As has been emphasized in the three preceding articles of this series, when a shear (S) wave propagates through a rock unit that has aligned vertical fractures, it splits into two S waves – a fast-S (S1) mode and a slow-S (S2) mode.
The S1 mode is polarized in the same direction as the fracture orientation; the S2 mode is polarized in a direction orthogonal to the fracture planes.
This month we translate the principles established by laboratory experiments discussed in the preceding articles of this series into exploration practice.
Figure 1 displays examples of S1 and S2 images along a profile that crosses an Austin Chalk play in central Texas.
The Austin Chalk reflection in the S2 image occurs later in time than it does in the S1 image because of the velocity differences between the S1 and S2 modes that propagate through the overburden above the chalk. Subsurface control indicated fractures were present where the S2 chalk reflection dimmed but the S1 reflection did not.
This difference in reflectivity strength of the S1 and S2 modes occurs because, as shown last month (June EXPLORER), when fracture density increases, the velocity of the slow-S mode becomes even slower. In this case, the S2 velocity in the high-fracture-density chalk zone reduces to almost equal the S-wave velocity of the chalk seal, which creates a small reflection coefficient at the chalk/seal boundary.
When fracture density is small, S2 velocity in the chalk is significantly faster than the S-wave velocity in the sealing unit, and there are large reflection coefficients on both the S1 and S2 data profiles.
Using this S-wave reflectivity behavior as a fracture-predicting tool, a horizontal well was sited to follow the track of a second S2 profile that exhibited similar dimming behavior for the Austin Chalk.
The S2 seismic data and the drilling results are summarized on figure 2.
Data acquired in this exploration well confirmed fractures occurred across the two zones A and B where the S2 reflection dimmed and were essentially absent elsewhere.
The seismic story summarized here is important whenever a rigorous fracture analysis has to be done across a prospect.
If fractures are a critical component to the development of a reservoir, more and more evidence like that presented here is appearing that emphasizes the need to do prospect evaluation with elastic-wavefield seismic data that allow geology to be imaged with both P waves and S waves.
The value of S-wave data is that the polarization direction of the S1 mode defines the azimuth of the dominant set of vertical fractures in a fracture population, and the reflection strength of the S2 mode, which is a qualitative indicator of S2 velocity, infers fracture density.
The Earth fracture model assumed here is a rather simple one in which there is only one set of constant-azimuth vertical fractures.
What do you do if there are two sets of fractures with the fracture sets oriented at different azimuths?
That situation will be discussed in next month’s article.