Monitoring geological gas storage sites with ambient noise interferometric methods: focus on seismic attenuation changes for gas movement detection.
Résumé
Recording the ambient seismic signal over an array of multiple sensors and applying ambient noise seismic interferometric (ANSI) methods can give access to the characterization of seismic wavefields which can in turn be analyzed to infer the seismic properties of the investigated medium and/or to monitor these properties over time. One advantage of those passive seismic methods-in addition to their relatively low cost of implementation-is that they do not require any active seismic source, which allows for their use even in areas where active source deployment is complicated (e.g., inhabited areas, dense industrial infrastructures), and can therefore be relevant to monitor natural gas or CO2 geological storage sites. In this paper, we analyze the temporal evolution of a seismic wavefield obtained with ANSI methods to monitor the structural changes and fluid distribution evolution occurring within a natural gas geological storage site where fluid movements induced by gas injection/production operations take place. The local seismic wavefield was reconstructed for the whole monitoring period and thoroughly analyzed. We addressed our effort into monitoring a specific attribute of the reconstructed seismic waves: the waveform coherency. This attribute reflects the variations of the subsurface attenuation properties, which are sensitive to the fluid distribution within the rock matrix and hence adapted to geological gas storage monitoring. Our results demonstrate the sensitivity of this passive seismic monitoring approach to gas movements within the reservoir. We note the existence of a relationship between a significant drop of the wavefield coherency values estimated at the reservoir level and a gas injection phase of about one month duration where significant pressure increase is observed. When no significant gas injection or extraction is performed, the coherency of the wavefield remains stable. This experiment hence confirms the interest of the wavefield coherency as a quantity suited to monitor geological gas storage sites evolution and potentially detect prospective gas leakage paths.
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