Tech Watch: Fractures Detected Without Shear Waves

The buzz words among oil and gas exploration companies around the world are “shale plays.” From Argentina and Brazil in South America to the US and Canada in North America to Poland, England, and Germany in Europe to China and India in Asia and elsewhere, shale plays are ubiquitous and attractive reservoirs with immense economical potential for oil and gas discoveries.

Many shale plays are expansive mild geologic structures that make the discovery process a relatively simple one. Explorationists in the US have commented that seismic is not necessary to find oil and gas in these unconventional shale plays – “… just drill anywhere and you will find pay.” Be that as it may, finding deposits and successfully exploiting and producing them are two different things.

A specific workflow is needed to construct FracMaps and schematics of the OVT migration. (Images courtesy of Geotrace)

The Importance Of Fractures

A critical ingredient in the successful exploitation and production of these prospects is understanding whether the reservoir is fractured. The orientation and density of fractures play an important role in the drilling and production strategy to follow. Several questions are asked. Is it best to drill horizontally? How do we design the most effective borehole trajectory to maximize production? Do we need to do hydraulic fracturing? How many frac stages are needed? How will the rock formation fracture?

The traditional way to answer some of these questions depended on the study of shear waves and the phenomenon observed in these waves called “shear wave splitting” or birefringence (also known as double refraction).

When shear waves pass through a fractured (anisotropic) medium, they split into two polarized shear waves that travel at different speeds and “orient” themselves with the fractures. The fast waves orient themselves parallel to the fractures, while the slow ones become perpendicular to them. By carefully measuring the orientation and speed of the arriving signal, it is possible to determine fracture density and orientation in the reservoir.

Shear (and converted) wave acquisition and processing are expensive and difficult and not yet part of mainstream seismic processing, especially when compared to compressional (P) data processing. Is there a simpler and less expensive way to detect fractures by using conventional P-wave processing? What could replace this shear wave splitting phenomenon? The answer is the newly developed concept of offset vector tile processing for wide-azimuth data.

An OMG is sorted into offset and azimuth displaying VT and HTI anisotropy.

The two necessary ingredients for this technology to work are a well-sampled wide-azimuth acquisition and a wide-azimuthal seismic processing system that includes an orthorhombic migration and velocity analysis.

It has been documented elsewhere that if seismic data are acquired with good azimuthal coverage such that reservoirs can be illuminated from different directions, enough information can be elucidated from the behavior of the compressional waves traveling through the rocks in different directions to help determine the orientation (and perhaps the density) of the fractures.

Similarly to the shear wave splitting effect, the compressional waves travel at different speeds, depending on whether they are moving along (fast) or against (slow) the fractures. Furthermore, it is possible to measure the propagation velocities of these compressional waves in different directions and determine the fracture orientation.

This new technology can be used to determine fracture orientation. It is based on the use of offset vector tiles (OVT), orthorhombic prestack time migration, and careful analysis of the generated offset migrated gathers (OMG).

OVT, Orthorhombic Migrations, OMG

When good azimuthal coverage exists in a seismic survey, it is possible to construct (minimal) datasets or OVTs that preserve the azimuthal and offset information after migration. When these OVTs are migrated, they generate OMGs. OMGs are the vector generalization of the traditional migrated gathers. They represent the subsurface as seen by different offsets from different directions (azimuths). If the velocity and anisotropy models are correct, these gathers should be flat. The deviation from flatness indicates the presence of an incorrect velocity, probably due to anisotropy in the vertical and horizontal directions.

A velocity ellipsoid describes orthorhombic anisotropy. There is one of these for every point and sample.

In traditional migrated gathers, vertical anisotropy (VTI) expresses itself as “hockey sticks” in the offset domain, requiring the introduction of an anisotropic correction into the migration traditionally called (eta) n .

The analysis of OMGs immediately demands the introduction of additional anisotropic corrections on top of VTI. The two parameters chosen are the horizontal propagation velocities in two different directions, referred to as Vfast and Vslow . The combined presence of horizontal anisotropy (HTI) and VTI anisotropy is orthorhombic anisotropy. It can be argued that this orthorhombic anisotropy is pervasive to the earth as it describes the natural symmetry (or lack thereof) occurring in a vertical depositional flat-layer environment that undergoes fracturing at a later stage.

Through a process called a surface-fitting algorithm, it is possible to find these velocities. One way to visualize these new corrections together with VTI is as a 3-D velocity ellipsoid with different sized semi-major and semi-minor axes and with different orientations. Every point in the survey has a tetrad of values (V, Vfast , Vslow ; and ) describing such an ellipsoid.

Zoomed-in FracMap (timeslice) shows fracture orientation and intensity (thickness of tick mark)

By selecting the orientation and the semi-major axis magnitude, it is possible to construct a simple representation of the anisotropy, called a FracMap. A time slice across one of these volumes shows a formation of directional patterns that can be interpreted as fractures. Waves move fast along this direction and slow perpendicular to them, hence the connection to fractures.

This velocity vector is used to determine fracture orientation in a reservoir and presents an attractive alternative to shear wave splitting. Although shear waves can produce similar information, the ease of processing wide-azimuth P-waves by well-known and understood techniques makes this a more desirable processing alternative.

REFERENCES

• Crampin, S., and Lovell, J. H. (1991), A decade of shear-wave splitting in the Earth’s crust: What does it mean? What use can we make of it? And what should we do next? Geophys. J. Int., 107, 387-407.
• Jenner A. E., Davis T., A new method for azimuthal velocity analysis and application to a 3D survey, Weyburn field, Saskatchewan, Canada, SEG 20, 102 (2001)
• Stein J. A., Wojslaw R., Langston T. and Boyer S., Wide-azimuth land processing: Fracture detection using offset vector tile technology, The Leading Edge, 29 , no. 11, 1328-1337, (2010)
• Wojslaw R. and Stein J. A., Orthorhombic HTI + VTI Wide Azimuth Prestack Time Migrations, SEG, Expanded Abstracts, 29 , no. 1, 292-296, (2010)