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Summary We present a finite-difference method for modeling seismic wave propagation in media in non-welded contact. Traditionally, two alternate approaches to finite-differencing have been used: the so-called homogeneous and heterogeneous approaches. In earlier work, non-welded contact has only been modeled using the heterogeneous approach; however, using the homogeneous approach offers possible advantages. The goal of this study was to investigate the application of the homogeneous approach to the problem of finite-difference modeling of non-welded contact. Introduction The subsurface is fractured on a wide range of scales, from microcracks to faults. Fracturing affects the mechanical properties of the medium such as reflectivity and anisotropy, as well as properties such as permeability. It is therefore of significant interest to investigate the seismic response of a fractured medium, as it may yield valuable information about a range of medium properties.
- Geophysics > Seismic Surveying > Seismic Modeling (0.49)
- Geophysics > Seismic Surveying > Seismic Processing (0.32)
Summary Laboratory experiments and numerical simulations have shown that static shear stress acting on a compliant fracture with irregular surfaces changes the scattering behavior of seismic waves (Nakagawa et al., 2000a). This effect is clearly seen for waves normally incident on the fracture that result in reflected and transmitted S-waves converted from a P-wave, and P-waves converted from an S-wave. For waves obliquely incident on the fracture, transmission and reflection coefficients of scattered waves are no longer symmetric about the normal incidence direction. These behaviors can be predicted by the seismic displacement-discontinuity boundary conditions (linear slip interface boundary conditions) by adding coupling terms to the fracture stiffness. Using this model, wave propagation is examined for a medium containing a single set of multiple parallel infinite fractures subjected to static shear stress.
SUMMARY An important issue in reservoir geophysics is to determine whether attenuation/dispersion measurements can be used to predict fluid filled fractures in tight gas sand reservoirs. To address this issue, we assume that fractures in a poroelastic medium can be represented by the permeability, squirt flow and stiffness constants tensors. We also assume that the principal axis of these tensors is aligned with the horizontal x-axis of symmetry. We construct models based on reservoir parameters from the Almond formation of the Siberia Ridge field, Wyoming. We predict attenuation responses by varying the permeability and squirt flow lengths in the plane of the cracks. We also vary the azimuthal and incident angles to predict the appropriate frequency range for detecting fractures. We analyze the sensitivity of the squirt flow length to attenuation and predict the contribution of fracture permeability in low permeability tight gas sands.
- North America > United States > Colorado (0.69)
- North America > United States > Wyoming > Sweetwater County (0.25)
- North America > United States > Wyoming > Siberia Ridge Field (0.99)
- North America > United States > Wyoming > Sand Wash Basin (0.99)
- North America > United States > Wyoming > Greater Green River Basin > Almond Formation (0.99)
- (8 more...)
- Reservoir Description and Dynamics > Reservoir Fluid Dynamics > Flow in porous media (1.00)
- Reservoir Description and Dynamics > Reservoir Characterization > Seismic processing and interpretation (1.00)
- Reservoir Description and Dynamics > Reservoir Characterization > Exploration, development, structural geology (1.00)
Summary Petrophysical parameters such as permeability and geophysical measurements such as sound velocity and attenuation are known to depend on porosity and the distribution of fractures. This research has investigated the effect of lithological parameters on fracture distribution and acoustic properties of synthetic sandstone core. The primary objective of this research was to develop a greater understanding of the relationships between fractures and petrophysical and geophysical parameters. The anisotropic properties observed were solely attributed to aligned fractures, which were observed both macroscopically and microscopically, with no evidence of grain alignment. Fractures within the synthetic sandstones were controlled by forming the core under a range of known forming stresses (sF), and after relaxation, studying the effects of the induced fracture system using petrophysical and acoustic techniques.
- Geology > Geological Subdiscipline > Geomechanics (1.00)
- Geology > Rock Type > Sedimentary Rock > Clastic Rock > Sandstone (0.83)
- Geology > Structural Geology > Tectonics > Plate Tectonics (0.71)
Shock Tube Experiments And the Observation of the Biot Slow Wave In Natural Rocks
Brown, Philip J. (Colorado School of Mines) | Batzle, Mike (Colorado School of Mines) | Peeters, Max (Colorado School of Mines) | Dey-Sakar, Samir (Colorado School of Mines) | Steensma, Gilein (Colorado School of Mines)
Summary A shock tube was used to study acoustic wave propagation in natural sandstone samples collected from the Lyons, Fox Hills, Berea, and Bentheimer formations. These samples had a permeability range of 0.3 to 3000 mD and a porosity range of 9 to 24 percent. Experimental results were obtained for both fully saturated and partially saturated specimens. Empirical information collected on wave velocities, attenuation and pore pressure are compared to a one-dimensional model. Such a comparison demonstrates that the model accurately predicts wave velocity but overestimates amplitude. For both the Berea and Bentheimer sandstone, the slow compressional wave was observed for permeabilities down to 200 mD. No slow wave was observed in any partially saturated samples. A fracture induced into one of the Berea samples parallel to the wave propagation direction produced inconclusive results.
- Europe > Netherlands (0.31)
- North America > United States (0.22)
- Geology > Rock Type > Sedimentary Rock > Clastic Rock > Sandstone (0.49)
- Geology > Geological Subdiscipline > Geomechanics (0.35)
Summary The Austin Chalk is a naturally fractured reservoir with predominantly vertical aligned fractures. In this study, we use a practical method to determine the principal fracture orientations in the Austin Chalk in Gonzales County, Texas. The database consists of nine 2-D P-wave seismic lines of different azimuths. The raw seismic data was processed through a sequence that preserved the relative changes of amplitudes with offset. The AVO gradient at every CDP in a seismic line was calculated. Then, the median AVO gradient of all the CDPs in a seismic line was chosen to represent AVO of the whole line. This procedure was repeated for all seismic lines. Well log data indicates that the Austin Chalk and its underlying formation, the Eagleford Shale, are fairly homogeneous in the study area.
