Induced microseismicity and procedure for closure of brine production caverns

Elf-Atochem has been solution mining a salt formation in southern France for the last twenty years and the problem arises of the disposition of deep caverns once brine production ceases. The regulatory authorities have required procedures for sealing off the caverns to make them environmentally safe and to monitor the post sealing behaviour of the caverns. This paper describes the deep downhole seismic monitoring carried out during the experimental studies conducted to determine the appropriate means of monitoring and to validate closure safety criteria. The results confirm the accuracy of downhole permanent seismo-acoustic technology to monitor the sealing processing, even in strong conditions (2000 meter depth and 100 degrees C). The microseismicity has been characterised and located using triaxial hodogram analysis, however the microseismicity shows many clusters of seismic "doublet" events allowing the use of relative analysis. The system has been installed before sealing and displays a high sensitivity. Under these conditions, the microseismicity has yielded useful data on the geomechanical behaviour of the site with respect to the final objectives. The seismicity is associated with the brine flow. The seismicity induced by the salt leaching process is a means to emphasize weak zones (fractures) that could again become active after closure. A closure test reveals that a direct relationship ex:sts between the seismicity and the bleeding off brine from the caverns. Microseismic monitoring results combined with field observations advance the development of the closure procedure.