3-D SEISMIC TECHNOLOGY: AN OVERVIEW

Based on a workshop sponsored by PTTC’s Eastern Gulf Region on September 18, 1998, in Jackson, MS.

BOTTOM LINE

A distinct advantage of 3-D seismic surveys over 2-D surveys is that they have potential to provide detailed information about subsurface rocks and fluids (including hydrocarbons), as well as the structure of a reservoir. A general knowledge of the criteria used to select a 3-D survey, and the considerations to properly design it, are critical to successfully applying this technology.

PROBLEM ADDRESSED

Recent advances in 3-D seismic imaging are at the forefront of reservoir characterization technologies. Today, 3-D seismic technology is widely accepted for imaging structures, but new applications providing the potential for directly measuring reservoir properties— such as rock type, porosity, and fluid content— are less known and accepted by operators. 

KEY WORDS:

3-D Seismic, Reservoir Characterization, Attribute Analysis, Survey Design

SPEAKERS

Bruce Hart
New Mexico Bureau of Mines & Mineral Resources

TECHNOLOGY OVERVIEW

Seismic surveys, especially 3-D surveys, are a significant improvement over conventional 2-D surveys for exploration and development, and reservoir characterization. Their advantages (including a continuity of subsurface information, extraction of new information from the data, and a common focus for integration of reservoir data from all sources) are explained in more detail below.

• First, 3-D surveys provide a continuous volume (100% coverage) of high-quality subsurface data. Both the structural and stratigraphic elements are continuously represented at a resolution not attainable in 2-D surveys. Subsurface features, including those associated with complex structures, can be accurately positioned; there is no inaccuracy of location due to “sideswipe” responses from off-line features, unlike 2-D surveys.
• 3-D surveys also offer a tremendous opportunity to extract additional information. Because data are handled digitally throughout the process, new information can be mathematically searched in the recorded seismic signals. This “attribute analysis” can yield additional details about important reservoir properties. Multiple regression and/ or geostatistical analy sis of seismic data can empirically relate signal characteristics (such as amplitude, reflection strength, and instantaneous phase and frequency) to physical properties of the reservoir. These include pore fluid types and amounts and reservoir pressure, as well as rock properties.
• In addition, digital 3-D seismic data allow information to be manipulated and displayed, enabling the best possible interpretation of subsurface geometries and properties. Autotracking selected horizons across large volumes of data can be an important time-saving aid to interpretation. Display options include: selective viewing at various scales, the use of color to display complex information, and the ability to view data in a wide variety of orientations (e. g., inline sections, crossline sections, arbitrary lines, multi-panel lines, horizontal time slices, and various perspectives of the data cube).
• A fourth benefit of 3-D surveys is that full-volume coverage can serve as a common interface for integrating geology, geophysics, and engineering analyses, particularly in reservoir characterization. This encouragement of disciplinary cooperation and synthesis may be one of its most important contributions.

However, 3-D seismic is not a panacea for independents; it may not be appropriate for every situation. A number of considerations are important before committing to implementing a 3-D survey. Factors that can affect the technical and/ or economic success of a 3-D survey include: 

Location: Desert areas are less expensive and less complex for surveys than farmlands or swamps. Potential environmental damage is also a location factor to assess, as is the complication of high noise levels near cities or seashores.

Depth of targets: Shallow surveys are cheaper than deep, but surveys with both shallow and deep targets may be even more costly.

Energy source: Vibroseis, dynamite, or heliportable dynamite are all options, based on the environment and survey objectives.

Time of year: Field conditions and survey crew availability/ cost will affect the timing of operations.

Additional effort and time should be spent properly designing the survey, which can be done with the help of a 3-D seismic service provider. If the target zones have high acoustic impedance contrast with the surrounding zones, thinner targets can be detected at greater depths. Survey frequencies may have to be adjusted to maximize target resolution. The general orientation of the project area will determine the line orientation geometry used. Consideration of depths, velocities, and X, Y dimensions of the target( s) will influence the record lengths, bin sizes, and filters needed. Adequate time also must be allowed for the design and permitting process.

LESSONS LEARNED

1. Cost is a consideration in choosing to apply 3-D seismicsurveys. Acquisition costs and the costs of hardware, software, and training, particularly for firsttime ventures into 3-D, need to be considered. The size of the target must justify the expense. On the other hand, costs of the survey should not be reduced to the point where it might jeopardize getting the necessary information.
2. Communicate. Plan early and well. Work closely with acquisition and processing groups, particularly for first-time ventures into 3-D surveys.
3. Work the data. There is more to data analysis than just obtaining a structure map. At the same, it’s important to remember that attribute analysis, because it is an empirical approach, presents some pitfalls. Any spurious correlations that are not statistically significant may be misleading. Operators should not become overconfident in numbers and should use test data sets whenever possible. They also should avoid overdependence on one technology by integrating information from other sources as a crosscheck.
4. Expect the unexpected. It’s important to expect a certain amount of trial and error and mistakes.

FIELD RESULTS

In the Gulf of Mexico, 3-D (and even 4-D) seismic surveys have given good results, better by far than with 2D, in pursuing development targets. In a Pleistocene offshore deltaic sandstone reservoir, multiple generations of 3-D seismic surveys (often referred to as 4-D seismic, with time as the fourth dimension) were used to look for changes in reservoir properties over time. The three surveys were normalized for differences in frequency content, phase, and static shifts. The remaining differences in the surveys were likely related to changes in pore fluids, since rock properties presumably would not change. Changes in amplitude were observed corresponding to the watering out of some parts of the reservoir and expansion of the natural gas cap. Such 4-D surveys have the potential for routine applications, as in monitoring the progress of enhanced oil recovery methods, locating bypassed pay zones/ compartments, and evaluating recovery efficiency.

A 3-D survey of a complex salt-dome structure in the Gulf of Mexico was able to resolve steeply dipping reflectors and faulting, which a 2-D survey would not have accomplished. A combination of amplitude difference analysis, coupled with other attributes of the seismic data, was used to locate small scale faulting adjacent to the salt and define untested reservoir compartments defined by those faults.

CONNECTIONS:

Bruce Hart
New Mexico Bureau of Mines & Mineral Resources
New Mexico Tech, 801 Leroy Pl., Socorro, NM 87801
Phone 505-835-5752, Fax 505-835-6333, E-mail hart@wiggle.nmt.edu

For information on PTTC’s Eastern Gulf Region and its activities contact:
Ernest A. Mancini, Professor of Geology, University of Alabama,
Box 870338, 202 Bevill Bldg., Tuscaloosa, AL 35487
Phone 205-348-4319, Fax 205-348-0818, E-mail emancini@wgs.geo.ua.edu

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