Summary Oil and gas production from tight, fractured reservoirs (sandstones, carbonates, shales) has increased considerably as commodity prices have risen over the past 5 years. A common aspect of these hard rock targets is an oriented stress field and open fractures. P-wave seismic processing that can identify these variations in the subsurface and take advantage of them will produce sharper images and improved fracture predictions. The following paper discusses various azimuthal processing techniques used to identify azimuthal velocity anomalies and azimuthal AVO anomalies. These techniques can impact the final stacked image and the prediction of open fractures.
Introduction Well data has long been an obvious measure of fracture presence in tight-rock reservoirs. Borehole breakout, FMI measurements and coring, have provided clear indications of oriented stress fields and open fractures. Prospecting for fractures with the bit is an expensive and often unrewarding endeavor. Multi-component acquisition, processing and interpretation may offer an opportunity for finding fractures with seismic but at an added time and financial cost compared to P-wave data. Early attempts to predict fractures with unmigrated P-wave data have been encouraging but the technique lacks spatial resolution. Sectored migration overcomes some of the limitations of operating on unmigrated data but often suffers from undersampled azimuths in the input dataset resulting in noisy results. This paper discusses the use of a full azimuth Kirchhoff migration approach for detecting azimuthal velocity anomalies. The variation in velocity versus azimuth is a potential indicator of open fracturing. With the measured azimuthal velocity field one then can effectively pursue Azimuthal AVO anomalies as an additional fracture indicator. The workflow can be modified for a depth migration approach to address overburden velocity complexity (not discussed in this paper).
Understanding Fractures and Seismic Natural fractures play a major role in determining the producibility of moderate-low permeability reservoirs. Intersecting fractures with properly placed vertical or horizontal well bores can provide access to greater reserves and provide for higher production rates. Fracture location, density, orientation and complexity will vary with the rock type and local stress. Field development requires a mapbased understanding of fracture patterns. Seismic data offers an indirect measure of the effects of fracturing either through measuring shear-wave splitting (multicomponent acquisition and processing), azimuthal variations in P-wave velocity, Azimuthal AVO or post-stack techniques like Curvature Analysis, Coherence or Inversion. There is no single technique that works best all the time and a combination of the above is usually the best approach for gaining confidence in a fracture prediction. (Figure 1).
Anisotropy To explore for fractures we need to identify changes in the earth related to Horizontal Transverse Isotropy or HTI- these are the velocity variations that occur as a function of azimuth. Most seismic processing is readily able to handle Vertical Transverse Isotropy (VTI) also known as Layered or Polar Anisotropy. VTI appears on NMO corrected unmigrated gathers and pre-stack migrated gathers as the well known "hockey stick" at the far offsets or high incidence angles (Figure 2). The velocity used to move-out or image the gather was generally correct for the near offsets but not the far so the event isn''t flat.