fractures will be used in conjunction with the azimuthal volumes in order to effectively map the fractures.

Elastic Properties of Four Shales Reconstructed From Laboratory Measurements At Unloaded Conditions

Summary The elastic properties of four shales (Barnett, Woodford, Muscogee, and Caney) are studied on 11 samples at the room conditions with the help of specially developed and constructed apparatus, allowing us to measure the elastic wave velocities (VP, VSH, and VSV) in different directions relative to the axis of cylindrical sample. The difference in the angular behavior of the velocities is attributed to the different mineral composition and microfabric of the shales. The stiffness tensors of shales are reconstructed from the measurements taking into account the experimental errors in velocities and rotation angle and possible disorientation of the sample axis relative to the elastic symmetry axis. The closest VTI stiffness tensors are inverted for all shale samples, and the accuracy of such an approximation is given in terms of relative average errors between experimental and theoretical velocities. These errors are shown to vary from 0.5% to 8%. These errors can be decreased if the tensor is approximated by the monoclinic symmetry in the laboratory coordinate system.

Comparison of Seismic Upscaling Methods

Random presence of various minerals, cracks and voids in different proportion make a rock inhomogeneous. These heterogeneities may be characterized by their different elastic constants. The size of heterogeneities can vary from micro-scale to macro-scale, and therefore the elastic properties of a rock become scale dependent. There has been great deal of efforts to estimate the effective properties of the heterogeneous materials, such as upper and lower bounds, self-consistent theory, differential effective medium theory; are based on the assumption that the inclusions occur in a particular shape. The mathematical formulations, based upon the solution of singularly perturbed linear equations approximation, partial differential equations, stochastic differential equations, ordinary differential equations and Markov chains, are used to estimate effective media properties . But these approximation methods, generally, do not take into account the interaction between heterogeneities. Our upscaling method is based upon pairand multispatial correlation function. Comparison of these two methods has been done with Backus averaging and simple running window averaging. Results indicate that the upscaling based pairand multicorrelation function are in good agreement to the original data.

Automated detection and location of hydrofracking‐induced microseismic event from 3C observations in an offsetting monitor well

The idea of construction of algorithm and results of developed software on the determination of microearthquake locations due to frac job is given. Influence of anisotropy on the accuracy of frac event locations is briefly discussed. Methodology of determination of horizontal component of geophones is developed based on perfshots and earlier located frac events data. Produced software works in real time and in automated regime.

3D Velocity Reconstruction In Shale Derived From Limited Number of Measurements

A technique allowing the determination of 3D distribution of both compressional and shear wave velocities in shales from limited number of experimental data has been developed. The technique involves an EMT (effective medium theory)-based inversion of shale’s microstructure (clay platelet orientation, crack shape and their connectivity). The knowledge of the microstructure and shale’s mineral composition makes it possible to find the shale’s stiffness tensor with the help of EMT methods. The stiffness tensor and density are necessary data to obtain the 3D distribution of velocities in shale. We demonstrate that this technique is applicable if only Vp is measured in some directions in the normal-to-bedding plane of shale, or Vp and Vs measurements are available only in the directions parallel and normal to bedding.

Determination of Stiffness Tensor of Shale From Logging Data

Summary A method is elaborated allowing a determination of the stiffness tensor of shales from logging data. The logging data includes sonic logs VP and VS (or VP, VS1 and VS2), density, porosity, and clay content. The stiffness tensor is inverted from this data with the help of effective medium theory (EMT). This method is applied to illite-rich shale of the Mississippian age. As a result a distribution of the components of stiffness tensor along the well is obtained at the sonic frequency (2 kHz). The symmetry of the shale’s stiffness tensor is found to be slightly orthorhombic, and taking into account the experimental errors, can be approximated by the VTI symmetry. For the productive interval of the shale formation, the average Thomsen

Behavior of Shear waves in water- and gas- filled cracks in anisotropic matrix

Summary Shales are complex porous rock materials, and can be considered as an aggregate of matrix and inclusions of various properties. The inclusions are primarily characterized by the contrast in elastic constants from the matrix. We have considered two types of shale samples: Huron shale (clay content = 62%), and Red bed member shale (clay content = 11.5%); for the modeling in which the clay minerals are mostly illite (87 %). The modeling has been carried out in mainly three steps: (a) define the clay matrix properties, K and μ for isotropic matrix, and five independent elastic constants for the anisotropic matrix with transversely isotropic symmetry (VTI), (b) introduce quartz and other minerals as inclusions, and (c) introduce fractures, either aligned, or randomly distributed. The general singular approximation (GSA) method, which takes in to account the connectedness of, has been used to calculate the effective elastic constants of the shale. Water and gas filled cracks have been considered in the modeling, and phase velocities for the two cases have been calculated. Modeling results show that the shale having water filled cracks shows singularity point in fast and slow shear waves, which is absent in shale with gas filled cracks.

Predicting S-Wave Anisotropy from P-Wave Anisotropy

Summary The developed method in this paper uses P-wave phase velocity to calculate the 21 independent elastic constants that define a general anisotropic medium. In this method we seek a solution close to a reference body and compute the deviation from the reference. Having calculated the elastic constants, we predict S-wave anisotropy. Forward modeling promises good results and we attribute this performance on the number of velocity measurements and the choice of a reference body. We inverted P-wave velocity measurements of the mantle rocks for stiffnesses in the frame work of a vertical transversely isotropic (VTI) medium and found these stiffnesses to be in good agreement with the published data based on mineralogy. We implemented this idea on 3D reflection seismic data that was processed for P-wave anisotropy. The predicted Swave anisotropy follows the P-wave anisotropy.