- North America > United States > Texas > West Gulf Coast Tertiary Basin > Eagle Ford Shale Formation (0.99)
- North America > United States > Texas > Sabinas - Rio Grande Basin > Eagle Ford Shale Formation (0.99)
- North America > United States > Texas > Maverick Basin > Eagle Ford Shale Formation (0.99)
- (9 more...)
VSP Fracture Imaging With Elastic Reverse-time Migration
Nihei, Kurt T. (Earth Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA.) | Nakagawa, Seiji (Earth Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA.) | Myer, Larry R. (Earth Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA.)
Summary Much of the effort to date on fracture detection and characterization has focused on methods based on an equivalent anisotropy descriptions of the fractured rock mass. However, with the growing use of downhole receiver arrays and higher frequency sources, new prospects may emerge for imaging discrete fractures and fractured zones. This paper examines the seismic signatures of an open, fluid-filled fracture numerically, and outlines an approach for elastic, prestack reverse-time migration of VSP data for imaging fractures. The results of this study highlight the importance of incorporating fracture-generated converted waves into the imaging method, and presents an alternate imaging condition that can be used in elastic reverse-time migration when a direct wave is recorded (e.g., crosswell and VSP acquisition geometries).
Summary Fractures in earth formations are a primary source of seismic anisotropy. Anisotropy measurement thus provides a means of fracture characterization. Using theoretical modeling and field results, this study demonstrates the feasibility of fracture measurements in open and cased boreholes using cross-dipole acoustic logging. Theory and field results are found to be in good agreement. However, orthogonal fracture sets may render a limitation for the anisotropy measurement. The interaction between the two fracture sets reduces the magnitude of anisotropy, which will be demonstrated by modeling and field results. Introduction Characterization of fractures in earth formations is an important task in hydrocarbon exploration because fractures provide conduits for reservoir fluid flow.
- North America > United States (0.47)
- Europe > Norway > Norwegian Sea (0.46)
- Research Report > New Finding (0.88)
- Research Report > Experimental Study (0.88)
- Geophysics > Borehole Geophysics (1.00)
- Geophysics > Seismic Surveying > Seismic Modeling > Velocity Modeling (0.49)
Fracture Roughness: The Key to Relating Seismic Velocities, Seismic Attenuation And Permeability to Reservoir Pressure And Saturation
Brown, Raymon L. (Oklahoma Geological Survey) | Wiggins, Michael (Department of Petroleum and Geological Engineering, University of Oklahoma, Norman, Oklahoma) | Gupta, Anuj (Department of Petroleum and Geological Engineering, University of Oklahoma, Norman, Oklahoma)
Summary The roughness of a fracture surface plays a role in the fracture permeability, the fracture compliance and the pressure dependence of the fracture aperture. Ultimately these properties of fractures are related to the time-dependent properties of a fractured reservoir. Since all reservoirs are fractured to some extent, these properties of fractures may have a more general application than is generally believed. It is proposed here that the fracture roughness also controls the seismic attenuation associated with fluid motion within fractures during the passage of a seismic P- or S-wave through a fractured reservoir. In fact, a shear wave can become more sensitive to fluid content than P-waves as a result of fracture roughness. A fracture modeling procedure is presented that can be used to relate seismic velocities and attenuation to the permeability and the pressure dependence of the permeability tensor for fractured reservoirs. Issues of scale and frequency dependence are discussed.
- Geophysics > Seismic Surveying > Seismic Processing (0.61)
- Geophysics > Seismic Surveying > Seismic Modeling > Velocity Modeling (0.61)
In contrast, the shallow dipping fracture sets identified with the purple Ribbon trend and fracture set 8 may represent synsedimentary slumping or gravity-driven sliding. This low-angle faulting is generally poorly imaged by FaultMagic, since the coherence algorithm operates in the vertical domain. This is a limitation of the technique and while predominantly vertical faults are imaged, this does not disprove the existence of low-angle faults. The faults identified by this work enhance and refine the Magnus slump structural model, and do not conflict with it. When integrated into the Magnus structural model, our work provides a powerful tool in understanding subsurface complexity. In particular it provides valuable input to engineering model data and helps explain communication pathways between some wells and possible upward migration of hydrocarbons through thin sand bodies. It also provides useful insights into predicting structural complexity and net sand presence at future well locations. Although not a complete solution, it forces a re-examination of existing geological and engineering models and in turn can have a real impact on oil production.
- Geology > Structural Geology > Fault (0.94)
- Geology > Geological Subdiscipline > Geomechanics (0.73)
- Europe > United Kingdom > North Sea > Northern North Sea > East Shetland Basin > Block 211/7a > Magnus Field > Kimmeridge Formation > Magnus Formation (0.99)
- Europe > United Kingdom > North Sea > Northern North Sea > East Shetland Basin > Block 211/7a > Magnus Field > Kimmeridge Formation > Lower Kimmeridge Clay Formation (0.99)
- Europe > United Kingdom > North Sea > Northern North Sea > East Shetland Basin > Block 211/12a > Magnus Field > Kimmeridge Formation > Magnus Formation (0.99)
- (2 more...)