- Copyright © 2001 Society of Exploration Geophysicists
Arguably, the cheapest and safest place to store gas is inside a reservoir. Over the years, Gaz de France has developed a unique expertise in this field. A brine-filled reservoir is used to store imported gas during summer and pump it out in winter according to market demand. Monitoring bubble expansion, field pressure, and gas saturation is critical to optimize field management and gas recovery. Several seismic monitoring experiments have been carried out over Céré-la-Ronde test site, with varying degrees of success. The latest attempt uses low-energy stationary seismic sources operating continuously in conjunction with vertical multicomponent receiver antennae. The system is fully automated and remotely controlled. The high density of data in the “elapse-time” domain makes it possible to observe and compensate for subtle near-surface variations. As a result, repeatability is increased by an order of magnitude compared to traditional surface time-lapse surveys. Minute changes in the seismic response can be measured and calibrated to direct reservoir measurements. This will lead the way to a new range of quantitative applications for seismic monitoring.
Storing gas in an underground formation is both safe and efficient, but maximum recovery must be ensured. Unfortunately, reservoir heterogeneity can be fully assessed only after gas injection, because the brine-filled pores are hardly visible seismically. Seismic monitoring therefore becomes a unique tool to improve understanding of the reservoir, demarcate the extension of the gas bubble, and measure pressure and fluid saturation changes. The Céré-la-Ronde gas storage facility was chosen as a laboratory for seismic monitoring before any gas injection (Figure 1). Located in the Loire Valley in central France, it is a water-bearing reservoir in a faulted anticline at a depth of 900 m with an average thickness of 20 m. Two seismic monitoring projects took advantage of this situation: Conventional 2-D seismic lines and …