The injected CO₂ that went missing

It is always good to speak to people at con­ferences and hear what is going on. That is how I learned about a CO₂ storage project that took place in Ja­pan a few years ago, named the Tomakomai Demon­stration Project. Within the framework of this project, which ran for more than 3.5 years, the injection of 300,000 tonnes of CO₂ took place into subsurface reservoirs.

It was the first CCS demonstration in Japan to test CO₂ injection at scale. Two injection wells were drilled from a position in the harbour of Tomakomai, a small town on the north­ernmost island of Japan, Hokkaido. Even though the surface location of the wells was onshore, the wells were drilled at a high angle with a TD location offshore. In addition, two observation wells were drilled, ocean bottom seismometers were placed for the detection of microseismic events, and re­peated 3D surveys were car­ried out to monitor plume migration.

Two very different reservoirs

The CO₂ was injected into two different reservoirs: The shallow Moebetsu Forma­tion of Quaternary age at a depth of around 1,000 to 1,200 m and the deeper Mi­ocene Takinoue Formation at 2,400 to 3,000 m.

Initially, the plan was to inject around 200,000 tonnes of CO₂ per year into each reservoir, but this had to be revised soon after drilling the injection wells. Especially the Miocene vol­canic reservoir turned out to have much lower injectivity of several nano-darcy, where the initial permeability was estimated at 9 to 25 md. It was the other way round for the shallow reservoir, though; anticipated permea­bilities were thought to be up to 7 md, but this amounted to several hundreds upon test. For that reason, the maximum injection rate for the Takinoue Formation was revised to 1,000 tonnes per year, whilst the Moebetsu target remained the same.

How much was injected at the end? The injection test ran for more than 3.5 years, but only 300,000 tonnes were injected in total, with slightly more than 300,000 tonnes in the shallow res­ervoir and only 98 tonnes in the deeper one. This low number overall must also be seen in light of the fact that the system was only opera­tional for less than half the time.

Monitoring

The extent of the CO₂ plume in the shallow Moe­betsu Formation could be monitored using the repeat 3D seismic surveys. How­ever, the small plume in the deeper volcanic “reservoir” could not be confirmed by the same technology, which was explained by the volume being too low to be detected.

Even though this is not a major surprise – there needs to be a minimum dis­turbance for a signal to be detected – it does show that it is not always guaranteed that monitoring will pick up the signal. Volumes need to be taken into account, as well as the probability that seismic won’t even be able to detect anything at all despite the volumes, as is the case with the planned Morcambe Bay CO₂ injec­tion project in the UK. In that case, gravity measure­ments are planned to mon­itor CO₂ plume migration instead.