Geologist Cristina Rodrigues, Professor at Portugal’s Fernando Pessoa University, came to Brazil to lecture at the Energy and Environment Institute (IEE/USP), at the invitation of the FAPESP Shell Research Centre for Gas Innovation (RCGI). She holds a degree from the University of Porto and is a researcher specializing in structural cartography, having obtained her Doctoral degree with work studying aspects of methane in coal layers. Ms. Rodrigues has an extensive amount of scientific production in this area, and took part in preparing and adapting the policies of the European Parliament and the Council of the European Union regarding CCS, which was published in 2009.

In this interview for the RCGI, the researcher states that coal layers are the best choice, in terms of CO2 Storage. She stresses that the CCS (Carbon Capture and Storage) technologies are absolutely necessary for achieving the goals stipulated at the Conference of the Parties (COP), in Paris, in 2015, for reducing emissions.

Geologist Cristina Rodrigues, Professor at Portugal’s Fernando Pessoa University


RCGI – In your presentation to the IEE/USP, you said that the coal layers are excellent places for storing CO2. Could you explain why?

CR – Sequestering and storing CO2 takes a number of variables into consideration, such as the storage capacity of geological structures and CO2 injection rates, and its feasibility in economic terms that, among other factors, depend upon the distance between the production sites and those places deemed adequate for storing CO2. Regarding storage capacity, coal layers undoubtedly are the best deposit areas of all that we are aware of, both conventional (including deep saline aquifers and depleted oil and gas reservoirs) and unconventional (coal layers and shale gas reservoirs). In economic terms, they are not necessarily the best, because those unconventional reservoirs have low permeability levels. The greater the permeability, the better the circulation of the gas in the reservoir. Therefore, we actually will spend more with the injection of CO2 in less permeable deposit areas. However, the technology has been evolving quite rapidly and that has allowed a greater use of unconventional reservoirs. They are already becoming economically feasible.


RCGI – What makes coal layers the best option in terms of storage?

CR – Coal is a sedimentary rock, in fact, of the most peculiar type, because it has a huge amount of organic material. In order to be called coal, a rock must be formed of at least 50% organic material. These layers vary in their thickness, depending on the conditions under which they were formed, and they can be interspersed with argillite, shale, and other sedimentary rocks. The more organic components these coal layers have, the better they will store CO2. This is because carbon dioxide has great affinity with the organic matter in coal, and storage takes place by adsorption: the fluid molecules cling to the pore walls of the organic matter of the coal, because of their high affinity with the organic structure of the reservoir. In these cases, the gas flow – actually a mixture of methane, nitrogen, carbon dioxide, and, in a smaller proportion, heavy hydrocarbons – depends on the concentration of each of the components of the gases in the gaseous blend, the gradient pressure of the different components of this mixture, and its interactions with the organic structure of the reservoir, as well as the effects of the organic property of shrinkage and swelling of the pores of the reservoir. And CO2 has a greater capacity for compression than methane, for example, which is a fluid that all of the world’s types of coal naturally produce and store.


RCGI – How would you classify the coal layers, in terms of safety?

CR – The best solution, in terms of safety, is coal. The link between CO2 and the organic structure is excellent. It is the adsorption process that links CO2 to the structure of the coal, physically attached to it, as long as sufficient pressure is maintained (critical desorption pressure), which will keep the natural gas permanently stored. It’s significant to note that 95% to 98% of the natural gas is stored in the coal in the adsorbed state and that only the remaining 2% to 5% are stored in the fractures, in the adsorbed or free state. Only this small percentage is susceptible to escaping back into the atmosphere, if the right retaining measures are not taken.


RCGI – Are there conditions under which a coal reservoir is “great” for carbon storage?

CR – I wouldn’t use the word “great”, but I can say that there are different qualities of coal. Those that occur in the Northern Hemisphere have thicker layers and less mineral components, while having more organic contents than those found in the Southern Hemisphere. The mineral components inhibit the circulation of the natural gas inside the reservoir and compete with the CO2 for space in the coal layer. In Brazil, the Lower Paraná Basin is where the reserves have a greater possibility of becoming repositories of CO2.


RCGI – Aside from the coal layers, what would be the best options for CO2 storage?

CR – We could say there are three options on the table: shale gas reservoirs, coal layers, and petroleum wells that are in the depressurization phase, in which it is necessary to inject fluids, in order for the well to continue to produce. At the present time, companies rarely sequester CO2 merely to store it – whether in “natural” reservoirs, like coal layers, or in those that are built specifically for this purpose, like salt caves, for example. In the first stage, it is more feasible to use the CO2 to recover more petroleum and more natural gas in reservoirs that need a “a bit of a boost”, in order to continue producing.


RCGI – In this case, what can be recovered by injecting CO2 in coal layers?

CR – The injection of CO2 into the coal layer can be used to recover methane gas contained there. That is done in three phases: first, the CO2 is captured from an emission source, then it is compressed, and, finally, it is injected into the coal layer to recover methane gas.


RCGI – You took part in writing and making cultural/reality adaptations to the CCS Directive of the European Parliament and the Council of the European Union. What position does the EU take regarding CCS?

CR – The fact is that it is impossible to achieve the emissions reduction goals that have been established, without applying the use of CCS. Especially in Europe, one of the storage options that the companies have highly considered are the saline aquifers. The CO2 is dissolved in the saline water. CCS planning is now imposed by the European Parliament. Those who have the highest emissions levels in all of the sectors will be required to do this. The European Commission has concluded that it is not possible to meet the targets without these initiatives. Even Carbon Capture and Utilization (CCU), which was banned until recently, is now being given incentives.


RCGI – How many CCS are now in operation worldwide?

CR – Well, if we think of megaprojects that work to neutralize carbon, we can count 25 to 30, most of which are in the U.S., Canada, Europe, and Asia. As for smaller projects, in the pilot stage, there are a little over 60.


RCGI – You also stated, when speaking of the unconventional reservoirs at an IEE event, that fracking is a technique that has been used since the 1970s. However, there are still many doubts and much reluctance on the part of society in general regarding fracking. Why do you think that prevails?

CR – In the first place, it is due to a lack of information. People think that if we use fracking, they will turn on the faucet in their homes and the water will catch fire as it pours out. That’s not how it works. Controlled fracking has been used since the 1970s in both conventional and unconventional wells, in order to “help” bring up the fuel being explored, and this is complementary to the reinjection of methane or some other fluid substance that helps remove the energy product from the reservoir. This is a technology used to make the fluid circulate better in reservoirs that have low levels of permeability, both the conventional and the unconventional types. But it is used more frequently in the unconventional reservoirs, because, as I have already said, their level of permeability is lower.


RCGI – Do you defend the use of fossil fuels?

CR – I believe that we will still need them for a long time – and, in fact, all of the forecasts tell us that we have at least 50 more years of utilization of fossil fuels. Above all, we need energy sources, because the demand for energy is continually growing. This is due to the world’s population increase and the quality of life of that population, requiring the use of technologies, gadgets, electronics…. All of this demands more energy availability. Therefore, I believe that we should use the possibilities that we have, while minimizing the problems caused to the environment. We do not have enough renewable technologies developed to meet the worldwide demand. Today, this is absolutely impossible. It could be that we will have them 60, 70, 80 years from now. Today, we need all of the energy sources. And all of them have their own impacts. The so-called “clean energies” do not exist. There are those whose effects can be seen by human beings, and those whose effects are not seen. The identification of these effects always depends on the scope of the observation.