Development of an innovative marine monitoring system for CO_2 leaks: system design and testing

摘要

A critical component of long term geological sequestration of anthropogenic CO_2 will be our ability to adequately monitor a chosen site to ensure public and environmental safety. Near surface monitoring is particularly important, as it is possible to conduct sensitive and direct measurements at the boundary between the subsurface and the biosphere (i.e. surface water or atmosphere). While discontinuous surface monitoring is often performed, continuous monitoring is preferable if one hopes to observe a leak in its early stages to allow for rapid remedial action. The geochemical signal that may result from a near-surface CO_2 leak might take the form of increased soil gas concentrations (on land) or changing pH, Eh, and aqueous chemistry (in groundwater or surface water), and thus continuous monitoring stations capable of analyzing for these parameters have great potential for early leak detection. In the framework of the EC-funded CO_2GeoNet and CO_2ReMoVe projects innovative monitoring systems have been designed and constructed for autonomous deployment in marine environments above geological CO_2 storage sites. The system developed within CO_2GeoNet was tested at a site in the Gulf of Trieste where there is no gas release; this site was chosen due to easy access and the presence of an existing oceanographic buoy onto which the monitoring station was mounted. Tests on this early prototype highlighted the various difficulties of working in marine environments, and this experience formed the basis for a new system developed for deployment at the Panarea test site within CO_2ReMoVe. This second site is located off the coast of Panarea Island, to the north of Sicily, where naturally-produced CO_2 leaks from the seabed into the water column. The advantage of this site is that the leaks occur in a relatively near-shore environment (<300m) and in water that is not too deep (<25m), thereby allowing for easy access by SCUBA divers for system testing and maintenance. This location allowed the unit to be connected via cable, rather than a buoy, which makes power supply and data transfer simpler. The system developed for this site consists of three monitoring points that are connected to a land-based control unit. Each point, located 100, 200, and 300m from shore in different CO_2 flux regimes, is able to measure dissolved CO_2 and CH_4, conductivity, pH, and temperature using low cost but sensitive sensors. The complete system consists of flexible solar panels, a central control unit and three monitoring points, and data download is conducted using a GPRS connection and a web server. Difficulties with the initial deployment in early April of 2008 has necessitated further development work, with the second deployment planned for early November. The following paper discussed the experience gained with these stations, and presents data analysis and anomaly recognition from a land-based monitoring station that has been collecting dissolved CO_2 data for over 18 months.