2.1.2 General operational risks related to CO2 injection

The main concerns regarding CO2 storage operations is connected to possible failure of pipelines, wells or other components that could lead to CO2 releases causing health and safety concerns for humans and/or the environment. CO2 is a substance occurring naturally in blood gases of the human body and is not classified as a hazardous chemical, but if inhaled in sufficiently high concentrations it can have toxicological effects. The dangers of breathing in elevated concentrations of CO2 are well known to people such as divers and anaesthetists. Outside these groups of specialists, knowledge about the impact of breathing elevated concentrations of CO2 is generally low. Moderately elevated levels of CO2 (2-6 %) induce adverse effects on humans such as headache, sweating, dizziness and difficulty in breathing. Very high levels of CO2 may cause confusion, unconsciousness, coma and death (asphyxiation by displacing oxygen in the air). A large leak of CO2 from a CCS operation has therefore the potential to be life threatening to people who might be caught within the subsequent CO2 cloud. CO2 cannot be seen or smelled, it provides no evidence of its presence that can be recognised by the senses.

The main technical considerations relating to risk of CO2 storage operations are the gas stream composition, temperature and pressure conditions. The effect of a potential CO2 release would, however, also depend a lot on several external factors. Such factors need to be taken into account during risk assessment and include the following: environment (subsea, subsoil, offshore, onshore, platform, vessel, confined space etc.), topography, wind and temperature conditions. Since CO2 is heavier than air, it remains close to the surface of the space in a deep or shallow pool, therefore, topography considerations need to be taken into account when planning CO2 injection infrastructures, i.e. pipeline routing.

Liquid CO2 is a powerful solvent that can have unwanted effects on some lubricants and is also highly invasive and capable of penetrating materials and causing damage. Seal elastomers are known to be vulnerable to explosive decompression damage, particularly when exposed to supercritical CO2. This means that careful selection of materials is very important for seals, flexible hoses, instruments, wire and cable insulators, controls and other safety-critical components (DNV, 2013).

Since CCS is a reasonably new technology, the industry and operational standards are still not generally accepted. Therefore, there is still a possibility that unknown effects and hazards could develop during the operational phase. Increase of experience and the stringent use of existing hazard management processes will reduce the likelihood of this to an acceptable level (DNV, 2013).

Besides the main technical considerations relating to risk of CO2 storage operations, the borehole operation itself may pose risk to the groundwater zone and proper well surface casing and cementing at appropriate depths is required to ensure isolation of protected groundwater sources and control of the well under maximum formation and operating pressures.

All site activities need be performed in a manner that avoids endangering protected groundwater sources. Surface pipe should be set to a depth sufficient to ensure control of the well under maximum formation pressures and operating pressures prior to the next casing interval. Casing the well begins with the large-diameter conductor pipe being driven or augured into the ground through the surface rubble or loam to hard pan, usually to a depth of 8 to 30 m. The conductor pipe prevents caving and washout at the rig base and provides containment of the cement for the surface casing at ground level. Once in place, the conductor casing is grouted with cement to maintain integrity around the casing and to prevent washouts.

The well should be drilled out through the conductor to below protected groundwater sources and the surface casing should be run and cemented back to the surface to protect any groundwater sources encountered. The well should be drilled to total depth and cased with the appropriate grade, weight, and size of casing to handle the operating parameters expected in the well and should be cemented back to the surface. At a minimum, the design of the casing should account for the internal yield strength of the pipe, casing collapse pressure, the pipe body yield, the required internal diameter of the pipe, and the corrosion resistance of the metallurgy.