For Fractures, P + S = Maximum Efficiency
In areas where fracture-producing stress fields have been oriented at different azimuths over geologic time, there can be fracture sets of varying intensities and different orientations across a stratigraphic interval.
This month, in the last part of our five-part series, we consider here how to use shear (S) seismic data to locate a fracture set oriented at a specific azimuth in a target interval that is embedded in a thick section dominated by a younger and more dominant fracture set oriented in a different azimuth.
The fracture orientations and two crooked-line surface profiles where compressional (P) and S-wave seismic data were acquired are illustrated on figure 1.
Two fracture trends are present:
- An older, pre-fold fracture set oriented approximately north-south.
- A younger, more dominant set, oriented approximately east-west, produced during a regional orogeny that fractured massive intervals of rock.
The older fractures can be open and gas-filled in a targeted unit at a depth of approximately 10,000 feet (3,000 meters).
The objective is to find this relatively thin interval with a north-south fracture set embedded in a thick section of more dominant, fold-related, and non-productive east-west fractures.
P-wave and SH-wave seismic data acquired along the two crooked-line profiles are shown as figure 2. The P-wave profiles tie at their intersection point, showing that P waves exhibit minor difference in velocity when they propagate parallel to and orthogonal to the east-west fractures that extend across a large part of the geological section.
This weak reaction of P-wave velocity to fracture orientation is one reason why P-waves have limited value for analyzing fracture systems.
A different behavior is observed for the SH data. SH reflections on Line 2, where the SH particle-displacement is aligned with the dominant east-west fractures (figure 1), arrive earlier than do their corresponding reflections on Line 1, where the SH particle-displacement vector is orthogonal to the extensive east-west fractures.
As has been described in the preceding articles of this series, the SH polarization along Line 2 is the fast-S mode for the east-west fractures, and the SH polarization along Line 1 is the slow-S mode for east-west fractures.
By comparing these P and SH images, we see hard evidence that S-wave velocity reacts more strongly to fractures than does P-wave velocity.
A valuable interpretation procedure is illustrated on figure 3, where the two SH profiles are depth registered across the reservoir target interval.
Here, the image on Line 1 is advanced in time to align key reflection events A and B above and below the targeted reservoir, the circled event at the tie point. If the desired north-south fractures are present within the reservoir interval, the reflection event will dim on Line 2, because the SH polarization on that profile would be the slow-S mode for a north-south fracture set.
As shown in parts three and four of this series (June and July EXPLORERs), slow-S velocity S2 decreases when fracture density increases, and thus S2 reflectivity weakens as shown in this example. In contrast, the reflection would remain bright on Line 1, where the SH polarization is the fast-S mode for north-south fractures.
That reflectivity behavior is what is demonstrated inside the circled target interval.
The exploration problem described here of locating a subtle fracture set hidden by a more dominant fracture set is one of the most challenging that can be encountered in interpreting fracture attributes from seismic data.
The fundamental principle illustrated by this case history is that multicomponent seismic data that provide both P and S data are far more valuable for fracture analysis than are single-component P-wave data alone.
Incidentally, this story and its illustrations are taken from a 20-year old U.S. patent (#4,817,061) – showing that good technology can be found in places other than technical journals.