Analysis of the Passive Seismic Monitoring Performance at the Rousse CO2 Storage Demonstration Pilot

AbstractIn January 2010, an integrated CO2 Capture and Storage (CCS) project, including CO2 capture, transportation and geological storage started in the Lacq-Rousse area, Southwest of France. Until March 2013, 51 thousands metric tons of CO2 were injected into a fractured dolomitic depleted gas reservoir at around 4500-m depth. Key objectives of the project were to develop, test, and validate the methodology and the technology required for managing safely such storage operations, and in particular to define an “optimal” monitoring program, economically and technically viable.A monitoring plan was designed, including the deployment of a hybrid multi-scale passive seismic monitoring network to address 3 monitoring objectives: watch seal integrity, distinguish natural seismicity from induced seismicity, and assess injection-induced seismicity. A near-surface network (Master Network), and a downhole array deployed in the injection well (Research Network), were installed between 2009 and 2011. In addition, detection of regional seismicity was also available through a seismometer.Data have been collected for 9 months prior to injection, during injection, and also after injection, as part of the post-injection monitoring program planned for 3 years. Starting from March 2011, as per end July 2014, the hybrid system allowed detecting about 2500 events. Over two third of those events were not energetic enough to be located. Over 600 events, with estimated magnitudes above -2.3 and not exceeding -0.5 were located in the reservoir. The monitoring therefore helped in concluding that the CO2 injection did not generate an adverse impact on reservoir integrity. Actual network performances have been in line with expectations. The effective Master Network location sensitivity is about magnitude -0.6 (completeness magnitude) in the volume of interest, confirming its ability to serve for an efficient seismic hazard monitoring.In this paper we show how the proper design of a passive seismic network consistent with the monitoring plan can benefit the different phases of a CCS project, and how passive seismic monitoring during the full duration of the injection can bring reliable evidence for reservoir integrity. The paper concludes that passive seismic monitoring with a limited number of near-surface sensors constitutes an effective alarm network. In addition passive seismic monitoring can provide a permanent seismic risk evaluation tool by continuously comparing seismic activity with the energy input into the reservoir. This second type of application is certainly not necessary for most oil and gas development, but it may constitute a precise element of monitoring if seismic hazards need to be closely monitored