A group of researchers linked to the FAPESP Shell Research Centre for Gas Innovation (RCGI) is investigating the possibility of opening immense caves for storing natural gas that has a high concentration of CO₂ in the salt layer that covers the pre-salt reservoirs. Opening caves in saline rock to store natural gases and other substances seems like something out of a sci-fi movie, but the technology is relatively, worldwide. The U.S., Canada, and Germany already are using this to store natural gas (CH₄) and other hydrocarbons, and China is beginning to investigate the technology. However, the storage of CO₂ in ultra-deep offshore salt caves is something new.

“Although the technology is not new, nowhere in the world is there an offshore facility, which is what we want to do. What exists are onshore deposits opened in thick saline layers. The United Kingdom has a project for developing a cluster of offshore caves in the Irish Sea, called the Gateway Project, but it is in shallow waters. In our case, we are talking about great depths,” explains Pedro Vassalo Maia da Costa, who is one of the researchers connected with the RCGI’s Project 34 (Development of Studies Regarding the Construction of Salt Caves for Separating CO₂ and CH₄ in the Pre-salt Region).

He did his Master’s and Doctor’s degrees in this area, to which he has dedicated the past six years. Costa says there are some 5,000 salt caves, throughout the world that are used for storing hydrocarbons and other substances. They are constructed via a leaching process: the saline rock is dissolved by injecting fresh water, or sea water that has less salt, thus opening a cave in the rock. “But not all saline rock is appropriate for construction caves. Halite is the most appropriate for this purpose, due to its purity and the deformation rate under the creep (cold flow) effect (deformation under constant pressure over a long period of time). Saline rock is an excellent geomaterial for the storage of liquids and gases, even under high pressure, and it has negligible porosity and permeability, compared to other types of rock.”

The high pressures registered at great depths are a challenge for the team, but not the only one. Generally speaking, the salt caves are used to store hydrocarbons, and not CO₂. However, research by the industry and by academia shows it is viable to definitively store CO₂ in a supercritical state in a salt cave.

“CO₂ has different chemical characteristics and compression factor from CH₄. They enter a supercritical state at different moments. Therefore, we are characterizing the physio-chemical properties of the behavior of CO₂ in this state.” The RCGI is setting up a laboratory for characterizing the physio-chemical properties of CO₂, natural gas, and a mixture of the two under high pressure, taking into consideration the temperatures expected for caves in ultra-deep waters.

Step by step – In his Doctoral thesis in the area of energy planning, Professor Costa analyzed various offshore deposit areas and chose to work with an area in the Espírito Santo basin, 50 km from the coast. It can handle the construction of 14 giant caves, 450 meters high and 150 meters in diameter. Via computer modeling, the Project 34 team is studying the possibility of opening these caves at that site.

Costa states that, for the present time, the idea is merely to store natural gas that has a high concentration of CO₂ in the caves, without intending to enable the monetization of CH₄ and other marketable gases. Pre-salt natural gas has a high CO₂ content, which makes it hard to use, since Brazil signed the Paris Agreement and has goals to meet for reducing greenhouse gas emissions.

“First, the objective is to inject the carbon-rich natural gas into the cave, in order to definitively store it. In the second phase of the project, the technology for the gravitational separation of CO₂ must be developed and, then, it will be possible to take commercial advantage of the natural gas. By keeping the cave under high pressure, the CO₂ changes to a supercritical state and settles out, to the lower part of the cave. The natural gas stays in the upper part of the cave, in a gaseous state. We, then, remove the natural gas and alleviate the pressure inside the cave to the point that the CO₂ goes back into a gaseous state. This cycle is repeated until the cave is totally filled with CO₂, when the cave will be sealed and abandoned,” Costa explained.

This technique is the focus of a patent filed by Álvaro Maia da Costa, in conjunction with Júlio Meneghini (RCGI’s Scientific Director), Kazuo Nishimoto (RCGI’s Director of the Carbon Abatement Program), and Cláudio Oller and Felipe Ruggeri, both of whom are connected with Project 34. The patent is registered as “Method for gravitational separation of natural gas in caves, the capture system and definitive storage of natural gas with CO₂ and system for the capture and definitive storage of CO₂.”

Pressure and monitoring – There are security procedures for ensuring that the cave behaves as expected. “We work with an experimental safety slab: the thickness of the saline rock above the cave must be at least 700 to 800 meters. It is one of the safest technologies for underground and geological storage in existence, since the porosity and permeability of the saline rock are negligible.”

However, the salt has a special characteristic: the creep, or cold flow, effect. The rock deforms under constant pressure maintained for a long period of time. “The pressure of the gas is calculated as a percentage of the lithostatic pressure on the top of the cave, that is, the weight exerted by layers of rock overlying the top of the cave and the water table. That pressure is exerted all around the cave: ceiling, sides, and bottom. Therefore, it must be closed slowly and monitored to ensure that the gas is sealed tightly, since it will also be compressed over time, and then stabilize.

“After the cave is filled, the next step is called ‘abandonment.’ That means that it will no longer be the target of new injections of storable substances. In order to avoid unnecessary risks, a well-planned project for abandonment must be made, and there must be monitoring to ensure that the cave stays sealed, without the risk of leaks. That can be done with equipment that monitors the temperature and pressure inside the cave.”