New and emerging technologies help discover/manage reservoirs - Brief Article

Advances in geophysics are furthering the seismic and remote sensing industries. The ability to gather and process increasingly large amounts of geophysical data has enabled data acquisition at ever-higher sensitivity and resolution. Innovative companies are finding ways to put these new exploration and reservoir tools to work for their clients.

The following articles illustrate some of these new technologies: 1) If three-component seismic data was not enough, surely nine-component data will be; 2) A new era in VSP acquisition allows better drilling decisions and improved reservoir migration; 3) Fiber optic seismic sensors, designed for permanent installation in wells, pass their first test with flying colors; 4) Remote sensing imaging spectroscopy takes off, as smaller/lighter equipment and better software help make it a valuable exploration aid.

A new technology: Nine-component seismic data

Three wave modes--one compressional (P) and two shear ([S.sub.V], [S.sub.H])--always exist in solid earth media regardless of whether the seismic source is dynamite, vibrator or a marine airgun. A principal difference between these wavefield components is the manner in which they cause rock particles to oscillate, Fig. 1. The seismic industry is currently wrestling with the problem of how many data components should be recorded to provide new rock/fluid information at an attractive price.

[Figure 1 ILLUSTRATION OMITTED]

To create optimal images of subsurface targets, a seismic wavefield must first be segregated into its component parts so that a P-wave image has minimal contamination from interfering [S.sub.V] and [S.sub.H] modes. Likewise, an [S.sub.V] image must have no interfering P and [S.sub.H] modes, and an [S.sub.H] image must be devoid of P and [S.sub.V] contamination.

In consolidated rocks, P-wave velocity is about twice as fast as either [S.sub.H]- or [S.sub.V]-wave velocity; this velocity difference aids in separating interfering wave modes. However, a more powerful technique for separating a seismic wavefield into its component parts is to concentrate on the distinctions in particle-displacement vectors associated with P, [S.sub.H], and [S.sub.V] modes, Fig. 1. For this reason, procedures that segregate these wave modes by their displacement-vector characteristics are called seismic vector-wavefield imaging.

Nine-component seismic. P, [S.sub.H], and [S.sub.V] particle displacements form an orthogonal coordinate system. A fundamental requirement for multicomponent seismic imaging is that reflection wavefields must be recorded with orthogonal three-component sensors that allow P, [S.sub.H] and [S.sub.V] particle motions to be recognized. Seismic data that is recorded with only single-component sensors (either hydrophones or vertical geophones) do not allow this.

In onshore seismic work, multicomponent seismic imaging can be further aided by using three oriented-vector sources that successively generate three distinct illuminating wavefields. The first source applies a vertical impulse to the earth; the second applies a horizontal impulse in a chosen direction; and the third applies a horizontal impulse that is orthogonal to that applied by the second source, Fig. 2.

[Figure 2 ILLUSTRATION OMITTED]

The wavefield produced by each of these sources contains P, [S.sub.H] and [S.sub.V] components, as has been emphasized. However, the vertical source produces a wavefield that has a P-wave component that is particularly robust, and the horizontal sources produce wavefields with strong, polarized shear-wave components.

When each of these three wavefields is recorded as successive field records by three-component sensors, the result is nine-component (9-C) seismic data, i.e., 3 sources x 3-C geophones = 9 components. When the reflected wavefields are recorded by a 3-D grid of three-component sensors, the data is called 9C3D data. If only one source is used with a 3-D grid of 3-C geophones, the data is referred to as 3C3D data, i.e., 1 source x 3-C geophones = 3 components.

Why 9-C imaging? Each particle-displacement vector (P, [S.sub.H] and [S.sub.V] in Fig. 1) reacts to rock properties in a different way and thus provides different information about rock systems, pore fluids and subsurface targets. By developing technology that allows P, [S.sub.H] and [S.sub.V] images to be constructed, much more information about subsurface geology, stratigraphic relationships, lithological distributions and pore-fluid properties becomes available.

To date, almost all seismic images have been made using only P-wave sources. The particle-displacement vectors P, [S.sub.H] and [S.sub.V] are defined more accurately with 9-C data than with 3-C data; thus, for technical reasons, 9-C data is preferred. However, 3-C data costs less than 9-C data, because less source activity is required to generate 3-C data. Still, many industry scientists are convinced that this new 9-C seismic information will be essential for better understanding of earth processes and for improved management of earth resources.

Exploration Geophysics Laboratory. An example of industry's commitment to further multicomponent seismic technology is its support of the Exploration Geophysics Laboratory (EGL). Located at The University of Texas at Austin, EGL's general objective is to develop new seismic technology that can improve the understanding and management of earth systems.

EGL deploys one or more 9C3D, onshore-seismic field crews on a full-time basis and investigates all aspects of 9C3D and 3C3D onshore-seismic technology and 4C3D marine technology. EGL research emphasizes new vectorized energy sources, innovative multicomponent field techniques, development of multicomponent imaging software, and demonstration of combined P, [S.sub.H] and [S.sub.V] seismic-data applications.

Detailed descriptions of new reservoir-characterization applications provided by combined P, [S.sub.H] and [S.sub.V] data are available from the Bureau of Economic Geology as publication RI 237 (phone: 512-471-1534; E-mail: amanda.masterson@beg.utexas.edu).

COPYRIGHT 2000 Gulf Publishing Co.
COPYRIGHT 2000 Gale Group