The Geophysical Corner is a regular column in the EXPLORER, edited by Bob A. Hardage, senior research scientist at the Bureau of Economic Geology, the University of Texas at Austin.
S-Waves and Fractured Reservoirs
In last month’s Geophysical Corner we showed that fracture orientation across fractured-reservoir intervals can be determined by azimuth-based analyses of S-wave velocities and reflection amplitudes.
This month, we return to the same 3C3D seismic data used last month and show how attributes determined from fast-S and slow-S data volumes allow patterns of relative fracture intensity to be determined in a qualitative, not quantitative, manner.
In Figure 1 of last month’s article we showed that in a fractured medium, a converted-S wavefield segregates into a fast-S mode and a slow-S mode, and that the azimuth directions in which these fast-S and slow-S modes orient their polarized displacement vectors differ by 90 degrees. Knowing the polarization directions of these two S-wave modes across this particular study area, we processed the 3C3D data to create a fast-S image volume and a slow-S image volume.
(The procedures used to segregate S-wave data into fast-S and slow-S images are exciting topics to geophysicists but are not appropriate to describe in this article.)
We show here in figure 1 a vertical slice from the fast-S volume and the corresponding vertical slice from the slow-S volume. The two fractured carbonate intervals A and B are labeled on each display, as well as several horizons interpreted near these two reservoir intervals.
Differences between these fast-S and slow-S images include:
- Reflection events A and B arrive approximately 50 ms earlier in the fast-S domain than they do in the slow-S domain.
- At certain image coordinates, there are differences between the magnitudes of fast-S and slow-S reflection amplitudes from targets A and B. Two of the more obvious examples are labeled SR1 and SR2.
- The fast-S time thicknesses across intervals A and B expand and contract in ways that differ from the expansion and contraction pattern of slow-S time thicknesses.
Some of these relative time-thickness changes are difficult to see by visual inspection of figure 1, but numerical analyses of the isochron intervals between interpreted horizons show numerous examples of such behavior.
Two locations where the time thickness of a reflection wavelet expands more in slow-S image space than in fast-S image space are labeled T1 and T2.
Local Difference: Reflectivity
The units bounding fracture intervals A and B have seismic impedances that are less than the impedances of fracture units A and B. This statement applies to most fractured targets and their bounding units.
Fast-S and slow-S reflectivities across targets A and B are controlled by the magnitude of the differences in impedances across the top and bottom boundaries of A and B. When fracture intensity and fracture openness increase locally, the difference between slow-S and fast-S velocities increases. Fast-S velocity changes little (usually not at all) when fracture intensity increases, but slow-S velocity decreases and becomes closer to the magnitude of the S-wave velocity of its lower-impedance bounding unit.
As a result, slow-S reflectivity diminishes, but fast-S reflectivity does not when fracture intensity increases.
To define locations where relative fracture intensity increases, we thus search the fast-S and slow-S volumes to find coordinates where S-wave reflection amplitudes diminish but fast-S amplitudes change little or not at all.
Two image coordinates where this type of reflectivity behavior occurs in figure 1 are labeled SR1 and SR2. The common interpretation of these differences in fast-S and slow-S reflectivities is that a relative increase in fracture intensity and/or fracture openness occurs at locations SR1 and SR2.
Local Variations: Interval-Time Thickness
When the slow-S interval-time between horizons aa and cc increases (figure 1b), two possible explanations are that (1) the thickness of reservoir A has increased or (2) reservoir A has a constant thickness, but slow-S velocity has lowered because of an increase in fracture intensity.
Other arguments may be proposed in different geological settings, but in this case, these two explanations were the most plausible.
Option 1 can be verified by measuring fast-S interval time between horizons aa and cc (figure 1a). If the reservoir interval thickens, fast-S interval time should increase. If fast-S interval time changes little, or not at all, then option 2 (increased fracture intensity) is accepted as the explanation for the increase in slow-S time thickness.
Two image coordinates where slow-S time thickness increases more than does fast-S time thickness are labeled T1 and T2. Increased fracture intensity is expected at each of these locations.
What we have demonstrated is that comparisons of fast-S and slow-S reflectivities and time thicknesses across fractured intervals allow locations of relative increases in fracture intensity and openness to be identified.
These S-wave behaviors indicate only qualitative variations in fracture intensity, not quantitative variations.
Proving the validity of predictions of fracture intensity requires extensive calibration of fast-S and slow-S attributes with reliable fracture maps across prospects. Such investigations are ongoing and will be reported in time.
For the present, we show you here the latest logic that seems to allow long-range, seismic definition of relative fracture intensity across multicomponent seismic image space.
Acknowledgment: This research was funded by sponsors of the Exploration Geophysics Laboratory at the Bureau of Economic Geology.