Abstract: Time-Lapse Seismic for Monitoring Gas in an Underground Storage Site

HUGUET, FREDERIC, JEAN-MARC MICHEL and DENIS MOUGENOT*

To track the injection of a gas bubble, 2D land seismic lines were acquired repetitively in the Paris basin, where a water-bearing sandstone is used to store gas. Despite accurate repositioning, differences in seismic noise and long wavelength statics occurred, due to the weathering zone.

Both sets of data were processed to obtain optimal noise-free preserved-amplitude migrated seismic traces. Before the difference sections were computed, the monitor lines had to be harmonized with respect to the base lines. The amplitude difference sections displayed a significant change in reflectivity, attributable to the expansion of the gas, which moves predominantly northwards along a sealed fault. Below the reservoir, the time-shift between the surveys is proportional to the thickness of the gas.

Tracking the saturation variations within the gas bubble proved to be more difficult, due to the limited accuracy of amplitude variations. Nevertheless, after computation of two sets of impedance sections and their calibration at the well locations, it was possible to link the difference in impedance to the main ranges of gas saturation.

This field scale study demonstrated the ability of surface seismic to track the expansion of the gas bubble away from the injection wells, even when acquisitions are not repetitive. Compared with controls made only at the well locations, this approach proved to be very useful in improving the exploitation of such an underground gas storage site.

AAPG Search and Discovery Article #90942©1997 AAPG International Conference and Exhibition, Vienna, Austria