S U M M A R YWe have carried out two seismic physical experiments to acquire wide-azimuth P-wave 3-D seismic data with a scaled down model (1:10 000) and scaled-up frequencies (10 000:1). Our aims are to verify the physical basis of using P-wave attributes for fracture detection, to understand the usage of these attributes and their merits, and to investigate the effects of acquisition geometry and structural variations on these attributes. The base model consists of a fractured layer sandwiched between two isotropic layers (Epoxylite). Inside the fractured layer there is a dome and a fault block for investigating the effects of structural variations. The two experiments were carried out using different acquisition geometries. The first experiment was conducted to maximize the data quality, with an offset-depth ratio of only 0.68 to the bottom of the fracture layer. For comparison, the second experiment was carried out to maximize the anisotropy effects, with the offset-depth ratio to the bottom of the fracture layer raised to 1.34.For each experiment, about 20 km 2 of wide-azimuth 3-D data were acquired with a P-wave source. The physical modelling confirms that the P-wave attributes (traveltime, amplitude and velocity) exhibit azimuthal variations diagnostic of fracture-induced anisotropy. For the first experiment with noise-free data, the amplitude from the top of the fracture layer yields the best results that agree with the physical model parameters and free of the acquisition footprint. The results from other attributes (traveltime, velocity, AVO gradient) are either contaminated by the structural imprint, or by the acquisition footprint due to the lack of offset coverage. For the second experiment, despite the interferences from multiples and other coherent noise, the traveltime attributes yield the best results; both the acquisition footprint and the structural imprint are reduced due to the increased offset coverage. However, the results from the amplitudes are affected by the noise and are less reliable. Analysis of the two experiments reveals that the offset-depth ratio to the target is a key parameter for the success of the P-wave techniques. Smaller offset-depth coverage may only be applicable to amplitude attributes with high quality data; whilst large offset coverage makes it possible to use traveltime attributes. A reliable estimation from traveltime attributes requires an offset-depth ratio of 1.0 or more.
Geophysicists have devoted great efforts to the problem of determining fluid saturation from seismic measurements, with some notable successes. It is commonly believed that fluid information is to be found in the P-wave data, with shear-waves being insensitive to fluid, and indeed almost all successful fluid-detection methodologies have been based on analysis of the P-wave.Nevertheless, when we deal with fractured reservoirs, we are faced with the phenomenon of seismic anisotropy, and the rock physics relationships relevant to fractured, anisotropic, rock are more subtle than those for the more familiar isotropic case. In particular, shear-wave splitting occurs, and is known to be sensitive to the fracture properties and fluid bulk modulus. It has long been hoped that analysis of shear-wave splitting in mulitcomponent data would be able to improve our ability to detect fluids.More recent theoretical advances in the area of frequency-dependent anisotropy have offered a new approach to this problem. These theories allow anisotropic dispersion and attenuation to be related to rock and fluid properties, typically through a fluid mobility parameter, defined as the ratio of permeability to fluid viscosity.The ability to detect a viscosity effect is of great potential relevance to the problem of oil-water discrimination. Oil and water have similar bulk moduli, and this fact has impeded efforts to tell the two apart from analysis of seismic data. The two fluids have markedly different viscosities, however, so if we can find a robust seismic signature of fluid viscosity we would greatly improve our chances to discriminate the two fluid saturations.In this paper we offer a theoretical analysis of wave propagation in vertically fractured rock which exhibits frequency-dependent anisotropy. For angles of incidence typical of seismic reflection data, we show that it is the slow-shear wave which suffers most attenuation and whose properties are most sensitive to fluid viscosity. Further to this, we demonstrate that the converted-wave amplitude in the fracture normal direction can be very sensitive to the fluid, even when the P-wave attributes are insensitive to fluid.Based on our analysis, we devise a processing strategy which is applied to 3D3C data from Shengli oilfield in China, which has undergone water flooding. We find amplitude and traveltime anomalies which correlate with known water and oil saturated zones, and which are consistent with the effects predicted by our theoretical modelling. We conclude that proper processing and interpretation of multicomponent data can help us to discriminate oil and water saturation in fractured reservoirs. Theoretical backgroundWe consider fracture distributions in porous rock which are approximately aligned and vertical. Traditional techniques for modelling the elastic properties of such systems make use of static equivalent medium theories in the long wavelength limit. Such theories have many features in common, and the simplest can be shown to be consistent with the notation of Sch...
