CO₂ Storage: Operation - injecting CO₂ underground

The process of actually injecting CO₂ into the storage formation is very well understood, both in terms of the equipment used, regulations, operations and safety practices, as a variety of gases are already injected underground for storage and waste disposal purposes. Compared to oil and gas, CO₂ is nonexplosive, non-toxic and non-flammable and more than 30 years of Enhanced Oil Recovery has given hydrocarbon companies extensive experience in how to safely compress CO₂ into a supercritical state and pump it down through dedicated well bores through the cap-rock and into a storage formation.

The CO₂ injection process is very stable, but there have been a few cases over the last 30 years where very limited amounts of CO₂ have come back up wells during injection. The causes are well understood (usually it has been due to a drop in pressure caused by maintenance work) and all were quickly capped and then fixed. Critically, the lessons learnt from these rare occurrences have been built in to existing and future CO₂ injection operations.

When CO₂ injection phase is taking place, a variety of site-specific monitoring, evaluation, reporting and verification (MERV) activities are conducted both at the injection site and in the surrounding area to ensure that the right amount of CO₂ is injected; that it is injected effectively and safely into the right part of the formation and that no unwanted migration or leakage of the supercritical CO₂ happens. By undertaking the right MERV activities, it ensures safe, secure storage over time that poses no harm to the environment and provides valuable feedback to continually improve current and future injection and storage activities.

MERV techniques have all been widely used in the oil and gas industry over the last 100-plus years – from seismic surveys, gravity measurements, well probes, groundwater and soil sampling to CO₂ sensors and alarms, airborne monitoring and satellite imaging. Click here for a diagram. Some of these monitor the movement of the CO₂ in the formation deep underground, and others will detect any well failures, leakages due to structural issues or indeed from other abandoned wells in the case of depleted oil and gas reservoirs. Because the planning and information gathering during site characterization on the storage site and environment prior to injection is so comprehensive, any unusual readings can be easily compared to these baseline results.

If MERV shows that any unplanned CO₂ migration or leakages are detected, even if absolutely minimal, the well planned remediation measures, learned from oil and gas industry, will swing into action depending on the cause – from capping abandoned wells with cement and steel to seal them, to stopping injection entirely. However, because of the very nature of the geological traps chosen for storage, the CO₂ is not a bubble of gas that can burst to the surface and so any CO₂ leaks from the storage formation will be slow and minimal, allowing plenty of time to make repairs and alterations to procedures. The MERV activities and data collection can also be used to refine the projections of CO₂ behaviour made during future planning.