Abstract

The cheapest and safest place to store gas is inside a reservoir. Over the years, Gaz de France has developed a unique expertise in that field. A brine-filled reservoir is used to store imported gas during summer, and pump it out in winter according to domestic demand. In this context, 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. Right at the beginning of the reservoir filling, in 1993, a baseline walkaway survey was shot using permanent downhole geophones. From that date and until 1997, five repeat surveys were shot with the same configuration and same vertical PDGs antennas. The seismic data were optimally processed. Travel times and impedance variations in the reservoir vicinity correlated with the gas volume as it increased and decreased aver the period. This technique allowed to follow the bubble extension and its contraction on a discrete basis and to track the changes in the reservoir properties.

The latest attempt uses low-energy stationary seismic sources operating continuously in conjunction with vertical multi-component 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. Tiny 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.

Introduction

There is no better, nor surer place to store natural gas than in an underground natural reservoir.

Gaz de France (GdF) has 14 sites where gas, delivered by pipeline from a variety of gas-producing countries, is stored in brine-filled reservoirs during the summer, then pumped out in the winter according to market demand. Then, monitoring gas bubble expansion, field pressure and saturation is critical to optimize field management and gas recovery. (Blondin E. and Mari J.L., 1986). Several seismic monitoring experiments have been carried out over the years at the Céré-la-Ronde test site, with quite interesting results. The latest attempt uses low-energy stationary seismic sources operating continuously in conjunction with vertical multi-component 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 ne

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