Advanced Surface Seismic for Natural Fracture Detection

Determining the nature of low-permeability fractured gas reservoirs is critical to the successful development of any given reservoir model. Historically, this information has come from sparsely distributed well information and has required interpolation in order to create a complete reservoir picture. With recent advances in the application of surface seismic data we can now provide some meaningful information in the intra-well spaces. This presentation will provide an overview of some of the techniques currently being investigated for the detection of natural fractures using surface seismic techniques. Examples demonstrating each technique will also be presented. The use of surface seismic data is not new to the exploration and exploitation of naturally fractured gas reservoirs. Many seismic surveys have been acquired, processed and interpreted in an effort to better understand the nature of these reservoirs. These efforts, however, have been primarily limited to indirect indicators requiring a good deal of interpretive input from the geologists, geophysicists and engineers in order to make some qualitative judgment about the fracture distribution within a field. More direct methods of fracture detection, while not new in theory, are being utilized in greater numbers due in large part to advances in data acquisition and processing technologies along with a better interpretive understanding of the results. These methods can be divided into two basic categories depending on whether we are using compressional (P) or shear (S) waves. In both cases, however, our goal is to identify and quantify the azimuthal anisotropy inherent in the rock properties of a fractured reservoir. This anisotropy produces a differential response of the seismic wave depending on the direction the fractures are encountered. In order to properly sample this azimuthal difference several propagation ray paths must be analyzed. To accomplish this, seismic surveys must be designed so that they are not only properly sampled in time and space but also in azimuth and offset. The first method involves the use of conventional P-waves and can often be accomplished with a small increase in data acquisition effort. This fact alone may be the reason several of our clients are pursuing this approach on new 3D surveys. Again, this method involves the acquisition of many propagation azimuths and the subsequent processing and interpretation of the different responses. The effects of natural fractures on P-waves are not, however, easily generalized. Our current tools are designed to measure differences in amplitudes (AVOAz) or travel-times (velocity) with seismic propagation azimuth. The technology will be illustrated on real 3-D P-wave data shot over gas fields in central Wyoming. Frequency attenuation is another property some are investigating. Ultimately, with a better understanding of the physics, we may be able to come up with a single inversion procedure that accounts for all the effects we observe on the seismic data. The use of shear waves for fracture detection is a relatively well understood science. It has been investigated for years, not only by those using surface seismic but also well log scientists who, long ago, realized that shear waves exhibit a very useful property called birefringence. What this means is that shear waves, which travel in orthogonally polarized pairs, tend to align themselves in the preferred stress directions and propagate at different velocities depending upon their orientation. These preferred directions often correspond to directions of fracturing and the difference in propagation velocity can be correlated to the magnitude or density of fracturing. By measuring the amplitude and travel-time differences of the various recorded shear waves we can then determine something about the nature of the fractured reservoir. Over the years there have been several successful projects using shear waves for fracture detection. These surveys involve both the creation of shear waves and the subsequent recording of their reflected energy. Three-component (X, Y, Z) sources and detectors are required. Most of these surveys have been limited to small-scale university projects or internal research experiments at larger oil companies. This limitation is usually due to the significant increase in acquisition effort and cost related to these types of surveys. Also, it is often difficult to generate shear waves with enough energy to investigate many deeper targets. The logistics of a shear wave source survey are often prohibitive and have led many to investigate the use of mode-converted seismic surveys. To accomplish this we use conventional P-wave sources (dynamite, vibrators or air guns) and record the shear wave energy created in the subsurface. This is a result of the mode conversion from P to S that takes place at any given seismic interface. The resulting upcoming shear wave can then be measured using many of the same tools used to analyze standard shear wave birefringence. This approach will be compared to the azimuthal P-wave-only technique discussed previously. Also, due to the fact that shear-waves do not propagate in fluids, conventional shear wave acquisition has not been available offshore. New acquisition technologies like ocean bottom 4-C cable recording, along with advances in processing and interpretation tools have allowed us to now offer valuable shear wave information utilizing a cost-effective acquisition approach. Advances in seismic acquisition, processing and interpretation technology now offer the explorer information about their fractured reservoir in areas where no well control exists. All of the direct methods, whether they use P-waves or S-waves, may provide a more detailed picture of the fractured reservoir, resulting in a better reservoir model and improved production. While conventional P-wave seismic has been an extremely effective tool for finding and delineating fractured reservoirs, we now have other options that can provide additional information about the nature of the subsurface. The difficulty comes, just as with other exploration tools, in balancing the value added from the increased information with the additional costs. Each geologic situation will have it's own set of problems which must be considered when choosing any of these methods.

Tags: kmd