Two years of hard research in STEMM-CCS have led us up to the main controlled release experiment, which will take place in the North Sea in May 2019. Our project team have been working hard to develop new knowledge, techniques and instrumentation that are critical to making this experiment a success. But what exactly are we planning to do?
The main objective of this experiment is to inject – in a carefully controlled manner – a small amount of CO2 gas below the seafloor sediments in the North Sea, and then see how and where the gas moves through the sediments and into the water column. This is an important experiment because in order for sub-seafloor carbon capture and storage (CCS) to be an acceptable means of locking CO2 gas away in long-term storage, we must be confident that the storage reservoirs are secure, and that requires being able to quickly and accurately detect any leak that might occur.
The experiment sounds simple enough, right? Stick a hose in the seafloor, blow some CO2 gas through it and wait to see the bubbles to pop out, right?
The challenges of doing this from a ship, in water more than 100m deep, in the North Sea where the weather conditions can be rather variable, are significant. The engineering alone required to make this experiment possible is epic in scale, and some very bespoke equipment has been built to enable it all to happen.
First, we need to get the ship, the team and all the equipment to the experimental site. The RRS James Cook will sail from her home port in Southampton to the Goldeneye experimental site in the North Sea. The first job is to perform a comprehensive seafloor and water column survey of the site, so we know exactly what it looks like before we start the experiment. Once this is done, a specially-made frame containing two large CO2 gas tanks and some smaller tracer gas tanks will be lowered from the ship onto the seafloor.
Next, we need to make a hole in the seafloor sediments so that we can release CO2 in just the right spot. A bespoke drill rig will be lowered into position on the seafloor, about 100m away from the gas tanks. The rig will push a pre-curved carbon steel pipe into the sediments, leaving the front end of the pipe about 1 metre below the sediment surface, and the tail end of the pipe sticking up just above the seafloor, ready for connection to the CO2 gas tanks. Once the pipe is in position, the drill rig is pulled back onto the ship.
A variety of sensing devices are put into position around the release site, ready to detect any leaked gas using visual, chemical and acoustic techniques. The remotely-operated vehicle (ROV) Isis attaches a hose that links the CO2 gas in the tanks to the end of the pipe, and – when everything is ready – we switch the gas flow on. The gas flow rate will be slow at first, and gradually increase up to maximum rate of around 100 litres per minute for about 2 weeks. The tanks hold 3 tonnes of CO2, to which chemically-inert tracer gases will be added in very precise concentrations before pumping down the release pipe. These tracer gases behave like chemical tracking devices and enable our sensors to detect and quantify the gas in the water column. Acoustic devices will “listen” for bubbles in the water, and we will have cameras to spot any visible gas bubbles emerging from the seafloor. As well as the static sensors, a range of chemical sensors will also be mounted onto an autonomous underwater vehicle (AUV), which will patrol the site on a regular basis, sniffing the water for signs of escaped gas.
As well as the RRS James Cook, the German research vessel RV Poseidon will be nearby to make measurements in the wider region surrounding the experimental site. This is so that we can be sure to spot any anomalies against “normal” background conditions, which the Poseidon will be monitoring throughout the experiment. Once the experiment is finished, the gas is turned off, and all the equipment has been removed, the team on board RV Poseidon will undertake a comprehensive survey of the release site before we leave the area at the end of May.