Geophysicists have devoted great efforts to the problem of determining fluid saturation from seismic measurements, with some notable successes. It is commonly believed that fluid information is to be found in the P-wave data, with shear-waves being insensitive to fluid, and indeed almost all successful fluid-detection methodologies have been based on analysis of the P-wave.Nevertheless, when we deal with fractured reservoirs, we are faced with the phenomenon of seismic anisotropy, and the rock physics relationships relevant to fractured, anisotropic, rock are more subtle than those for the more familiar isotropic case. In particular, shear-wave splitting occurs, and is known to be sensitive to the fracture properties and fluid bulk modulus. It has long been hoped that analysis of shear-wave splitting in mulitcomponent data would be able to improve our ability to detect fluids.More recent theoretical advances in the area of frequency-dependent anisotropy have offered a new approach to this problem. These theories allow anisotropic dispersion and attenuation to be related to rock and fluid properties, typically through a fluid mobility parameter, defined as the ratio of permeability to fluid viscosity.The ability to detect a viscosity effect is of great potential relevance to the problem of oil-water discrimination. Oil and water have similar bulk moduli, and this fact has impeded efforts to tell the two apart from analysis of seismic data. The two fluids have markedly different viscosities, however, so if we can find a robust seismic signature of fluid viscosity we would greatly improve our chances to discriminate the two fluid saturations.In this paper we offer a theoretical analysis of wave propagation in vertically fractured rock which exhibits frequency-dependent anisotropy. For angles of incidence typical of seismic reflection data, we show that it is the slow-shear wave which suffers most attenuation and whose properties are most sensitive to fluid viscosity. Further to this, we demonstrate that the converted-wave amplitude in the fracture normal direction can be very sensitive to the fluid, even when the P-wave attributes are insensitive to fluid.Based on our analysis, we devise a processing strategy which is applied to 3D3C data from Shengli oilfield in China, which has undergone water flooding. We find amplitude and traveltime anomalies which correlate with known water and oil saturated zones, and which are consistent with the effects predicted by our theoretical modelling. We conclude that proper processing and interpretation of multicomponent data can help us to discriminate oil and water saturation in fractured reservoirs. Theoretical backgroundWe consider fracture distributions in porous rock which are approximately aligned and vertical. Traditional techniques for modelling the elastic properties of such systems make use of static equivalent medium theories in the long wavelength limit. Such theories have many features in common, and the simplest can be shown to be consistent with the notation of Sch...
Gaussian beam migration (GBM) is an effective imaging method that has the ability to image multiple arrivals while preserving the advantages of ray-based methods. We have extended this method to linearized least-squares imaging for elastic waves in isotropic media. We have dynamically transformed the multicomponent data to the principal components of different wave modes using the polarization information available in the beam migration process, and then we use Gaussian beams as wavefield propagator to construct the forward modeling and adjoint migration operators. Based on the constructed operators, we formulate a least-squares migration scheme that is iteratively solved using a preconditioned conjugate gradient method. With this method, we can obtain crosstalk-attenuated multiwave images with better subsurface illumination and higher resolution than those of the conventional elastic Gaussian beam migration. This method also allows us to achieve a good balance between computational cost and imaging accuracy, which are both important requirements for iterative least-squares migrations. Numerical tests on two synthetic data sets demonstrate the validity and effectiveness of our proposed method.
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