Subsalt caverns can store CO₂

By Marcos de Oliveira  |  FAPESP Research for Innovation – The development of offshore subsalt oilfields in Brazil has fostered the creation of new technologies to circumvent the difficulties of producing in deep water and far from the coast. One such technology has been produced by researchers affiliated with the Research Center for Gas Innovation (RCGI) in São Paulo and has recently resulted in a patent application to the National Industrial Property Institute (INPI), the government agency responsible for granting and managing industrial intellectual property rights. Hosted by the University of São Paulo’s Engineering School (POLI-USP), RCGI is an Engineering Research Center (ERC) funded by FAPESP and Shell.

The innovation is a system that separates carbon dioxide (CO₂) from methane (CH₄) by gravity. These two gases are dissolved in the crude oil extracted from wells, especially in subsalt oilfields where the seabed is between 2,000 and 3,000 meters below the surface.

The researchers’ solution indicates that CO₂ separation and retention can be performed in caverns in the salt layer. The caverns would be formed by leaching, the high-pressure injection of seawater to dissolve salt, leaving spaces up to 450 m high by 150 m wide. Each cavern would be large enough to store 8 million metric tons of CO₂.

CO₂ has a market equivalent to slightly more than 1% of the total emissions of the gas. It is used in the food and drink industry (in carbonated soft drinks, for example, among other products) and in the petrochemical industry. When released into the atmosphere, it is environmentally harmful. Indeed, CO₂ is the main cause of global warming and climate change. With the technique described in the patent application, it could be stored in offshore salt caverns forever.

Crude oil extracted from subsalt reservoirs is mixed with natural gas in liquid and gaseous form, with CO₂ levels ranging from 20% to 60%. “The technique currently in use to separate CO₂ and natural gas involves membranes on the drilling rig. This system is costly to operate. Twenty years ago, CO₂ was released into the atmosphere, and methane [the main component of natural gas] was burned off in flare stacks on the rig itself or at the refinery,” said Julio Meneghini, coordinator of RCGI and Full Professor at POLI-USP, in an interview given to Agência FAPESP.

Salt caverns have been used to store hydrocarbons (oil, gas, etc.) since the 1960s. Today, there are more than 4,000 such caverns all told.

The RCGI researchers’ invention combines the use of storage caverns in rock salt with CO₂-methane (CH₄) gravity separation. Following extraction from the well, a mixture of CO₂ and CH₄ is injected at high pressure into a deep-sea cavern with the aid of a layer of synthetic fluid that separates gas and seawater, prevents them from mixing, and acts as a sort of repellent to both gas and salt water.

The innovative concept is the use of pressures in the range of 500-600 atmospheres (atm) to keep the CO₂ in a supercritical thermodynamic state, in which its density is similar to a liquid’s but its viscosity is lower than that of the gaseous state.

The separation is due to this difference in weight. The CO₂ is heavier and sinks to the bottom, while the natural gas (mostly methane, with relatively small amounts of ethane and propane) stays at the top of the cavern and can easily be removed for monetization.

“Without CH₄, the pressure inside the cavern can be lowered, and the CO₂ can be converted into gas. As a result, there will be room to store more CO₂. When the cavern is full, it can be sealed and abandoned,” Meneghini explained. Even in extreme circumstances, such as earthquakes, for example, the gas will stay in the cavern because saline rock self-repairs quickly without cracks.

Intellectual property

The caverns can be formed in the salt layer found between the seabed and the oil deposit below. This layer is approximately 3,000 meters thick. The equipment can be the same as that used to inject seawater into oil wells when they are almost depleted. High-pressure injected water blasts the remaining crude out of the rock.

The caverns will have to be formed near drilling rigs, which will supply electricity to run pumps sitting on the seabed to inject water at rates ranging from 200 to 1,000 cubic meters per hour. Other structures called risers will be responsible for injecting the gas. Pressure and gas sensors will monitor the procedures performed inside the caverns.

The RCGI researchers are currently working on the Espírito Santo basin some 50 kilometers offshore. Based on computer modeling, they estimate that 14 caves can be made there.

Natural gas siphoned off from the caverns can be used to generate electricity for oil production platforms or stored as a strategic reserve. Another advantage is the ability to store CO₂ at sites far away from population centers.

The following researchers signed the patent application: Álvaro Maia da Costa, Pedro Vassalo Maia da Costa, Cláudio Oller, Felipe Rugeri, Júlio Meneghini, and Kazuo Nishimoto.