One of the technologies that prevents the emission of carbon dioxide molecules into the atmosphere is called Carbon dioxide Capture & Utilization or Storage, better known by the acronym CCUS. How does CCUS work? In simple terms, it involves two stages: in the first stage, particular technologies are used to capture and separate the carbon dioxide molecules – for example those found in the fumes from fossil fuel combustion – from the other molecules with which they are mixed; at this point, the second phase comes into play, in which the carbon dioxide is stored in safe places (storage) or is used to produce other substances (utilization) undergoing chemical transformation. In both cases, however, its dispersion into the atmosphere is avoided. In the case of storage, the captured carbon dioxide is transported to sites where it can be injected into the subsoil, in safe “deposits” (geological storage, such as depleted hydrocarbon deposits or saline aquifers), capable of containing it without leakage in the longer term. 

To prevent the risk of accidents and ensure the maximum safety of these plants, CO₂ geological storage projects are preceded by in-depth studies on the characteristics of the reservoirs used as storage sites, to certify their suitability. The projects also envisage a system for monitoring the stored carbon dioxide continuously to ensure that there is no leakage or migration from the storage site over time. Carbon dioxide is, after all, not a toxic gas: the human body breathes and produces it, emitting it in exhalation. Only in extremely high concentrations, of more than 10 times those currently found in the atmosphere, does carbon dioxide prevent the correct supply of oxygen to the body and thus begin to cause health problems. Carbon dioxide could also be stored in the depths of the oceans, where it would remain confined due to the high pressure of the water column above it. This option, however, has never been applied and its impact on the marine ecosystem is still being tested on a small scale. The alternative to storing the captured carbon dioxide is to use it. Many processes use carbon dioxide as a raw material to produce chemical intermediates, plastics, fuels, to feed the algae from which biofuels are obtained and to produce carbonates that can be used in construction. There are many possible uses; the problem is that transforming carbon dioxide into products requires a lot of energy and expensive plants. If the energy needed for these processes is produced from fossil fuels, much of the benefits in terms of reducing carbon dioxide emissions could be lost. 

Internationally, CCUS is considered an important tool for the decarbonization of the energy system. The same International Energy Agency (IEA) in a recent report (CCUS in Clean energy Transitions) stated that the capture, utilization and storage of carbon dioxide must be a fundamental pillar of the efforts required to eliminate net greenhouse gas emissions within this century. In addition, many of the countries that have already declared they intend to achieve this goal have shown great interest in this class of instruments, considering them useful to reaching this ambitious target. As regards the theoretical underground storage capacity, the OGCI (Oil & Gas Climate Initiative) is currently carrying out a survey of suitable sites and has recently provided a rough estimate for the projects considered so far (512 in 12 countries or regions) totaling more than 12,000 billion tons of carbon dioxide. Taking into account this potential (albeit estimated) and the fact that annual anthropogenic carbon dioxide emissions today total nearly 35 billion tons, the option of storage is not just a solution that will soon be available, but it could also be adopted for many years to come, pending the development of zero-emission energy sources. 

The technologies that capture and store or utilize carbon dioxide are still too expensive and energy intensive to be adopted on a broad scale. In particular, if the goal is to capture a large proportion - close to or greater than 90 percent - of the CO₂ contained in a gas mixture, the costs and energy consumption rise exponentially. The hope is that technological innovation and economies of scale and learning acquired during the first major projects will enable the reduction of costs and energy intensity, making these options viable on a commercial scale. In fact, new more efficient technologies are currently under research and development, while the first large-scale carbon dioxide capture and sequestration projects that use technologies already proven and available are underway. Eni is also making efforts in this field. The various initiatives under research include two that would successfully combine Eni's vast knowledge of reservoir dynamics with these new technologies: the first is the construction of one of the largest carbon dioxide capture and storage hubs, using as storage site the now depleted natural gas fields in the Middle Adriatic, off the coast of Ravenna (Italy); the second initiative under consideration concerns the UK. In October 2020, Eni obtained a license from the UK Oil and Gas Authority for a storage project in Liverpool Bay, in the eastern Irish Sea. Finally, in the field of the utilization of carbon dioxide, Eni is evaluating some possibilities of integration with the activities it already performs in the natural gas and green chemistry sectors. All the projects under research are described on the Eni website (www.eni.com).

Natural Resources Studies & Analysis, General Manager Natural Resources Eni