Geophysics: The Potential and Limits for Reservoir Description and Monitoring

Use of geophysics, particularly 3D seismic surface measurements, as an essential element for reservoir description has come into prominence in the last decade. This use has been driven by the shift from exploration to exploitation in many parts of the world. This occurrence is particularly widespread in the United States where most of the new reserves are generated by infill and extension drilling based on the detailed knowledge of the reservoir characteristics.

Tracking the movement of fluid saturations is a logical extension of the geophysical technology being applied to reservoir description. Several successful thermal drive reservoir monitoring projects have been reported in the literature. Because of the large number of reservoirs under gas injection or water-flood, there is a strong economic incentive to extend the application of the monitoring technology to these drive mechanisms. A few such projects have been published, some with mixed results.

Several studies show that incremental recoveries of less than 0.5% will recover the monitoring costs of many small reservoirs; and, for larger reservoirs, the comparable figure is in the neighborhood of 0.1 %.

Examples from the United States, where accurate descriptions of the compartmentalization of the reservoir is the key factor, and from the North Sea, where monitoring of the fluid fronts is a major economic issue, show that substantial progress has been made in analyzing and understanding complex reservoirs. A critical factor is the linkage and integration of all the geological, geophysical, and engineering information across the various technologies in a form which makes it readily available to those who are analyzing it.

Examples show that feasibility studies which evaluate the sensitivity of seismic, petrophysical, and engineering parameters can be useful in estimating the reliability of surface seismic methods for tracking the movement of fluid fronts during the production of a reservoir.

A gross estimate of the potential market for geophysics in reservoir description and monitoring can be made by searching for the number of reservoirs exhibiting the range of physical properties which might be reasonably detected directly or which might be detected as changes in response over appropriate time intervals. Using conservative criteria, such an estimate suggests that the size of the geophysical market in another decade could be several multiples over what it is today.

As the dense spatial coverage provided by 3D seismic measurements become better integrated with other information technologies, the potential range of conditions in which reservoirs can be successfully described and monitored will expand, even with today’s technology. Currently, there are perhaps two major limiting factors. One is the cost of 3D, three component, surface seismic measurements, a technology essential for estimating lithology and fracture orientations. The other is the degree to which seismic amplitudes can be reliably transformed into lithology and porosity attributes.

Significant progress continues to be made in steadily reducing these barriers, and the expectations are that geophysical methods will become a larger part of a rapidly growing reservoir management industry.