- Copyright © 2005 Society of Exploration Geophysicists
Enhancing oil and gas production from fractured reservoirs increases the need to image fracture systems through the study of wide-azimuth, surface, P-wave seismic data. Although it is understood that reservoir complexity could be diagnosed through seismic anisotropy studies, a remaining technical challenge involves maximally exploiting seismic attribute sensitivities to improve mapping of the spatial distribution of subsurface fractures.
Here, we present a case study of mapping a possible fracture network using wide-azimuth ocean-bottom cable (OBC) seismic data recorded over a Middle East offshore production field. The results shown in this paper demonstrate the potential for obtaining detailed fracture information and predicting reservoir flow anomalies through seismic studies.
Detecting fractures by extracting seismic azimuthal anisotropy
The presence of vertical fractures and/or aligned porous grains alters a rock's physical properties and is a major cause of azimuthal anisotropy. In such rocks, seismic waves will sense various degrees of stiffness and compliance of the rocks depending on the direction of wave propagation through them. P-waves are relatively insensitive to the presence of fractures at near offsets but can be very sensitive at large offsets. The common practice of fracture detection using P-wave data is to extract seismic azimuthal anisotropy in which the long and short axes of an ellipse are used to predict fracture orientation and the ratio of anisotropy which could, in turn, be a useful indicator of fracture intensity. Inversions of attributes associated with amplitude, velocity, and/or arrival time are often performed to accomplish this task. However, the effectiveness of such methods for fracture detection depends on each attribute's sensitivity to the presence of fractures.
In this case study, we mapped a possible fracture network of a test area within a production field by using seismic anisotropy. The task was carried out in two steps: (1) construction of multi-azimuthal attribute maps and (2) description of the fracture network …