A review of continuous soil gas monitoring related to CCS – Technical advances and lessons learned

Abstract One of the most vigorously discussed issues related to Carbon Capture and Storage (CCS) in the public and scientific community is the development of adequate monitoring strategies. Geological monitoring is mostly related to large scale migration of the injected CO 2 in the storage formations. However, public interest (or fear as that) is more related to massive CO 2 discharge at the surface and possible affects on human health and the environment. Public acceptance of CO 2 sequestration will only be achieved if secure and comprehensible monitoring methods for the natural habitat exist. For this reason the compulsory directive 2009/31/EG of the European Union as well as other international regulations demand a monitoring strategy for CO 2 at the surface. The variation of CO 2 emissions of different soil types and vegetation is extremely large. Hence, reliable statements on actual CO 2 emissions can only be made using continuous long-term gas-concentration measurements. Here the lessons learned from the (to the authors’ knowledge) first world-wide continuous gas concentration monitoring program applied on a selected site in the Altmark area (Germany), are described. This paper focuses on the authors’ technical experiences and recommendations for further extensive monitoring programs related to CCS. Although many of the individual statements and suggestions have been addressed in the literature, a comprehensive overview is presented of the main technical and scientific issues. The most important topics are the reliability of the single stations as well as range of the measured parameters. Each selected site needs a thorough pre-investigation with respect to the depth of the biologically active zone and potential free water level. For the site installation and interface the application of small drill holes is recommended for quantifying the soil gas by means of a closed circuit design. This configuration allows for the effective drying of the soil gas and avoids pressure disturbance in the soil gas. Standard soil parameters (humidity, temperature) as well as local weather data are crucial for site specific interpretation of the data. The complexity, time and effort to handle a dozen (or even more) single stations in a large case study should not be underestimated. Management and control of data, automatic data handling and presentation must be considered right from the beginning of the monitoring. Quality control is a pre-condition for reproducible measurements, correct interpretation and subsequently for public acceptance. From the experience with the recent monitoring program it is strongly recommended that baseline measurements should start at least 3 a before any gas injection to the reservoir.