Group investigates the viability of injecting CO2 into shale gas reservoirs in the Paraná Sedimentary Basin and in offshore turbidites of the Santos Basin

A group of researchers from the FAPESP Shell Research Centre for Gas Innovation (RCGI) is studying a method for verifying the possibility of storing CO2 in two types of rock formations: shale in the Paraná Sedimentary Basin, which is found from the State of Mato Grosso and down to Uruguay, and the offshore turbidites of the Santos Basin, where petroleum is pumped.  The purpose is to verify whether those types of rock have the capacity for storing CO2 and, if possible, in the case of shale, inject the CO2 at the same time that the natural gas it contains (commonly called shale gas) is removed. The result will be published in the CCS Atlas of the Southern and Southeastern Regions of Brazil, which will come out at the end of the project, two-and-one-half years from now.

The Atlas will present all of the geological information of the units studied, the data obtained for them, and the location of the areas most favorable for Carbon Capture and Storage (CCS) in the south and southeast of Brazil. The project will also assess the economic feasibility of CCS in geologically selected sites. And it will present criteria for assessing the environmental and socioenvironmental impacts of storing CO2 at those sites. Once all of this information is gathered, it will be possible to determine if an area can or cannot safely receive CO2.

Samples – To that end, the team of the RCGI’s Project 36 (Carbon Storage in Geological Reservoirs in Brazil: Perspectives for CCS in Non-conventional “Onshore” Petroleum Reservoirs and in “Offshore” Sedimentary Basins in Southeastern Brazil), selected methods used worldwide for assessing the capacity for storing CO2 in geological reservoirs, in order to validate those methods for the Brazilian sedimentary basins studied or, due to the distinctive characteristics of the rock, whether or not it is necessary to adapt them to local basins. The group is coordinated by geologist Colombo Celso Gaeta Tassinari, Director of the Institute of Energy and the Environment (IEE/USP) and Professor in the Institute of Geosciences of USP (IGc/USP).

He explains that the shale of the Paraná Basin occurs in two geological formations: Ponta Grossa, which is the oldest and deepest, and Irati, which is the newest and shallowest. “In the areas where these rocks occur, there are several coal-powered plants. This is a highly industrialized zone, where the level of CO2 is relatively high,” Professor Tassinari points out. To study those rocks, scientists will collect samples from the shale beds in stone quarries and in drill holes that were already made by the Geographical Service of Brazil (Cia. de Pesquisas de Recursos Minerais-CPRM) and Petrobras.

The turbidites of the Santos Basin are sedimentary rocks are sedimentary rocks of variable granulometry that contain intercalated layers of clay. “In this case, we will attempt to receive samples from the National Petroleum Agency (NPA). If we are unable to do so, we will attempt to study analogous samples, that is, samples from another place, but that are similar to those of the Santos Basin. And, also if it is impossible to identify analogous samples, we will make the assessment based on geological information that we have at hand in the literature,” Tassinari explains.

Step by step – The samples will be subjected to laboratory studies, to analyzer their characteristics and composition. “We want to know such things as: what minerals they contain, how porous and permeable they are, what is the composition of the clay, what is their total organic carbon content, what are the types of organic material that they contain…? Based on these studies, we will characterize them according to the parameters used, worldwide, to qualify rock as appropriate for storing CO2. These international parameters vary from place to place, and that is one more challenge to overcome.”

One of the main characteristics is how porous it is: the rock must have space for CO2 to enter, and capacity to retain it for at least one thousand years. Another is the degree of water saturation in the pores of the rock, because wherever there is water the CO2 cannot enter. The composition of the minerals and of the organic material of the samples is also relevant, because if they are clay, and depending on the type of clay, the capacity of CO2 retention within the rock can be greater. “The more organic material the rock has, the greater its capacity to retain CO2, especially by adsorption.”

The geologist explains that, furthermore, it is necessary to check the geological conditions of the location of the rock. “It must be at least one thousand meters below the surface; we also need to know if the formation is thick enough (100 meters thick, or more); and the region in which the layer of rock is located must be checked as to its favorability, having now geological fractures, because CO2 can escape under those conditions. This work is done based on remote sensing techniques, for example, various types of satellite images, geological maps, and other available information.”

Moreover, in the turbidites of Santos, it is necessary to note the occurrence of sealing rock on top of the rock being studied – those that block the escape of the CO2. “There may be clay or basalt, for example, that is, impermeable rock.”

Finally, Tassinari emphasizes the importance of isotherms – tests performed on rock to determine its capacity for the adsorption of CO2. “That can be simulated for specific temperatures and pressures. If we know the depth at which the layer of rock is located, we can estimate the temperature and the pressure to which it is subjected and model those conditions on the isotherms, in order to determine the  adsorption capacity of the rock (in grams of CO2 per ton of rock).”

Shale – According to Tassinari, shale holds a large advantage over other rock, in terms of CO2 retention: they are rocks that retain gas, without need of a sealing rock, unless they are completely fractured. The other advantage is that shale rock contains shale gas. “It is possible to inject CO2 and remove the natural gas that is there. For each molecule of CO2 injected, at least two molecules of CH4 are extracted,” Tassinari explains. “In the case of wanting to build gas-powered thermoelectric plants, for example, it would be possible to think of a closed system: place the thermoelectric plant over a shale gas deposit, remove the gas from the shale rock for the plant and inject the CO2 emitted, where it would be a sustainable  process from the environmental standpoint, since it is a closed and localized system that is easy to monitor.”