Monitoring Station

One way to allay public concern regarding CO2 storage safety is to develop early warning systems which can monitor gas compositions and concentrations in the near-surface environment. Increased concentrations of CO2 or other associated gases in these phases, above natural biogenic variations, would then trigger a warning. One such station has recently been developed and deployed in the San Vittorino area of central Italy. Although the monitoring station (jointly funded by the EC NASCENT project and the Civil Protection Agency - Region of Lazio) was originally deployed to monitor for dissolved gas precursors related to CO2-induced sinkhole formation, it could easily be deployed to monitor soil gas above a CO2 sequestration site (for example above abandoned wells).

The San Vittorino plain is a triangular-shaped intramontane basin filled with fluvial-lacustrine sediments and local travertine deposits, and is surrounded by mountains formed by carbonate and siliciclastic successions thrusted onto various bedrock lithologies (Centamore and Nisio, 2003). The plain is bordered by normal and/or strike-slip faults and is also cross-cut by other regionally important features. The fault segments that cross the plain are not easy to identify due to the plastic nature of the filling sediments, however evidence of their existence is given by the occurrence of flowing springs, mineralised waters, gas bubbling vents and generally elevated soil gas and gas flux values throughout the valley. It has been the interaction of these deep acidic gases, like CO2 +/-H2S, with the spring and ground waters which have resulted in the formation of more than 30 sinkholes in the central and eastern sectors of the plain

The monitoring station uses gas permeable tubing to sample gases dissolved in groundwater via diffusion. As the internal volume of the tube represents a "head space", equilibrium concentrations will be governed by Henry's Constant. The use of gas permeable tubing has been shown to be effective for other gas species (e.g. Jacinthe and Groffman, 2001; Jacinthe and Dick, 1996). The gas permeable tubing is placed in a piezometer (installed above a microgravity minimum in the town of Vasche) and is linked at surface with the monitoring station, which houses CO2, H2S and H2 sensors, a continual power supply and an external data relay system. The in-house developed software system is programmable to sample at different intervals, however a sample time of once every 4 hours was chosen to allow the sampling tube to re-equilibrate with the surrounding water. Recorded data can be down-loaded from the laboratory via modem.

Figure 3. Dissolved CO2 results from the monitoring station installed in Vasche (after Annunziatellis et al., 2004). Dots indicate individual values while the running average is a black line. The vertical bars are earthquakes near the station registered by the Istituto Nazionale di Geofisica e Vulcanologia in Italy.

Figure 3. Dissolved CO2 results from the monitoring station installed in Vasche (after Annunziatellis et al., 2004). Dots indicate individual values while the running average is a black line. The vertical bars are earthquakes near the station registered by the Istituto Nazionale di Geofisica e Vulcanologia in Italy.

The geochemical monitoring station began to collect data on December 20, 2002, and during the intervening period it has collected approximately 2 years worth of dissolved gas data. Some of the data collected during the monitoring period is shown in Figure 3 (note that the values are not dissolved gas concentrations but rather the concentration in the silicone tubing which has equilibrated with the groundwater). The general decreasing trend of the line chart also shows several sharp drops in the signal, which could be linked to different environmental variables (such as water and atmospheric temperatures affecting the sensors) or to the occurrence of earthquakes. The first case cannot be verified because this prototype station was not equipped with temperature sensors. The second hypothesis can be studied by comparing the dissolved gas data with the occurrence of earthquakes in an area surrounding the station site, as shown in the figure. In particular there were about 4 magnitude-3 earthquakes in the region during the monitoring period. The line chart shows abrupt decreases in correspondence with earthquakes which occurred in the Reatini Mts, the Sibillini Mts and at Gran Sasso Mt. There are, however, other CO2 decreases which cannot be correlated with seismic events in the region and thus there may be multiple processes which contribute to the signal.

Was this article helpful?

0 0

Post a comment