Friday, December 22, 2017

Interpretation of Actual AVO

How do real AVO gathers compare with synthetics? The real gather observed at the Texas gas well and carefully processed shows the same AVO signature as the synthetic gather generated using log data from the well. Both gathers show a small negative at normal incidence that becomes more negative with offset. This signals hydrocarbons, and sure enough, the well did produce gas. The gradient and intercept are both negative, and their product positive. A section composed of product traces from every gather in the seismic line shows a zone of positive product. A second well drilled in the zone confirmed the presence of gas.

 Because the synthetic was built from log data - density, compressional velocity and shear velocity - rather than estimated values, it closely matches the observed gather. Often estimated is shear velocity, and this creates a common stumbling block to AVO modeling. Dramatic AVO effects appear in gas sands where the shear velocity is often too slow to be measured with conventional sonic tools. Introduction of the DSI tool removes this impediment.

Once the AVO signature of hydrocarbons is known, seismic data can be examined for fluids. For example, what would AVO analysis have revealed about the two bright spots - one from gas, the other from basalt?  

Carefully processed gathers from the well locations show the difference between the AVO signature of gas and that of high velocity basalt. Gas shows the now-familiar increase of amplitude with offset, while basalt shows a decrease.

AVO effects may also be tracked across a reservoir to delineate a fluid contact. A technique developed by Ed Chiburis, while at Saudi Aramco , has had remarkable success delneating Saudi Arabian oil reservoirs. In 26 of 27 cases, the technique predicted the presence or absence of oil, which was later confirmed by drilling. 

The technique identifies changes in AVO behavior along a seismic line, and associates those changes with changes in fluid compostion. Once a given fluid has been identified in a well, the AVO behavior of the gather at the well is defined as the standard to look for elsewhere in the section. 

To overcome the lack of true amplitude processing in most data, Chiburis developed a normalization technique using another reflection that shows consistent amplitiude in the section as a reference. Peak amplitudes of the target reflection in each AVO gather are picked interactively on a workstation and normalized trace by trace to the reference event. Use of a reference event removes or minimizes amplitude distortion associated with flaws in acquisition and processing.  The technique also circumvents the need for synthetics. The measured AVO response at the well serves as the standard. The major limitation of this method is that the geology and stratigraphy must be well-known in order to associate changes in AVO with changes in fluid type. 

Another clever AVO analysis technique practiced by Amoco is to display and compare seismic sections made up of partial stacks. Here, AVO information - or fluid discrimination information -maske by a full stack, is retained in a partial stack of the far offsets. A partial stack is similiar to a full stack, except that each trace is the average of trace in a small range of offets rather than all offsets in the CMP gather. 

Where is AVO going?

Some companies use AVO routinely in an attempt to reduce risk associated with potential drilling locations. Others have tried the technique and found the processing too time-consuming or too difficult. An increasing number of practitioners is insisting on quantitative agreement between synthetic and observed data before they will use the technique. Currently, most examples of AVO interpretation are qualitative.  In the Royal Oil & Gas example, the qualitative match between the two is good, but quantitatively the synthetics predict a 100% increase in amplitude with offset while the data show an increase of more than 200%.

Eliminating the discrepancy between observed and synthetic data is therefore a focus of AVO-related research , and touches on five main topics- processing , synthetic modeling, petrophysics, interpretation and inversion.

Researchers seek a true amplitude processing scheme to produce AVO data traces that can be compared quantitatively to computer-perfect synthetics. Conventional processing for structural imaging does not preserve amplitudes. Researchers are revisiting basic processing steps such as deconvolution, velocity analysis and migration with a view to AVO applications.

Current research in synthetic modeling addreses a wide range of topics. Synthetics are only as good as what goes into them. How should logs sampled every 6 inches (15 cm) be "averaged" , or blocked, to produce layered earth models? Different blocking technique produce different synthetics. What is the effect of layer thickness on AVO synthetic? The right combination of layer thickness and seismic wavelength gives rise to reverberations in the layer that alter reflected amplitude. Can seismic energy be modeled as simple rays, or is it better to use seismic wave theory? In the examples presented above, ray theory was enough. But when angles becomes large and velocity variations complex, more computer-intensive wave theory is necessary. How does velocity anisotropy affect AVO? As angle of incidence increases, differences between horizontal and vertical velocities cannot be ignored in earth models.

In general, petrophysics is the link between earth models and any seismic interpretation, but it is particularly important in AVO interpretation. Changes in porosity, mineralogy, cementation, stress, compaction or other properties that modify the velocity or density of the rock, can give rise to AVO signatures that mask fluid effects. Changes in fluid saturation, on the other hand, may exhibit no change in AVO signature. For example, in shallow or unconsolidated sands, or overpressured zones, the AVO response is about the same for all saturations. Drilling will confirm the presence of gas, but it might be just "fizz water." Laboratory and field measurements on reservoir rocks, and especially nonreservoir rocks, under in-situ conditions, are crucial to the construction of a reliable earth model. Improved understanding of rock properties at core, log and seismic scales will lead to more unambigous AVO interpretation. 

Standard AVO interpretation fits reflection amplitudes to straight line approximations of Zoeppritz prediction curves. More refined interpretations quantify goodness-of-fit or other statistical analyses of the fit. Work is under way to abandon the straight line approximation and fit the real curve. 

A great deal of research is devoted to inversion, the attempt to derive a likely earth model starting with real data - the inverse of synthetic modeling. To date, results indicate that knowledge of the Vp/Vs ratio is required for stable inversion. Sometimes Vp can be estimated from seismic stacking velocities, but Vs cannot. Full inversion of AVO data for material properties continues to intrigue researchers, but it has yet to be proven feasible. 

What is the future in AVO? One hot topic is three-dimensional (3D) AVO. Many operators have already successfully interpreted 3D seismic data sets for AVO by assembling two-dimensional (2D) AVO sections in series. Few have tried real 3D AVO, that is, considering source-receiver paths in different azimuths. This requires knowing velocity anisotropy in the horizontal plane. 

Time-lapse AVO is another topic that show promise. As a reservoir is produced, fluid contacts will move. Seismic surveys shot at different times can be analyzed for fluid changes using AVO techniques. Information about drained and undrained volumes can affect development and production plans.

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