Monitoring of geological carbon sequestration (GCS) sites will primarily depend upon the tools and technology developed over many decades in support of the oil and gas industries for ensuring the safe injection and storage of CO2 in the subsurface. For more than thirty years CO2 has been compressed and transported in pipelines for injection as part of enhanced oil recovery projects. However, the aims of CO2 injection for carbon sequestration have significant differences from EOR injections, where the motivation is enhanced production. While the regulatory environment that will govern CO2 storage in the subsurface is still evolving, operators of GCS sites will need to demonstrate protection of drinking water resources, accountability of the volume injection and permanence for the CO2 emplaced. The US Department of Energy has coined the acronym MVA to describe the requirement to monitor the movement of CO2 into, through, and out of the targeted geologic storage area, verify the location of CO2 that has been placed in geologic storage, and account for the CO2 that has been transported to a geologic storage site. To accomplish this, advanced well-based technologies will be needed to meet regulatory and technical requirements. Recent demonstration projects have incorporated permanently emplaced sensors including pressure, distributed fiber-optic, temperature, seismic, electrical resistivity, and geochemical sampling to name a few. These are often installed so that they are compatible with periodic wireline operations. In this paper we will examine integrated well-based monitoring programs that were part of CO2 storage demonstration programs and will discuss requirements for future commercial installations.
Introduction
The oil and gas industries have been using the concept of the smart well - a well that contains permanently emplaced sensors, valves, and chokes - to increase the quantity and quality of information available and to provide a means to actively manage wellfield operations. Downhole zonal control systems allow operators to access individual zones using valves that are operated either by a wireline or hydraulically (Sun et al., 2006). Monitoring flow from individual sections can be carried out using venturi flow meters (Ong et al., 2007). Permanent monitoring including pressure, temperature, and seismic sensors is increasingly common, as is the installation of distributed fiber-optic temperature and strain measurement (Rambow et al., 2007; Wilson et al., 2008). Because the draft rules and guidance that have been promulgated for monitoring of CO2 storage sites incorporate significant data collecting requirements, many of the technologies that have evolved for monitoring oil and gas field will be needed, along with additional technologies to support long-term monitoring.
The European Union directive 2009/31/EC of the European Parliament and of the Commission states "Monitoring is essential to assess whether injected CO2 is behaving as expected, whether any migration or leakage occurs, and whether any identified leakage is damaging the environment or human health. To that end, Member States should ensure that during the operational phase, the operator monitors the storage complex and the injection facilities on the basis of a monitoring plan designed pursuant to specific monitoring requirements." Furthermore the directive goes on to state that "After a storage site has been closed, the operator should remain responsible for maintenance, monitoring and control, reporting, and corrective measures pursuant to the requirements of this Directive on the basis of a post-closure plan submitted to and approved by the competent authority as well as for all ensuing obligations under other relevant Community legislation until the responsibility for the storage site is transferred to the competent authority." Monitoring of sites is expected to continue under the EU directive for at least 30 years past the operational life of the reservoir.
